Soil Specimen Preparation for Laboratory Testing

SOIL SPECIM EN PREPARATION FOR LABORATORY TESTING A symposium presented at the Seventy-eighth Annual Meeting AMERICAN SOCIETY FOR TESTING AND MATERIALS Montreal, Canada, 22-27 June 1975

ASTM SPECIAL TECHNICAL PUBLICATION 599 D. A. Sangrey, symposium co-chairman R. J. Mitchell, symposium co-chairman List Price $35.00 04-599000-38

,4N~L ~L~/~AMER~CAN SOCIETY FOR TESTING AND MATERIALS 1916 Race Street, Philadelphia, Pa. 19103

qi]|lY

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(~) BY AMERICAN SOCIETY FOR TESTING AND MATERIALS 1976 Library o f Congress Catalog Card Number; 76-704

NOTE The society is not responsible, as a body, for the statements and opinions advanced in this publication.

Printed in Bahimore, Md. June 1976

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Foreword The symposium on Soil Specimen Preparation for Laboratory Testing was presented at the Seventy-eighth Annual Meeting of the American Society for Testing and Materials held in Montreal, Canada, 22-27 June 1975. Committee D-18 on Soil and Rock for Engineering Purposes sponsored the symposium. D. A. Sangrey, Cornell University, and R. J. Mitchell, Queen's University of Kingston, presided as symposium cochairmen.

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Related ASTM Publications Performance Monitoring for Geotechnical Construction, STP 584 (1975), $14.00, 04-584000-38 Field Testing and Instrumentation of Rock, STP 554 (1974), $18.75, 04-554000-38 Analytical Methods Developed for Application to Lunar Sample Analysis, STP 539 (1973), $15.00, 04-539000-38

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A Note of Appreciation to Reviewers This publication is made possible by the authors and, also, the unheralded efforts of the reviewers. This body of technical experts whose dedication, sacrifice of time and effort, and collective wisdom in reviewing the papers must be acknowledged. The quality level of ASTM publications is a direct function of their respected opinions. On behalf of ASTM we acknowledge with appreciation their contribution.

A S T M Committee on Publications

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Editorial Staff Jane B. Wheeler, Managing Editor Helen M. Hoersch, Associate Editor Charlotte E. DeFranco, Senior Assistant Editor Ellen J. McGlinchey, Assistant Editor

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Contents Introduction Effect of Water Saturation History on the Strength of Low-Porosity Rocks--G. BALLIW, B. LADANYI,AND D. E. GILL Testing Equipment Rock Types Specimen Preparation Testing Procedures Experimental Results Conclusions

4 5 7 8 11 12 19

Four Factors Influencing Observed Rock Properties-P. G. CHAMBERLAIN,E. M. VAN EECKHOUT,AND E. R. PODNIEKS Discussion of Critical Factors Summary

21 22 34

Trimming Device for Obtaining Direct Shear Specimens from Samples of Stiff Fissured Clay Shale--G. N. DURHAM Residual Shear Test Procedures Waterways Experiment Station Residual Shear Testing WES Direct Shear Trimming Device Specimen Preparation Discussion

37 38 38 39 40 42

Effects of Specimen Type on the Residual Strength of Clays and Clay Shales--F. C. TOWNSENDAND P. A. GILBERT Previous Investigations Materials and Equipment Specimen Preparation Test Results and Analyses Conclusions

43 44 45 47 49 63

Effects of Storage and Extrusion on Sample Properties-ARA ARMANAND S. L. MCMANIS Literature Survey Sampling and Field Testing Laboratory Tests and Results Selection of Representative Specimens General Conclusions

66 67 68 69 80 85

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Transportation, Preparation, and Storage of Frozen Soil Samples for Laboratory Testing--T. H. W. BAKER Factors Affecting Laboratory Tests on Frozen Soils Frozen Soil Samples Machining and Preparation of Specimens for Testing Rough Cutting Methods Finishing Methods Storage and Protection During Laboratory Testing Conclusions

88 89 89 97 98 98 104 111

Temperature-Controlled Humid Storage Room-MICItAEL BOZOZUK Design Closed Flow Conditioning System Handling and Preparation of Samples for Storage Effect of Storage Time on Test Results Summary

113 115 119 122 122 125

Effect of Storage and Reconsolidation on the Properties of Champlain Clays--P. LA ROCHELLE, J. SARRAILH,AND F. A. TAVENAS Characteristics of the Cemented Clays Water Migration Following Sampling Influence of Reconsolidation Influence of Storage Time Conclusion

126 128 130 137 140 144

Pore Water Extraction and the Effect of Sample Storage on the Pore Water Chemistry of Leda Ciay--J. K. TORRANCE Soil Material Storage Procedures Pore Water Extraction Results and Discussions Conclusions and Recommendations

147 149 149 150 151 155

Variation in Atterberg Limits of Soils Due to Hydration History and Specimen PreparationmD. A. SANGREY,D. K. NOONAN, AND G. S. WEBB Test Program Conclusions

158 160 167

Effect of Specimen Preparation Method on Grain Arrangement and Compressibility in SandmARsHUD MAHMOOD,J. K. MITCHELL, AND ULF LINDBLOM

Soil Fabric One-Dimensional Compressibility

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169 170 171

Experimental Investigation Fabric Results Compression Test Results Conclusions

171 178 180 190

A Technique for the Preparation of Specimens of Loose Layered Silts--V. A. NACCIAND R. A. D'ANDR~A Soil Description Specimen Preparation Typical Testing Procedure and Result Conclusions

193 195 195 198 200

Shrinkage of Soil Specimens During Preparation for Porosimetry Tests--T. F. ZIMMIEAND L. J. ALMALEH Equipment Experimental Work Conclusions

202 204 211 214

Compaction and Preparation of Soil Specimens for Oedometer Testing--A. R. BOOTH Choice of Compaction Method Construction of Mold Method of Compaction Adjustment of the Degree of Saturation Comparison of Specimens Effect on Results Conclusions

216 217 218 219 221 223 224 225

Laboratory Preparation of Specimens for Simulating Field Moisture Conditions of Partially Saturated Soils--T. Y. CHH AND S. N. CHEN Review of Current Methods for Pretesting Treatment Development of Equipment and Procedures for Pretesting Treatment Test Results and Discussion General Conclusion

232 236 243

Scalping and Replacement Effects on the Compaction Characteristics of Earth-Rock MixturesmR. T. DONAGHE AND F. C. TOWNSEND Procedure Test Results and Discussion Conclusions

248 249 257 274

Study of Irregular Compaction Curves--P. Y. LEE Laboratory Investigation

278 281

229 230

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Discussion of Test Results Conclusions

Importance of Specimen Preparation in Microscopy-J. E. GILLOTT Microscopic Methods Specimen Preparation for Fabric Analysis Specimen Preparation for Analysis of Particle Size and Shape Ion Bombardment Replication, Shadowing, and Coating Discussion Conclusions

282 287

289 291 293 299 300 302 304 305

Use of Ultrasonic Energy for Disaggregation of Soil SamplesmA. I. JOHNSONAND R. P. MOSTON Ultrasonic Equipment Testing Methods Summary

308 308 311 312

Soil Drying by Microwave Oven--P. V. LADEAND H. NEJADI-BABADAI Heating with Microwaves Effects of Heating Clay Mineral Systems Preliminary Investigations Determination of Water Content Effects of Microwave Heating on Soil Characteristics Summary and Conclusions Discussion

320 321 322 323 324 330 333 335

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S T P 5 9 9 - E B/J u n. 1976

Introduction

A laboratory test run on an inappropriate specimen is often worse than no test at all. Certainly there are tests for which the preparation does not significantly change the measured soil property; but the far more common situation is to have a real or potential variation in the measured soil property as a result of alternative specimen preparation techniques. The objective of this symposium was to collect and exchange information on this problem. Hopefully this will lead to improvements in our specimen preparation methods or at least a better understanding of the influence of our preparation methods on final test results. The entire question of adverse effects on test results through specimen preparation needs to be examined in the context of the use being made of the test results. In some cases, accepted practice or a certain design method are based on a test result involving a particular specimen preparation technique. If newer, and clearly better, specimen preparation methods are proposed for this test, there will often be reluctance on the part of users to change, simply because they are accustomed to the older methods and have a strong empirical experience to account for the poorer specimen preparation. Another common situation is that there are some test parameters which can be used in different design methods. For some of these design methods, the specimen preparation is very important, while for others it is much less important. What general principles, if any, can be applied to the preparation of soil specimens for laboratory testing? In general, laboratory soil testing should be done on specimens as nearly identical to field deposits as possible. For natural soils, this means a minimum of disturbance, contamination, and alteration. For artificially prepared, Or reconstructed, soils, the objective is to duplicate the in situ structure and state of the soil, at least in those ways that would influence test results. An overall objective should be to define methods of specimen preparation and testing which achieve the smallest variability in the end result. Some methods are inherently less variable than others, and these will produce more accurate and more predictable end results. An objective of the ASTM symposium is to provide a forum for the exchange of information on a topic of interest or concern. The morning session of this symposium was separated into two major topic areas. The first group of papers was concerned with rock as an engineering material, with stress-strain and strength behavior being the main subjects. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 9 Downloaded/printed byby ASTM International www.astm.org Copyright 1976 University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

2

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

From the interest shown for this session it is clear that geotechnical engineers are becoming increasingly sensitive to the problems of rock engineering and the role of geotechnical engineering methods in solving rock mechanics problems. Storage, extrusion, and predrying effects were covered during the second part of the morning session. The overall conclusion to be drawn from this group of papers is that there exists a great potential for change in soil and rock specimens and their measured properties if samples are handled poorly and stored for long periods of time. Results from this group of papers have a direct bearing on present ASTM standard methods and, in some cases, clearly indicate a need for reconsideration of existing specifications. Three major topic areas were included in the afternoon session. Methods for preparing reconstructed loose cohesionless soil specimens in the laboratory were discussed in the first group of papers. These specimens were intended for studies of liquefaction potential and similar large deformation response. This very current subject was of particular interest to a large part of the symposium audience. Preparation of compacted soil specimens was a second topic area dealing with reconstructing soil specimens in the laboratory. Papers in this part of the symposium were primarily concerned with the problems of preparing a laboratory specimen which represented the field situation. As in the case of the second half of the morning session, there were some direct implications for present ASTM standard methods indicated in these papers. The final session of the symposium was appropriately concerned with recent techniques applied to laboratory preparation of soil specimens. All of the papers described new equipment or new techniques for preparing and testing soil specimens. None of the methods described are presently covered by ASTM standard methods, but it is reasonable to expect a need for standards in the near future if there is more widespread use of these new techniques. A number of present ASTM standard methods were included in the studies reported in this special technical publication. In several cases, the results of these research studies indicated a need to reconsider the present specification, or at least some of its details. Whether it is appropriate to change an existing standard method or add a method is an important decision which cannot be based on a single research study; however, users of specifications should be aware of potential problems even if the specification is not changed. The listing in Table 1 summarizes the papers included in this special technical publication and the ASTM standard methods to which they apply. Only the major associations are noted and there are numerous minor specification references which have not been included. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

INTRODUCTION

3

TABLE 1--ASTM standards and relevant papers. ASTM Designation D 421 D 422 D 423 D 698 D 1140 D 1557 D 1587

D 2049 D 2216 D 2217 D D D D

2664 2936 2938 3080

RelevantPapers from This Symposium

Sangrey, Noonan, and Webb Johnson and Moston Johnson and Moston Sangrey, Noonan, and Webb Chu and Chen Donaghe and Townsend Lee Johnson and Moston Chu and Chen Donaghe and Townsend Lee Arman and McManis Bozozuk LaRochelle, Sarrailh, Roy, and Tavenas Torrance Mahmood,Mitchell, and Lindblom Lade and Nejadi-Babadai Sangrey, Noonan, and Webb Johnson and Moston Ballivy, Ladanyi, and Gill Chamberlain,Van Eeckhout, and Podnieks Durham Townsend and Gilbert

This ASTM special technical publication contains a group o f symposium papers addressing a broad range of materials and testing methods. It is clearly shown that in most cases the methods of specimen preparation have a pronounced influence on the subsequent test results. In a few cases, the opposite conclusion is drawn, for example, in the paper by Townsend and Gilbert, but it is equally important to know about minor effects as it is major ones. Several o f the papers present results and conclusions which have a direct bearing on present ASTM standard methods. Collecting such information is a major reason for having a symposium and special technical publication supported by ASTM. The responsibility for critically reviewing these research studies and, where appropriate, making modifications to existing standards rests with the A S T M committee structure.

D. A . S a n g r e y Cornell University, Ithaca, N.Y.; symposium co-chairman.

R . J. M i t c h e l l Queen's University at Kingston, Ontario, Canada; symposium co-chairman.

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G. Ballivy, ~ B. Ladanyi, 2 a n d D. E. Gill 2

Effect of Water Saturation History on the Strength of Low-Porosity Rocks

REFERENCE: Ballivy, G., Ladanyi, B., and Gill, D. E., "Effect of Water Saturation History on the Strength of Low-Porosity Rocks," Soil Specimen Preparation for Laboratory Testing, ASTM STP 599, American Society for Testing and Materials, 1976, pp. 4--20. ABSTRACT: The purpose of the tests described in this paper was to investigate how the mechanical properties of rock observed in the tests are influenced by the whole saturation history of the specimen prior to testing. Three aspects of the saturation history were studied in this paper: the effect of drying and resaturating the specimen prior to testing, the effect of resaturation method, and the effect of the chemical nature of the resaturating fluid. Three rock types were used in the tests: a gneiss, a cemented sandstone, and a fine grained limestone. All three rocks had apparent porosities below 2 percent. Results of triaxial and splitting tests are reported in the paper. One series of specimens was brought from the site in its natural saturated state and tested without drying while the others were either air or oven dried and then resaturated prior to testing. The resaturation was performed either by immersing the specimen in water under vacuum, or by injecting the saturation fluid, under pressure, through a thin channel drilled along the specimen axis. Either distilled or seawater were used as the resaturating fluid. The results show that the inclusion of a drying and wetting cycle prior to testing has a clear overconsolidation effect on the rock behavior, that is, it increases its apparent strength. On the other hand, the channel saturation technique gives a better saturation of the specimen and results in a strength decrease. Finally, the results show that the chemical composition of the saturation fluid has also a significant effect on the measured rock strength. The practical conclusion to be drawn from this study is that representative rock samples, taken in connection with a given project, should, from the moment of coring until they are tested, be held under environmental conditions that are as close as possible to those which will prevail after the completion of the project. This implies that no drying and wetting cycles should be included if they are not expected to occur in practice. If this condition cannot be met, specimens should be saturated using a natural saturation fluid and using an efficient saturation technique such as the described axial channel saturation method. KEY WORDS: soils, rock mechanics, rock sampling, splitting tests, triaxial tests, saturation methods, pore pressure ~Geotechnical engineer, Lalonde, Girouard, Letendre and Associates, Montreal, P.Q., Canada. ZProfessor and associate professor, respectively, Department of Mineral Engineering, Ecol6 Polytechnique, Montreal, P.Q. Canada. 4 Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 9 Downloaded/printed byby ASTM International www.astm.org Copyright 1976 University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BALLIVY ET AL ON LOW-POROSITY ROCKS

5

It is well known [1-5] 3 that the strength of rock depends, in large measure, on the degree of saturation of specimens at the monent of testing. In fact, it has been stated, in ASTM Test for Triaxial Compressive Strength of Undrained Rock Core Specimens Without Pore Pressure (D 2664-67) "that the field moisture condition of the specimen should be preserved until time of test . . . or should be tailored to the problem at hand." The wetting of specimens prior to testing has a strength reduction effect which is generally very large for all kinds of rocks. This effect results essentially from a reduction of the free surface energy of the material [3]. This phenomenon is well known, but its physical and thermodynamic aspects have not been completely clarified to date [6]. The intent of the present study is to illustrate more specifically the effect of water saturation history on the strength of low apparent porosity rocks. Three types of rocks have been tested, namely, a cemented cambrian sandstone, a lithographic ordovician limestone, and a charnokite (archaean granito-gneiss). As it is not usual to preserve the field moisture conditions of the specimen until laboratory testing, this paper also examines the effects of various resaturation processes on rock strength. Study of these processes included development of new resaturation equipment, special specimen preparation, and the use of two different pore fluids.

Testing Equipment The rock cutting was performed with a circular watercooled diamond saw blade. Whenever required, the ends of the specimens were ground flat on a lathe. The specimens were weighed on electronic balances, and calipers were used to measure their final dimensions. The oven used for drying the specimens was built in such a way that the air, heated to 40.5~ as it entered the oven, was forced to circulate throughout it; the total volume of air in the oven was renewed every minute. Resaturation by fluid injection was done with the apparatus shown schematically in Fig. 1. It consists essentially of a pressure vessel (A), through the cover of which eight specially prepared specimens can be connected to eight tubes; these tubes are all connected to a second pressure vessel (B) which acts as a saturation fluid reservoir. A nitrogen gas bottle (C) pressurizes, through a regulator, the saturation fluid contained in the vessel (B); this pressurized fluid is injected, by means of the tubes, through a channel drilled along the axis of each of the specimens. A second nitrogen gas bottle (D) applies a pressure, also 3 The italic numbers in brackets refer to the list o f references appended to this paper.

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6

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. l--Apparatus for saturation of rock specimens by radial divergentflow from a central channel; (a) view of the apparatus; (b) scheme of the saturation system. through a regulator, on the saturation fluid contained in the vessel (A), thus providing a constant fluid pressure at the outside surface of the specimens. The saturation fluid pressure gradient results from the difference between the pressures produced by the nitrogen gas bottles C and D. This gradient produces radial divergent flow within the specimens. The apparatus was constructed in such a way that all steel surfaces coming into contact with the saturation fluid were lined with plastic material. For the tests results reported in this paper, the pressure in C was 300 Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BALLIVY ET AL ON LOW-POROSITY ROCKS

7

psi (2.07 MPa), and in D, 50 psi (0.34 MPa). The photograph in Fig. 1 shows the apparatus just described. Both a standard testing machine and, more often, a programmable universal testing machine (Tinius Olsen) were used for loading the specimens to failure. Diametral splitting tests were conducted between rigid plattens. Triaxial tests were performed in a modified Hock cell, in which the confining pressure was supplied by a pump (Structural Behavior Engineering Laboratories, Model 100 LP + 100 P). To test the specimens which had been resatured by fluid injection, the plattens were modified as shown in Fig. 2b. As far as the pore pressure is concerned, a nitrogen gas bottle, combined with a pore fluid reservoir, was used whenever the back pressure to be maintained during testing was less than 500 psi (3.45 MPa); otherwise, the back pressure was provided by the pump. Figure 2a is a photograph of the Hoek cell with modified plattens. Specimen deformations were measured with electromechanical extensometers, in which sensors were linear potentiometers. The longitudinal deformation of the triaxially tested specimens was measured outside the cell; the signal output by the measuring devices was recorded against the load applied by the testing machine on a standard X - Y recorder. In the case of the splitting tests, the changes in both the vertical and the horizontal diameter were measured and recorded, as for the triaxial tests.

Rock Types The tests were performed on three types of rocks from various locations in the province of Quebec.

Cambrian Cemented Sandstone (Potsdam Group) It is predominantly a white to off-white orthoquartzite with dolomitic cement; the apparent porosity of this bed is less than 2 percent.

Ordovician Sublithographic Limestone (Trenton Group, Tdtreauville Formation) It consists of beds of dense bluish-black limestone up to 6 in. in thickness, separated by shale partings; it has a lithographic stone appearance [7]. The total porosity of this rock is low (1.2 to 1.9 percent) [8], and its apparent porosity is less than 1 percent.

Charnokite This is an Archaean granito-gneiss from the Quebec City area. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

8

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 2--Hoek's cell with modified plattens; (a) view of the cell and the plattens; (b) modified bottom platten.

Specimen Preparation General

All the specimens were prepared from NX (diameter: 2~ in. or 5.38 cm) core samples. The present study involved two types of samples which have been cored below the water table. Saturated Samples--Samples selected were kept immersed in water at the drilling site and delivered to the laboratory, where they were submitted to various procedures, including drying and resaturation. Air-Dried Samples--In the other cases, the samples selected at the drilling site were kept under ambient conditions and delivered to the laboratory where they were submitted to various procedures. In all cases, the core specimens were cut to the desired lengths a short time after delivery, and the effect of wetting caused by the cooling water during the cutting was considered insignificant as far as mechanical properties are concerned. The control of specimen saturation was made by a periodical weighing. Usually, a specimen was considered to have reached a given saturation degree when periodical weighings showed constant weight for at least three consecutive days. Specimens f o r Tensile Splitting (diametral compression) Tests

As a rule, specimen disks submitted to splitting tests were about 90 of Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BALLIVY ET AL ON LOW-POROSITY ROCKS

9

an in. (1.90 cm) thick. The following groups of specimens were prepared from the two previously described types of samples (see Table 1). TABLE l--Groups of specimens prepared by different methods

and number of tests in each group.

Group

Tension Splitting Tests

Triaxial Compression Tests

Air-Dried Specimens A AO AS AOS AOSC AOC

42 9 19 17

50 97 80 8

Saturated Specimens S SO SOS

57 23 42

From saturated samples (S): Group S--The specimens of this group, prepared from saturated samples, were obtained by cutting specimens of core into disks which had been temporarily removed from their water bath. The specimens were then measured, weighed, and tested. Group SO--The specimens of this group were the same as in group S, except that they were oven dried before testing. Group SOS--This group is the same as group S, except that the specimens, after being measured, were oven dried and resaturated by immersion before testing. From air-dried samples (A): Group A--The specimens of core, selected from air-dried sample lots, were cut into disks. The specimens were measured, weighed, and tested. Group AO--The specimens of this group were prepared as were those in group A, except that they were oven dried before testing. Group AS--These specimens were prepared as those in group A, except that they were resaturated by immersion prior to testing. Group AOS--The specimens in this group were prepared as were those of group A, except that they were oven dried and subsequently resaturated by immersion prior to testing.

Specimensfor Triaxial Compression Tests Specimens submitted to triaxial compression tests were about 4 88 long (10.80 cm) cylinders. They were all prepared from the samples of

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10

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

Type A, that is, the air-dried samples, and they fall into one of the following groups: Group AO--This group is the same as AO, described for specimens for tensile splitting. Group AOS--This group is the same as AOS, described for specimens for tensile splitting. Group AOSC--The specimens in this group were prepared as were those in the splitting tests, except that a 89 hole was drilled along each of their axes for about 80 percent of their length [11]. No cooling fluid was used during this operation. Figure 3 shows a specimen into which a hole has been drilled, as

FIG. 3--Specimens with central channel; (a) radially saturated sandstone specimens with cut fitting, ready for testing; (b) section of a specimen with complete fitting.

described previously. This figure shows also the brass fitting that was cemented to the collar of the channel, in order to enable the specimen to be mounted on the resaturation apparatus described previously. This fitting covers the hole wall for a length equal to about 30 percent the specimen length, leaving an unlined cylindrical channel in the central portion of the specimen; the length of this cavity is then about 60 percent of that of the rock specimen. Figure 3a is a photograph of such a specimen. Note that the threaded part of the fitting was cut away before testing. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BALLIVY ET AL ON LOW-POROSITY ROCKS

11

Group AOC--The specimens in this group were as in group AOSC, except that they were not submitted to resaturation, although they were provided with a central channel. A review o f the groups and the tests made in each group is presented in Table 1.

Testing Procedures Splitting Tests Specimens were mounted on the testing machine in such a way that the loading could be performed along two diametrically opposite lines on the lateral surface o f the disks. The electromechanical extensometers were then mounted and set to zero. The loading proceeded at such a speed that the minor principal stress increased at a rate o f 100 psi/s. Tensile strengths were calculated from the usual formula 2P To

-

nDL

(1)

where

To = tensile strength, P = maximum load applied, D = diameter of the specimen, and L = thickness o f the specimen. Only the tests in which failure started at the center o f the cross section o f the specimen, and in which failure plane coincided with the loaded diametrical plane, were considered to be valid.

Triaxial Compression Tests Jacketed specimens were mounted in the triaxial cell with proper spherically seated plattens. The cell was placed subsequently into the testing machine, and the extensometer was installed. The loads were applied at such a speed t h a t the major principal stress within the specimens increased at a rate of 100 psi/s. Prior to the test, the confining pressure was applied to the specimen by increasing simultaneously the axial and the cell pressure. Whenever required, back pressure was raised to the desired level (200 psi = 1.38 MPa with limestone specimens, 300 psi = 2.07 M P a with charnokite specimens, and up to 5000 psi = 34.5 MPa with sandstone specimens). Pore pressure was applied from the bottom plattens, and the testing was started only when the same pore pressure could be read at the top plattens. Similar procedures have been reported in the literature already [8]. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

12

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

The extensometer was then set to zero, and the specimens were brought to failure by increasing the axial stress.

Experimental Results Check o f Pore Pressure Distribution in Triaxial Specimens Before starting the study of the effect of specimen preparation history on its strength, a check of the system of pore pressure application in the triaxial cell was made by a series of triaxiai tests on cemented Potsdam sandstone. All the specimens in these tests were of the AOSC group, that is, they had a central hole and were resaturated by immersion after being air and oven dried. The triaxial tests were conducted according to the Heck's procedure [9], but the pore pressure u was kept constant during the tests. Such a system, if working well, that is, if resulting in a uniform distribution of the applied pore pressure (back pressure) throughout the specimen at failure, should result in essentially drained test conditions. According to the concept of effective stress, failure strengths of such a series of tests, plotted against the effective confining pressure, should fall on a single failure line [10]. Figure 4 shows that, with the usual scatter of results, this assumption was found to be valid in the tests with the Potsdam sandstone. These results illustrate that the pore pressure application system used in the tests was quite effective, even for rocks of such a low porosity.

Effect o f Specimen Preparation History on the Results o f Tension Splitting Tests Figures 5 and 6 show the results of a large series of tension splitting tests carried out on specimens of Trenton limestone, prepared according to various procedures. The results show clearly that tensile strength is affected very much by the specimen preparation history. In fact, three different groups of results can be seen in Figs. 5 and 6. The lowest strengths were found for specimens saturated without over drying (groups S and AS), the highest, for those that were air and oven dried (group AO), while the strengths of those tested air or oven dry (A and SO), as well as those tested after having been saturated following air or oven drying (AOS and SOS), were located between the two extremes. These results lead to the conclusion that an oven drying, or a severe air drying, produces a clear overconsolidation effect on the strength of rock, and that the effect is irreversible, that is, it cannot be eliminated by subsequent resaturation of specimens. On the other hand, Fig. 5 shows that

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BALLIVY ET AL ON LOW-POROSITY ROCKS

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4O0

/

I-z w

40.

~35~( LEGEND:

t,~176 ,o] , L,o

Test with opplied e pressure u= 8 0 0 psi Specimen:Cemented sondstone~ group AOSC

20 3

MPo ksi

EFFECTIVE CONFINING PRESSURE O''5

FIG. 4--Results of triaxial compression tests with Potsdam sandstone.

the overconsolidation effect seems to affect much less the modulus of elasticity of rock, because the peak strength points, shown in Fig. 5, are distributed around a mean straight line without any systematic trend.

Effect of Specimen Preparation History on the Results of Triaxial Tests Figure 7 shows, in terms of principal stresses at failure, a summary of all triaxial test results obtained with specimens of Trenton limestone and charnokite prepared by various described methods. In addition, the median failure line for AOSC specimens of Potsdam sandstone is shown also in the figure for comparison. The tests with Trenton limestone were made at different confining pressures varying from 500 to 5000 psi, while those with charnokite were all at an effective confining pressure of 1000 psi (6.9 MPa). Some results for the latter are summarized also in Table 2, together with the corresponding water contents after immersion in the distilled and the seawater, respectively. The test results lead to a number of interesting conclusions concerning Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

14

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

psi

MPa

1500

/

I0

/

A0 bJ Ico I000,

'AOS,

-6 w

I

-4

/

500-

-2

/ 0

/

LEGEND

IM P a [ ~ e d l o n

/

vol~e of To ond F~v

0.5% 19 ~ ' ~ - s t a n d o r d

deviotion

number of tests

0'5

Ii0 VERTICAL

STRAIN

1',5 210 8v = A.~I

,

25

, II, per cent

I

5--Effect of mode o f saturation on splitting tensile strength and failure strain of Trenton limestone. For the definition o f symbols, see Table 1. FIG.

the effect of preparation history on the failure behavior of rocks under triaxial test conditions. The results show clearly that: 1. The strength of dry specimens is from 20 to 30 percent higher than that of comparable specimens when tested saturated, at any confining pressure. 2. The presence of a central hole in the specimen improves and accelerates considerably the resaturation of the specimen. This is seen clearly in the results obtained for the charnokite. This method, however, could not have been applied to the specimens of Trenton limestone, which had a tendency to fracture along bedding planes during pressure saturation. The limestone, therefore, was soaked in water, and a back pressure of 200 psi was applied through the central channel. This saturation method was clearly less effective, which explains the much smaller difference between Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BALLIVY

Initml conditions:

ET A L O N

S

or

LOW-POROSITY

A -~...._....~

ROCKS

15

A

@l To. (29Opsi) ,,o~s, S

,

TO= 740psi

@

~e,ts

AS

/ on saturated /4-- samples ~

(160psi)

TO = 780psi

~

(260psi)

SO

AO ~

tests on oven dried samples

TO = 1150psi

(;500 psi)

ID @

TO= 1290psi (~90psi)

AOS

SOS 4---

TO= 1090psi

L

tests on resaturated samples

(280psi)

LEGEND=

5~

9

(150psi)

(160psi) =standard

number of tests

deviation

To = median tensile strength

Ips== 6.9 kPo

FIG. 6--Effect of drying and saturation history on tensile splitting strength To of Trenton limestone. For the definition of symbols, see Table 1. /,

psi

-~-

%,o / 9 AOS(sea

I-

=-

water)

/

50

-300

AOSC j POTSDAMSANDSTONE

/

MPa - 400

~z 9 A/0SC (sea water)

(~

. . Q.~

3o-L2oo/ I" ~ o _ ~ ~

~''

t,,-

"EOENO

~oo

,

J l J

/ O

-

-

~T~,~"5 =median effective stresses ~. R,H.=relotive humidity 5 ~ number of tests

ioJ

L/~30(016~1 :median water content=0.30% ~ standard devJation=O.16%

,o

~ 2

5

4

~

5

,-~-=~ ksi

EFFECTIVE CONFINING PRESSURE 0"5

FIG. 7--Effect of mode of saturation on triaxial compression strengths of Trenton limestone and charnokite. For the definition of symbols, see Table 1.

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

24

13

16

AOS

AOSC

Oven-dried specimens 53 800 (12 000)

43 100 (7 600) 35 700 (8 300)

ol ult psi

s

3.17 (0.6)

2.9 (0.4) 2.7 (0.5)

070

0.22 (0.9) 1.11 (0.5)

W , 070

14

24

n

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

NOTE--n = number o f tests. (7 600 psi) = standard deviation. 1 psi = 6.9 kPa. Period of saturation: 8 to 42 days. ol ult = median ultimate axial strength. o3 = 1 000 psi for all the tests. Eult = median peak deformation. W, % = water content in percent.

n

Mode of Saturation

Distilled Water

50 500 (7 100) 43 800 (5 900)

s

~0

3.1 (0.4) 2.8 (0.3)

Seawater Ol ult, psi

Fluid of Saturation

strength of charnokite.

0.28 (0.11) 0.97 (0.37)

W , 070

TABLE 2--Influence of mode of saturation and composition of pore fluid on the triaxial compression

z

m (D

O .<

-4

-n

O

2O r-

O

Z 11

-4

"U 1'I m

z

m

m

t-

_o

..L

BALLIVY ET AL ON LOW-POROSITY ROCKS

17

AOS and AOSC lines for Trenton limestone. The difference increases slightly, however, with increasing confining pressure, which is due probably to an increase in the degree of saturation because of increasing consolidation of the rock under confining pressure. 3. For an intermediate saturation in a 75 percent relative humidity atmosphere [12], the strengths are located, as expected, between those of dry and completely saturated specimens. However, for the reason outlined previously, there is a tendency for consolidation under a high confining pressure to increase the degree of saturation; this leads to an apparent decrease of strength at higher pressures.

Effect o f Type o f Pore Fluid on the Results o f Triaxial Tests As shown by Colback and Wiid [2], the type of pore fluid in the rock may have considerable effect on strength. This phenomenon is thought to be due to a combination of thermodynamic and mechanical effects. In fact, Colback and Wiid have shown that the type of fluid has an effect on the free surface energy in rocks. On the other hand, there is no doubt that the fluid viscosity plays also an important role in determining the level of pore pressures at failure, as shown by Brace and Martin [13]. The results of triaxial tests with specimens of charnokite soaked in distilled and seawater (with 3.4 percent salt content), respectively, are shown in Fig. 7 and summarized also in Table 2. It is seen clearly that the seawater has the effect of increasing apparent strength of this rock by about 20 percent, no matter which method of saturation is used. However, the saturation by radial flow remains still much more effective than by simple immersion, leading to about five times higher water contents and to a corresponding decrease of strength of the order of 20 percent.

Effect o f the Time o f Saturation on the Water Content Figure 8 shows a plot of water contents achieved after different periods of continuous saturation of charnokite specimens, using two different methods. The results indicate that simple immersion of specimens leads in a short period to an upper level of saturation, which remains practically constant with time, even after six weeks of immersion. On the other hand, saturation by radial flow from a central channel shows a continuous increase in water content for a long period of time, attaining a maximum only after about three weeks. Since this maximum water content is about five times higher than that after simple immersion, these results indicate clearly the importance of using a proper saturation method if one wants to obtain a true saturated strength of rock. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

j

J

I

TIME OF SATURATION

loo Isl

21 doys

. . . . . . . . .

[o.tT(e)

42doys

- -

-

1

t"

I

~ SEA WATER

WATER

( 8 ) : 8 TESTS

SEA

AOS~ ~ D,ST,LLEDWATE~

AOSC

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

FIG. 8--Variation o f water content with time o f saturation in charnokite specimens, using two different saturation techniques: A O S = air dried, oven dried, resaturated; A O S C = air dried, oven dried, saturated through a central hole.

0

'~176

2%

water content

8 days

(~ DISTILLEDWATER

LEGEND :

--L

if)

m .-~

-n -<

0

0

ffJ

z "11 02D

5

> -t

m z 1) :TJ m -o ),

o_

(I) "o m

r

oo 0

Go

BALLIVY ET AL ON LOW-POROSITY ROCKS

19

Conclusions This experimental investigation shows that the specimen preparation history has a great influence on the strength of ordinary rocks o f low porosity. Three factors affecting strength have been identified: the oven drying, the final water content, and the chemical composition of pore water. In particular, oven drying leads to an irreversible overconsolidation effect, which may be due to a modification in the structure of hydrosilicates, resulting in petrification o f bonds. On the other hand, an ordinary air drying at 60 to 70~ and a relative humidity o f less than 45 percent has a much smaller overconsolidation effect, as found in the tension splitting tests. This means that, for ordinary rock testing purposes, air drying with subsequent resaturation may be acceptable, but oven drying should be avoided. The water content o f rock at failure was found to be an important factor in determining its strength. Since the water content attained by radial flow and back pressure through a central channel can be up to five times higher than that after a simple immersion of the specimen, it is not sufficient to indicate only that the specimen was saturated during the test; its water content at failure should also be identified. In general, it is clear that the results closest to reality will be obtained if the specimens are tested at their natural, or maximum expected, water content, without being subjected to drying prior to testing. Finally, these results show that even small amounts o f chemicals in the pore water, such as the salts in the seawater, may affect the strength o f rock considerably. This result indicates that using distilled water for rock saturation is not always the best method for finding its expected strength. Acknowledgments

The experimental work described in this paper was carried out at the Rock Mechanics Laboratory o f Ecole Polytechnique, Montreal, and was supported by the National Research Council of Canada Grants 1801 and 3412.

References [1] Robinson, L. H., Quarterly of the Colorado School of Mines, Vol. 54, No. 3, July 1959, pp. 177-200. [2] Colback, P. S. B. and Wiid, B. L. in Proceedings, Third Canadian Symposium Rock Mechanics, Toronto, 1965, pp. 65-83. [3] Wild, B. L., "The Influenceof Moisture upon the Strength Behaviourof Rock," Ph.D. thesis, Universityof the Witwatersrand, Johannesburg, South Africa, 1967. [4] Morlier, P., "R61e des Fluides en M~canique des Roches," Proceedings of the 2nd Congress of the International Societyof Rock Mechanics, Belgrade, 1970. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

20

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

[5] Mellor, M., "Strength and Deformabifity of Rocks at Low Temperatures," Research Report 294, U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, N.H., 1971. [6] Chenevert, M. E., in Rock Mechanics-Theory and Practice, W. H. Somerton, Ed., Society of Mining Engineers of The American Institute of Mining, Metallurgical, and Petroleum Engineers, New York, 1970, pp. 599-627. [7] Clark, T. H., "Montreal Area," Geological Report 152, Minist~re des Richesses Naturelles, Qu6bec, 1972. [8] Durand, M., "Etude de propri~t~s Physiques et Chimiques de Calcaires de la R6gion de Montreal," Thb,se M.Sc.A., Ecole Polytechnique de Montr~.al, Montr6al, Qu6bec, 1969. [9] Heck, W. J. in Basic and Applied Rock Mechanics, K. E. Gray, Ed., Society of Mining Engineers of The American Institute of Mining, Metallurgical, and Petroleum Engineers, New York, 1972, pp. 243-266. [16] Robinson, L. H., on Quarterly of the Colorado School of Mines, Vol. 54, No. 3, July 1959, pp. 177-200. [11] Bernaix, J. Etude G~otechnique de la Roche de Malpasset, Dunod, Paris, 1967. [12] Dunn, J. R. and Hudec, P. P., "The Influence of Clays on Water and Ice in Rock Pores," Part II, Physical Research Report RR 65-5, Department of Public Works, State of New York, 1965. [13] Brace, W. F. and Martin, R. J., III, International Journal of Rock Mechanics and Mineral Science, Vol. 5, No. 5, Sept. 1968, pp. 415-426.

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

P. G. Chamberlain, ~ E. M. Van Eeckhout, 2 and E. R. Podnieks'

Four Factors Influencing Observed Rock Properties

REFERENCE: Chamberlain, P. G., Van Eeckhout, E. M., and Podnieks, E. R., "Four Factors Influencing Observed Rock Properties," Soil Specimen Preparation for Labora-

tory Testing, ASTM STP 599, American Society for Testing and Materials, 1976, pp. 21-36. ABSTRACT: This paper presents a review of the effects of four factors in specimen

preparation that influence mechanical property tests--moisture content, orientation with respect to rock fabric, fractures, and specimen tolerances--and discusses methods developed during various Bureau of Mines testing programs for controlling these factors. Bureau and other research is cited to present problems in obtaining suitable property data arising from these factors and to define solutions to the problems. Rock mechanics researchers dealing with field sampling and laboratory testing should find the material helpful in establishing specimen handling and preparation procedures for obtaining relevant test results. KEY WORDS: soils, rocks, rock properties, specimen preparation, laboratory tests,

moisture content

The U. S. Bureau o f Mines has measured numerous rock properties for a variety of field projects over the years. Many factors influencing mechanical p r o p e r t y testing h a v e been i d e n t i f i e d a n d investigated. This p a p e r discusses f o u r critical f a c t o r s - - m o i s t u r e c o n t e n t , o r i e n t a t i o n with respect to r o c k f a b r i c , f r a c t u r e occurrence, a n d s p e c i m e n t o l e r a n c e s - - a n d presents m e t h o d s for alleviating p r o b l e m s arising f r o m these f a c t o r s . O b v i o u s l y , these a r e n o t the o n l y critical f a c t o r s influencing r o c k p r o perties; t h e r e m a y be several features o f a n y p a r t i c u l a r field site which exert g r e a t c o n t r o l over the b e h a v i o r o f r o c k at t h e site, yet which c a n n o t be simu l a t e d in a l a b o r a t o r y testing p r o g r a m . T h e f o u r factors p r e s e n t e d are, h o w ever, b o t h significant a n d c o m m o n . A l l can be c o n t r o l l e d t h r o u g h p r o p e r I Geophysicist and supervisory mechanical engineer, respectively, Twin Cities Mining Research Center, Bureau of Mines, U. S. Department of the Interior, Minneapolis, Minn. 55111. 2Research engineer, Mining Research Division, Continental Oil Co., Ponca City, Okla. (formerly Geophysicist, Twin Cities Mining Research Center, Bureau of Mines, U. S. Department of Interior, Minneapolis, Minn. 55111). 21

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by by ASTM International www.astm.org Copyright9 1976 University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

22

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

specimen handling procedures. The order of factor presentation does not imply relative importance since, in different rocks, under given circumstances, the ranking of factor importance would change. Discussion of Critical Factors M o i s t u r e Con ten t

There is little doubt that moisture content affects the mechanical properties of various rock types, sedimentary rocks in particular [1-5] .3 This effect is usually a lowering of strength with increased moisture, as well as an increase in mechanical compliances in certain rocks. For example, Table 1 shows how T A B L E 1--Strength and Young "s modulus o f shale as a function o f relative humidity.

Source Beatrice mine

Matthews mine

A R M C O mine

Relative Humidity, ~ 0 48 100 0 48 100 0 48 100

Relative Strength a

Relative Young's Modulus ~

1.0 0.52 0.31 1.0 0.68 0.32 1.0 0.83 0.48

1.0 0.72 0.39 1.0 0.86 0.41 1.0 0.93 0.90

"All specimens compressed paraUd to bedding; values represent average o f two to six tests.

strength and Young's modulus values for coal mine shales varied after stabilization at three humidities [3]. Further examples are shown in Figs. 1,2, and 3. It follows that, if meaningful data are to be obtained in the laboratory, samples should not lose or gain water in transit from the field. Care must be taken also in the laboratory to prevent changes in moisture content during storage and specimen preparation. Many methods are available for stabilizing water content in samples transported from the field to the laboratory. The methods range from waxing the sample to enclosing it in airtight steel drums [6] or dipping in cellulose acetate butyrite [7]. The Bureau of Mines has encased some large coal samples in concrete, but the shrinkage characteristics of certain cements make this potentially harmful for structurally weak specimens. Although plastic bags are the easiest sealant to use, they are difficult to seal and tend to rip easily. During a recent coal sampling trip to Illinois, representative samples 3 The italic numbers in brackets refer to the list o f references appended to this paper.

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

CHAMBERLAIN ET AL ON OBSERVED ROCK PROPERTIES

9,000 - -

1

I

i

I-

23

-

~ Dried 8,000 Sandstone

7,000

9~

c~ E

6,000

z 5,000

% ~ - ' ~ = ~ ' ~ Sample 1005/31

4,000 w ~E

o 5,000

~

Submerged in woter

idity Sample 1 0 0 5 / 7

Submerged in water

50 pct relative [ humidity as zero datum

2,000

1,0OO

0 -2.0

I O

I 2.0

I 4.0

I 6.0

8.0

MOISTURE CONTENT, wf-pct FIG.

l--Effect of moisture content on compressive strength [l ].

of coal wrapped in plastic bags lost 1 percent in weight during three days transportation to Minnesota. This weight was regained upon placing the coal in a 100 percent relative humidity chamber. Such weight changes vary, of course, with initial moisture content and climatic conditions during transportation. If convenient, it is best to coat samples with wax or other vapor sealant. Once the samples have been transported to the laboratory, the question of storage until specimen preparation arises. If there is a good vapor sealant on the samples, there is no problem unless the sealant cracks. Samples without a proper sealant should be stored at a humidity which maintains the as-collected moisture content in the rock, as evidenced by weight measurements. The samples can be stored in an enclosure where the air above a saturated salt solution is stabilized at the desired humidity (see Winston and Bates [8] for a list of possible chemicals) or in an enclosure containing the proper mixture of wet and dry air [9]. A problem arises if the equilibrium humidity for maintaining the original moisture content is not known for a given rock. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

24

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

20,000

I

I

I

+ 0 ~. 0

I 5,000

I

Illinois Illinois Illinois Illinois

I

No. 6cool No.6 cool No, Scool No. 6cool

I

I

(Inland

Mine)

(Orient

M i n e No. 4)

I

( S a h a r a M i n e No. 20) (Old Ben M i n e )

x Illinois No. 6cool (Montery Coal Co.)

"7. 0. ~E

Z W CE I0,000

§

x

x

0. oo

5,000

I I0

0

I 20

I 30

I 40

I 50

I 60

I 70

I 80

I 90

I O0

RELATIVE HUMIDITY, pct

FIG. 2--Effect of humidity on compressive strength of coal [4]. LATERAL STRAIN, / = i n / i n 1,000 1,500 I 1

5 O0 !

0 5,000 Dry lateral

Wet lateral

Dry axial

a

2,000 I

2,500

Wet axial ~A

,J

4,000

~" 3,000

/," S" / ; / ' ..::.-' . / y . ..,s;-' ,/I J ' ..,:.S,-"

2,000

/,'/s/

~anderasa._d,,on.

I,OCO ~ 0

"S~ 1,000

I 2,000

I 3,000

I 4,000

5,000

AXIAL STRAIN, /=in/in

FIG. 3--Effect of moisture content on deformation of sandstone [5]. Equilibrium humidity can be determined by placing samples in a variety of dessicator-stabilized humidities and monitoring the weight changes. For example, see Table 2 for the weight changes an Illinois coal gave after one Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

CHAMBERLAIN ET AL ON OBSERVED ROCK PROPERTIES

25

TABLE 2--Moisture content changes in coal as a function o f humidity environment. Relative Humidity, 070

Specimen Weight Loss, ~

100 78 48 12 0

0.0 1.2 5.0 6.3 6.8

month at the indicated humidities. This particular coal needed to be stored at nearly 100 percent humidity to maintain moisture content stability, and a plastic enclosure was built subsequently for this purpose. Relative humidities of less than 100 percent will be required to maintain original moisture content for some shales and other rocks which can absorb water in higher humidity environments [3, 4]. During specimen preparation in the laboratory, water is used most often as a coolant and flushing agent for the drilling and cutting operations. Although contact between rock and water is generally brief, rock types very sensitive to water should be cut dry or with a substitute liquid. After preparing specimens from the collected samples and before testing, specimens again need to be stored at equilibrium humidity. Dessicators stabilized at the proper humidity as described earlier are ideal for this purpose. Testing at a room humidity other than the stabilization humidity will have little or no effect on test results if the test is of short duration (less than 30 min).

Orientation Although it has been well established that mechanical properties of rock vary with specimen orientation [10-12], many engineers still do not realize the importance of preparing test specimens properly oriented with respect to geologic structure to obtain valid test results. Rock property anisotropy was demonstrated in one study [13] which showed that compressive strength of granites varied approximately 20 percent, while Young's modulus varied up to 100 percent with direction (Fig. 4). Another study [3] showed that Young's modulus of coal mine shale was 30 to 80 percent higher whenspecimens were loaded parallel to the bedding rather than perpendicular to it. Tensile strength results varied several hundred percent with direction for shales and other layered sedimentary rocks [14] (Fig. 5). Indirect tensile strength of coal varied threefold with different orientations in the horizontal plane [15]. Shear strength of a schistose gneiss, as determined from confining pressure tests (triaxial compression tests), varied significantly as a function of direction [16] (Fig. 6). Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

26

SOIL SPECIMEN

800

/

PREPARATION

I

7oaf ~.6OO I'/

FOR LABORATORY

I

e

J. Hordwoy / / " /-/ / /

Stonsteod granite - - -

,//

,oo

//~

~,oo

I1"

~-i.iL-_-4

i

J. Groin.*'~ ~" j~'/

.// /C-~

.L Rift~.~ /// . // //

./I""

.-"

-/

500

..

/

_

."

/'//

I I I ,OOO 1,5OO AXIAL STRAIN, Fin/in

.....

O

TESTING

i 2,000

2,500

F I G . 4--A nisotropic deformationol characteristics of granite [13].

2,000

[

I

I

I

I

I

I

Idaho pS p r i n q s gneiss r I

Green River Pshole (rich)

1,500

Green River shale (leon) Z 1,000

Sandstone

500

Blockhowk

Z tAi p-

.____._x/-'x

i

,.

I

15

"X-

I

I

I

I

I

30 45 60 75 90 INCLINATION OF LAYERING TO ~3' de?

I

105

FIG. 5--Effect o f orientation on tensile strength [14].

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

120

CHAMBERLAIN ET AL ON OBSERVED ROCK PROPERTIES

27

o"3

T 15

J

f

I

l

J

t

i

i

I

-,r iw i1c

oc

O

30 60 /~'-SCHISTOSITY ANGLE, (:leg

90

FIG. 6--Effect of orientation and confining pressure on shear strength [16]. Consequently, techniques for orienting specimens are very important to ensure that measured properties are relevant. Orientation begins at the field site, where samples of rock to be sent to the laboratory are marked with a cardinal compass and vertical directions before being removed from their in situ positions for shipment [17]. Cores being drilled in the field can be oriented by drilling with special core barrels which cut a pattern of grooves along the length of the core. The field orientation must be, of course, maintained throughout the specimen preparation process to be meaningful. How does one determine the direction in which specimens should be cored for laboratory tests from the samples collected in the field? Generally, the directions of interest are the principal mechanical property axes Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

28

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

which coincide usually with axes of petrofabric symmetry [13,18]. Sometimes visual examination of rock samples lets the investigator establish the axes; frequently, however, this is impossible. For these rocks, acoustic wave velocity measurements are made on an oriented sphere cut from the rock sample [19], with the resultant velocities plotted on a stereonet or an equal area net [20]. The principal velocity directions determined from the nets coincide with preferred orientation of rock fabric [20], which, in turn, correlates with principal mechanical properties. For example, the stereonets in Fig. 7 show a direct comparison between principal velocity H=Hlgh value axis M=Medium value axis L=Low value axis

H=4.75 km/sec M=4.45 km/sec L=5.44km/sec

H=59.0 GN/m2 M=52.9 GN/m 2 L=20.1 GN/m2

H=20.4 GN/m2 M= 17.4 GN/m2 L=16.4 GN/m2

Pulse velocity

Young's modulus, compression

Young's modulus, tension

FIG. 7--Stereonet plots of principal velocity and mechanicalproperty directionsfor shale [3].

directions and principal moduli of elasticity poles in a roof shale from Consolidation Coal Company's Matthews mine, Middlesboro, Kentucky [3]. Since the sphere is oriented with respect to the rock sample from which it was cut, specimens can be prepared from the rock sample with their axes of symmetry oriented according to the directions determined from the acoustic measurements on the sphere. Tests on these properly oriented specimens will provide the principal mechanical properties from which properties in any other direction can be transformed mathematically. Although preparing and testing a sphere may seem unnecessarily complicated, the entire process is quite routine and inexpensive. It is the best method of assuring properly oriented specimens for any mechanical property testing program. With the increasing use of anisotropic rock properties in analyzing rock mechanics problems, significant time and cost savings can result from using the techniques described.

Fractures The fact that rock is cracked or fractured has long caused consternation to people attempting to characterize a rock mass using laboratory property tests. Fractures or joints can provide a domineering influence on rock Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

CHAMBERLAIN ET AL ON OBSERVED ROCK PROPERTIES

29

properties; the effects of fracture systems are similar to those of bedding planes or other planar features but are frequently of significantly higher magnitude [21,22]. Characterizing rock from a particular site requires a distribution of fractures in the specimen that represents the distribution of fractures at the field site. Obtaining representative rock blocks from a test site in fractured rock is, however, no easy matter, since the fractures tend to separate easily. Large fracture spacing may preclude obtaining representative samples that can be handled readily. Preparing specimens from fractured samples borders on the impossible frequently. For most researchers, the usual procedure is to gather intact portions of rock from a site and to characterize the site based on the intact rock properties. While such characterizations are valid for certain applications, they do not reflect accurately the strength or deformability of the entire rock formation. The difficulties in collecting samples and testing representative fractured rock specimens sometimes lead to research on blocks of artificial material containing joints [23-25]. This does not, however, solve the problem faced by the design engineer--characterizing a specific field site. Collecting rock blocks in the field for characterizing a site should be preceded by geologic reconnaissance to determine the fracture distribution at the site [It)]. Following this, rock blocks containing as representative a fracture system as possible are selected for the laboratory testing program. Transporting samples must be done carefully to minimize alterations in the fracture system. For example, a recent shipment of cores from a field site in Arizona, packed in the usual core boxes, suffered significant changes in the size distribution of intact pieces, which indicated introduction of new fractures during transit (Fig. 8). Where it is of interest to preserve the

,51 20

II

Before shipment

Eg After shipment

w E

~ 5

0

I to 2

l

ll ll

to 4

4 to 5

5 to 6

6 to 7

? to 8

8 to 9

to I0

I0 to Id

n

II 12 13 14 15 16 to to to to to to 12 13 I~ 15 16 17

, 17 18 19 20 to to to to 18 19 2 0 21

CORE PIECE LENGTH, in

FIG.

8--Core piece size distribution before and after transportation.

original fracture pattern, the cores should be packed in foam rubber or newspaper inside wooden core boxes. Large blocks should be strapped to

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30

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

pallets placed on old tires or other shock absorbent materials in the truck bed. Specimens prepared from the collected samples should be large enough to include a unit cell or one repetition of the fracture pattern as measured during the geologic reconnaissance. If this would produce a specimen too large for available testing systems, specimens must be cut as large as feasible from selected locations in the sample that will provide realistic characterization of the site. If a desirable specimen size cannot be determined by examining fractures, preliminary tests can be run on various size specimens. Strength and moduli decrease generally with increased specimen size up to a size containing a representative fracture system [26-28]. The size above which the properties measured in the preliminary tests do not change, if the size range includes that specimen containing a representative fracture distribution, would then be the desired specimen size for the testing program. Fracture separation is frequently a problem during specimen preparation. Generally, larger specimens are less prone to separation along fracture planes than are small specimens. Careful attention must be given to specimen preparation details such as drilling or cutting speed, torque, flushing fluid flow etc. to minimize vibrations and reduce stresses induced in specimens during the preparation process. Structural integrity of specimens with a tendency to separate can be maintained sometimes with a thin " b e a d " of cement along one side. Although tension tests cannot be run on such specimens, triaxial or uniaxial compression test results are not affected significantly by the cement. Material that is very friable--coal for example--can sometimes be cut from original samples with a bandsaw, thus minimizing separation of preexisting fractures by eliminating the torques associated with coting. Sawing specimens with an appropriate bandsaw blade can be done without water coolant, thereby preventing undue damage to cracks filled with water soluble minerals or clays. Casting rough blocks of coal in concrete or hydrostone before sawing helps prevent damage during cutting also. Where a testing program is being conducted for theoretical studies of rock behavior, precautions must be taken to ensure that all specimens are similar. This uniformity can be accomplished by testing specimens with: (1) no flaws; (2) similar natural fracture distributions; or (3) uniform induced fracture systems. Flawless specimens are obtained by drilling slender cores and discarding those segments containing fractures. Commercially available dye-penetrant flaw detecting techniques can be used to find defects not otherwise visible [9]. Obtaining specimens that all contain similar natural fracture distributions is difficult, especially since no acceptable method exists for judging their similarity. Generally, a rock sample containing a uniform fracture system throughout is selected; identically oriented specimens are then prepared from this sample. Care must be taken while Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

CHAMBERLAIN ET AL ON OBSERVED ROCK PROPERTIES

31

preparing these specimens to prevent alterations in the fracture system. Specimens with a uniform distribution of induced fractures can be prepared by line-loading a rock block at intervals corresponding to the desired fracture spacing. The pieces of the rock block are then cemented back together for additional preparation such as coring, sawing, or grinding. The cement is then dissolved with an appropriate solvent which does not affect the rock [29] (Fig. 9). With servo-controlled testing machines, it is possible to load Load o pplicotion

~Frocture

FIG. 9--Preparingspecimenscontainingfractures. intact specimens beyond the peak load, inducing a fracture regime in the specimen, then to remove the load before complete loss of cohesion. Subsequent testing can then be performed on the fractured specimen.

Specimen Tolerances Failure to adhere to prescribed specimen tolerances during specimen preparation can yield misleading test results. Excessive side roughness, end roughness, end parallelism, and end perpendicularity to the core axis (or sides if prismatic specimens are used) can all cause problems (Fig. 10). End roughness greater than 0.001 in. can create nonuniform stresses near the ends and decrease the apparent strength of the specimen [30] (Fig. 11). Ends which are not parallel to each other or perpendicular to the core axis within 0.25 deg can also alter the apparent strength and deformation moduli [9,30]. Figure 12, for example, shows affects of excessive end inclination on typical load-deformation curves. Side roughness becomes important Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

32

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

II I1 l

End porollelism

End roughness

End inclination

Side roughness

FIG. lO~Critical specimen tolerances. because stress concentrations may develop along rough sides and because it creates difficulties in preparing the ends parallel and perpendicular to the axis. ASTM Test for Unconfined Compressive Strength of Rock Core Specimens (D 2938-71), Test for Direct Tensile Strength of Rock Core Specimens (D 2936-71), Test for Triaxial Compressive Strength of Undrained Rock Core Specimens Without Pore Pressure Measurement (D 2664-67), Test for Flexural Strength of Concrete (Using Single Beam With ThirdPoint Loading) (C 78-67), and the Bureau guidelines [31] spell out acceptable specimen tolerances. Adherance to nominal dimensions is generally not critical. Gross changes in length-to-diameter ratios (L/D) can, however, make significant differences in property test results due to end constraint [26] (Fig. 13). Hawkes and MeUor [32], after surveying a variety of studies on length-to-diameter ratio effects, have recommended a L / D of 2.5. For competent rock types, it is fairly easy to circumvent problems of poor specimens. If cylindrical specimens are desired, cores can be ground on their cylindrical surface to remove side roughness. It is then simple to clamp the cores in a V-block to surface grind the rough-sawn ends parallel to each other and perpendicular to the core axis. Lapping techniques are not acceptable, due to the complete lack of control on end parallelism and perpendicularity to the core axis. Prismatic specimens can be machined

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33

CHAMBERLAIN ET AL ON OBSERVED ROCK PROPERTIES

48

I

0

I

i

I

8o o o

0

0 0

0 Granite

4O

0

0 0

,, 32 Q.

0

I..z

~ 24 ic~ Q

000 I ~ So ndsto~ne

p-

0

0 0

w

00

0

Marble

A

A

0

(..)

8

0 ~:~Limestone O0

Sm t _

r o

Medium

0

..L

Rough

7-

I

I

I

T y p i c a l end s u r f a c e texture variation measurement

i

4,000 1,000 2,000 3,000 W, END SURFACE TEXTURE VARIATION, /=in

I 5,000

F I G . 1 1 - - E f f e c t of specimen end surface texture variation on compressive strength [30].

accurately by damping the rough-sawn specimens in a f'~ture that will hold a surface perpendicular to a vertical reference plane of the surface grinder. Accurate specimens of friable rocks are harder to prepare. Clamping must be done delicately, if at all. Extremely incompetent rocks must be tested in the rough-sawn condition. Such materials usually have such variable properties that the effects of poor tolerances are overshadowed. These weaker materials exhibit also greater ability to adjust internally to irregularities in the surface, thus minimizing stress concentration around such irregularities. Gypsum compound caps may be cast on the end of poor specimens of weak rock to create acceptable end conditions. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

34

SOIL

SPECIMEN

PREPARATION

FOR

LABORATORY

DEVILIS HILL DACITE

ST CLOUO GRAY GRANODIORITE 55,

7,

6l

-,~G_

d

o<25

5

~ 2o

4

i

i

1 _

,

0(. = . 0 8 ~

3

0(-2= .35'

~ ~o

2

~

5

I

x <

0

g

FIG.

TESTING

(Z = 53 ~

5.0 I0.0 15.0 O AXIAL DEFORMATION. IO-3 m

5.0

IO.O

150

12--Effect of end inclination on the load~deformation behavior of rock

~ 1.6

I

b

I

|

I

-.r I~ 1.4

I

I

o'CN = c o m p r e s s i v e

I

strencjth

[9].

I

normalize?

I~ 1.2

~ 1.0 O r N

.8

..J

0

I

I

I

I

I

I

I

I

I

0.2

0.4

0.6

0.8

I .0

1.2

1.4

1,6

1.8

L/D,

FIG. t3--Effect

lencjth-to-diam

2.0

ratio

of length-to-diameter ratio on compressive strength

[26].

Summary The increased use of computer modeling in the design of openings in rock and foundations on rock has made it more and more important to know relevent and accurate rock properties. From the time of sampling a field site to the completion of property tests on the rock in the laboratory, many factors in specimen handling may alter their apparent properties. Moisture content, orientation, crack or fracture occurrence, and specimen tolerances are four factors that need to be considered in any laboratory testing program. The negative influences of these factors can be minimized, however, by giving careful attention to sampling, transporting, storing, and preparing specimens.

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CHAMBERLAIN ET AL ON OBSERVED ROCK PROPERTIES

35

References [1] Colback, P. S. B. and Wild, B. L. in Proceedings, 3rd Canadian Symposium on Rock Mechanics, 1965, pp. 65-83. [2] Parker, J., Transactions, Society of Mining Engineers, American Institute of Mining, Metallurgical, and Petroleum Engineers, Vol. 247, June 1970, pp. 142-144. [3] Van Eeckhout, E. M., "The Effect of Moisture on the Mechanical Properties of Coal Mine Shales," P h . D . thesis, University of Minnesota, Minneapolis, Minn., Dec. 1974. [4] Aughenbaugh, N. B. and Bruzewski, R. F., "Investigation of the Failure of Roofs in Coal Mines," Final Report on Bureau of Mines Contract HOI 11462, Oct. 1973. [5] Mann, R. L. and Fatt, I., Geophysics, Vol. 25, No. 2, April 1960, pp. 433-444. [6] "Recommended Practice for Core-Analysis Procedure," API RC 40, American Petroleum Institute, 1960. [7] Oitto, R., Zona, A., and Stears, J., Coal Mining and Processing, Vol. 7, No. 12, Dec. 1970. [8] Winston, P. W. and Bates, D. H., Ecology, Vol. 41, No. 1, 1960, pp. 232-237. [9] Podnieks, E. R., Chamberlain, P. G., and Thill, R. E. in Proceedings, 10th Symposium on Rock Mechanics, Austin, Tex., 20-22 May 1968, American Institute of Mining, Metallurgical, and Petroleum Engineers, pp. 215-241. [10] McGill, E. and Raney, J. A., Geological Society o f America Bulletin, Vol. 81, Oct. 1970, pp. 2949-2958. [ll] Hock, E., South African Institute Mining and Metallurgy Journal, Vol. 64, No. 10, 1964, pp. 510-518. [12] Jaeger, J. C., GeologicalMagazine, Vol. 97, 1960, pp. 65-72. [13] Douglass, P. M. and Voight, B., Geotechnique, Vol. 9, No. 3, 1968, pp. 376-398. [14] Youash, Y., Geological Society of America Bulletin, Vol. 80, Feb. 1969, pp. 303-306. [15] Komar, C. A., Overbey, W. K., Jr., and Pasini, J., III, "Directional Properties of Coal and Their Utilization in Underground Gasification Experiments," Technical Progress Report 73, Bureau of Mines, Nov. 1973. [16] Deklotz, E. J. and Brown, J. W., "Tests for Strength Characteristics of a Schistose Gneiss, Mechanical Anisotropy," Technical Report 1-67, Department of Army, Missouri River Division, Corps of Engineers, Omaha, Neb., May 1967. [17] Steckley, R. C., Podnieks, E. R., Chamberlain, P. G., and Pugliese, J. M. in Field Testing and Instrumentation of Rock, ASTM STP 554, American Society for Testing and Materials, 1974, pp. 100-115. [18] McWilliams, J. R. in Testing Techniques for Rock Mechanics, ASTM STP 402, American Society for Testing and Materials, 1966, pp. 175-189. [19] Vickers, B. L. and Thill, R. E., Journal of Science Instruments, Series 2, Vol. 2 Oct. 1969, pp. 901-902. [20] Thill, R. E., Willard, R. J., and Bur, T. R., Journal o f Geophysical Research, Vol. 74, No. 20, Sept. 1969, pp. 4897-4909. [21] Mogilevskanza, S. E. M. in Proceedings, 2nd Congress of the International Society for Rock Mechanics, Vol. 1, Privredni pregled, Belgrade, Yugoslavia, 1970, pp. 333-339. [22] Lane, K. S. and Heck, W. J. in Proceedings, 6th Symposium on Rock Mechanics, University of Missouri-Rolla, 1964, pp. 98-108. [23] ErgQn, I. in Proceedings, 2nd Congress of International Society for Rock Mechanics, Vol. 1, Privredni preglad, Belgrade, Yugoslavia, 1970, pp. 497-507. [24] Brown, E. T., Journal ofSoilMechanics, Vol. 96, No. SM6, Dec. 1970, pp. 1935-1949. [25] Einstein, H. H., Nelson, R. A., Bruhn, R. W., and Hirshfeld, R. C. in Rock Mechanics Theory and Practice, Proceedings, l l t h Symposium on Rock Mechanics, New York, American Institute of Mining, Metallurgical, and Petroleum Engineers, 1970, pp. 83-103. [26] Obert, L., Windes, S. L., and Duvall, W. I., "Standardized Tests for Determining the Physical Properties of Mine Rock," Report of Investigations 3891, Bureau of Mines, 1946. [27] Bieniawski, Z. T. in Proceedings, 10th Symposium on Rock Mechanics, American Institute of Mining, Metallurgical, and Petroleum Engineers, 1972, pp. 409-427. [28] Dhir, R. K. and Sangha, C. M., Colliery Guardian, VoL 220, No. 1, Jan. 1972, pp. 75-78.

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36

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

[29] Wawersik, W. R. and Brown, W. S., "Creep Fracture of Rock," University of Utah Report, UTC-ME 73-197, Final Report on Bureau of Mines Contract HO220007, July 1973. [30] Hoskins, J. R. and Horino, F. G., "Effect of End Conditions on Determining Compressive Strength of Rock Samples," Report of Investigations 7171, Bureau of Mines, 1968. [31] "Bureau of Mines Test Procedures for Rocks," Information Circular 8628, U.S. Bureau of Mines, 1974, pp. 120-126. [32] Hawkes, I. and MeUor M., Engineering Geology, Vol. 4, No. 3, July 1970, pp. 177-285.

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G. N . D u r h a m I

Trimming Device for Obtaining Direct Shear Specimens from Samples of Stiff Fissured Clay Shale REFERENCE: Durham, G. N., "Trimming Device for Obtaining Direct Shear Specimens from Samples of Stiff Fissured Clay Shale," Soil Specimen Preparation for Laboratory Testing, ASTM STP 599, American Society for Testing and Materials, 1976, pp. 37-42. ABSTRACT: A device has been developed at the U. S. Army Engineer Waterways Experiment Station for obtaining direct shear specimens of hard, brittle soils, the use of which has resulted in a reduction in man-hours for specimen preparation from 3 or 4 h to 50 or 60 min per specimen. This device, which has proved to be superior to any previous apparatus used for obtaining direct shear specimens of such soils, is fitted to the base of a commercially available unconfined compression apparatus of the hand-operated type. Samples found untrimmable heretofore can now be tested, because a sample is held rigidly during trimrning operations, and the time of exposure to air drying (and subsequent deterioration) has been reduced substantially. KEY WORDS: soils, soil tests, shear properties, clays, shales, residual stress

The concept of residual strength suggested by Skempton 2 has contributed greatly toward a better understanding of the magnitude and mechanism of shearing resistance of highly overconsolidated clays and clay shales. Typically, clay shale materials exhibit significant differences in the effective shear strength parameters, c and +, corresponding to peak strength and residual strength. The shearing resistance of a clay shale undergoing displacement will decrease substantially after failure and ultimately stabilize at a constant residual shear strength. The laboratory determination of the residual strength requires unconventional apparatus and techniques for producing a large displacement. Two methods are most prevalent for measuring residual shear strength--annular torsion tests and repeated direct shear tests. Townsend and Gilbert 3 reported t Research civil engineer, Mobility and Environmental Systems Laboratory, U. S. Army Engineer Waterways Experiment Station, Vicksburg, Miss. 39180. 2Skempton, A. W., Geotechnique, Vol. 14, No. 2, June 1964, pp. 77-101. 3Townsend, F. C. and Gilbert, P. A., "Engineering Properties of Clay Shales; Residual Shear Strength and Classification Indexes of Clay Shales," Technical Report S-716, Report 2, U. S. Army Engineer Waterways Experiment Station, Corps of Engineers, Vicksburg, Miss., Aug. 1974.

37

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38

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

the results of residual shear strength tests performed on samples of highly overconsolidated clay shales from numerous geological deposits. With regard to this discussion, two of their more significant findings were: 1. The variation of residual friction angles measured by repeated direct shear tests, annular shear tests, or rotational shear tests were very small and well within the range of experimental error. 2. Residual shear strengths were found to be independent of specimen preparation procedures with comparable values being obtained from intact, precut, and remolded specimens of a clay shale sample.

Residual Shear Test Procedures Annular shear apparatus tend to be expensive and require elaborate procedures and a trimming device for intact specimen preparation. Hence, annular shear devices are usually found only in research-oriented or advanced soil testing facilities. However, most facilities for testing physical properties of soil are equipped with direct shear testing equipment that may be adapted to residual direct shear testing with perhaps only minor modifications. A controlled strain device, in which the direction of shear can be reversed, should be used to apply the horizontal shear force to the specimens. The shear surface or zone can be formed by (a) shearing an intact specimen, (b) cutting a plane surface through an intact specimen or (c) shearing a completely remolded specimen. Practical laboratory considerations favor the use of a precut shear plane, since producing a plane surface by shearing an intact stiff-to-hard specimen may be difficult. Any irregularities of the shear surface would introduce an added resistance that would not be a measure of the shear strength of the material. More often than not, stiff and hard clays and clay shales are jointed or fissured or both. In addition, shales and clay shales often contain faults and shear zones. These formations usually contain partings of soil, silt, sandstone, or siltstone, and, occasionally, partings or layers of bentonite. These characteristics of clay shale formations tend to complicate the trimming of intact direct shear specimens, resulting in a time-consuming, tedious task.

Waterway Experiment Station Residual Shear Testing Since 1968, the U. S. Army Engineer Waterways Experiment Station (WES) has become extensively engaged in residual shear testing of clay shale. The only previous method, of many tried at WES, that produced satisfactory 3 by 3-in. (76 by 76-mm) square direct shear specimens of hard soils consisted of the following steps: with a bandsaw, a horizontal slice ~ in. (3.2 mm) thicker than desired was c u t from a 5-in. (127-mm)-diameter undisturbed sample; the horizontal surfaces were made parallel and brought to the desired Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

DURHAM ON SHEAR SPECIMENS OF STIFF FISSURED CLAY SHALE

39

thickness by fitting the slice in a 6-in. (152.4-mm)-diameter ring and planing the surfaces with the knife edge of a hardened steel straightedge. A square approximately 388 in. (77.8 mm) on the side was cut with the handsaw, and the sides were trimmed with the steel straightedge and stationary cutting guides to form an exact 3 by 3-in. (76 by 76-mm) specimen. This process took approximately 3 to 4 h, with a high rate of specimen breakage, especially in final stages of preparation. These lost specimens were probably due to the large amount of handling and the deterioration of the soil during the 4 h of air drying (even though trimming was done in a room with controlled temperature and humidity). Earlier attempts to prepare direct shear specimens consisted of forcing a standard direct shear specimen cutter into an approximate 3 by 3-in. (76 by 76-mm) horizontal slice with the aid of an unconfined compression testing frame. However, the ~-in. (3.2-mm)-thick walls of the cutter deformed outwardly during these operations, resulting in a specimen of irregular shape. Furthermore, because of the stresses imposed by the walls of the cutter, the specimen tended to break when extruded from the cutter into the shear box. The foregoing problems and experiences resulted in the development of a trimming device that has thus far proved to be far superior to any of the previous apparatus used at WES for obtaining direct shear specimens of hard, brittle soils.

WES Direct Shear Trimming Device A direct shear trimming device was designed and fabricated at WES in 1971. The trimming device (Fig. 1) is fitted to the base of a commercially available unconfined compression apparatus (Soiltest Model U-160; a handoperated device). The loading frame of this apparatus was removed, with the gearing arrangement located in the inclosed base used to push the specimen through the specimen cutter blade and into the specimen holder. The trimming device consists of two principal components, the specimen holder and the specimen cutter blade. The two parts are detachable for trimming procedures (reasons discussed later) and for convenient sharpening or replacing of a worn cutter blade. The specimen holder consists of four sides fastened with machine bolts; this not only facilitates fabrication but also allows the prepared specimen to be extracted without extrusion from the specimen holder. Two specimen holders were fabricated for an 0.25 (6.4-mm) or an 0.50-in. (12.8-mm)-thick specimen. Normally, two 0.25-in. (6.4-mm) specimens are trimmed from a single sample to form the upper and lower halves of a direct shear specimen with a precut shear plane. U. S. Army Engineer Manual EM-1110-2-19064 suggests that a direct shear 4"Engineeringand Design:LaboratorySoilsTesting,"EngineeringManualEM 1110-2-1906, Appendix IX(A), Officeof the Chiefof Engineers, Headquarters, Departmentof the Army, Washington,D. C., 30 Nov. 1970. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

40

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 1-- WES direct shear trimming device. specimen should be 3 by 3 in. (76 by 76 mm) square, trimmed from a 5-in. (127-mm)-diameter undisturbed sample. For direct shear devices equipped to test smaller circular specimens, a device similar to that in Fig. 1 could be constructed, probably with a less complicated design and more efficient and less expensive fabrication procedures.

Specimen Preparation A horizontal slice of soil, approximately ~ in. (3.2 mm) thicker than the desired final height and approximately 388 by 388 in. (77.8 by 77.8 mm) Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

DURHAM ON SHEAR SPECIMENS OF STIFF FISSURED CLAY SHALE

41

square, is placed on a soft wood block about 4 by 4 by 88in. (102 by 102 by 6.4 mm), which rests on a piston (Fig. 2a). By means of a hand crank, the piston pushes the soil vertically upward through the cutter blade until the blade comes into contact with the wood block. The wood block is used to prevent corners of the specimen's bottom surface from shearing away under the force of the cutting action. The piston is then lowered slightly and the

FIG.2-- Typicalprocedurefor trimming directshearspecimenfrom clay-shalesample. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

42

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

block removed; wall friction prevents the specimen from moving. The piston is then brought into contact with the specimen and moved upward until the upper surface of the soil extends slightly above the specimen holder. The excess soil above the specimen holder is removed with a hardened steel straightedge (Fig. 2b). The cutter blade is disconnected from the specimen holder, which, in turn, is removed from the stanchions and fastened inverted to a 89 (12.7-mm)-thick machined steel plate. The excess material from the bottom surface is then removed (Figs. 2c and d), the sides of the specimen holder are disconnected, and the specimen is placed in the shear box for testing. Discussion

Use of the WES direct shear trimming device has resulted in a reduction in man-hours for preparation from 3 to 4 h to 50 to 60 min per specimen. Samples heretofore found untrimmable can now be tested for residual strength parameters because the sample is held rigidly during trimming operations and the time of exposure to air drying (and subsequent deterioration) has been reduced substantially.

Acknowledgment Acknowledgment is made to W. J. Hughes, civil engineer technician, and J. P. Wislocki, mechanical engineer, both of the WES, for their significant contributions in the development and design of the WES direct shear trimming device. The author wishes to thank M. L. Doiron for her editorial review of this paper and K. M. Jennings, who typed and assembled the paper.

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F. C. Townsend' and P. A. Gilbert'

Effects of Specimen Type on the Residual Strength of Clays and Clay Shales

REFERENCE: Townsend, F. C. and Gilbert, P. A., "Effects of Specimen Type on the Residual Strength of Clays and Clay Shales," Soil Specimen Preparation for

Laboratory Testing, ASTM STP 599, American Society for Testing and Materials, 1976, pp. 43-65. ABSTRACT: This investigation was an examination of the effects of testing apparatus, specimen type and density, normal stress, and sequence of loading on the residual strength of various clays and clay shales. Annular shear and repeated direct shear tests were conducted on intact shale specimens with precut failure planes, while comparison rotational shear tests were conducted on the same shale material subjected to various degrees of remolding. Test results indicate that comparable residual strengths can be obtained, regardless of equipment type and procedure or degree of remolding. Supplemental annular shear tests on residual clay and silty sand show that precutting of the failure plane or the specimen density has no effect on the ~/value. The residual friction angle is independent of stress history and normal load [when greater than 2 kg/cm 2 (1.96 X 105 Pa)]. KEY WORDS: clays, shales, residual shear stress, direct shear tests, annular shear tests, soils The concept o f residual strength has contributed greatly towards a better u n d e r s t a n d i n g o f t h e m a g n i t u d e o f shearing resistance t h a t is o p e r a t i v e in o v e r c o n s o l i d a t e d clays a n d c l a y shales a f t e r large d i s p l a c e m e n t s h a v e o c c u r r e d . This residual s h e a r i n g resistance is quite low in c o m p a r i s o n to p e a k strengths determined b y conventional l a b o r a t o r y procedures a n d equipm e n t , a n d special a p p a r a t u s a n d t e c h n i q u e s c a p a b l e o f p r o d u c i n g large displacements are required for the accurate l a b o r a t o r y determination o f the r e s i d u a l s h e a r strength. T y p i c a l l y , a n n u l a r s h e a r o r r e p e a t e d direct s h e a r tests using intact specimens with o r w i t h o u t p r e c u t failure p l a n e s o r specim e n s o f r e m o l d e d m a t e r i a l a r e u s e d to e v a l u a t e the r e s i d u a l strength o f a m a t e r i a l . T h e tests a r e o f t e n c o n d u c t e d in m u l t i s t a g e f a s h i o n ; t h a t is, a Research civil engineers, Soil Mechanics Division, U.S. Army Engineer Waterways Experiment Station, Vicksburg, Miss. 39180. 43 Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by by ASTM International www.astm.org Copyright9 1976 University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

44

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

single specimen is tested under various normal stresses in order to define the residual envelope. Currently, only a few investigators [1-3] 5 have examined various facets o f equipment and procedural effects in residual strength. The objective o f this investigation was to examine the effects o f equipment type and associated procedure, specimen type, normal stress, and loading sequence on the residual strength of various clay shales and clays. These effects were evaluated by conducting annular shear and repeated direct shear tests on shale specimens with precut failure planes. Comparison rotational shear tests were conducted on remolded specimens subjected to different processing procedures. Additional annular shear tests on a clay and micaceous silty sand were used to evaluate the effects of precutting and initial specimen density.

Previous Investigations Equipment The repeated direct shear method using conventional apparatus was developed by Skempton [4] and has been adapted as a uniform procedure by the Corps of Engineers. This method involves shearing the specimen a limited distance in one direction and then either reversing the direction o f shear or relieving the normal load and pushing the movable half o f the shear box back to its initial position before shearing again. These operations are repeated until sufficient displacements are accumulated to reach a residual condition. A characteristic stepped displacement-shear stress curve is produced in which the succeeding peaks are due to particle reorientation or to the formation of new slip planes with each cycle. 3 The less common ring torsion method was used first in the 1930's by early investigators [5,6]. The method provides continuous displacements in one direction of an annular specimen; this feature is desirable in research evaluation of residual strength [3,4, 7].

Specimen Type A general agreement exists in the literature [3,8] that residual shear strength is independent of stress history and that the alignment o f clay particles in a failure zone o f remolded material controls the value o f +r ' Consequently, three basic specimen types, (a) intact, (b) precut, and (c) resedimented or remolded-reconsolidated, are being tested by various laboratories. Testing on intact specimens o f clay shale poses several problems, 2The italic numbers in brackets refer to the list of referencesappended to this paper. 3Skempton himself recognizedthat the method was imperfect and suggested that ideally the displacement should be continuouslyin one direction, as occurs in field situations. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

TOWNSEND AND GILBERT ON CLAYS AND CLAY SHALES

45

namely, (a) preventing slippage between the specimen and torque plate and (b) having equipment capable of applying high shearing stresses to initially shear the material and yet sensitive enough to measure low residual strengths. As a result of these difficulties and Skempton's [9] observation that precutting ensures the lowest shearing resistance and most conservative result, the present trend in testing clay shales is to use precut specimens. Alternatively, since any effects of stress history of the sample on +r ' are obliterated by the remolding action occurring in and along the failure zone, the use of remolded samples offers another method for specimen preparation in residual strength testing. Tiedermann [5], Skempton [4], and Petley [7] demonstrated that the residual strength of clays normally consolidated from a slurry was equivalent to that of undisturbed overconsolidated clay.

Effects of Loading Sequence Based upon the assumption that ~br' is independent of stress history (previous loading and unloading), a laboratory practice of using multistage loading tests has developed, that is, a single specimen is tested under various normal stresses in order to define the residual strength envelope. Kenney [10] presents results of tests under o, ' from 0.8 to 8.0 and 0.6 to 6.0 kg/cm ~ (0.78 x 105 to 7.8 x 105 and 0.58 x 105 to 5.8 x 105Pa)thatshowed that ~r' values were essentially the same for a given o~' value, regardless of loading sequence. Bishop et al [3] likewise stated that, based on ring shear tests on Weald and brown London clay, ~r ' is independent of stress history. Materials and Equipment

Materials Various clay shales from foundations at Corps of Engineers' projects were tested using repeated direct shear, annular shear, and rotational shear equipment and techniques. These were: (a) Dawson shale, Chatfield Dam, South Platte River, Colorado, (b) Taylor I shale, Laneport Dam, San Gabriel River, Texas, (c) Strawn shale, Proctor Dam, Leon River, Texas, (d) Kincaid shale, Cooper Dam, Sulphur River, Texas, (e) Bearpaw shale, Fort Peck Dam, Missouri River, Montana, and (1") Pierre shale, Oahe Dam, Missouri River, South Dakota. Since shearing an intact specimen of clay shale by annular shear equipment is virtually impossible, the effects of precutting and compacted density were evaluated on softer materials. These were: (a) Residual clay, Volta Grande, Brazil, (b) Micaceous silty sand, Rio Paraibuna, Brazil, and (c) Micaceous silty sand, Rio Paraitinga, Brazil. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

46

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

Table 1 presents the classification indexes of the materials tested. The classification indexes of clay shales are greatly influenced by test procedures [11], so all specimens reported in Table 1 were subjected to the three Corps of Engineers processing procedures for clay shales, namely, (a) undried, (b) air dried, and (c) blenderized. Complete descriptions of these procedures are published elsewhere [11-13]. TABLE 1--Classification indexes of clay shales and clays tested for this study.

Material

Processing Method"

Liquid Limit

Plastic Limit

Activity Plasticity Plasticity <2/am, Index Index % <2/am, ~

1.17

Dawson shale Chatfield Dam

undried air dried blenderized

61 65 84

20 23 25

41 42 59

35 50 56

0.84 1.05

Taylor I shale Laneport Dam

undried air dried blenderized

65 65 79

22 21 21

43 44 58

47 50 57

0.91 0.88 1.02

Strawn shale Proctor Dam

undried mr dried blenderized

51 52 62

23 21 24

28 31 38

47 48 64

0.59 0.64 0.59

Kincaid shale Cooper Dam

undried mr dried blenderized

88 78 113

22 23 22

66 55 91

35 46 52

1.88 1.17 1.75

Bearpaw shale Ft. Peck Dam

undried air dried blenderized

95 128 161

29 31 29

66 97 132

61' 64

1.59 2.06

Pierre shale Oahe Dam specimen U-22

undried air dried blenderized

83 91 140

41 40 38

42 51 112

34 47 56

1.23 1.08 2.00

Pierre shale Oahe Dam specimen U-28

undried air dried blenderized

178 297 351

34 34 31

144 263 320

75 80 85

1.92 3.28 3.76

39

28

11

12

0.92

Micaceous silty sand Rio Paraitinga Micaceous silty sand Rio Paraibuna

..."

33

27

6

5

1.20

Residual clay Volta Grande

...~

96

38

58

40

1.45

* In accordance with procedures in EM 1110-2-1906 [13].

Equipment The apparatus used for determining the residual shear strength of the shales consisted of an annular shear device, a repeated direct shear machine, and a rotational shear machine. Brief descriptions of the equipment follow. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

TOWNSEND AND GILBERT ON CLAYS AND CLAY SHALES

47

Waterways Experiment Station Low-Capacity Annular Shear Apparatus-The annular shear apparatus used in these tests is based on previous designs by Hvorslev. 4 The annular specimen is 4.00 in. (10.16 cm) outside diameter, 2.03 in. (5.16 cm) inside diameter, and 0.75 in. (1.91 cm) thick. The normal stress is applied by a pneumatic stress cell which pulis on a cable bearing against the loading platen on top of the specimen. A maximum normal stress of 15 kg/cm ~ (14.7 • 105 Pa) can be applied. The shear stress (maximum 2.5 kg/cm ~ (2.45 • 105 Pa)) is measured by a torque plate through torque arms mounted on the top loading platen. An electrical motor and gear box rotate the lower half of the specimen. The specimen is confined between upper and lower confining rings, and the gap can be adjusted by differential screws as the specimen consolidates during shear. Geonor Direct Simple Shear Machine--The reversal direct shear tests were conducted in a Geonor, direct-simple shear apparatus which was modified to accept a 2.78-in. (7.06 cm)-square direct shear, 0.59-in. (1.50 cm)-thick specimen. The lower half of the shear box is fixed, and the top half can be displaced 0.3 in. (0.76 cm) to either side of center. The upper half of the shear box has knife edges; the lower half is Teflon coated to minimize any metal-to-metal friction should the gap close. The gap is adjusted manually by inserting a feeler gage. To minimize geometric effects and area corrections, only those shear stresses measured when the box is centered are used in plotting residual strength against deformation curves. Harvard Rotational Shear Apparatus--The rotational shear apparatus was developed at Harvard University by La Gatta [8]. The apparatus tests annular specimens (2.8 in. (7.11 cm) outside diameter, 2.0 in. (5.08 cm) inside diameter, and 0.14 in. (0.36 cm) high) of remolded material prepared from the trimming scraps of the annular shear specimens and processed by the Corps of Engineers [13] undried, air dried, or blenderized procedures. Shear stresses are generated by rotating a porous disk at the bottom of the specimen and using two electrical force transducers to measure the couple required to keep the top half of the specimen stationary.

Specimen Preparation Testing of clay shale materials was conducted either on specimens of intact material with precut failure planes or on specimens composed of remolded material. Specimens with precut failure planes were prepared by separately trimming the top and bottom specimen halves from the shale cores and placing them in confining rings or shear boxes. These specimen halves were then placed in their respective shearing devices (annular shear 4Construction or modificationwas accomplished at the WaterwaysExperiment Station under the generalsupervisionof B. N. Maclver. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

48

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

or direct shear) and allowed to consolidate under the applied normal stress. This method of preparation facilitates reproducible results and ensures the lowest possible shear resistance that can be mobilized by the shale [9]. Annular shear specimens of Volta Grande clay for investigating the effects of precut failure planes were prepared by either trimming intact specimens from a block sample or separately trimming the top and bottom specimen halves in the procedure used for shale specimens. Annular shear specimens of micaceous silty sand for investigating the effects of density were prepared by trimming intact specimens from an undisturbed block sample or by compacting specimens, prepared from trimming scraps of the intact specimens, in the confining rings to the selected density and water content. Compaction was accomplished using a hand-held pneumatic tamping miniature compactor (similar to the spring-loaded Harvard miniature compactor) with a Teflon section-shaped foot. Loosely or densely compacted specimens were prepared at 90 or 100 percent, respectively, of maximum standard density.

Annular Torsion Tests Conducted in Waterways Experiment Station Low-Capacity Annular Shear Apparatus Annular shear specimens were allowed to consolidate or swell under selected normal stresses until equilibrium was achieved (usually two to three days). Shear was then initiated at a slow rate of displacement5 (1.75 cm/day) until the peak strength had been achieved (usually when only several millimetres of displacement had occurred). The rate of displacement was then increased to 14.0 cm/day until an apparent ~r' condition was reached. Then the rate of displacement was reduced to its original value. It was anticipated that using a rapid rate of displacement to produce large displacements, followed by a reduced rate of displacement for the final evaluation of d~r', would prove to be a satisfactory and less time-consuming method of determining ~r' than the use Of a constant low rate of displacement method [14]. After 4,' had been determined under a specific normal stress, shear was terminated. The specimen was then either consolidated or allowed to rebound under a new normal stress. After equilibrium had been reestablished, shear was reinitiated at a rate of displacement consistent with that used under the initial normal stress.

Repeated Direct Shear Tests Conducted in Modified Geonor Direct Simple Shear Apparatus Repeated direct shear tests are the most common method presently used for measuring ~ ' . However, the validity and effects of repetitive 5Displacement is measured along the circumferencecalculated from the mean specimen radius. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

TOWNSEND AND GILBERT ON CLAYS AND CLAY SHALES

49

back-and-forth movement as opposed to continuous one-directional shear have not been fully investigated. Accordingly, this series of tests was made for comparison of d~,' values measured by annular tests. The specimens were precut, consolidated to equilibrium under various normal stresses corresponding to those used in the annular tests, and generally sheared at a slow rate of displacement (1.16 cm/day). After ~ / h a d been clearly defined, as shown by a ~ / versus logarithm displacement graph, the specimen was returned to its center position, a new normal stress was applied, and shearing was reinitiated after consolidation equilibrium had been achieved.

Rotational Shear Tests Conducted in Harvard Apparatus This series of tests evaluated the effects of material processing on ~r' values of remolded specimens prepared through use of the three procedures: undried, air dried, and blenderized. The remolded specimens at water contents above the liquid limit were consolidated under a normal stress of 6 kg/cm 2 (5.88 x 105 Pa) until observation of vertical displacements indicated that equilibrium had been reached. To prevent excessive extrusion during consolidation, loads were applied incrementally until a normal stress of 6 kg/cm 2 (5.88 x l0 s Pa) was reached. A pilot test series was conducted to determine the effects, if any, of displacement rate d on d?r'. Based upon these pilot tests, the rate of displacement used for the tests discussed herein was 69.1 cm/day. Each specimen was displaced an additional several centimetres after de/ had been reached, as determined by a dCr' versus log displacement plot [6, 8].

Test Results and Analyses Tables 2 and 3 summarize the annular shear, repeated direct shear, and rotational shear test results. The order of normal stress listing refers to the sequence in which the normal stresses were applied during multistage tests.

Effect of Equipment Type Figure 1 presents the residual strength envelopes of companion annular shear, repeated direct shear, and rotational shear tests. Since the residual shear stress is quite small, tile vertical scales of the graphs have been exaggerated to provide better comparison between types of tests. From these results, it is evident that very little difference exists between the residual shear strength values of clay shale specimens with precut failure planes, whether measured by annular shear or repeated direct shear tests (compare circles and triangles). Herrmann and Wolfskill [1] Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

H A 288

6.1 6.1 12.1 12.1 3.1 3.1 1.6 1.6 1.6

1.75 14.03 1.75 14.03 1.75 14.03 1.75 14.03 1.75

~

~

69

12.8 10.6 10.6 11.7 10.3 8.8 8.6 8.2 8.3

6.8

7.0

7.3

4.9 4.6

4.9

5.7

Residual Shear Strength, ~r ', deg

13.0 61.0 0.5 13.0 11.3 68.3 0.2 35.4 3.3

19.5

31.9

73.1

8.5 4.8

38.4

Displacement per Stage Required to Achieve ~r ', cmb (not cumulative)

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AS

blenderized

H A 310

Taylor I AS-20

6.0

air dried

H A 316

6.0

6.0

RS undried

1.163 0.593

1.75

6.1 6.0 6.0

1.75

Rate of Displacement, cm/day

6.1

RDS

AS

Test Type~

DS-16

Dawson ASI3

Shale/Test

Normal Stress, kg/cm 2 Comments

residual was not yet achieved

residual was not yet achieved residual was not yet achieved

cyclic annular shear tests; sample sheared for 1.75 cm in one direction; normal load removed and sample manually returned to initial starting point continuous annular shear, one direction

T A B L E 2--Summary o f residual shear strength tests on clay shales,

0

.-t

-t m

.<

0

-t

W 0..n

0

Z "11

5

--4

m -u

m z -u

r" r -o m

o

r

tl~ 0

RS undried

air dried

blenderized

H A 323

H A 321

H A 292

69

6.0

1.75 14.03 1.75 1.75 14.03

69

1.6 1.6 1.6 3.1 3.1

69

6.0

0.59 0.59

6.0 12.0 6.0

1.75 14.03 1.75 1.75 14.03 1.75 1.75 14.03 1,75 1.75 14.03 1.75

69

1.16 1.16

6.1 6.1 6.1 12.1 12.1 12.1 3.1 3.1 3,1 1,6 1.6 1.6

6.0

6.0 12.0

14.6 5.7 5.5 5.3 5.0

7.2

9.1

9.6

9.2 9.8

9.5 9.5 10.0 9.3 9.3 9.3 9.2 9.4

8.2 8.5 8.7

9.8

9.6 10.3

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AS

RDS

DS-5

Kincald AS-25

AS

RS, blenderized

H A 285

Strawn AS-23

RDS

DS-17

2.0 58.4 3.4 12.6 28.0

1.4

6.0

91,2

3.0 0.35

1.7 17.1 1.5 1.7 18.5 1.7 3.9 37.7 3.3 5.3 28.0 5.0

20.9

33.0 10,0

residual not developed prior to increase in rate of displacement

residual not achieved prior to increase in rate of displacement

residual not achieved prior to increase in rate of displacement peak not achieved prior to increase in displacement rate

tests on material prepared by undried and air-dried procedures could not be conducted as peak strengths overloaded force transducers

=,.k

Grl

fm co

-r

.<

z

0r'-

Z

o

~0 m 30 -4

F

z o

m z o

Z

-4 O

AS

RS, blenderized

RDS

Test Type"

CopyrightbyASTMInt'l(allrightsreserved);WedDec2214:00:00EST2010 Downloaded/printedby UniversityofBritishColumbiaLibrarypursuanttoLicenseAgreement.Nofurtherreproductionsauthorized.

Bearpaw AS-26

DS- 15

Shale/Test

1.75 14.03 1.75 1.75 14.03 1.75 1.75

14.03 1.75

14.03 1.75

6.1 6.1 6.1 12.1 12.1 12.1 3.1

3.1 1.6

1.6 1.6

0.59 0.59

1.75 1.75 14.03 1.75 14.03 1.75 1.75 14.03 1.75 14.03 1.75

3.1 6.1 6.1 12.1 12.1 12.1 3.1 3.1 1.6 1.6 1.6 6.0 12.0

Rate of Displacement, cm/day

Normal Stress, kg/cm 2

7.1 8.2 8.0 8.0

7.4 6.5 6.9 6.6 6.8 6,6 7.4

5.5 5.3

4.7 4.8 4.9 5.2 5.8 5.5 5.7 5.7 5.9 5.9 5.6

Residual Shear Strength, ~, ', deg

14.0 3.5

5.2

75.0

2.4 55.7 6.8 3.5 J4.0 7.0 7.6

25.3 28.5

3.8 14.2 13.2 2.3 25.9 6.4 8.8 28.1 6.0 55.9 ll.O

Displacement per Stage Required to Achieve ~r ', cm b (not cumulative) Comments

0.75 N solution of Na2SO4 used as saturating fluid

rotational shear tests could not be successfully completed due to "grittiness" of material

T A B L E 2--Summary o f residual shear strength tests on clay shales--Continued,

-4 l'rl

,-n .,<

o

-4

o

03

r"

:23

o

z "rl

-4

:23

"o :D m "13

m Z

o_

I"11

r" f/J "u

fD

_o

t.~

RS, air dried RS, blenderized with distilled water

RS, blenderized in 0.75 N Na2SO4

H A 197

H A 222

RDS

6.1 12.1 12.1 12.1 6.1 6.1 6.1 3.1

6.0 12.0

6.1 6.1 6.1 12.1 12.1 12.1 6.1 6.1 6.1 3.1 3.1 3.1

1.75 1.75 14.03 1.75 1.75 14.03 1.75 1.75

0.59 0.59

1.75 14.03 1.75 1.75 14.03 1.75 1.75 14.03 1.75 1.75 14.03 1.75

69.0

69.0

6.0

6.0

69.0

0.59 0.59

6.0

6.0 12,0

3.0 3.6 3.8 4.0 3.7 3.5 3.5 3.7

7.1 7,1

10,4 10.2 7.7 8.3 7.8 8.1 8.6 7.5

7.0 7.1 6.9

12.2 2.3 41.7 1.2 1.7 30.2 6.1 3.5

25.7 2.4

21.0 43.0 1.2 8.8 62.8 4.1 5.3 68.5 4.2 33.3 5.0 1.7

8.9

15.0

4.7

6.4

8.6

15.7 3.6

5.0

6.3 6.2

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high plasticity AS-29 AS

DS-12

Pierre shale low plasticity AS-28 AS

RDS

DS-10

peak not achieved prior to increase in displacement rate

0.75 N solution of NazSO4 used as saturating fluid

O1

fro

I

0

Z

0

0 Z

0o m ~D .-I

Z

m Z

Z

RS, undried

RS, air dried

RS, blenderized

H A Ii5

H A 122

H A 119

69

69

6.0

6.0

0.59 0.59

6.0 12.0

69

1.75

1.6

6.0

14.03 1.75 1.75 14.03

3.1 3.1 1.6 1.6

3.2

3.3

2.0

3.1 3.6

3.6 3.1 3.7 3.8 3.7

Residual Shear Strength, ~r ', deg

5.3

480.4

191.2

15.0 6.0

1.7

57.6 3.5 35.1 5.1

Displacement per Stage Required to Achieve ~r ', cm ~ (not cumulative) Comments

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NOTE--All specimens except those tested in the rotational shear apparatus were intact except that the failure surface was precut prior to testing. ~ AS = annular shear; RDS = repeated direct shear; RS = rotational shear. b If residual not achieved for stage, displacement shown is total displacement for stage at specified displacement rate.

RDS

Test Type"

DS-!3

Shale/Test

Rate of Displacement, cm/day

Normal Stress, kg/cm 2

T A B L E 2--Summary o f residual shear strength tests on clay shales--Continued.

Z fi3

--4 m

0 -<

~D

O

t-

O Z -11 O :D

"U ), :]0 3> --I

i"1'1

-m

ITI Z

0~ O I" fD "U

01

TOWNSEND AND GILBERT ON CLAYS AND CLAY SHALES

55

TABLE 3--Summary o f results o f annular shear tests.

Normal Stress, kg/cm 2

Rate of Displacement, cm/day

Residual Shear Strength, d~r', deg

Displacement per Stage Required to Achieve ~r ', cmb

Paraitinga, undisturbed block, 94.7 lb/ft 3

3.1 3.1 3.1 1.1 1.1 1.1

1.75 7.02 1.75 1.75 7.02 1.75

31.3 31.2 31.1" 33.6 33.1 32.9"

8.7 49.8 11.7 9.6 29.4 10.4

Paraibuna, undisturbed block, 100.9 lb/ft 3

3.1 3.1 3.1 1.1 1.1 1.1 9.1 9.1 9.1

1.75 7.02 1.75 1.75 7.02 1.75 1.75 1.02 1.75

29.5 28.7 28.5" 27.4 27.8 27.4" 28.4 27.4 26.7"

9.8 48.7 8.5 9.6 29.4 10.9 16.1 51.5 16.1

Paraitinga, bag specimen, loosely compacted, 95.5 lb/ft~

1.1 1.1 1.1 3.1 3.1 3.1

1.75 7.02 1.75 1.75 7.02 1.75

34.5 33.5 32.7" 29.4 27.8 27.7~

25.3 28.1 12.3 19.0 47.1 15.4

Paraitinga, block specimen, loosely compacted, 96.1 lb/ft3

1.1 1.1 1.1 3.1 3.1 3.1 5.1 5.1 5.1

1.75 7.02 1.75 1.75 7.02 1.75 1.75 7.02 1.75

32.0 30.8 31.2" 30.5 29.6 30.0" 29.5 28.6 28.2"

17.7 33.1 5.9 3.8 48.7 2.8 13.8 79.9 6.4

Paraibuna, block specimen, loosely compacted, 98.0 lb/ft3

1.1 1.1 1.1 3.1 3.1 3.1

1.75 7.02 1.75 1.75 7.02 1.75

30.1 29.5 29.3 a 28.5 28.0 26.9 a

28.0 36.8 2.7 13.8 79.9 6.4

Paraitinga, block specimen, densely compacted, 106.8 lb/ft 3

1.1 1.1 1.1 3.1 3.1 3.1 9.1 9.1 9.1

1.75 7.02 1.75 1.75 7.02 1.75 1.75 7.02 1.75

33.8 32.3 32.4 ~ 30.8 29.8 29.6 a 28.1 28.1 27.7"

15.6 35.8 2.7 13.7 79.9 6.4 23.7 21.2 28.5

Paraibuna, block specimen, densely compacted, 108.9 lb/ft 3

1.1 1.1 1.1 3.1

1.75 7.02 1.75 1.75

28.3 28.1 28.5* 29.6

38.2 14.0 1.7 15.5

Specimen Description

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56

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

T A B L E 3--Summary of results of annular shear tests--Continued.

Normal Stress, kg/cm 2

Rate of Displacement, cm/day

Residual Shear Strength, ~, ', deg

Displacement per Stage Required to Achieve ~r ', cmb

3.1 3.1 9.1 9.1 9.1

7.02 1.75 1.75 7.02 1.75

28.6 28.4* 27.9 27.4 27.1 a

52.7 8.4 15.1 44.3 2.3

Volta Grande precut failure plane

3.1 3.1 3.1 1.6 1.6 1.6 6.1 6.1 6.1 12.1 12.1 12.1

1.75 14.05 1.75 1.75 14.05 1.75 1.75 14.05 1.75 1.75 14.05 1.75

8.9 9.5 8.6 9.3 10.0 9.1 8.3 9.2 8.6 8.6 ... 10.7 c

29.6 72.9 80.9 10.5 70.8 83.1 12.1 49.3 73.0 8.7 8.7 94.5

Volta Grande intact specimen

3.1 3.1 3.1 6.1 6.1 6.1 12.1 12.1 12.1 6.1 6.1 6.1 3.1

1.75 14.05 1.75 1.75 14.05 1.75 1.75 14.05 1.75 1.75 14.05 1.75 1.75

8.8 8.8 8.5 7.9 9.3 8.0 8.1 10.6 9.5 c 9.0 ~10 9.7 9.7

24.4 72.9 80.9 10.5 70.8 83.1 11.0 50.6 69.4 7.4 88.9 94.5 19.3

Specimen Description

* Residual strength for a given normal load, taken at slowest displacement rate at greatest cumulative a m o u n t of displacement. b Not cumulative. c Considered incorrect.

likewise demonstrated that residual shear strength values for repeated direct shear and annular shear tests on resedimented Cucaracha shale were similar. On the other hand, in a series of companion repeated direct shear and annular shear tests on five different over-consolidated finegrained soils, Bishop et al [3] observed that only in the case of remolded Cucaracha shale were residual shear strength values approximately the same. They reported that residual shear strengths of stiff clays determined by repeated direct shear tests were 13 to 58 percent higher than values measured by annular shear tests. In addition to the fundamental difference between the two tests relative Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

1

o

2.5

.5.0

I

I

I

A~

.

a

IO.O

planes.

12.5

0

0

2..5

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12.5

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7.5

EFFECTIVE NORMAL STRESS ~ r

2.5

KG/CMZ

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f . PIERRE S H A L E (LOW P L A S T I C I T Y ) O A H E DAM

~

I I I I I 2S s.o 7.5 ~0.O Iz.s e. BEARPAW S H A L E , FT. PECK DAM

|

NOTE: TESTS CONDUCTED IN O,75N NclzSO 4 .

d. KINCAID S H A L E j

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,

2

0

As|

~|

NOTE

N U M B E R S IN S Y M B O L S INDICATE S E Q U E N C E OF L O A D I N G FOR M U L T I S T A G E LOADING TESTS. SYMBOLS CORRESPOND WITH T E S T TYPE. I~ V A L U E S GIVEN A R E VALID W H E N P L O T T E D TO E Q U A L SCALES,

9 ROTATIONAL SHEAR (UNDRIED} 9 ROTATIONAL SHEAR (AIR-DRIED) 9 ROTATIONAL SHEAR (BLENDERIZE0)

REPEATED DIRECT SHEAR "~ PRECUT O ANNULAR SHEAR ~ SPECIMENS

LEGEND

9" PIERRE S H A L E (HIGH P L A S T I C I T Y ' ) OAHE DAM

,

1--Residual strength envelopes o f various clay shale specimens with precut failure

PROCTOR DAM

7.5

,

I I I 7.5 ~o,o Jz.5 L A N E P O R T DAM

FIG.

c. STRAWN S H A L E ,

0

I I 2,5 s.o b. TAYLOR I S H A L E ,

2

I

2..5 s.o 7.s ~o.o ~z.s a. DAWSON S H A L E , CHATF'IELD DAM

I

jb~#:7o*

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

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SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

to direction of displacement along the failure plane, repeated direct shear tests on soft materials may be impeded by testing difficulties, for example, excessive extrusion, box tilt, and failure to control the gap between the top and bottom halves of the shear box, which might lead to a premature termination of the test. However, these problems do not manifest themselves to such a degree in tests of stiff clay shales for specimens with precut failure planes. These results (summarized in Fig. 2) show that repeated direct shear test results fall within or near the range of { / v a l u e s measured by annular shear tests. MATERIAL TESTED

RANGE OF r O

VALUES FOR VARfOUS TEST TYPES AND TECHNIQUES

2

4

6

8

1

IO

12

DAWSON SHALE TAYLOR I SHALE STRAWN SHALE K INCAID SHALE SEA RPAW SHALE PIERRE SHALE (LOW PLASTICITY)

RA•""'NGE

PIERRE SHALE (HIGH PLASTiCITy)

OF VALUES FOR ANNULAR SHEAR TESTS i

LE, ,END O A N N U L A R SHEAR {3 REPEATED DIRECT SHEAR A ROTATIONAL SHEAR

I. EFFECTS OF O'n IGNORED, s CM/DAY 2. REPEATED A N N U L A R TEST - N O T ONE- DIRECTIONAL SHEARING

FIG. 2--Range in ~r' values measuredby various test techniques and methods. Effect o f Remolding

Figure 1 indicates that comparable residual shear strengths are measured by annular shear or repeated direct shear tests on intact specimens with precut failure planes and by rotational shear tests on specimens of remolded material. Although remolded material was not tested in annular shear or repeated direct shear in this study, Bishop et al [3] and LaGatta [8] demonstrated that nearly identical residual shear strength values were measured on remolded and intact specimens. These results suggest that the use of remolded material offers an alternative method of specimen preparation for determining ~r' values. However, the problems associated with soft materials tested in repeated direct shear tests mentioned in the preceding paragraph may seriously restrict this procedure unless the overestimation of ~,' by repeated direct shear tests is acceptable. The results listed in Tables 1 and 2 and presented in Fig. 1 demonstrate that, although the classification indexes vary dramatically with processing procedure, the method of processing has little effect on measured Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

TOWNSEND AND GILBERT ON CLAYS AND CLAY SHALES

59

residual shear strengths. Generally, blenderization produces somewhat lower residual strength values than those measured for undried or air-dried material. Apparently, blenderization causes a greater disaggregation of the soil particles than is achieved during shear strains. It may be observed that greater displacements are required to achieve the residual condition in tests using less disaggregated (undried and air-dried) materials than in tests using well-disaggregated (blenderized) materials. It appears, therefore, that while the processing method has little effect on measured +r' values, greater disaggregation reduces the test duration. LaGatta [8] observed similar results on crushed versus uncrushed Cucaracha shale; tenfold displacements were required for the uncrushed material to reach the same ddr' as that obtained for the crushed material. Although determination of residual strength by testing remolded specimens using the Harvard rotational shear apparatus is attractive, due to the ease of specimen preparation and short time required for testing, that is, three to four days versus the three to four weeks required for annular shear or repeated direct shear, the apparatus has one basic limitation, in that materials with fine sand or coarse silt sizes cannot be tested. For example, the Kincaid shale contained approximately 45 percent montmorillonite, which normally would facilitate rotational shear testing. However, hydrometer analyses revealed that the blenderized material contained only 52 percent minus 2/am size particles, and reliable results of rotational shear tests were unobtainable due to the grittiness of the material. The effects of coarse particles are quite evident because very thin specimens (0.2 cm thick) are tested in the Harvard rotational shear apparatus. Waterways Experiment Station experience indicates that results of tests on blenderized material containing less than 55 to 60 percent minus 2/am sizes may be questionable.

Effect of Precut Failure Plane The results of annular shear tests on both precut and intact specimens of a soft, Volta Grande Brazilian clay that could be sheared intact in the annular shear apparatus are listed in Table 3 and presented in Fig. 3. The excellent agreement of the results for the intact and precut specimens indicates that precutting has little effect on residual strength and at least ensures determination of the lowest possible residual value.

Effect of Density The annular shear test results for dense and loosely compacted intact specimens of micaceous silty sand are listed in Table 3 and presented in Figs. 4 and 5. These results show that ddr' is independent of initial specimen density. Obviously, the large deformations required to achieve a residual Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

60

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

3.0

LEGEND SPECIMEN TYPE 0

I 0

z

NOTE: NUMBERS BESIDE DATA POINTS REFER TO SEQUENCE OF LOADING.

~NTACT PRECUT

~

= 8.7 ~ WHEN PLOTTED TO EQUAL SCALES.

2.0

+;

m ~

J

g

8,s

zj~ o

I

a.s

~ I

5.o NORMAL STRESS ~

Z 7.5

I

~o.o

I

~z.5

9 KG/CM 2

FIG. 3--Residual strength for annular shear tests on precut and intact specimens of soft, Volta Grande Brazilian clay. condition alter any structure imposed by compaction, and the final void ratio in the failure zone is dependent only upon the effective normal stress, that is, the critical void ratio. E f f e c t o f N o r m a l Stress

The residual strength envelopes presented in Figs. 1 and 3 are satisfactorily represented by straight lines through the origin. Thus, for these materials, with the testing conditions and range of normal stresses involved, ~r' is independent of normal stress. However, the micaceous silty sand residual envelopes shown in Figs. 4 and 5 exhibit a slight curvature at lower normal stresses. This slight curvature at lower normal stresses is in agreement with the literature [3,8,15]: that up to a stress level of about 2 kg/cm 2 (1.96 • 105 Pa), ~r' decreases as the normal stress increases (that is, the failure envelope is curved at lower stresses and ~?r' is dependent upon the normal stress). This decrease in 4 / w i t h increasing normal stress is possibly due to the higher normal stresses forcing the clay platelets into a more oriented structure. Under lower normal stresses, a more random orientation exists. The degree of dependence of d~r' on normal stress appears to vary with material; for example, annular shear tests indicate that, for all practical purposes, blue London clay and Studenterlunden clay exhibit an independence of ~ ' with respect to normal stress, whereas brown London clay and Weald clay exhibit markedly curved residual strength envelopes [3]. Repeated direct shear test results at the Norwegian Geotechnical Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

o

o

9 9 9 9

LEGEND

[

2

5 i

6

NORMAL STRESS O-'n} KG/CM 2

4

i

o

7

F I G . 4 - - R e s i d u a l shear strength e n v e l o p e f o r Paraitinga material.

3

LOOSELY COMPACTED: FROM BAG S A M P L E LOOSELY COMPACTED: FP,OM BLOCK SAMPLE DENSELY COMPACTED: FROM BLOCK SAMPLE UNDISTURBED SAMPLE

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

(,q &J rr

D o

.J

I to

tO

O3

to to M

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r-

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-t

62

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

--

O

It

',,(

r W 6J J d D_ 0_

t~

UU O0 d J mnn

z

~"~I O 0 ~3

zE~ "9 W w~

\

uo~:

\

O~Z

I

\

I

I

I

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O O

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

TOWNSEND AND GILBERT ON CLAYS AND CLAY SHALES

63

Institute [10] also indicated that d?~' for Bearpaw shale varies only slightly with normal stress, while similar tests on Pepper shale show a marked dependence of ~ ' on o, '. Kenney [16] suggested that this dependence of ~r' upon the normal stress would be greater for highly montmorillonitic clays than for soils composed of massive minerals.

Effect of Loading Sequence The residual shear strength envelopes presented in Figs. 1, 3, 4, and 5 for annular shear tests indicate that sequence of shearing under various normal stresses has little or no effect and thus that ~r' is independent of stress history. Although it might appear that ~r' values determined by subsequent rebounding under lower normal stresses would be prejudiced by the residual structure preimposed under the higher normal stresses, this apparently is not the case. Evidently the large displacements required to achieve a residual state under each normal stress completely destroy any structure preimposed at a higher normal stress, and the practice of multistage testing is feasible. It was observed in these tests and those by Bishop et al [3] that, although a residual condition had been achieved for a particular normal stress, a further displacement of several centimetres was required to reestablish the residual condition under a subsequent lesser normal stress. Hence, it is imperative in multistage tests that sufficient displacements be achieved to reestablish a residual condition, particularly for rebound conditions. Conclusions

Within the bounds of this investigation (the testing methods employed and the procedures and equipment used) and from published information of similar studies, the following conclusions are drawn. (a) The difference in residual friction angles, ~r ', measured by repeated direct shear tests, annular shear tests, or rotational shear tests for different hard clay shales, varied from a minimum of 1.0 to a maximum of 3.3 deg and averaged 2.4 deg for the various materials tested. These variations include effects of different testing procedures and types of tests and are considered to be small. However, other investigators have indicated greater differences between results of repeated direct shear tests and annular shear tests on soft clays. (b) The residual friction angle is, for practical purposes, independent of specimen preparation procedure and density, and comparable residual shear strengths were obtained on intact, precut, and remolded specimens. The residual friction angle is also independent of ~nor~al stress; the residual strength envelope is accurately described by a straight line through

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64

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

the origin. However, some investigators have indicated that, for normal stresses less than 2.0 kg/cm 2 (1.96 x l0 s Pa), the residual envelope is curved; that is, the residual friction angle is dependent upon normal stress. (c) The sequence of application of normal stresses in a given test has no significant effect on the residual strength envelope. However, several centimetres of additional displacement may be required to establish residual conditions under reduced stresses. (d) The Harvard rotational shear device offers a rapid, satisfactory, and low-cost means for determining ~r' for many, but not all, clay shales. Tests in which the Harvard rotational shear apparatus is used should be restricted to shales with a minus 2/am content (blenderized) greater than 55 to 60 percent because of the adverse effects of the coarser particles.

A cknowledgments The tests described and the resulting data presented herein, unless otherwise noted, were obtained from research conducted under the Civil Works Investigation Program of the U. S. Army Corps of Engineers by the Waterways Experiment Station. Permission was granted by the Chief of Engineers to publish this information. References [1] Herrmann, H. G. and Wolfskill, L. A., "Engineering Properties of Nuclear Craters; Residual Shear Strength of Weak Shales," Technical Report 3-699, Report 5, U. S. Army Engineer Waterways Experiment Station, Corps of Engineers, Vicksburg, Miss., Dee. 1966. [2] De Beer, E. in Proceedings, Geotechnical Conference, Vol. 1, Oslo, Norway, 1967, pp. 83-88. [3] Bishop, A. W. et al, Geotechnique, Vol. 21, No. 4, Dec. 1971, pp. 273-328. [4] Skempton, A. W., Geotechnique, Vol. 14, No. 2, June 1964, pp. 77-101. [5] Tiedemann, B., Bautechnik, Vol. 15, Nos. 30 and 33, 1937, pp. 400-403 and 433-435. [6] Hvorslev, M. J., "Uber die Festigkeitseigenschaften Gestorter Bindiger Boden," Ingeniorvidenskabelige Skrifter A-Nr. 45, Danmarks Naturvidenskabelige Samfund, Copenhagen, Denmark, 1937 ("Physical Properties of Remolded Cohesive Soils," Translation No. 69-5, June 1969, U. S. Army Engineer Waterways Experiment Station, Corps of Engineers, Vicksburg, Miss.). [7] Petley, D. J., "The Shear Strength of Soils at Large Strains," Ph.D. dissertation, University of London, London, England, 1966. [8] LaGatta, D. P., "Residual Strength of Clays and Clay-Shales by Rotation Shear Tests," Contract Report S-70-5, U. S. Army Engineer Waterways Experiment Station, Corps of Engineers, Vicksburg, Miss., June 1970; prepared by Harvard University under Contract Nos. DACW39-67-C-0024 and DACW39-69-C-0028 (Harvard Soil Mechanics Series No. 86, July 1970). [9] Skempton, A. W. in Proceedings, Sixth International Conference on Soil Mechanics and Foundation Engineering, Vol. III, Discussion, Session 8, Montreal, 1965, pp. 551-552. [10] Kenney, T. C., "Direct-Shear Tests on Samples of Pepper and Bearpaw Shales," Internal Report F.24-13.01, Norwegian Geotechnical Institute, Oslo, Norway, May 1965. [11] Townsend, F. C. and Banks, D. C., "Preparation Effects on Clay Shale Classification

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

TOWNSEND AND GILBERT ON CLAYS AND CLAY SHALES

[12]

[13] [14]

[15] [16]

65

Indexes," Preprint 2140, American Society of Civil Engineers National Meeting on Water Resources Engineering, Los Angeles, Calif., Jan. 1974. Hdey, W. and MacIver, B. N., "Engineering Properties of Clay Shales; Development of Classification Indexes for Clay Shales," Technical Report S-71-6, Report 1, U. S. Army Engineer Waterways Experiment Station, Corps of Engineers, Vicksburg, Miss., June 1971. "Engineering and Design, Laboratory Soils Testing," Engineer Manual EM 1110-2-1906, Headquarters of the Department of the Army, Corps of Engineers, Washington, D. C., Nov. 1970. LaGatta, D. P., "The Effect of Rate of Displacement on Measuring the Residual Strength of Clays," Contract Report S-71-5, U. S. Army Engineer Waterways Experiment Station, Corps of Engineers, Vicksburg, Miss.; prepared by Harvard University under Contract No. DACW39-69-C-0028,Aug. 1971. Chattopadhyay, P. K., "Residual Shear Strength of Some Pure Clay Minerals," Ph.D. dissertation, 1972, University of Alberta, Edmonton, Canada. Kenney, T. C. in Proceedings, Geotechnical Conference, Vol. 1, Oslo, Norway, 1967, pp. 123-129.

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A r a A r m a n 1 a n d K . L. M c M a n i s ~

Effects of Storage and Extrusion on Sample Properties

REFERENCE: Arman, Ara and McManis, K. L., "Effects of Storage and Extrusion on Sample Properties," Soil Specimen Preparation for Laboratory Testing, ASTM

STP 599, American Society for Testing and Materials, 1976, pp. 66-87. ABSTRACT: The shear strength of specimens obtained with thin-walled tubes decreased as a result of long-term storage. The preconsolidation pressure of stored tube samples also followed a similar trend. Specimens obtained from hand-cut blocks did not show a decrease in strength or preconsolidation pressure with time. Samples stored by wrapping in plastic film and aluminum foil were found to be preserved as well as those stored after coating with paraffin. Stresses applied to samples for extruding from thin-wailed tubes were found to be in excess of the shear strength of the soils. X-ray radiographs of specimens obtained with thin-walled tubes showed excessive disturbances caused by sampling and extrusion. KEY WORDS: soils, storage, tubes, shear strength, radiography, penetrometers (radiation)

I n g e o t e c h n i c a l engineering, the decision m a k i n g a n d design is o f t e n d e p e n d e n t o n the p r o p e r t i e s o f the f o u n d a t i o n m a t e r i a l available. I n these cases, it is o f u t m o s t i m p o r t a n c e t h a t the p h y s i c a l p r o p e r t i e s o f t h e soil m a t e r i a l s be a c c u r a t e l y d e t e r m i n e d regardless o f the s o p h i s t i c a t i o n a n d accuracy of the analytical methods. The structural integrity and economy o f t h e final s t r u c t u r e will d e p e n d largely o n the j u d i c i a l choice o f representative s a m p l e s a n d specimens a n d o n t h e m e t h o d s o f testing. S a m p l e d i s t u r b a n c e occurs g e n e r a l l y d u r i n g t w o essential stages o f the s a m p l i n g o p e r a t i o n : p o s s i b l e m a j o r d i s t u r b a n c e s w h e n t h e s a m p l e s are r e m o v e d f r o m their p a r e n t d e p o s i t s (not discussed in this p a p e r ) ; a n d d i s t u r b a n c e due t o e x t r u s i o n , p r o t e c t i o n , storage, t r a n s p o r t a t i o n , a n d s p e c i m e n p r e p a r a t i o n . This p a p e r p r e s e n t s s o m e t y p i c a l results f r o m a c o m p r e h e n s i v e s t u d y c o n c e r n i n g the effects o n test specimens due to s t o r a g e , e x t r u s i o n , m e t h o d s o f selecting r e p r e s e n t a t i v e specimens, a n d t Professor, Department of Civil Engineering, Louisiana State University, Baton Rouge, La. 70803, and Dean of Engineering, Delgado College, New Orleans, La. 70119, respectively. 66 Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by by ASTM International www.astm.org Copyright9 1976 University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

ARMAN AND McMANIS ON STORAGE AND EXTRUSION

67

inherent variations within samples. Six sample sites, representing several typical geologic formations in Louisiana, provided about 1800 samples for more than 6500 laboratory and 1700 field tests of stiff pleistocene clays, soft silty marine clays, and highly organic silts and clays.

Literature Survey Hvorslev [1],2 in his classical work on subsurface exploration, listed the requirements for a suitable, undisturbed laboratory specimen: no disturbance of the soil structure, no change in water content or void ratio, and no alteration in constituents or chemical composition. At the same time, he acknowledged the fact that such criteria are impossible to meet. Most geotechnical engineers, realizing the limitations of sampling, use test data with caution. Often, however, uncertainties arising from sample disturbance become glossed over by sophistications involving the analysis and design of foundations. Various methods and tools for borehole sampling have evolved, and cores obtained with thin-walled tubes have been accepted universally. In spite of warnings, such as one by Terzaghi and Peck [2], that, " I f tube samples have been taken . . . . it is always desirable to investigate the extent to which the consistency of the clay has been affected . . . . " little attention is paid to the extent of disturbance in most daily work. Bozozuk [3] showed that the long-term storage of marine clays reduced the measured preconsolidation pressure by 4.8 percent. Thus, consolidation tests should be conducted as soon as possible after sampling. Kallstenius [4] found a general lowering of the strength of samples with storage time. Many investigators have found that tube sampling causes disturbance, resulting from excessive friction along the wall of the tube. It is logical to expect, therefore, that similar disturbances also occur during the extrusion of samples. Lang [5] measured the forces acting on a stiff clay in an open-drive, thin-walled tube sampler and, by two different methods, determined the ratio of peak forces (inside force/outside force) to be 0.48 and 0.28, respectively. The average friction along the tube wall was 64 percent of the shear strength of the soil. The peak axial stress in the core had a mean value of 13.4 times its uniaxial shear strength. Sone [6] found the pressure required for extrusion of an alluvial clayey silt to be several times larger than its unconfined compressive strength, and the compression during extrusion was almost equivalent to the failure axial strain measured in the unconfined compression test. Nuclear techniques to determine changes in the bulk density of soil samples resulting from sampling and extrusion showed only minor changes resulting from extrusion [7]. 2The italic numbersin brackets refer to the list of referencesappendedto this paper. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

68

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

Various techniques have been used to determine visually the extent of structural disturbances. Terzaghi, Hvorslev, and others sliced and slowdried their specimens to enhance the structural features. Even though this technique is very effective, it not only destroys the sample, but also affords visual examination of only one plane. Radiography has been used successfully for geotechnical investigations [8] and for studying varved clays [9]. The selection of typical specimens, as well as specimen size, is still largely left to personal preference and convenience. Past work indicates that small specimens trimmed from large samples provide better test results. One recommendation is that the trimmed surfaces should be as small as possible in relation to the original size of the specimen [5]. To avoid disturbance caused by remolding of the outer surfaces, large samples should be obtained and trimmed so that any remolded material will be removed [10]. The shear strength of Leda clay was shown also to be affected by the sample diameter [11]. It was shown that sample size is the most important factor influencing the shear behavior of specimens [121. Several investigators have shown that hand-cut (bench-cut) block samples are less disturbed structurally at almost all stages of the sampling operation. Block samples are often uneconomical or cannot be physically obtained to the required depths.

Sampling and Field Testing Penetration Tests The ASTM Penetration Test and Split-Barrel Sampling of Soils (ASTM D 1586-67) were made for visual classification and determination of natural moisture content to assure that all later tests would be performed in the same soil deposit. The ASTM Thin-Walled Tube Sampling of Soils (D 1587-67) was made with 2.8-in. tubes to obtain cores for several experiments. Except for certain samples to be used for extrusion tests in the laboratory, the cores were extruded in the field with a hydraulic ram, classified visually, cut into 10 in. lengths, and then wrapped or coated. The samples for extrusion testing were sent to the laboratory in sealed tubes.

Sample Protection Because two methods of sealing the extruded cores were to be compared, identical samples were eitheff immersed completely in melted paraffin or wrapped first in household polyethylene film and aluminum Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authoriz

ARMAN AND McMANIS ON STORAGE AND EXTRUSION

69

foil and then sealed in household polyethylene bags. All samples were transported in specially molded styrofoam boxes (Fig. 1).

FIG. 1--Tubesamples and special container. B l o c k Samples

Hand-cut block samples (1 ft cubes) were obtained from only two of the sites because of the difficulty of obtaining such samples from deep layers. One set of these was obtained from a deposit of stiff pleistocene clay in southeast Louisiana at depths of 15 to 35 ft. A second set was obtained from a soft highly-dessicated fat clay (recent Mississippi River alluvial deposit) at depths of 5 to 15 ft. Standard penetration tests and tube sampling were performed at the natural ground surface within a radius of 15 ft of the block sampling sites. The samples were cut by chain saw and hand tools from active borrow pits which were excavated immediatley preceding the tube sampling. They were covered with several layers of aluminum foil and paraffin and placed in specially fabricated boxes with 1 in. of sawdust insulation all around.

Laboratory Tests and Results For all the soil tests except radiography and photoelastic analysis, the following standard procedures were used: ASTM Laboratory Determination of Moisture Content of Soil (D 2216-66); ASTM Particle-Size AnalyCopyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

70

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

sis of Soils (D 422-63); ASTM Test for Liquid Limit of Soils (D 423-66); and ASTM Test for Plastic Limit and Plasticity Index of Soils (D 424-59). Long-term storage effects required, in addition: ASTM Tests for Unconfined Compressive Strength of Cohesive Soils (D 2166-66); ASTM Test for Unconsolidated, Undrained Strength of Cohesive Soils in Triaxial Compression (D 2850-70); and ASTM Test for One-Dimensional Consolidation Properties of Soils (D 2435-70). The effects of core extrusion from thin-walled tubes were studied from the unconfined compressive strengths, consolidation, and radiographs.

Sample Protection The coating of undisturbed samples with paraffin has been accepted universally as one method of preserving sample integrity. However, earlier laboratory observations indicated that even experienced and careful technicians had difficulties in removing the hardened paraffin without damage to the samples, Sweating under the paraffin was noted also. A sample from a depth of 40 ft has a temperature of about 65~ (18~ When it is wrapped in foil and dipped in hot paraffin 120~ (49~ considerable sweating results, with an increase of moisture content in a zone adjacent to the outer surface of the sample. The potential sudden effects of water migration on the pore pressure and moisture distribution, as well as the homogeneity of the soil, would be highly undesirable. The use of aluminum foil and plastic film as an alternative protective coating was studied. Approximately 100 tube samples (about 18 in. long) of three types of soil were split immediately after sampling to make three specimens. One of these, to be used for determining the natural moisture content, was wrapped in a plastic bag. The others were used to determine the effectiveness of alternative protective coatings. One slice, after being wrapped in foil, was placed in a 6-in. round ice cream carton which, in turn, was filled with melted paraffin (the usual field procedure). The other specimen was wrapped in aluminum foil and sealed with household plastic film. The three specimens from one sample were then placed in a clear plastic bag that was sealed partially with a twist wire tie and sent immediately to the laboratory in a styrofoam container. The natural moisture contents were determined immediately upon arrival at the laboratory. Both the paraffin-coated and the foil/plastic-wrapped specimens were stored at 100 percent relative humidity and 72~ (22~ After random storage periods of 14 to 33 days, the two specimens of a set were tested for moisture content. The results indicated that the foil/plastic wrapping maintained the moisture content of a specimen just as well as the paraffin coating (Table 1). After these tests, the Louisiana Department of Highways adopted

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ARMAN AND McMANIS ON STORAGE AND EXTRUSION

71

TABLE 1--Test o f sample protection methods (typical soft and stiff heavy clays)--14 to 31 days at 72~ and lOOpercent humidity. Average Moisture Content, ~ Natural

Paraffin Coated

Foil Plastic Wrapped

29 27 39 35 39 51 53 57 22 22 24 64 67 19 21 17 20 13 12

26 29 37 35 38 48 54 53 21 24 19 64 65 16 21 18 20 13 12

27 30 38 31 40 53 53 55 21 22 24 64 67 17 20 19 20 11 12

the use o f foil/plastic protection instead o f paraffin. Samples are stored and transported in a specially designed insulated styrofoam container (Fig. 1). S i m u l a t e d F i e l d Storage

Earlier observations had shown that sampling crews remain in the field usually about five days before bringing samples to the laboratory. These samples are generally extruded in the field, wrapped, placed in the special s t y r o f o a m boxes (Fig. 1), and stored in covered trucks. The temperature in the storage part o f these trucks often rose to and remained at I I 0 ~ (43 ~ for about 10 h each day. To study the effect o f such storage on moisture content, 10-in.-long samples (2.8 in. diameter) were brought in plastic bags to the laboratory immediately after extrusion. A Y2-in.-thick specimen was cut f r o m the middle to determine the natural moisture content. The remaining two end sections (about 4.5 in. long) were then rewrapped and placed in the styrof o a m containers. One set of containers was kept in a forced-draft oven at l l 0 ~ (43~ the other set at 72~ (22~ and 100 percent relative humidity. After five days, the specimens were removed and the moisture contents determined f r o m the entire 4.5 in. length. Because no appreciable difference in moisture content was found

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72

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

among the three sets, it is evident that the storage methods and the two protective coatings caused no changes in the moisture content (Table 2). Thus, any observed differences in the moisture content of samples are inherent--probably due to silt and calcareous intrusions, etc. TABLE 2--Simulated field storage o f extruded cores. Moisture Content, % Soil Type

Natural

5 Days at 100~

Heavy clay Heavy clay Heavy clay Heavy clay Heavy clay Heavy clay Heavy clay Silty clay Light silty clay Clay Clay Clay Clay Clay Clay

56.0 69,0 69.0 66.0 67.0 64.0 64.0 19.3 29.0 23.0 56.0 69.0 66.0 67.0 64.0

63 63 69 69 71 64 64 19 29 23 63 70 69 72 64

5 Days at 72 ~ and 100 Percent Humidity 62 71 70 66 65 67 63 14 3O

Long- Term Storage The effects of long storage times on the shear strength and consolidation characteristics were determined from three types of samples: 12-in. hand-cut cubes, 5-in. cores, and 2.8-in. cores. All samples were sent to the laboratory immediately after protective coatings had been applied. Initial testing was performed on 2.5-in.-diameter cylindrical specimens trimmed from identical companion samples to determine the moisture content, unconfined compressive strength, undrained triaxial shear strength, and consolidation characteristics without long-term storage effects. The remaining samples were stored for different times at 72~ (22~ and 100 percent relative humidity in the field-applied wrappings. In the case of the unstored samples, the initial (four to seven day) undrained triaxiai shear strengths of specimens from the 5-in. cores were slightly lower than those from the large hand-cut blocks. Specimens from 2.8-in. cores had much lower strengths (Fig. 2). Such effects must be attributed to disturbances of the outer zones during tube driving and core extrusion. At the end of randomly chosen periods, the stored samples were removed from their containers, trimmed to cylindrical specimens (2.5 in. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

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Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

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74

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

diameter), and tested for unconfined compressive strengths, undrained triaxial shear strength, and consolidation characteristics. Only samples with similar moisture contents, densities, and classifications were used. Up through the first ten days of storage, the decrease in shear strengths for specimens from the three sizes of samples was small--practically indiscernable. However, the specimen strength of both the 2.8 and 5-in. cores deteriorated at an increasing rate after the first ten days (Fig. 2). Any attempt to analyze this strength deterioration versus time numerically would be of no value and might be misleading, because each type of soil reacts differently to storage time, as well as other disturbances. However, it is of the utmost importance to know that extended storage affects the strength and that it should be avoided. The derivation of a universai correction factor or formula does not appear feasible due to all the variables involved. The relaxation of overburden stresses, changes in pore pressure, and unavoidable migration of water within the sample, as some of the reasons for changes in the measured soil characteristics, are supported by the lack of strength reduction in specimens from the block samples (Fig. 2). The results agree with those of Kallstenius [5]. Extended storage times also bring about a reduction of the preconsolidation pressure by as much as 30 percent (Fig. 3) for specimens from the tube cores. By contrast, no change occurred in the block samples, except for some scatter after 100 days. Bozozuk [3] found much less reduction (only 4.8 percent). Evidence of disturbance in 2.8-in. cores is shown in Fig. 4. The void ratio/log pressure curves approach those obtained for the same soils after they had been remolded and compacted under static loads that approximated the original overburden pressures. The average moisture contents during extended storage remained unchanged. Conceivably, some change in moisture content or dry density should occur also. However, all determinations of these properties showed only small variations, due to natural inherent scatter.

Extrusion Effects Several methods are used to extrude soil cores from sampling tubes. Pressurized water, as well as hydraulic rams, are used in the field or laboratory. Each method affects the soil properties, depending on the soil type, its condition, and the type of sampling tube, etc. Several replicate samples were obtained with a 2.8-in. thin-walled tube from each type of soil deposit. Some cores were extruded hydraulically in the field and placed in a protective coating. Others were sealed in the tubes. All samples were transported without delay to the laboratory.

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ARMAN AND McMANIS ON STORAGE AND EXTRUSION

s

75

Biock - trimmed to 2.5 in. r A

2.5 in. # sample trimmed from 2.81n. ~4 o v LU 0E

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LOG TIME (DAYS) FIG. 3--Storage time versus preconsolidation pressure.

The field-extruded samples were tested for unconfined compressive strength after being radiographed. The sealed samples were used to determine the extrusion pressure and the resulting strains. At first, strain cells attached to the piston of the hydraulic ram were used to determine the extrusion forces. More consistent results were obtained later with a calibrated pressure gage attached to the oil cylinder. The displacements at the ends of the cores were measured with two extensiometers. After a core had been extruded, a portion of it was trimmed and tested for unconfined compressive strength. Another section was sliced for X-ray radiography. Some test specimens were radiographed also before the unconfined compression test.

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76

SOIL SPECIMEN

PREPARATION

FOR LABORATORY

TESTING

1.2

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During core extrusion, the end of the sample in contact with the piston began to show measurable displacements before the opposite end. Thus, internal displacements were occurring within the tube. The maximum strain (the strain at the piston end before steady movement at the opposite end) varied from 0.001 to 0.005. The average strain was 0.003 (Table 3). In all cases, the applied stress exceeded the unconfined compressive strength of the soil to a maximum of 900 percent. This is somewhat less than the 1340 percent measured by Sone [5]. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

ARMAN AND McMANIS ON STORAGE AND EXTRUSION

77

TABLE 3--Typical extrusion stresses and unconfined compressive strengths (heavy stiff clay at 35 f t depth).

Sample Length, in. 22 33 32 33 33 33 25 33

Max Extrusion Stress, psi 69 150 137 94 129 159 78 198

Max Extrusion Strain 0.005 0.004 0.004 0.003 0.003 0.003 0.001 0.003

Unconfined Composition Strength," psi 17 12 20 20 14 20 6 28

Unconfiried Composition Strength,b psi "14"' 18 19 ... o

H

9 ...

=Laboratory extruded. bField extruded.

Radiography

Radiographs showed two distinct distorting effects caused by sampling and extrusion processes. The first type of distortion, observed in all cores, was a gradual bending of the soil layers, with a m a x i m u m at the tube surface and decreasing toward the center. The bending was a symmetrical dome-like effect around the longitudinal axis. This effect was more pronounced in the soft marine deposits (Fig. 5) tested than it was in the stiff pleistocene clays (Fig. 6). A comparison o f the layers in the hand-cut samples (Fig. 7) and the tube samples (Figs. 5 and 6) demonstrates the effects of tube driving and extrusion. Note that reference grids were superimposed photographically on the X-ray films to aid in visual analysis. A second, more serious type of disturbance shown by the radiographs was a definite failure plane pattern in some of the untested stiff clays (Fig. 8). These failure planes, either at a 50 deg angle or with a classical cone shape, occurred at intervals along the longitudinal axis of some specimens. To determine if the failure planes had occurred during either sampling or extrusion, radiographs o f specimens in tubes were made also. H o w ever, because o f parallax effects caused by the roundness o f the sample and shadowing of the high-intensity X-rays necessary to penetrate the steel tubes, the tests were inconclusive. Thus, it could not be determined clearly when these failures occurred. However, edge turning could be observed in the radiographs of unextruded c o r e s - - a n indication that at least some o f the edge distortion took place during tube driving. H o w ever, it is more likely that most of the failure planes appeared during extrusion because the sample was in compression.

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78

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 5--Edge turning in marine clays.

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ARMAN AND MCMANIS ON STORAGE AND EXTRUSION

79

FIG. 6--Edge turning in stiff clays. Photoelastic Analysis

To obtain a qualitative analysis of the stress patterns developed during sampling, a photoelastic analysis was performed with a thin-walled tube that had two opposite 90 deg arcs of the wall removed. A gelatin/water mixture was poured into a 4 by 5-ft by 3-in. glasswalled tank with cross polorizing filters on the walls. After the gelatin had set, the tube sampler was driven in slowly by hand. The developed stress patterns (resulting from bi-refringence of gelation) were recorded on color movie film and color still photographs (Fig. 9). These patterns, Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

80

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 7--Lack of distortion in block speciman (stiff clay). covering the full volume of the driven tube, were very similar to the bent layers in soft soils that had been observed in the radiographs (Fig. 6). However, no failure planes developed. Although this experiment showed that distortion occurs during sampler driving, it did not disprove the earlier stated contention; friction, due to adhesion of the soil to the tube wall, increases the distortion during extrusion.

Selection of Representative Specimens Importance f o r Design

The selection of a specimen to represent a specific soil deposit is critical to soil data acquisition. Once the choice has been made, the designer Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

ARMAN AND McMANIS ON STORAGE AND EXTRUSION

81

FIG. 8--Failure plane in 5-in.-diameter specimen.

receives only the data obtained from that particular point in the ground, or for the whole deposit, from a few selected samples. In special cases, when all the samples are used for testing, any errors that may result from specimen choice can be eliminated. In order to observe and analyze the specimen selection process, the authors made long-term observations in several soil laboratories, as well as testing samples obtained from various sources. A review of the soil profiles at several agencies indicated that the soil characteristics were plotted as neatly divided 3-ft layers. If one considers that thin-walled tube samples are 3 ft long, such plots indicate arbitrary specimen selection in the laboratory. However, soils do not always become deposited as homogeneous 3-ft layers. Without the benefit of the observations of a wellqualified soil exploration technician or engineer, a high risk factor may be introduced into geotechnical engineering. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

82

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 9--Photoelastic simulation of tube sampling. Variations in 36-In.-Long Cores

Figure 10 shows the erratic physical variations along a typical 36-in.long tube sample obtained from a soil that is considered locally to be uniform. (It should be noted that the core was classified as a silty clay for its full length.) All the specimens showed similar variations in the Atterberg limits. With the unconfined compressive strength varying from 1100 to 2700 psi, the arbitrary selection of a representative single specimen from this sample could result in either underdesign or overdesign. It should be emphasized that some of these variations are probably due to sampling and extrusion disturbance. However, slickensides not visible to the naked eye, very thin sand and silt layers, calcareous nodules, and marine shells were observed in radiographs of parts of these specimens (Fig. 11). P o c k e t Penetrometers

The use of pocket penetrometers to assist the laboratory technicians Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

ARMAN AND McMANIS ON STORAGE AND EXTRUSION

83

Top of somple 0

rm

l

i

|

i

i

I

I

I

I

I000

2000

3000

4000

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~m ~ 24 3O

36 I

50

I00 DRY U N I T W E I G H T ( I b / f t 3)

I

150 30

40

I

50

MOISTURE CONTENT (%)

0

UNCONFINED STRENGTH ( I b l f t 2)

FIG. lO--Variations in sample properties, or engineer in specimen selection has been studied also. (This instrument is presently in use by some agencies for this purpose.) One hundred and fifty strength tests, obtained from one site by unconfined compression, undrained triaxial compression, and pocket penetrometer tests (Fig. 12), show wide scatter, with the plots distributed unequally above a 45 deg line (representing one-to-one agreement). Because of inherent scatter in both the triaxial and unconfined compressive strengths of soils, all the scatter in Fig. 12 should not be attributed to the pocket penetrometer. Statistical analysis shows the error distribution of pocket penetrometer readings to be a band, in which they were at least one half the laboratory strengths 97 percent of the time and not more than one and one-fourth the laboratory strengths 94 percent of the time (Fig. 13). On the average, the errors vary from 90 to 120 percent. Based on this evaluation, the pocket penetrometer has been considered satisfactory for selection, but not for testing. It may reduce the probability of gross errors, but it will not solve completely the problem of selecting typical specimens. Field N o t e s

Many errors in the selection of a representative specimen can be eliminated by a thorough study of the field notes taken by a well-qualified, observant exploration engineer or technician. The penetration test record, comments on the terrain, as well as careful notes (for example, of sample separation after extrusion because of slickensides or very thin silt or sand Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

84

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 11--Embedded shells and silt planes in 2,8-in. tube sample.

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ARMAN AND McMANIS ON STORAGE AND EXTRUSION

85

FIG. 12--Pocket penetrometer versus laboratory strength.

lenses, or obnoxious odors) serve as a warning to the laboratory technicians and the engineer who makes the final selection of the most representative specimens. General Conclusions Undisturbed tube samples do not have to be coated with paraffin for protection. A wrapping of household polyethelyene film covered by aluminum foil, followed by sealing in common household polyethelyene Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

86

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 13--Error distribution in strength. bags, is satisfactory protection, provided that they are stored also in containers insulating them against impact and direct exposure to the elements. Long-term storage of soil cores results in serious deterioration of the shear strength and lowering of the measured preconsolidation pressure. Tube samples should be tested within 15 days after sampling to prevent erroneous results, due to the deteriorating effects of long-term storage. The extrusion of soil cores from thin-walled tubes required friction stresses and axial compression greatly in excess of the unconfined shear strength. Soil reorientation, as well as internal failures, may occur during tube driving and core extrusion. Immediate attention should be given to the development of a thin, low-friction lining for sampling tubes. Radiography is useful for determining the extent of the disturbances, as well as the presence of anomalies, in soil cores. It should be made part of the soil testing program in critical foundation projects. The pocket penetrometer is a useful, inexpensive, and a simple tool for identifying and selecting representative soil specimens. Soils should be tested within two weeks after sampling to prevent test errors due to the deteriorating effects of long-term storage. If practical, large (at least 5-in.) diameter tube samples should be obtained. They Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

ARMAN AND McMANIS ON STORAGE AND EXTRUSION

87

could then be trimmed to small specimens, in which the trimmed-down distorted outer portions have been eliminated. However, for critical designs involving soft soils, in situ tests must be made in addition to laboratory tests and should be given prominence when the acquisition of large tube or block samples is impractical or uneconomical. Acknowledgment This study was conducted as part of a research project financed by the Louisiana Department of Highways and the U.S. Department of Transportation, Federal Highway Administration. The authors express their gratitude to the Louisiana Department of Highways for the personnel and equipment they provided. Thanks are due to Charles Hill, engineering editor, and Norma Duffy, draftswoman, for their invaluable assistance. The dedicated work of many student workers who spent their time collecting data is gratefully acknowledged. References [1] Hvorslev, M. J., Subsurface Exploration and Sampling of Soils for Civil Engineering Purposes, reprint, Engineering Foundation, 1965. [2] Terzaghi, K. and Peck, R. B., Soil Mechanics in Engineering Practice, 2nd Edition, Wiley, New York, 1968. [3] Bozozuk, M. in Samplings of Soil and Rock, ASTM STP 483, American Society for Testing and Materials, 1970, pp. 121-131. [4] Kallstenius, T. in Proceedings, Specialty Session on Quality in Soil Sampling, Fourth Asian Conference, International Society for Soil Mechanics and Foundation Engineering, Bangkok, July 1971. [5] Lang, J. G. in Proceedings, Fourth Asian Conference, International Society for Soil Mechanics and Foundation Engineering, Bangkok, July 1971. [6] Sone, S. in Proceedings, Fourth Asian Conference, International Society for Soil Mechanics and Foundation Engineering, Bangkok, July 1971. [7] Shackel, B. in Proceedings, Fourth Asian Conference, International Society for Soil Mechanics and Foundation Engineering, Bangkok, July 1971. [8] Krinitzsky, E. L., Radiography in the Earth Sciences and Soil Mechanics, Plenum Press, New York-London, 1970. [9] Kenny, T. C. and Chan, H. T., Canadian Geotechnical Journal, Vol. 9, 1972, p. 195. [10] Holtz, W. G. in Laboratory Shear Testing of Soils, ASTM STP 361, American Society for Testing and Materials, 1963, p. 417. [11] Coats, D. F. and McRostie, G. C. in Laboratory Shear Testing of Soils, ASTM STP 361, American Society for Testing and Materials, 1963, p. 459. [12] Lo, K. Y., Seychuk, J. L., and Adams, J. I. in Sampling of Soil and Rock, ASTM STP 483, American Society for Testing and Materials, June 1970, pp. 60-71.

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T. H. IV. Baker'

Transportation, Preparation, and Storage of Frozen Soil Samples for Laboratory Testing

REFERENCE: Baker, T. H. W., "Transportation, Preparation, and Storage of Frozen Soil Samples for Laboratory Testing," Soil Specimen Preparation for Laboratory Testing, ASTM STP 599, American Society for Testing and Materials, 1976, pp. 88-112. ABSTRACT: Increased construction activities in cold regions require a better understanding of the behavior of frozen soil. Although in situ testing procedures are being improved or developed, the highly variable and complicated conditions usually encountered in the field make it difficult to formulate interpretations that are useful in practice. Controlled laboratory tests on frozen soils are helpful, therefore, in determining behavior indicative of diverse field conditions and will assist in establishing design information. This paper is a review of the procedures used and the difficulties encountered in the transportation, preparation, and storage of naturally and artificially frozen soil samples required for testing in the laboratory. The effective control of sample disturbance by temperature variation and sublimation and the need for further investigation as to their effect on the mechanical properties of frozen soil are discussed.

KEY WORDS: soils, frozen soils, transportation, preparation, preserving, sublimation, thermal degradation, mechanical properties, laboratory tests

Procedures for transporting, preparing, and storing the naturally and artificially frozen soil samples that are required for laboratory testing are still in the formative stages. This paper reviews the current practices used for naturally frozen field samples from the time of sampling, and for artificially prepared laboratory samples from the start of the molding operation. A review of the literature is included, as well as the experiences of industry, university groups, and government departments and agencies. 'Research officer, Division of Building Research, National Research Council of Canada, Ottawa, Ont., Canada. 88 Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 9 Downloaded/printed by Copyright 1976byASTMInternational www.astm.org University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BAKER ON FROZEN SOIL SAMPLES

89

Factors Affecting Laboratory Tests on Frozen Soils Laboratory tests are performed on naturally and artificially prepared frozen soil samples to investigate (a) strength and creep deformation behavior and (b) volume change. Tests conducted under (a) include: stress and strain controlled unconfined compression tests, stress and strain controlled triaxial shear tests, ring shear tests, Brazilian tests, tension tests, load-controlled indentation tests, flexural strength tests, dynamic loading tests, and torsion tests. Tests conducted under (b) include: uniaxial consolidation upon thawing under constant stress, triaxial consolidation upon thawing under constant stress, and uniaxial heave during freezing. The strength and deformation properties of frozen soil samples are known to be affected by sublimation, evaporation, and thermal disturbance. Their effect is in the redistribution and ultimate loss of moisture from the specimen as the result of a temperature gradient or low humidity environment or both. Loss of moisture reduces the cohesion between soil particles and may reduce the strength (which is dependent on temperature). The effects of moisture redistribution in frozen soil are not known, but are thought to change the strength and creep behavior. Thermal disturbance of a frozen sample refers not only to thawing, but also to temperature fluctuations. Soil structure may be changed completely if the sample is thawed and then refrozen. Temperature fluctuations can set up thermal gradients, causing moisture redistribution and possible change in the unfrozen moisture content. Care must be taken, therefore, to ensure that frozen soil specimens remain in their natural or prepared states and that they are protected against the detrimental effects of sublimation and thermal disturbance until testing is completed. The type of laboratory test to be conducted will determine the degree of care required in providing this protection. Frozen Soil Samples

Naturally Frozen Undisturbed block and core samples of frozen soil must be protected from thawing and loss of moisture, from the time they are taken from the ground, throughout the period of transportation, storage, machining, and testing. In most cases, the sample must be maintained under the same thermal and moisture conditions existing at the time of sampling. The implications and practicality of this will be discussed later. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

90

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

Methods of handling frozen soil samples in the field have not changed greatly in recent years. When the samples have been examined visually and logged at the field site, most investigators wrap the frozen samples in cellophane and place them in polyethylene bags (6 mil) (Fig. 1). All air

FIG. 1--Large block sample of frozen soil delivered from the fieM in a polyethylene bag (scale in tenths of afoot).

must be evacuated from the bag, and it must be sealed to prevent sublimation. This is accomplished best by using a vacuum pump, but can be done by forcing most of the air out by hand before sealing. Humidity can be maintained by placing some snow or crushed ice in the bag. Heat sealing the plastic bag after de-airing gives the most permanent seal, but a locking nylon tie (of the type used for bundling electric cables) is adequate if it is fastened properly. The seal should be checked regularly; any air that has entered the bag should be removed, and the bag should be resealed. This packaging procedure keeps moisture loss from the sample within acceptable limits and has been used successfully by the author to Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BAKER ON FROZEN SOIL SAMPLES

91

store frozen samples for periods of eight months or more. To minimize still further the moisture loss due to transfer of vapor through the plastic, Roggensack [1] 2 suggested that the wrapped samples should be packed in snow or ice chips. This will reduce vapor pressure gradients. Even with these precautions, some water loss occurs as vapor leaves the sample and crystallizes on the inside of the plastic wrapper. This will be discussed in the section on sublimation. After the sample has been packaged to keep sublimation to a minimum, some facility is required at the site to prevent thermal disturbance. For example, if unfrozen samples have been obtained for frost heaving experiments, they must not be allowed to freeze so that they can be tested in truly undisturbed conditions. If ambient temperatures are below 0~ these samples should be placed in well-insulated boxes and kept in heated tents or some other warm enclosure until they are shipped. On the other hand, when frozen soil samples are obtained for thaw consolidation, strength, and creep tests, the in situ temperature must be maintained. As ground temperatures vary with depth during the year, the temperature should be determined at the time of sampling. In addition, depending on the tests to be conducted, it may be necessary to determine the annual amplitude of the temperature fluctuations at the sample depth. Maintaining the in situ temperature of the frozen samples has been attempted, using several methods. Portable refrigerators powered by 110-V generators have been used successfully at field sites to store frozen soil samples. These can control adequately the thermal environment and can be set to maintain the temperature measured at the field site when the sample was obtained. Insulated boxes packed with ice, dry ice, or freezer "jelly" packs have been used also to prevent thawing of frozen samples in the field. This is considerably less expensive than the use of portable refrigerators, but, of course, temperature cannot be controlled accurately. A typical insulated box used for field storage and shipping by the University of Alberta [1] is shown in Fig. 2. When using solid slabs of dry ice, thin sheets (25 mm) of insulation must be placed between the dry ice and the soil samples to minimize "burning" effects caused by contact with carbon dioxide. The dry ice is placed between the insulation and the top, bottom, and sides of the box. If freezer packs are used, they can be arranged in the same manner. A double-walled insulated box has been used, where dry ice is placed in the space between the two walls. The temperatures of samples, packed in this manner, are not constant; they will undergo a fairly rapid drop in temperature, the rate depending on how well protected they are by insulation. The dry ice evaporates slowly, and the temperature of the box (and consequently of the samples) will ~The italic numbers in brackets refer to the list of references appended to this paper. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

92

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

STRONG LATCH WITH PROVISION LOCKING FOR

~

,

~ ~'~ 1

1

~

' ~ , [ i i ~ [

RTV CAULKING IN

INBULATIO N

i

JOINTS

i

i q

SASH CHAIN TO PREVENT LID PROM FALLING BACK MINIMUM OF THREE HINGES ALONG BACK 3" (760ram) HIGHOENSITY URETHANE iNSULATION INSIDE ~" (12ram) GOOD QUALITY PLYWOOD

LARGE D-HANDLES BOLTED TO BOX i:

METAL BANDING FOR REINFORCEMENT 2"x4" (S0mm BY 100ram) BOLTED TO BOX BOTTOM SO IT CAN BE AS A PALLET (ALSO KEEPS BOX BOTTOM AWAY FROM MOISTURE)

FIG. 2--Typical insulated box used for field storage and shipping.

increase. When ice or freezer packs are used, there is no drastic cooling of the samples, but, in time, the temperature of the insulated box and samples will increase unless the ice is rel~laced periodically. If the boxes are to be stored for any length of time (for example, a few weeks), it is advisable to pack snow or crushed ice around the sample bags to prevent sublimation and to fill any air spaces with crushed newspaper to provide added insulation and reduce movement of air in the box. The cover of the box should be fastened securely and made airtight by applying a thin bead of room temperature vulcanizing silicone rubber (RTV) caulking compound around the edge of the lid before the box is closed. Although not entirely satisfactory, tempory measures to protect samples in the field can be used in some instances. During the winter, frozen soil samples, packaged in polyethylene bags, can be buried in snow drifts, or, if the ambient air temperature is below 0~ they can be left in the open air but must be protected (shaded) from the sunlight. During the summer, frozen samples have been stored successfully for up to three days in shallow pits dug in the frozen ground or put into plastic sleeves and stored in the boreholes from which they were taken. The pit should be lined and covered with some insulating material. Moss serves quite well for this purpose. The top of boreholes, used to store samples, should be covered or plugged with insulation to prevent thawing and closing of the hole. If adequate storage facilities are not available at the field site and the samples are to remain frozen, they must be transported immediately to a Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BAKER ON FROZEN SOIL SAMPLES

93

temperature- and humidity-controlled storage area. The frozen soil sampies must be shipped in refrigerated containers or insulated boxes. Power requirements for shipping a typical refrigerated unit by aircraft are outlined in Fig. 3 [2]. A competent person should accompany frozen samples FIELD SITE GAS DRIVEN GENERATOR I IlOV

l

SINGLE PHASE

PORTABLE REFRIGERATED UNIT I O.3m3 (10 fl 3) 10V l AIRCRAFT

SINGLE PHASE

DYNAMOTOR I 28V l

DC

I RECTIFIER I I0V l

3 PHASE

l AIRCRAFT POWER I FIG. 3 --Power for refrigerated units.

transported by commercial airlines from the field to the laboratory. (Refrigerated units not attended could be left unplugged on the parking apron or in a storage area, and thawing or undesirable temperature changes of the samples might occur.) Arrangements can be made to have the insulated boxes placed in airport freezers between flights. Cold storage areas are not always available, but restaurants and cafeterias at airports are usually most helpful in allowing their facilities to be used for this purpose if the containers are not too large or too many; arrangements for storage en route should be made prior to shipment. The author has found that insulated boxes packed properly can be left without auxiliary freezing up to 8 or 9 h without apparent deterioration of the samples. The samples can be left in the insulated containers and placed in cold storage upon arrival at the laboratory until required for testing. Preparing and transporting naturally frozen undisturbed soil samples for laboratory testing is a very difficult and costly operation. In most cases, and for most purposes, insulated boxes have been used successfully to transport frozen samples to the laboratory, provided care has been taken in packaging, storing, and handling the containers.

Artificially Frozen The physical properties that affect the mechanical behavior of soil, for example, homogeneity, grain size, moisture content, compaction, and, to Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

94

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

some extent, ice lens structure, should be closely controlled. This can be achieved in the laboratory by using special molds and controlled methods of freezing. The steps followed in the preparation of laboratory frozen soil specimens are: 1. 2. 3. 4. 5.

Assembly of the mold. Compaction of the soil sample. Saturation of the sample. Freezing. Disassembly of the mold and removal of the sample.

The geometry of the mold is determined by the desired shape of the test specimen. Rectangular, cylindrical, and dumbbell specimen shapes commonly are used, depending on the test to be performed. As unidirectional freezing of the sample is usually desirable, the mold is fabricated from or covered with materials having good insulating properties. Use of split molds with all joints ,fastened by easily removed connectors greatly facilitates specimen removal after freezing. All interfacing surfaces of the mold and all threads on metal connectors should be coated with a lubricant to ensure a tight fit for evacuating the air from the samples and to aid in extracting the sample from the mold in the cold room. Care must be taken to prevent the lubricant (especially the silicone types) from coming in contact with the sample. Air-dried soil usually is placed into a mold in 30-mm-thick layers and compacted by a drop hammer or vibration action, following standard soil mechanics procedures (for example, Modified Proctor, American Association of State Highway Officials' Standard). Each layer should be scarified to a depth of approximately 5 mm before the next layer is placed. If the soil has a wide range of grain sizes, vibration may cause the particles to separate into layers of uniform size. For sands, the vibratory technique is the better compaction method. The following method of evacuation and saturation has been found by Haynes [3] to give a fully saturated compact sample. After the sample has been compacted, caps can be placed on each end of the mold. Both end caps should be fitted with hose connections, which include porous stones, to prevent the loss of soil during the air evacuation and water saturation phase. Plastic tubes are connected to both of the end caps. The tube from the top cap is connected to a vacuum pump; the tube from the bottom cap leads to an o n / o f f valve and then to a reservoir of distilled water. A vacuum pump is also connected to the water reservoir. The on/off valve allows for separate deaeration of the mold and the reservoir. A photograph of the apparatus is shown in Fig. 4. A vacuum is applied to the top of the mold with the o n / o f f valve closed. At the same time the distilled water reservoir is placed under a Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BAKER ON FROZEN SOIL SAMPLES

FIG.

95

4--Apparatus for molding and saturating artifical samples.

vacuum to remove as much air as possible. After about 2 h, the vacuum pressure in the reservoir is decreased, and the valve is opened. A differentia/ pressure between the mold vacuum and the water tank vacuum is maintained so that the water travels up through the sample and saturates it at a slow steady rate. Fast saturation must be avoided to prevent: (a) piping o f water through channels along the outside o f the sample; (b) a " q u i c k " condition disturbing the dense packing obtained in compaction; or (c) incomplete evacuation of air. When the samples appear to be saturated (that is, water can be seen in the top tube), the vacuum pumps are turned off, and the sample is allowed to saturate under a 0.5-m head of water for about 1 h. Goughnour and Andersland [4] and H o o k e et al [5] prepared sand-ice specimens by mixing natural snow uniformly with a selected proportion o f precooled sand. This mixture was compacted in the mold, and water, precooled to 1 ~ was admitted under pressure. Little snow melted during this process, and the sand particles remained dispersed during freezing. Using this procedure, frozen sand specimens were obtained with large ice contents. Vyalov [6] reported that frozen soil without ice segregation can be obtained by freezing samples rapidly at temperature below - 3 0 ~ To obtain Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

96

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

specimens having a layered or reticulate structure (that is, horizontal or randomly oriented in layers, respectively), the soil samples should be frozen at temperatures of about - 6 ~ or above. A layered, ice-lensed structure is obtained by cooling one end of the sample and providing free access to a constant supply of water at the other end. Tsytovich and Sumgin [7] investigated the influence of the freezing method on the unconfined compressive strength. The results indicated that a freezing temperature between - 1 2 and - 1 7 ~ gave a relatively lower strength if the freezing time was less than three days for sand and four days for clay. They also found that the strength properties were not affected significantly if the freezing occurred from all sides or only from the top down. To achieve a uniform moisture content throughout a sample of Fairbanks silt, Haynes [3] froze specimens unidirectionally in a cold chamber at a temperature of - 37 ~ The bottom mold connection was attached to a water supply; the top of the mold was exposed to freezing temperatures. Loose insulation (granulated cork) was placed under and around the mold to reduce radial heat loss and to allow the sample to freeze from the top down. Vermiculite can be used for this purpose instead of granulated cork (Fig. 5). A constant supply of water was maintained at the bottom of the mold to replenish the water that migrated upward from the unfrozen zone

FIG. 5--Vermiculite placed around the mold to allow the sample to freeze unidirectionally. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BAKER ON FROZEN SOIL SAMPLES

97

to the freezing front. Thermistors, placed at locations along the center line of the specimen, recorded the freezing rate. Cold rooms, refrigerated cabinets, and constant temperature baths have been used by many investigators to control the freezing rate of remolded soils. Many cooling systems have slow response times, so that rapid adjustment of the rate of heat extraction from the sample is usually very difficult. More accurate control of the freezing process can be obtained, using a thermoelectric cooling plate which operates on the Peltier effect [8]. The design of a cooling plate (Peltier module) is described in detail elsewhere [9,10]. The temperature of the plate can be maintained with an accuracy of at least _+0.02~ As heat removal from the sample is uniaxial, this method is well suited for unidirectional freezing of a soil specimen. Sampies of various sizes, with variations in moisture content of + 1.5 percent by dry weight, have benn frozen succesfully under many different test conditions, using a thermoelectric plate at the Division of Building Research (DBR), National Research Council of Canada (NRC) [9]. Rapid sample freezing minimizes redistribution of soil moisture. Freezing too rapidly, however, can cause entrapment of air bubbles in the specimen and tensile failures due to thermal shock. Typical freezing rates used by Sayles and Haines [11,12] on small specimens (69 mm diameter by 140 mm high) were: Ottawa sand (20 to 30 mesh) Manchester fine sand Hanover silt Suffield clay

38 mm/day 40 mm/day 80 mm/day 170 mm/day

Methods of freezing large remolded soil specimens have been reported by Warder and Andersland [13] (125 mm diameter by 300 mm high) and Laba [14] (100 mm diameter by 241 mm high), but no reference was made to their homogeneity. It is not known whether it is possible to produce large uniform samples using existing freezing techniques. Samples frozen in the laboratory are molded normally in the test apparatus and frozen just prior to testing, thus eliminating the difficulties associated with the storage of frozen specimens.

Machining and Preparation of Specimens for Testing The machining and preparation procedures used for frozen soils depend upon the size and shape of the specimen required, the type of soil, and the particular test being performed. Similar procedures are followed for cutting and machining both naturally frozen and artificially frozen samples. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

98

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

Frozen soil samples should be handled with gloves and all tools and equipment kept in the cold room to avoid sample damage by localized thawing. A temperature of - 5 _+1 ~ is the most suitable ambient temperature for machining with respect to material workability and personal comfort. At warmer temperatures, surface thawing is a problem, and cutting tools must be cleaned frequently, for they become coated and clogged with frozen soil, reducing their cutting efficiency. Working at colder temperatures is uncomfortable and slow. The soil is also difficult to work with because of increased hardness; cracks can be formed easily in it, due to increased brittleness. Any cracking induced in the specimen during preparation will reduce the load-carrying capability and influence the test results. Thermal disturbance, due to frictional melting by a cutting tool, may be insignificant when preparing specimens for thaw consolidation tests, but it may be very significant for strength tests. Extreme care must be taken when machining frozen soil samples for testing to keep disturbance to an absolute minimum. Various cutting and grinding techniques have been used to machine specimens to the shape required for the test to be performed.

Rough Cutting Methods A number of methods for cutting frozen soil, including using a hand, band, or chain saw, forcing a cutting edge into the frozen soil, and using a hot wire have been investigated at NRC. Others have found rotary diamond saws satisfactory. The advantages and disadvantages of these methods are summarized in Table 1.

Finishing Methods After being cut roughly to the required dimension, rectangular specimens are finished usually by one, or a combination, of the following methods, listed in increasing order of precision: 1. Hand shaving with a sharp, straight cutting edge (for example, a draw knife). 2. A coarse wood rasp or file. 3. Grinding with several grades of emery paper or grinding stone. 4. Milling machine equipped with heavy-duty cutters. 5. Drill press (heavy-duty) equipped with an end milling tool. These methods have been used on a wide range of frozen soils, and the particular application of each is dependent upon the required specimen tolerances. The allowable specimen tolerance is dependent on the type of test to be preformed and is discussed next. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BAKER ON FROZEN SOIL SAMPLES

99

Cylindrical specimens are machined either on a metal working lathe or cut carefully with a coring tube in the laboratory. A lathe is being used extensively at the Division of Building Research to obtain well finished cylindrical specimens of most types of frozen soils up to 150 mm in diameter. Soils containing pebbles are extremely difficult to machine on a lathe. If pebbles are not plucked out by the lathe, the specimen becomes jammed on the cutting tool and breaks between the tool post and the chuck. The various steps in the preparation of specimens on a lathe are outlined pictorially in Figs. 8-10. Samples are cut initially to eight-sided cylinders on the bandsaw. A V-block is used to hold the specimen so that the ends can be squared on the handsaw. The dimensions of the eight-sided cylinder are chosen so that the specimen can be turned to a right cylinder with both ends faced. A finished specimen 75 mm in diameter by 150 mm long is turned from an eight-sided cylinder 85 mm across the end and 200 mm long. The eight-sided cylinder is placed on the lathe, with one end supported in a four-jaw chuck and the other end supported by the tallstock. The tailstock is separated from the specimen by the round wooden block which has sandpaper glued to one side and a cone seat (for the tailstock) on the other. Pressure applied to the specimen by the tailstock is distributed evenly over the face of the specimen by the wooden block. Best results are obtained when the frozen soil specimen is turned at 690 rpm and the carriage feed set at 30 mm/36 revolutions. The maximum depth of cut should be limited to 0.38 mm. A tungsten carbide cutting tool, with a minimunrback clearance of 45 deg, gives the best results. For clean cuts, the tool has to be sharpened often, as the abrasive action of the soil dulls the edge quickly. A tool post grinder can be used for dressing coarse-grained soils. Grinding efficiency for fine-gra/ned soils is reduced by buildup of material on the grindstone. A special holder that screws onto the spindle of the lathe and allows the ends of the specimen to be squared and faced with the cutting tool is shown in Fig. 10. Cylindrical samples can be cored from block samples in the laboratory, using a diamond set core barrel and a large industrial drill press. The block is held to the drill press flame by clamps. This procedure has been used in Alaska to produce sand and gravel specimens with less than 0.5 percent variation in diameter and for silt and clay samples with less than 0.4 percent variation [15]. The drill press was fitted with a gas swivel through which pressurized liquid nitrogen vapor was passed to blow the cuttings away. The configuration of the diamond matrix and vents on the core bit was varied in order to core each type of soil most effectively. To obtain specimens of consistent diameter, the internal diameter of the core bit must be checked often and the diamond matrix replaced if the allowable tolerances are exceeded. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

inadequate due to slow progress and poor geometric control; for effective cutting the blade had to be continually cleaned to remove partially thawed clay and to prevent binding

a swede saw was used for rough cutting large blocks of frozen clay (Fig. 6); use of a miter box improves geometric control

excellent for rough cutting large blocks of frozen peat and silt which have high ice contents

very satisfactory for cutting fine-grain specimens; quite adequate for coarse sands (Fig. 7) fast cut straight cut

Hand saws Miter saw Hack saw Swede saw

Chain s a w Electric powered 254 mm arm Gas powered 381 mm arm

Band saw 1/2 hp, 1725 rpm

5 mm blade, 2 teeth/cm

10 mm blade, 1 tooth/cm

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

largest sample that could be cut was 159 mm thick, but with height attachment up to 311 mm blade wanders when cutting specimens 60 to 200 mm thick cuts slowly and dulls very quickly due to the abrasive action of the frozen soil; tooth life very short with no significant

ineffective in cutting frozen clay with the teeth and chain becoming clogged with clay and the blade binding in the cut

a cheese cutter frame was rigged with a hot wire, but not enough pressure could be applied by hand to obtain efficient cutting; it is suggested that this method not be used to cut laboratory specimens of frozen soil due to its thawing action

wire passed quickly through 10-cm specimen of frozen clay

Hot wire (piano) 23 kg weights 2to4V

Disadvantages poor, as the ice in front of the cutting edge chipped, leaving voids in the sample

Advantages

Sharp edge Axe (for example)

Method

TABLE 1--Rough cutting methods.

if)

--4

.--t m

---I 0

"11

0

0

"TI

Z

-4

~D

~D m "0

z

m

o_

O) "0 m

1.-

_o

Q

in conjunction with table and guide, it is the most desirable method for cutting frozen soils; very clean straight cut even in coarse-grain pebbly soils

depth of cut restricted by the radius of the blade; clayey soils greatly reduce cutting efficiency of the saw blade as the blade must be cleaned regularly

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

Diamond saw

difference between ferrous, non-ferrous and wood cutting blades

m

I-"

"o

o') 0

m z

N

"r -n O

Z

O

2O

Ill

7~

102

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 6--Use o f a swede saw f o r rough cutting large block samples.

At the University of Alberta, a core barrel attached to a milling machine was used to core cylindrical specimens from block samples [1]. An 80 mm length of steel tubing (67 mm inside diameter) was fabricated with a cutting edge on one end. The cutting edge was formed by rolling the lip of the tube inward and sharpening it to an inside diameter of 64 mm. The core barrel, clamped in the drive chuck of a milling machine, was rotated at 300 rpm. Only short specimens were prepared for testing. Long specimens break during coring, particularly if the frozen soil has a high ice content. Dumbbell-shaped specimens for tensile testing are prepared in molds or by machining samples on the lathe, using these procedures. Greater care is necessary in machining naturally frozen samples, as they

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BAKER ON FROZEN SOIL SAMPLES

FIG. 7--Use

103

o f band saw to cut smaller samples.

may contain large ice inclusions (lenses, etc.) that are very fragile and easily chipped. Unexpected occurrences of pebbles can ruin many hours of machining if care is not taken when they are detected. Many samples, obtained at high cost in the field, have not been tested because they were too difficult to machine. These combined cutting and coring processes are used to prepare frozen soil specimens for all forms of load testing. Dimensional tolerances for uniaxial thaw-consolidation test specimens are particularly critical, as the sample must fit tightly into the consolidometer ring. The nature of the other tests is such that the dimensions of specimens are less critical. It is important that the ends of the specimens are parallel and plane, so that intimate contact occurs with the loading platens. Seating errors have been observed on many loading curves [4]. Eccentric loading can give lower strength values [3], but these are often ignored. Although the effects of thermal disturbance induced by machining procedures have not been investigated, every effort should be made to eliminate or reduce them, especially on clay samples. Surface thawing has been noticed on several occasions when machining specimens of frozen day.

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

104

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 8--Eight-sided

cylinder cut on band saw.

Storage and Protection During Laboratory Testing Careful control of temperature and humidity in storage areas is necessary to protect the frozen sample from sublimation and thermal disturbance, from the time it has been molded or machined or both until testing has been completed. Protection methods are similar to those used when transporting frozen samples from the field. Samples should be wrapped in an impermeable material and stored in a refrigerated room or freezer. Khomichevskaya, one of the earlier Russian investigators, wrapped naturally frozen samples in insulation and stored them in underground rooms for up to seven days with good results [16]. Sublimation of samples can be minimized by controlling the humidity and reducing air flow around the specimen. Most small capacity freezers do not have air circulation fans in the freezing compartments, and air Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BAKER ON FROZEN SOIL SAMPLES

105

FIG. 9--Specimen turned on lathe to required diameter. flow is limited to that caused by convection currents. Large walk-in freezers rely on fans to circulate the air and control the temperature effectively throughout the storage chamber. Frozen samples can be protected from these air currents by wrapping them in cellophane and placing them in plastic bags. Snow or ice shavings can be placed in the bags to raise the humidity around the sample. The bags should then be evacuated to remove as much air as possible and heat sealed. These procedures are shown in Figs. 11 and 12. In studies of ways to store and preserve frozen samples, Livingston [17] found that samples lost considerable weight, due to surface evaporation, when the storage temperature ranged from 0 to - 6 ~ To study the effect of the storage temperature on surface evaporation and sublimation, small specimens of frozen Keweenaw silt (approximately 25 mm cubes with polished surfaces) were prepared and stored at various temperatures. The results indicated that storage at temperatures between - 8 and - 12~ reduced the rate of surface evaporation to an acceptable value. Various methods were investigated to reduce sublimation further, as failure to control this moisture loss affects test results. Synthetic resins, silicone oil, varnishes, natural oils, and other commercially available products were sprayed or painted on the surface of the specimens. Vegetable and minCopyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

106

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. lO--Facing specimen ends on the lathe using special holder. eral waxes could not be used as they must be heated. Although many of the surface coatings reduced the rate o f surface evaporation, none o f the materials investigated provided satisfactory protection for more than one week. Frozen soil specimens at the University of Alberta have been coated with a thin layer of ice by applying a fine spray of supercooled distilled water [I]. This film o f ice prevents sublimation of the sample and is removed easily prior to testing. Yershov et al [18] published information on the sublimation o f frozen soils. As shown in Fig. 13, different soil types have different sublimation rates. Finer grained clay soils have a higher evaporation rate than do coarser sands. The rate o f sublimation is affected greatly by the ambient temperature (Fig. 14). The parameters studied were temperature, relative humidity, and air velocity, but no details were given on how the experiments were undertaken nor on the apparatus used. A similar study was undertaken by the author to determine the rate of sublimation on three soil types: Ottawa sand (20 to 30 mesh), Niagara silt, and Leda clay. Four specimens of each soil type were prepared by adding 50 g o f water to equal volumes o f each soil and placing the mixture in Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BAKER ON FROZEN SOIL SAMPLES

107

FIG. 11--Evacuatingplastic bag to prevent sublimation. plastic boxes 1.9 • 10-2 m deep, having a surface area o f 3.31 • 10-3 m 2. Two samples of each soil type had their surfaces covered with cellophane; the others were left uncovered. All the samples were weighed and placed in the cold room to freeze at a temperature o f - 4 . 4 ~ and a relative humidity of 37 percent. The specimens were weighed after 8 h and then once a day for 23 days. Results are presented in Fig. 15. The sublimation rates of the frozen soils investigated by the author are within the same range as those reported by Yershov et al [18]. Similarly, it was found that the finer clay soils have higher sublimation rates than the coarser sands. This is because the larger particle surface area in fine-grained soils allows for film flow, which brings the water to the surface o f the sample. Samples that were covered with cellophane had significantly lower sublimation rates than those that were left uncovered. Many investigators [5,7,17,19-21] have found that the compressive strength of frozen soil is greatly dependent on its overall moisture content. Compressive strength increased with increasing moisture content. It Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

108

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 12--Heat sealing the plastic bag.

reached a maximum when the soil was saturated completely and then decreased to the strength of ice. Any loss of moisture from a specimen, therefore, will cause a change in its compressive strength. The compressive behavior of frozen soil is also affected greatly by temperature and by temperature fluctuations. It is imperative, therefore, that specimens be stored and tested in a freezing chamber that has only a small temperature fluctuation to minimize thermal disturbance. The effect of fluctuations in temperature can be reduced by enclosing the specimen in an insulating jacket during storage and testing. Vyalov [6] suggested the following permissible temperature variations when storing and testing frozen soils within different ranges: Temperature, ~ Permissible deviation

up t o - 2 _+0.1

-2to -5 _+0.2

-5to-10 __.0.5

below-10 ___1.0

Guarded hot plate experiments carried out by Hoekstra [22] using Fairbanks silt (see Fig. 16) show that any thermal gradient in the sample, even at temperatures below freezing, will cause moisture redistribution in fro-

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

0

10

2(1

I

100

L

0

I

200

I

I

I

I

I

TIME,

300

h

400

I

ISANDY

500

I

LOAM

J

600

I

v

BENTONITE

KAOLIN

M ~

GLUKHOV

-1.6deg C REL. HUM,: 65% A I R V E L . : 4 . 2 m/sec

TEMP.:

700

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FIG. 13--Sublimation rates o f frozen soils ( Yershov et al [18])

D

z 0

<

~E

x 30 2

? o

I

0

10

20

0

__

\

I -2

O

-4

I

-8

TIME

TEMPERATURE,

-6

9......

SANDY

LOAM

I

deg C

-10

= 600h

I

LOAM

-12

HUM.: 60% VEL.: 4 . 5 m/see

MOSCOW

9

CLAY

KIEV

9

O MOSCOW

I

-14

FIG. 14--Sublimation and temperature ( Yershov et al [18])

z o

I

s

o

I

O (.O

if)

tin

-o

>

I--

0

q')

Z

N m

O z "11 ~o O

m ..n

go >

110

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

4o t

I

I

I

I

I

I

TEMP.: -4.4 de9 C REL. HUM.: 37% ONE SIDE EXPOSED 3O

2

2O z o

t

CELLOPHANE COVERED(ALL THREESOILS)

.[ _ I

I00

200

I

I

300 400 TIME, h

500

600

700

FIG. 15--Sublimation in N R C / D B R cold room.

0.35

I I

I

I

k

E

0 30

~-" Z Z

I

I

.ICE

I~'x ~''"/,'~',k ~

I

I

[

I

I

I

LAYER

INITIAL ~

WATER CONTENT

S = 0.73 r t40 TIME =

0.25

MIN

R _-,

1 6 0 _ - - -B'" - --'

u < 0.20

/\~X//~NNOTE: ~

\I\/A.

~/~bX-'

t

ARROWS POINT-_

TO BOUNDARY

BENT~ R ~ E NFRsO~,ELN A N g -

I 2 3 4 5 DISTANCE FROM COLD PLATE, inches

FIG. 16--Effect o f temperature gradient on moisture redistribution in frozen Fairbanks silt ( Hoekstra [22]).

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BAKER ON FROZEN SOIL SAMPLES

111

zen soil. This moisture movement will increase the sublimation/evaporation rate, as well as change the fabric of the specimen. Many investigators [19,23] have found that the unfrozen moisture content will influence greatly the compressive strength of frozen soils. Williams [24], using calorimetric methods, found that there was a hysteresis effect in the temperature dependence of the unfrozen water content. The unfrozen water content of a frozen soil, at a given temperature, is dependent on whether the temperature is reached by cooling or warming. If temperature fluctuations occur, even though the specimen remains below 0 ~ the unfrozen moisture content will change. This is of great concern when testing frozen soils at temperatures above - 5 ~ The cold rooms at DBR/NRC provide good examples of well controlled environments. The temperature of the cold rooms can be controlled accurately by a reheat system to within ___0.1~ at any location in the room. Each refrigeration unit is operated by a two-stage compressor with a capacity of 3.5 tons of refrigeration circulating Freon 22. The cooling capacity is adjusted automatically to be in excess of that required, and the temperature is controlled by electric reheating. The reheat unit has a capacity of 12 kW and is operated by a solid state controller. Two evaporation units are located in each of the cold rooms so that one unit can be defrosted without shutting the cold rooms down. A separate auxiliary compressor and control system is available in case one of the main systems becomes inoperative. In addition, a freezer of 1 m 3 capacity is available also in the laboratory as a backup system. Samples can be transferred to the freezer in case of emergency shutdown. The need for an alarm system to provide warning of equipment failure and for several backup systems is appreciated only after samples are lost due to thawing because of unexpected equipment breakdowns.

Conclusions The important factors to be considered in transporting, preparing, and storing frozen soils for laboratory testing are the control of the effects of sublimation/evaporation and minimization of thermal disturbance. Their effect on the ice content, fabric, and unfrozen moisture content of the specimen, and, therefore, on the strength and deformation behavior of the material, is of great significance. Further evaluation of existing procedures and development of new or improved techniques are required before standard methods can be proposed for the laboratory testing of frozen soils.

Acknowledgments The author wishes to acknowledge the assistance of W. D. Roggensack, University of Alberta, M. S. King, University of Saskatchewan, and W. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

112

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

A. Slusarchuk, Northern Engineering Services Ltd. (formerly of the Division of Building Research) in the development of this reveiw paper. References [ll goggensack, W. D., University of Alberta, personal communication, 1974. [2] Slusarchuk, W. A., Northern Engineering Services Ltd., Calgary, Alta., personal communication, 1974. [3] Haynes, F. D., U.S. Army Cold Regions Research and Engineering Laboratory, personal communication, 1974. [4] Goughnour, R. and Andersland, O. B., Journal of the Soil Mechanics and Foundation Division, American Society of Civil Engineers, Vol. 94, No. SMI, 1968, pp. 923-950. [5] Hooke, R. L., Dahlin, B. B., and Kauper, M. T., Journal of Glaciology, Vol. 11, 1972, pp. 327-336. [6] Vyalov, S. S., "Methods of Determining Creep, Long-Term Strength, and Compressibility Characteristics of Frozen Soils," National Research Council Technical Translation, T'r-1364, Ottawa, 1969. [7] Tsytovich, N. A. and Sumgin, M. I. "Principles of Mechanics of Frozen Ground," U.S. Army, Snow, Ice, Permafrost Research Establishment, Translation No. 19, 1959. [8] Lechner, W., "Peltier Cooling," Philips Technical Review, No. 27, 1966, pp. 113-130. [9] Williams, P. J., Canadian Geotechnical Journal, Vol. 5, No. 4, 1968, pp. 264-266. [10] Webb, E. L. R., "Equivalent Circuit Model for Thermoelectric Temperature Controllers," Bulletin No. 18, Radio and Electrical Engineering Division, National Research Council of Canada, Ottawa, 1968. [11] Sayles, F. H., "Creep of Frozen Sands," U.S. Army, Cold Regions Research and Engineering Laboratory, Technical Report No. 190, 1968. [12] Sayles, F. H. and Haines, D., "Creep of Frozen Silt and Clay," U.S. Army, Cold Regions Research and Engineering Laboratory, Technical Report No. 252, 1974. [13] Warder, D. L, and Andersland, O. B., Canadian Geotechnical Journal, Vol. 8, No. 1, 1971, pp. 46-68. [14] Laba, J. T., Highway Research Record, No. 360, 1971, pp. 26-36. [15] Shuster, J. A. in Proceedings o f the Symposium on Cold Regions Engineering, American Society of Civil Engineering, University of Alaska, Vol. 1, 1971, pp. 73-117. [16"1 Khomichevskaya, L. S., "Compressive Strength of Permafrost and Ice in Their Natural States," U.S. Army, Arctic Construction and Frost Effects Laboratory, Translation No. 20, 1951. [17] Livingston, C. W., "Explosion Tests in Keweenaw Silt," U.S. Army, Snow, Ice, Permafrost Research Establishment, Report No. 30, 1956, pp. 25-40. [18] Yershov, E. D., Gurov, V. V., and Dostovalov, B. N., "Ice Sublimation in Fine-Grained Soil of Various Cryogenic Structure During its Interaction with an Air-Current," U.S. Army, Cold Regions Research and Engineering Laboratory, Translation No. 439, 1973, pp. 2477_251. llgl Yong, R. N. in Proceedings of the Permafrost International Conference, National Academy of Sciences-National Research Council, 1963, pp. 315-319. [20] Kaplar, C. W., "Some Strength Properties of Frozen Soil and Effect of Loading Rate," U.S. Army, Cold Regions Research and Engineering Laboratory, Special Report No. 159, 1971. [21] Andersland, O. B., The Northern Engineer, Vol. 3, No. 3, 1971, pp. 4-6. [22] Hoekstra, P., Water Resources Research, Vol. 2, No. 2, 1966, pp. 241-250. [23] Sanger, F. J., "Foundations of Structures in Cold Regions," U.S. Army, Cold Regions Research and Engineering Laboratory, Monograph No. III-CA, 1969. [24] Williams, P. J. in Proceedings of the Permafrost International Conference, National Academy of Sciences-National Research Council, 1963, pp. 225-229.

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

Michael Bozozuk'

Temperature-Controlled Humid Storage Room

REFERENCE: Bozozuk, Michael, "Temperature-ControlledHumid Storage Room," Soil Specimen Preparation for Laboratory Testing, ASTM STP 599, American Society for Testing and Materials, 1976, pp. 113-125. ABSTRACT: The design of a soil sample storage room that has been in operation since 1954 is described. The room provides a storage environment with a constant temperature of 55 ~ and relative humidities of 90 to 97 percent. It is of wood frame construction, lined with copper sheeting, forming a watertight interior. The desired temperature and humidities are obtained by running water down the walls of the room and circulating it through a heat exchanger. The preparation of samples for storage and the effect of storage times on softs are discussed. KEY WORDS: soils, design, storage room, constant temperature, humidity, samples, storage time

The purpose of a storage room for soils is to retain soil specimens in their original condition at time of sampling until testing can be undertaken. The functional requirements for such facilities will depend upon the nature and quality of the samples to be stored, as well as the use to be made of them. Granular soils obtained for grain size or compaction tests can be kept in ordinary burlap sacks and stored almost any place for years, even in an unheated shack. On the other hand, high quality undisturbed samples of sensitive marine clays, obtained for consolidation, undrained triaxial strength, or other special engineering tests, require careful handling and ideal storage conditions. These samples should be wrapped, waxed, labelled, and stored properly, free of shocks or vibrations, in a humid environment at a constant temperature near that which prevailed in the ground at the time of sampling. Unless these storage requirements are met, high quality undisturbed soils will deteriorate, and the results of the special engineering tests would be questionable. The storing of soils in a warm, humid environment is conducive to high rates of oxidation causing chemical changes in the soil, 1Research officer, Geotechnical Section, Division of Building Research, National Research Council of Canada, Ottawa, Ont., Canada. 113 Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed byby ASTM International www.astm.org Copyright 9 1976 University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

114

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

deterioration of the sealing wax, and formation of water blisters (Fig. 1), and sometimes breeds bacteria that feed upon the soils or the wax. High temperatures permit dissolved gases in saturated clays to come out of solution, causing expansion and deterioration of the soil skeleton. If stored in a dry and warm environment, soil moisture may escape, causing

FIG. 1--Poorly stored samples. the soils to shrink; on the other hand, samples stored submerged in water containers may absorb water and swell. Consequently, the clays could change both physically and chemically so that they would no longer exhibit the engineering properties of the in situ soil they are supposed to represent. In 1954, the Division of Building Research, National Research Council of Canada, designed and constructed a temperature-controlled humid storage room (Fig. 2), to reduce or minimize the effects of improper storage; it has operated continuously since then. The room is approximately 13 ft long, 6 89ft wide, and 9 ft high. A removable wooden floor provides a relatively dry walking area and supports the storage shelves and working table. This paper discusses the design of the room, the handling and preparation of samples for storage, and the effect that long storage times can have on engineering properties. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BOZOZUK ON TEMPERATURE CONTROLLED HUMID STORAGE ROOM

115

FIG. 2a--Constant temperature humid storage room.

Design The storage room was designed as a necessary accessory to laboratory soil testing facilities. Its purpose is to keep the soil samples in the best possible condition, that is, to bridge the period from the time they are obtained in the field until they are tested and to provide a conditioned environment for performing certain special engineering tests, when required. The principle of operation is as follows. Water is directed to flow down the inside walls of the room to the floor, where it is ponded. Natural evaporation from the wetted walls and the reservoir that covers the entire floor area provides relative humidities from 90 to 97 percent. The temperature control is obtained by circulating the ponded water through a heat exchanger located outside the room. After it is cooled, it is pumped back to the top of the room, where it is directed against the walls through nozzles to complete the flow cycle. Any heat gain through the walls is picked up by the flowing water and dispelled through the heat exchanger. The storage room is essentially a copper tank enclosed in an insulated wood frame. A plan view (Fig. 3) shows the layout of the sample racks and worktable, relative to the door and window. It was constructed on 2 by 2-in. wood sleepers laid on the concrete floor of the laboratory. Set at 12 in. centers and running parallel to the length of the room, they were covered with 88 plywood and nailed together (Fig. 4). The Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

116

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 2b--Mechancal plant: (a) chilled liquid and (b) heat exchanger.

purpose of the sleepers was to provide an air space and ventilation for the floor and adequate support for the superstructure, in case it had to be moved. The plywood was then covered with a vapor barrier that projected about 2 in. beyond the perimeter. To facilitate circulation of the ponded water, the floor was sloped 1 in. to the drain located at the back o f the room by tapering the 2 by 6-in. floor joists. They were placed across the sleepers to form the floor of the room. The wall frames, made o f 2 by 3-in. studs, were erected on the joists, which, in turn, supported the frame roof made up of 2 by 4-in. joists. All joists and studs were placed at 16 in. centers whenever possible. The spaces between the studs and joists in the floor and ceiling were insulated with 2-in.-thick mineral wool insulation. The exterior of the room was first sheathed with a layer o f 5/16-in. plywood, then covered completely with a vapor barrier that overlapped at the ends and corners. It was, in turn, protected with a second layer of 5/16-in. plywood. The walls, floor, and ceiling inside the room were covered with Y2-in.thick fir plywood. Finally, it was covered completely with 20-gage copper sheeting to form a watertight interior. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BOZOZUK ON TEMPERATURE CONTROLLED HUMID STORAGE ROOM

117

FIG. 2c--Interior of humid storage room.

12'- I0'~

L.

_1

PUMP a MOTOR HEAT EXCHANGER

t~"FLOOR

~

l

CHILLED --WATER SUPPLY

DRAIN /

/SAMRLE

?

j 3 ' x 4' THERMOPANE

RACKS

WORK TABLE

r I I

6L(

I ~_~---REMOVABLE

e-~-r~-(///i

FLOOR-..~

W~NOOW

~%"2" OIA. ACCESS HOLE

SECTIONS

I t~NSITE BAFFLE

SPRAY NOZZLES

INTERIOR SHEET COPPER

" /

SEAL'~WOOD FRAME WALL W{TH 2"x 3" STUDS ~ 16"C.C.

3'x7' REFRIGERATOR DOOR

FIG. 3--Floor plan o f storage room.

Two thermopane windows were provided for lighting. A 3 by 4-ft window installed at one end of the room allowed ordinary light from the laboratory to illuminate the worktable (Fig. 3). A second 2 by 4-ft window was installed in the ceiling to permit artificial lighting of the room, using a fluorescent ceiling lamp suspended above it (Figure 4). For Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

118

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

.•(fEXJSTING CEILING

LABORATORY

IN

; LU~RES~ENTT LIGHTsO C) 0

2"x4"@ 16" C.C.-~ /COPPER~,

=

-J" THERMOPANE / WINDOW

WATER INLET

T'I"

i,l.._~_ CL A M p

2" INSULATION

c r-

I/4" TRANSITE

6' - I0 5/B"

BAFFLE

/I/2"

FIR PLYWOOD

8'- I/4"

~ 2 " x 3" STUDS @

=6 "

Cl C.

~TWO LAYERS 5/16" PLYWOOD

20 GAUGE COPPER TO FORM WATERPROOF INTERIOR.~

WITH VAPOUR BARRIER

IN BETWEEN.

~2 _

1/2"

5~L_

4'- 11Y4"

;I

5 / 8 II X

~

I 5/8"

12' C.C.

~/

3/4" SYLVAPLY PLYWO ~' / 2 "

Fo,~ LEVELLING JO,ST

JL,~ , ~ ~ ------CEOA,, , "r 2" DIA. S/B"nnnn/J TAPEREB JO,ST ""-,/2" FIR PLYWOOD ~,,/'~/'~'.-I-':: ~"OUTLET # EXISTING FLOOR IN LABORATORYj

pq

I~

E,~

LSLEEPERS

II. [X]

X~l/4"

D..r-Ji

PLYWOOD

FIG. 4--Design section o f storage room. safety reasons, no electric outlets or fixtures were permitted inside the room. If power is required, it is brought in through the 2-in. diameter access or service hole through the wall, 5 in. from the ceiling above the window (Fig. 3). The hole is left open to dissipate the air cushion effect every time the door was opened or closed. A 3 by 7-ft refrigerator door with a two-way latch, insulated and covered with sheet copper, was installed on the outside to close against a rubber seal placed around the doorway. Three removable floor sections were constructed to provide the walking area and support for the racks and table above the reservoir (Fig. 3). These consisted of 90 sylvaply plywood sections nailed to 2 by 2-in. cedar sleepers, which, in turn, were nailed to 2-in.-thick tapered cedar Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BOZOZUK ON TEMPERATURE CONTROLLED HUMID STORAGE ROOM

119

levelling joists (Fig. 4). The removable floor facilitates maintenance when required. A 5 in. clearance was provided, adjacent to the sprayed walls (Figs. 3,4), to ensure an adequate circulation of air under the floor.

Closed Flow Conditioning System The desired storage temperature and relative humidity are obtained by continuously circulating the ponded water from the reservoir through a heat exchanger and back into the top of the room, where it is directed to flow down the walls. This is achieved by pumping the preconditioned water into a copper tube header, fastened to the ceiling about 89 in. away from the walls, and covering most of the room perimeter (Fig. 3). Originally, the header was fitted with eleven flat atomizer jets spaced relatively uniformly, but this mixed the air and water to such an extent that it created fog. Any objects placed in the room collected water immediately. Even beakers filled up within a short time. Such an environment was satisfactory for storing and curing concrete specimens but was not considered satisfactory for soil samples. Consequently, the atomizer jets were replaced with 10 in. lengths of 88 copper tubing. It was possible, by bending the tubing to direct the flow along the walls, to create the flow pattern illustrated in Fig. 5, and to wet the walls below 24"C C.

PER TUBE NOZZLE,lO"

LO?0 GBE ,IZ0 0,REGT LOW

l

//

/

RUNNING

DOWN WALLS

S, TTooLgVEL PONDED WATER

9 FLOOR OF RESERVOIR

,/

FIG. 5 Flow pattern o f water along walls o f storage room.

the header completely. A 88 transite (asbestos) baffle, installed in 4-ft-wide sections and set 2 89 in. away from the walls, prevented splashing of the interior (Fig. 4). It also increased the wetted surface area from which water evaporated into the room. The bottom of the baffle was Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

120

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

kept about 2 in. above the surface of the ponded water to ensure proper circulation of air. The storage room requires about 75 Imperial gallons of water to create a pond 1 89 in. deep at the shallow end. Slimacide V-10, with a pine root oil base, is added to the circulating water to prevent bacterial growth. The water is pumped from the reservoir, through a strainer, to the heat exchanger, as shown in Fig. 6, where it is cooled, then back to the room E__-~

.....

~_--_~_--_~

~NOZZLES

TEMPERATURE CONTROL

CONDITIONED WATER TO HUMID ROOM

t [ VALVF,I E Jil ;

I

I

~

VALVE

HEATEXCHANGER

ALVE THERMOMETER

BYPASSVALVE/

SENSOR

ENSOR

THERMOMETER

WATER IL " ~ SUPPLY ~ ~ -~ .j I TEMPERATURE

HERMOMETER TEMPERATURE

SERVO MOTOR

CHILLED ~

~ i

/

PRESSURE GAUGE

~'I

/

/ THERMOMETER

CENTRIFUGAL

~

!

TEMPERATURE SENSOR

WATERFROMROOM

PUMP

\STR%ER DRAIN

FIG. 6--Conditioning equipment and controls. at a rate of 15 Imperial gal/min, under a pressure of 16 psi. A single phase l l0-V 60-cycle 3450-rpm V2-hp motor powers the centrifugal pump. The temperature of the chilled water supply feeding the heat exchanger is about 41 ~ it is exhausted at about47 ~ The temperature o f the storage room is regulated by the control panel and three temperature sensors installed at the locations shown. The panel operates the servo motor driving the by pass valve mounted on the chilled water supply line, thus controlling the amount of cooling required to maintain a room temperature of 55 ~ which is about the mean annual ground temperature in the Ottawa region. Four thermometers, located as shown, provide an easy check on the operation of the system. In assembling the plumbing system, air bleed valves were provided at all high points and drains at all low points. To facilitate maintenance and flushing of the room, it is connected permanently to a floor drain in the laboratory. The storage room is furnished with four sample racks and a worktable

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BOZOZUK ON TEMPERATURE CONTROLLED HUMID STORAGE ROOM

121

laid out as shown on Fig. 3. The support frames are made of aluminum angles and the shelves of asbestos sheeting. The design details are given on Figs. 7 and 8.

~

ALUMINUM FRAMING IlL'ix I I,/21'X3/f6" THK.

~21

/47

821

ASBESTOS BOARD SHELF 5/8" , 12"X I14'l THK,

,//

===:

~ALUMINUM BASE PLATES ~ UNDER EACH UNIT t25/8"x II~'x 3/16" THK,

48" FIG.

12"

7--Storage room racks.

SECURE SYLVAPLY TO FRAME USING N-I/4'L20 BRASS BOLTS ~ NUTS 8r SCREW I/2" ASBESTOS SHEETING WrTH VB" F/HD BRASS SCREWS.~

SYLVAPLY G6"

|

26"

/

J

I

~3"

--

112" ASBESTOS SHEETING BOLTED TO SHELF FRAME WtTH 4-I~L20 BOLTS B NUTS',,

15,

--i

112"

35' 14' I

_ ALUMINUM FRAMING [12" x ~ ~/2" X ~ / 4 II THK.

r

,4

)"

LUMINUM BASE PLAT[ ~ 25"x 3 " x 3/16" THK.

FIG.

8--Worktable.

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122

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

Handling and Preparation of Samples for Storage Careful handling and protection of high-quality undisturbed samples start in the field. As soon as the sample tubes are removed from the borehole, they are numbered, sealed and capped, and placed in special carrying cases lined with styrofoam and foam rubber. Consequently, the samples are protected from extreme temperature changes, shocks, and vibrations until they reach the laboratory. At the laboratory, they are recorded immediately and placed in the storage room. If the sample tubes are made of stainless steel, the soil cores can be left in them until needed. If rusting or corrosion is a possibility, or if the tubes are needed for continuing the sampling program, the soils should be extruded and waxed for storage. The cores are extruded vertically, using a hydraulic extruder. They are extruded in lengths of 4 or 8 in. at a time, cut o f f with a wire saw, covered with thin plastic film and coated by dipping them carefully several times into melted wax. For very weak or soft soils, the cores are wrapped with cheesecloth to provide lateral support as they are extruded, then cut off, wrapped in plastic film, and waxed. The wax used in the laboratory is a Grade 175 microwax (Imperial Oil Co.), that has a congealing temperature of 78 ~ Labels are applied and coated with wax before the samples are placed on the storage racks. Block samples are prepared for storage in a similar way, except that the wax is applied by brush.

Effect of Storage Time on Test Results Grain size analysis and Atterberg limits of sensitive marine clays do not appear to be affected significantly by long periods of storage, even though the samples change color frequently, indicating that some chemical changes have occurred. La Rochelle et al, 2 on the other hand, found that the shear strength of some Champlain clays was reduced after long periods of storage. Similar results were obtained from strength tests performed on marine clays f r o m Canadian Forces Station (CFS) Gloucester, which had been stored for seven years. The results, summarized in Table 1, show that generally the degree of saturation and wet density decreased, and the water content increased somewhat by extending the storage time f r o m one to seven years. There was also a reduction in shear strength, averaging 11.2 percent and a small decrease in strain at failure. Some investigators ~'4 have found that long storage times reduce the 2La Rochelle, P., Sarrailh, J., Roy, M., and Tavenas, F. A., this publication, pp. 126-146. 3Leonards, G. A. and Altschaeffl, A. G., Journal o f Soil Mechanics and Foundations Division, Vol. 5, Sept. 1964, pp. 133-155. 4Bozozuk, Michael in Sampling o f Soil and Rock, A S T M STP 483, American Society for Testing and Materials, 1971, pp. 121-131.

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

0.24 0.31 0.39 0.47 0.51 0.58 0.75 0.86

o',

67.7

99.5

0.348 0.546 0.493 0.706 0.716 0.818 0.774 0.734

0.642

Change, ~

102.1

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

0.83

0.93 1.03 0.68 0.56 1.03 0.84 0.86 0.67

6Q, ~s, 07o kg/cm 2

Average

96.4 107.6 95.4 94.5 97.3 108.5 99.7 117.8

y, lb/fP

. . . . . . . . . . . . . . .

80.7 52.1 85.3 88.7 79.1 50.0 70.5 35.6

w, 070

99.3 99.8 99.8 99.8 100.0 98.9 99.2 99.3

S, 070

Tested, Nov. 1968

in situ vertical effective stress. effective confining pressure. degree of saturation at start of test. water content at start of test. wet density at start of test. maximum deviator stress at failure, A(m - o~)y. axial strain at failure (rate of strain 2070/h.

0.38 0.50 0.62 0.76 0.74 0.95 1.31 1.39

7.2 16.2 25.5 36.9 43.0 48.8 57.0 63.0

NOTES---O 'o = o'c = S = w = y= /IQ = ~, =

Oo'

Depth, + 0 . 3 ft

Consolidation Stress, k g / c m ~

-1.5

98.0

97.8 100.3 96.5 99.8 97.1 96.7 98.9 96.8

S, 070

+3.4

70.0

77.9 76.7 95.8 84.5 77.2 49.8 61.7 36.5

w, ~

-2.1

100.1

96.3 96.4 90.5 95.6 96.0 107.2 102.8 115.6

-11.2

0.570

0.300 0.273 0.460 0.621 0.812 0.772 0.650 0.672

-2.3

0.81

0.91 0.69 0.66 0.38 1.07 1.23 0.93 0.58

r. 6Q, Es, 07o lb/ft a kg/cmz

Tested, Dec. 1974

tube samples o f marine clay, obtained from CFS Gloucester, Dec. 196Z

TABLE 1--Effect o f storage on triaxial consolidated anisotropically undrained strengths, measured on undisturbed

r

0 0

O 22 fi) Ill :Ij

O "1" C

irm

0

0 0 z-H

m

c

"u rn nn

O Z -I m

N

c

O

124

SOIL

SPECIMEN

PREPARATION

FOR L A B O R A T O R Y TESTING

measured preconsolidation pressure, whereas others a found no effect. To investigate this point further, additional consolidation tests were performed on undisturbed samples of clay from CFS Gloucester that had been stored in the temperature-controlled humid storage room from five to seven years. In Fig. 9, the results are compared with the original tests reported by Bozozuk and Leonards. ~ The measured preconsolidation VERTICAL EFFECTIVE STRESS TONSIFT2, kglcm 2 0 5

I0

15

SOIL PROFILE ...... BLACK ORGANIC TOPSOIL TAN COLOURED FINE SAND AND SILT

0.5 I

1.0 I

1.5 I

2.0 I

2.5 I

3.0 1

3.5 O

\

SOFT GREY-BROWN SILTY CLAY WITH OCCASIONAL DECAYEDROOTS, ROOTHOLES, AND SOME SMALL FLAT STONES SOFT GREYSILTY CLAY WITH SOMECLAM SHELLS

25

GREY SILTY CLAY WITH SOME SHELLS

351

O IN I~

DESICCATED GREY-BROWN SILTY CLAY

20

30

%

~

PRECONSOLIDATION PRESSURE MEASURED I TO 4 MONTHS AFTER SAMPLING (BOZOZUK AND LEONARDS, 19/21

E

PRECONSOLIDATION PRESSURE, MEASURED 5 TO I YEARS AFTER SAMPLING

GREY CLAY WITH BLACK MOTTLING, AND OCCASIONAL SMALL FLAT STONES

CONSOLIDATION TESTS MEASURED 5 TO 7 YEARS AFTER SAMPLING

l0 w

40 45

IN SITU VERTICAL EFFECTIVE STRESS

[5

50 55 60

GREY SILTY CLAY WITH BLACK MOTTLING, OCCASIONAL SHELLS AND SMALL STONES

I

I

I

\

I

I

I

I

F I G . 9--Effect of storage time on preconsolidation pressure measured on undisturbed samples obtained from CFS Gloucester.

pressure was reduced only slightly in the lightly overconsolidated clay formation from 6 to 18 ft (1.8 to 5.5 m). Below this depth, the measured overconsolidation pressure of 0.25 kg/cm 2 was 40 percent lower than the originally measured value of 0.42 kg/cm ~. Below the 50 ft (15.2 m) depth, the reduction was about 35 percent. As the loading schedules in both ~Bozozuk, Michael and Leonards, G. A . in Proceedings, Specialty Conference on Performance of Earth and Earth-Supported Structures, American Society of Civil Engineers, Purdue University, West Lafayette, I n d . , Vol. 1, P a r t 1, 11-14 J u n e 1972, pp. 229-317.

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BOZOZUK ON TEMPERATURE CONTROLLED HUMID STORAGE ROOM

125

testing programs were identical and the test specimens were the same size, the observed reduction in preconsolidation pressure must be attributed to storage time. The evidence concerning the detrimental effects of storage time on the engineering properties of soils is accumulating but is still not entirely conclusive. Detrimental effects, as noted previously, have been observed under the best storage conditions, and it is known that a relaxation of stresses with time may cause changes in soil structure. In planning a soil testing program, therefore, consolidation, undrained triaxial strength, and other special engineering tests should be performed as soon as possible after the samples are obtained. Atterberg limits, grain size analysis, and other classification or index tests may be performed after long periods of storage without affecting the results seriously.

Summary In 1954, a controlled-temperature humid storage room was designed and constructed in the Geotechnical Laboratory of the Division of Building Research, National Research Council of Canada. The room is a wood frame construction, lined completely with sheet copper on the inside. The desired temperature and humidity are achieved by running water down the walls to the floor, where it is ponded below a removable floor. Heat gain into the room is removed by pumping the water through a heat exchanger fed by chilled water. The paper discusses the design, the preparation of samples for storage, and the effects that long storage times may have on test results.

A ckno wledgmen ts Thanks are due to K. R. Solvason, Research Officer, Building Services Section, who designed the mechanical plant for the storage room. This paper is a contribution from the Division of Building Research, National Research Council of Canada and is published with the approval of the Director of the Division.

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P. L a Rochelle, ~ J. Sarrailh, ~ M. R o y , 1 a n d F. A . Tavenas ~

Effect of Storage and Reconsolidation on the Properties of Champlain Clays

REFERENCE: La Rochelle, P., Sarrailh, J., Roy, M., and Tavenas, F. A., "Effect of Storage and Reconsolidation on the Properties of Champlain Clays," Soil Specimen Preparation for Laboratory Testing, ASTM STP 599, American Society

for Testing and Materials, 1976, pp. 126-146. ABSTRACT: A testing program has been made on samples of sensitive cemented clays at different times after sampling. When comparing results of unconfined compression tests performed on the field immediately after sampling or in the laboratory the following week, it is observed that there is a decrease in the measured strength which may be attributed to water migration; however, this effect was minimized or eliminated completely by keeping the samples in the tubes or by trimming the samples immediately after sampling. The reconsolidation of the samples to the in situ stresses restored an appreciable part of the strength lost by sampling disturbance; this effect is negligible in the case of good quality samples. Tests made on block samples which had been stored for many years in a humid room have shown a decrease in the shearing of the clay, but the preconsolidation pressure has not been affected.

KEY WORDS: soils, cohesive soils, tests, samples, preservation, storage T h e h i s t o r y o f a c l a y s a m p l e p r i o r t o testing m a y have a n a p p r e c i a b l e effect o n the p r o p e r t i e s o f t h e c l a y m e a s u r e d in the l a b o r a t o r y . I n t h e case o f the soft sensitive clays, o b s e r v a t i o n s b y d i f f e r e n t a u t h o r s h a v e s h o w n t h a t t h e s a m p l e s s h o u l d be m a n i p u l a t e d with great c a r e d u r i n g t r a n s p o r t a t i o n , p r e p a r a t i o n , a n d t r i m m i n g so as to a v o i d a n y shocks, v i b r a t i o n s , o r stress c o n c e n t r a t i o n s , w h i c h a r e b o u n d to d i s t u r b the clay s t r u c t u r e a n d affect the measured properties. In spite o f all due p r e c a u t i o n s with regard to m a n i p u l a t i o n , the p r o p erties o f the clay s a m p l e s m a y b e altered b y o t h e r p h e n o m e n a , s u c h as the w a t e r m i g r a t i o n within the s a m p l e . T h a t p r o b l e m was r e c o g n i z e d b y H v o r s l e v [1]z w h o suggested t h a t " s e r i o u s l y d i s t u r b e d parts o f t h e s a m p l e Professor, graduate student, and associate professors, respectively, Department of Civil Engineering, Universit6 Laval, Quebec, Canada. 2The italic numbers in brackets refer to the list of references appended to this paper.

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LA ROCHELLE ET AL ON STORAGE AND RECONSOLIDATION 127

should, as far as possible, be separated from the undisturbed parts in order to avoid migration of pore water from the disturbed to the undisturbed parts." More recently, Bjerrum [2] has reported on some observations made at Norwegian Geotechnical Institute (NGI) on Norwegian clays. Clay samples were tested immediately after sampling and after a storage time of three days; during this relatively short storage time, water migration resulted in water contents which were 3 to 4 percent higher in the core than in the outer zone of the samples; moreover, a reduction of 15 percent of the undrained shear strength was observed even if the samples had been reconsolidated to the field stresses. The reduction was attributed to the internal swelling which had occurred within the sampling tube during that fairly short period of time. Following such observations, many questions arise concerning the behavior of the sensitive clays of eastern Canada. These clays are known to have a strongly bonded structure [3-7], which is a major difference when compared to the Scandinavian clays. Is it then possible that the cementation bonds have the ability to preserve the properties of the clay during a change in stresses, thus preventing the clay from swelling as suggested by Bjerrum [2]? Is it of essential importance that the samples be reconsolidated to the same pressure they carried in the field in order to yield representative test results? And finally, what is the influence of the storage time on the cementation bonds, and how does it affect the test results? This paper presents the results of a testing program which was carried out with the purpose of providing answers to these questions. Samples of the cemented Champlain clays were used to study the occurrence of the water migration and its influence on the undrained shear strength, the importance of the reconsolidation to the in situ stresses, and the effect of prolonged storage time on the characteristics of the bonds. The present study is then aiming at determining differences in soil properties resulting from some variations in sample treatment. It must be realized that, in the case of natural soil deposits, such differences may be fairly small when compared to the natural scatter of the soil properties, further amplified by the variability of the sample quality. Hence, a very large number of tests would have to be performed in order to arrive at definite conclusions based on sound statistical analysis; this point has not always been fully realized by previous authors involved in similar testing programs. In the present study, if the large number of tests required to reach definite conclusions had been performed, the publication of the results would have been delayed considerably; however, the authors believe that the available results show strong tendencies which are of some importance to the engineering profession and should be known at this time, pending confirmation by further studies.

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128

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

Characteristics of the Cemented Clays The geology and the general physical and mechnical properties of the Champlain clay deposits have been discussed by many authors [3-71. The deposits which are found in the Saint-Laurent, Ottawa, and Saguenay river valleys have been known, for many decades, to be composed mainly o f an extremely sensitive clay, but it is only recently that the presence and significance o f the cementation bonds in the clay structure was fully appreciated. Although their exact nature is not completely understood, the cementation bonds are known to be the main cause for the relatively high rigidity and strength of that clay; similarly, the fact that the preconsolidation pressures pc measured on the clay samples are appreciably higher than the maximum loads which the deposit has ever carried in nature has also been explained by the presence o f the cementation bonds. The intensity o f cementation bonding in the clay may vary considerably from one area to the other; for the purpose o f the present study, three sites were chosen.

Saint-Aiban

The site o f Saint-Alban is located 80 km from Quebec City at the northern fringe o f the Champlain clay deposits in that area. The clay found on that site, which has been described previously in the literature [8,9], is very soft and sensitive below a desiccated crust 2 m thick; the strength, measured by means o f a Nilcon vane apparatus, increases from 10 kPa (210 lb/ft 2) below the crust at 2 m depth to 21 kPa (440 lb/ft 2) at 7.5 m depth. As seen f r o m Table 1, the water contents, the limits, the indices, and the clay contents decrease with depth. In terms of the previous loads carried by the deposit, the clay is nearly normally consolidated;

TABLE 1--Typical properties of the clays studied. Saint-Alban Soil Properties Water content, % Plastic limit, ~ Liquid limit, % Plasticity index, % Liquidity index Clay content, % Sensitivity(by lab. vane) Field vane strength, kPa pc - po, kPa

Saint-Jean2 m depth 7.5 m depth Saint-Louis Vianney 95.0 27.0 55.0 28.0 2.4 80.0 14.0 10.0 18.0

50 20 35 15 2 35 22 21 45

69.0 27.0 50.0 23.0 1.8 80.0 50.0 43.0 102.0

42.0 18.0 29.0 11.0 2.2 60.0 >500.0 240.0 640.0

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LA ROCHELLE ET AL ON STORAGE AND RECONSOLIDATION

129

however, the difference (Pc - Po) between the preconsolidation pressure Pc and the effective overburden pressure Po, as given in Table 1, indicates that the clay has gained a pseudo-preconsolidation which may be attributed to a delayed consolidation during the aging of the deposit [10] and which is locked into the clay structure by the cementation bonds.

Saint-Louis The properties of the d a y at Saint-Louis, Yamaska, which is located 160 km southwest of Quebec City, have also been discussed in detail in previous papers [6,11]. The typical values given in Table 1 correspond to the elevation at which block samples were cut out from a trench dug in a landslide crater. The clay plasticity on that site is more uniform with depth than in Saint-Alban, and the vane strength at block level is 43 kPa (900 lb/ftz). The clay is slightly overconsolidated by previous loads, but the major part of (pc - po) is estimated to be due to delayed consolidation and bonding. In terms of undrained strength and bonding, this clay may be considered representative of the average clay encountered in the SaintLaurent lowlands.

Saint-Jean. Vianney The village of Saint-Jean-Vianney, which was located approximately 200 km north of Quebec City on the northern side of the Saguenay river valley, was the site of a disastrous landslide in 1971, which was reported in the literature [12]. In many regards, the clay on that site is similar to the Toulnustouc clay reported on by Conlon [4]. The typical properties listed in Table 1 correspond to the elevation of block samples cut out from a trench dug in the crater of the landslide. The clay is very stiff, the vane strength being 240 kPa (5000 lb/ft2); it has a low plasticity and is extremely sensitive. When compared to the average Champlain clay deposits, the clay at Saint-Jean-Vianney is highly overconsolidated; the measured preconsolidation pressure at the level of the block samples is 900 kPa (8.4 T/ft 2) and it is estimated that half the value of (Pc - Po) given in Table 1 is due to delayed consolidation and bonding. When comparing the properties measured on the three sites, it becomes obvious that the clay of Saint-Alban is the least cemented; as a matter of fact, of all the Champlain clay deposits studied, that clay probably lies close to the lower end of the scale in terms of intensity of cementation. As the cementation bonds are believed to prevent swelling following a stress release, the clay from Saint-Alban was chosen for the study since it was considered to be the most susceptible to water migration within the clay samples after sampling. On the other hand, the effect of storage time on the bonds was evaluated on block samples from the two other sites where the cementation is more important. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorize

130

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

Water Migration Following Sampling In order to find out whether any water migration takes place within the samples o f cemented clays following sampling, two types of checks were made: the first one consisted o f a direct comparison of the results of unconfined compression tests made at different times after sampling, and the second check consisted o f measuring the water contents across the clay samples a few days after sampling. The samples were taken at the site o f Saint-Alban by means o f a NGI thin-wall stationary piston sampler; the stainless steel tubes used had a diameter of 73 mm, a length of 1 m, an area ratio o f 9 percent, and an internal clearance of 0.25 percent. Tube samples were taken in adjacent boreholes over an area of 4 by 4 m at two different elevations (Fig. 1); 0

I0

Cu-kPo 20 30

o

40

50

T S T-AL BAN

I

~ 1~o 2

I tube somples (upper level)

3 4 E

6

~~c

I tube somples 1 (lower level )

7

8

o vane I o vane 2 e,--e average

9 -

,o FIG.

I

i

l--Undrained strength profile at Saint-Alban.

the useable length o f sample was on the order of 80 cm per tube. Five tubes were taken at depths between 2.1 and 3.0 m, and nine tubes were between 6.1 and 7.0 m.

Unconfined Compression Tests The undrained strength at different times after sampling was measured by unconfined compression tests made on specimens trimmed to a diCopyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

LA ROCHELLE El" AL ON STORAGE AND RECONSOLIDATION 131

ameter of 3.8 cm and a height of 7.6 cm. A press, a triaxial cell, and the necessary equipment for extruding and trimming the samples were installed in a shanty on the site, so as to make compression tests immediately after sampling. In order to study the effect of short storage time on the undrained strength, four series of tests were made on the samples taken at the two different levels:

Series A--The samples were extruded, trimmed, and tested immediately after sampling on the field. Series B--The samples were extruded, trimmed, and paraffined on the field and tested in the laboratory about one week later. Series C--The samples were extruded and paraffined on the field, and they were trimmed and tested in the laboratory about one week later. Series /)---The tube samples were transported to the laboratory and stored in the humid room; the samples were extruded, trimmed, and tested the following week. All compression tests in the field and in the laboratory were made by the same operator, using the same apparatus. It may be worthwhile to mention that the so-called paraffined samples were, in fact, wrapped in plastic films which were sandwiched between layers of a mixture of paraffin and vaseline; this technique was found to be very efficient in preventing any measurable loss or gain of moisture during storage periods of more than three years in a humid room. Comparison o f the Results--When comparing the results in such a study, problems arise due to the variation of strength of the specimens, even when tested under exactly the same conditions. In the present case, two types of variations are encountered: the first one is the natural variation of the undrained strength profile along the length of the tubes, and the second is the variability in the quality of the tube samples and also of the specimens within one tube, resulting from the disturbance due to the sampling operation. Strength Profile--From previous studies in Saint-Alban, the strength profiles are known to be fairly uniform throughout the site; nevertheless, two vane profiles were determined 2 m apart in the central part of the sampling area, and the results are given in Fig. 1. It is seen that both profiles coincide fairly well. On the same figure, the positions of the tube samples have been drawn for the two levels studied; it may be observed that the vane strength varies by an appreciable amount in the depth increase, corresponding to the length of the tubes. At the upper level, the vane strength is 10 kPa (210 lb/ft 2) at the elevation of the top of the tube and 12.5 kPa (260 lb/ft 2) at the elevation of the bottom, thus increasing by 25 percent. At the lower level, there is an increase in strength of about Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

132

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

15 percent from the top to the middle of the tube sample and then a slight decrease in the bottom half. In order to make the strength results of the compression tests comparable, they were all referred to the vane strength measured at the elevation of the specimens within the tube and were expressed by the ratio C~

c~, = c . ~ / c .

(l)

where C~r is the undrained shear strength given by the unconfined compression test at failure, and C,v is the vane strength measured at the same elevation. The vane strength was taken as a reference because it indicated, in a very consistent manner, the variation of undrained strength along the profile. Any elaborate discussion on the significance and value of the vane strength is beyond the scope of the present paper. Variability o f the Quality o f Samples--In spite of all the care taken during the sampling operations in the field, there is an appreciable variation in the quality of the samples obtained in such deposits of soft sensitive clays. Figure 2 gives the results of unconfined compression tests top 0

,

0.2

4

1.4

1.6

1,8

f ~ I

I

f i

--

strength ratio, Cfv 0.6 0.8 1.0 1.2

!

2

}-

0.4

tube

/

I '

/

[

bo~'m

FIG. 2--Variability of the quality of tube samples. made on specimens trimmed from two tubes taken in two adjacent holes at the lower depth (Fig 1). It may be seen that the results from Tube I are much higher than those from Tube H, although the field technique, the sampling apparatus, the operators, and the testing method were the same for both tubes. Moreover, the strength of the specimens within the same tube sample varies appreciably, depending on the position of the

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LA ROCHELLE ET AL ON STORAGE AND RECONSOLIDATION 133

specimens in the tube; this variability may be attributed to sampling disturbance [13]. Of the eight 10-cm-long specimens, which can be cut from each tube, Specimen I, located at the top, was systematically eliminated as being disturbed; compression tests on Specimen 1 would give only 20 to 30 percent of the vane strength at that elevation. Even Specimens 2 and 3 (Table 2) give results which are questionable. Hence, the variability of the quality of the tube samples and of the specimens within each tube requires that the results be analysed carefully. Analysis of the Results--The results of the comparative study of the four test series are given in Table 2. Three different approaches were used to analyse the results. In a first approach, all tests were included to compute the average CM thus, the corresponding values of N given in Table 2 represent the total number of tests made for each series in this program. As some tests were giving exceptionally low values of C~, and others, exceptionally high values, the second approach, called the statistical approach in Table 2, consisted in eliminating the results lying outside _+1 standard deviation from the average, and a new average value was recalculated with the results that were not rejected; their number and the corresponding average C~ are given in Table 2, for each case. The third approach was based on an arbitrary rule whereby only the best results were used, up to a number equal to half the total number of tests available in each case. When comparing the results of the three different approaches given in Table 2, it is seen that, as expected, the first two approaches yield average values of C~ which are nearly identical; as for the third approach, it is quite normal that the average values of C~ be appreciably higher. However, for the purpose of the present study, it is not so much the absolute magnitude of Civ which is of interest, but rather its variation from one series to the other. In this respect, it is interesting to note that the three approaches used to analyze the results give essentially the same tendency. As the number of tests at either of the sampling levels is rather small in some of the series, and as the ratio C~ takes into account the variation of strength with depth, it is believed that the averages computed for both levels combined are more representative and should be used for the comparison of the different series. Moreover, as the three approaches are equally justifiable and show essentially the same tendency, the overall averages of the values of the three different approaches may be used for the sake of simplicity in the discussion; these averages are given at the bottom of Table 2. Discussion--It is seen that the same strength values are obtained from the unconfined compression tests in both Series A and B; in both cases, the samples were trimmed on the site, immediately after sampling, thus preventing water migration from the disturbed zone to the undisturbed core of the sample. As the compression tests were made on the field for Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authori

upper lower

Statistical

both

Overall average

4 14 18

8 15 23

9 28 37

Na

0.96

1.14 1.12 1.12

0.78 0.95 0.89

0.85 0.87 0.87

C/~ avg

4 3 7

5 4 10

8 6 14

N

B

0.97

1.17 1.14 1.16

0.88 0.83 0.85

0.92 0.87 0.90

C~ avg

3 3 6

3 3 6

5 6 11

N

C

0.80

0.85 0.86 0.85

, 0.73 0.82 0.77

0.73 0.81 0.78

C/v avg

4 10 14

4 14 19

7 20 27

N

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aN is the number of test results used to c o m p u t e the corresponding average value of Cir.

upper lower both

Best tests

both

upper lower both

Sampling Level

All tests

Choice of Tests

A

Series

T A B L E 2--Comparison of unconfined compression tests.

D

0.91

1.01 1.06 1.05

0.81 0.84 0.85

0.81 0.84 0.83

C/~ avg

-I

--I m

O -<

O

-i"1 O

z

5

-t

m -u

"u

m

m tD

r-

O w

LA ROCHELLE ET AL ON STORAGE AND RECONSOLIDATION 135

Series A and in the laboratory for Series B, the identical results confirm that the testing conditions were comparable. The strength values obtained in Series C are appreciably lower, by about 17 percent, than in Series A and B; in this case, the samples were extruded and paraffined on the field, and they were trimmed only one week later, just before being tested in the laboratory. Thus it would seem that under the conditions of complete stress release, and given sufficient time, the undrained strength may be affected by water migration within the sample. However, if the confining stresses are not released completely, the effect on the strength does not seem to be as important. This is illustrated by the fact that the strength measured in Series D is only about 5 percent lower than in Series A and B and about 12 percent higher than in Series C; in Series D, the samples were left in the stainless steel tubes and stored in the humid room for two weeks or more before being tested. It should be noted that the internal clearance ratio of the tubes is very small, on the order of 0.25 percent, and could then efficiently confine the samples. These data show a strong indication that the strength of a clay sample decreases appreciably in a short time after sampling, when stored under conditions of complete stress release; following the observations made by other authors [1,2], it is reasoned that water migration within the sample might be responsible for that decrease.

Water Content Measurements It may be possible to check whether water migrates within a soil sample by measuring the water content in a cross section of the cylindrical sample. This was attempted by cutting 2 cm-thick discs from samples and taking the water contents on the outer 1-cm ring (Wo), on the central 3-cm-diameter core (we) and on the intermediate ring (wi). These measurements were made on samples which had just been extruded from their tubes and on samples which had been stored for a certain time after extrusion. The water content measurements are given in Table 3, together with the history of the samples. When comparing the water contents of the core and of the outer ring, A(wc - Wo), it can be seen that there is no consistent tendency for the core to have a higher water content. However, it is interesting to note that, in the case of the samples which have been stored for two to three years after being extruded from their tubes, the water contents of the intermediate rings are all higher than in the outer rings and, except for one case, also higher than in the central core. It would then seem that, under conditions of complete stress release, a measurable migration of water from the outer more disturbed zone towards the center o f the sample takes place, but that it does not affect the core as much as the intermediate zone. If this is confirmed by further

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2.1

6.1

4.0

5.6

4.3

M

M'

C-4-6

C-26-5

B-12-G

05/06/73

12/09/72

20/1 0/ 72

21/06/74

21/06/74

21/0 6/ 74

Sampling

12/06/73

18/10/72

29/ 01 /73

13/07/74

24/06/74

24/06/74

Dates o f Extrusion

07/ 01/ 75

07/01/75

07/ 01/ 75

13/07/74

24/06/74

24/06/74

Water Content

0.0 +0.5

-0.8 +0.4

-1.5 -0.6

a(w~ -

51.2 51.5

65.0 51.1 +0.9 +0.4

+ 2.2 + 0.3

73.3 + 0.3 . . . . . .

70.2 69.3

66.8 62.5

69.2 67.7

Wo~

Wo)

52.1 51.9

67.7 51.4

73.6 70.3

70.2 69.8

66.0 62.9

67.7 67.1

wi a

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awo = water content o f the outer 5-mm ring, we = water content o f the 3-cm-diameter central core, and wl = water content o f the intermediate ring.

6.1

Depth, m

N'

Tube No.

-0.1 - 1.8

- 1.3 - 1.9

+0.6 -0.6

-0.3 + 0.3

+0.9 -0.6

-0.6 +0.7

A(w, - w~)

TABLE 3--Water content measurements across samples (from Saint-AIban).

52.0 50.1

66.4 49.5

74.2 69.7

69.9 70.1

66.9 62.3

67.1 68.8

w~a

+0.8 - 1.4

+ 1.4 - 1.6

+0.9 . ..

-0.3 + 0.2

+0.1 -0.2

-2.1 + 1.1

A(wc - wo)

-.I 0 -< ..q t'rl 6o ---I

z

-t

m

..~

rrl z

nl

6o 0 t-co

CO

LA ROCHELLE ET AL ON STORAGE AND RECONSOLIDATION

137

studies, it would constitute an additional argument in favor of the use of larger diameter tube samplers.

Influence of Reconsolidation

It is generally agreed that the detrimental effect of water migration and of some limited sampling disturbance may be remedied partly by reconsolidating the samples to the in situ stresses [2]. The beneficial effect of reconsolidation is twofold: first, it squeezes out the excess water resulting from swelling, and second, it restores the effective stresses which were acting in the soil element in nature. The main difficulty of such a procedure lies in the choice of the correct stress values corresponding to the field stresses. In order to do so, the value of the coefficient of earth pressure at rest, Ko, should be known; however, that coefficient remains one of the most elusive parameters of clay deposits. Many methods have been suggested for the determination of the Ko values; a recent study [14], comparing different methods of Ko measurements made on the clay deposits of Salnt-Alban, has shown that the Ko value may be taken anywhere between 0.4 and 1.2, a probable lower limit being 0.56. For the purpose of studying the influence of reconsolidation on the clay of Saint-Alban, three types of tests were made: 1. Unconsolidated undrained (UU) tests under confinement pressures equal to half the total vertical overburden pressure. 2. Isotropically consolidated undrained (CIU) tests, with isotropical consolidation pressures equal to 0.8 times the vertical effective overburden pressure. 3. Anisotropically consolidated undrained (CAU) tests, with vertical and horizontal consolidation pressures equal to 1.0 and 0.56 times the vertical effective overburden pressure, respectively. In the CIU tests, the consolidation pressures were kept below the vertical effective overburden pressure to prevent any overstressing of the clay structure [15] which could affect greatly the strength parameters. For the CAU tests, a K value of 0.56 was arbitrarily adopted, as this value corresponds to the modified [16] Jaky's expression (0.95 - sin d~r'), where ~,' is the friction angle measured on remolded clay. All tests were made at the same controlled strain rate of 0.3 mm/min. From the results given in Table 4, the following observations may be made: 1. The UU tests give strength values (C~, Table 4) which are generally higher than those obtained by unconfined compression tests (C~, Table 2). Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

138

SOIL SPECIMEN PREPARATION

FOR LABORATORY TESTING

TABLE 4---Influence o f reconsolidation. Type of Test

Sampling Level No. of tests

AVJV, %

et, %

Cs~

UU

upper lower

3 4

... ...

3.10 0.92

1.19 1.46

CIU

upper lower

4 4

4.1 1.2

1.60 1.00

1.89 1.99

CAU (K = 0.56)

from 2.3 m to 6.4 m

4

2.2

0.87

1.45

2. The anisotropic consolidation under K = 0.56 does not seem to improve appreciably the measured strength; this may possibly indicate that the actual value of/to is higher than 0.56. 3. The CIU tests give strength values which are considerably higher than the values obtained by UU tests and are nearly twice the vane strength. The stress-strain curves for UU and CIU tests are compared in Fig. 3; it can be seen that both the peak strength and the rigidity are improved.

II ~o

8

_o x

o

13_

7

~6 b J

b-~

o

o

2

4 6 8 E,deforrnation,%

I0

12

FIG. 3--Comparison o f UU and C I U tests.

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LA ROCHELLE ET AL ON STORAGE AND RECONSOLIDATION

139

Discussion--From these results, it is evident that a reconsolidation in the isotropical conditions restores the strength to an appreciable degree. Questions may be raised as to the significance of the fairly large change of volume (AVJV) encountered during consolidation (Table 4) when compared to the small amount of water migration observed. However, the increase of strength, which may seem quite important, is not outside the order of magnitude of the undrained strength, which is expected on these soils, when compared to the vane strength. Previous studies [6], have shown that the undrained strength obtained by compression tests, U or UU tests, on good quality samples amounts to 1.5 times the value of the vane strength measured at the elevation of the samples, or more. Hence, it should be realized that these observed differences between UU, CAU, and CIU results for cemented clays are due to the fact that the clay structure has suffered from the sampling operation. For good quality samples, such differences may not be so evident; this is illustrated by the results of UU and CAU tests (Fig. 4) made on samples which

T _0 x

g

b

0

2

4 6 8 ~, d6formation, %

I0

12

FIG. 4--Comparison o f UU and CA U tests on good quality samples.

have been taken by means of a new tube sampler developed at Laval University. It is seen that the stress-strain curves are comparable, and the undrained strengths measured for the CAU and UU tests are 1.80 and 1.85 times the vane strength, respectively. Although all these results emphasize the importance of good sampling techniques, they also Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

140

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

show that it is possible, after sampling, to restore part of the characteristic of brittleness in the sensitive cemented clays, provided that the clay structure has not been affected too much. It should be emphasized that this testing program has been performed on the clay of Saint-Alban, which is among the least cemented clays encountered in the Saint-Laurent lowlands, as may be appreciated from the values given in Table 1. The detrimental effect of water migration and stress release should normally be less pronounced in more intensely cemented clays; however, no data are available to confirm that assertion.

Influence of Storage Time The influence of storage time on the characteristics of undisturbed soil samples is of some importance for research or even commercial laboratories which get involved in long-term testing programs. Unfortunately, very little is known of that influence on the sensitive cemented clays. Bozozuk [17] has reported a reduction of 4.8 percent of the preconsolidation pressure in samples of the sensitive clay from the Ottawa Sewer Plant, after 17 months of storage; no data are available on the variation of the strength with storage time. In the present testing program, compression and consolidation tests were made on block samples taken from two sites and stored for long periods of time. Block samples were preferred to tube samples for such a study because the tests are reproducible with very small dispersion, and, if the blocks are properly wrapped and stored in a humid room, no water migration should take place and affect the test results appreciably. For such samples, it was thought that any change in the strength of consolidation characteristics would result in a chemical change, which would mainly affect the cementation bonds. The typical properties for the clay of these two sites, Saint-Louis and Saint-Jean-Vianney, are given in Table 1. In both cases, the block samples were cut from a trench dug in the craters of landslides; they were wrapped in plastic sheets, sandwiched between layers of a mixture of paraffin and vaseline, and were transported on a foam mattress to the laboratory, where they were kept in a humid room at a temperature of 70~ and a relative humidity of 95 percent. There were occasional failures of the climatizing apparatus of the humid room, which resulted in spells of three to five days with a temperature of 75 ~ and relative humidity of 60 percent. The sample from Saint-Louis has been stored for more than six years and that from Saint-Jean-Vianney for more than three years. The first test on the Saint-Louis blocks were made five months after sampling, and on Saint-Jean-Vianney blocks, within six weeks after sampling. From the data available from adjacent blocks taken at the same time, Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

LA ROCHELLE ET AL ON STORAGE AND RECONSOLIDATION

141

no noticeable change in the water content could be detected after these long periods of storage. It was observed that a green moss had formed on the face of the blocks under the paraffin; consequently, a 2.5-cm-thick layer of soil was removed from the faces of the blocks before cutting the test specimens. All cutting and trimming operations were made by a thin wire saw.

Undrained Strength In order to examine whether the undrained strength has been affected by the long period of storage time, CIU tests were performed under exactly the same testing conditions as similar tests which were made on adjacent blocks, within a few weeks after sampling. The CIU tests were chosen, as they are considered to be an efficient means of evaluating the intensity of bonding in cemented clays [4,6,18]. From the comparative results given in Table 5, the following observations may be made: 1. The change of volume during consolidation has increased considerably and has more than doubled in some cases; this may be indicative of a certain weakening of the clay structure. 2. The deviator stress at failure (ol - o3)t has decreased by 10 to 15 percent for Saint-Louis clay and by 14 to 21 percent for Saint-JeanVianney. 3. The strain at failure ~r has increased appreciably in the case of SaintLouis but not in the case of Saint-Jean-Vianney. 4. The pore pressure generated by shear deformation has a tendency to be lower at failure for the Saint-Louis clays. Figure 5 gives the stress-strain and pore pressure curves obtained from CIU tests performed on block and tube samples after five months of storage by Lefebvre [19] and on block samples after six years of storage; this comparison indicates that the effect of storage is somewhat similar to the disturbance resulting from tube sampling but not as severe.

Consolidation Characteristics The consolidation characteristics, and especially the preconsolidation pressure pc, may be considered as an indication of the presence of cementation bonds in the sensitive clays, where the magnitude of pc is appreciably larger than the maximum load which has ever been carried by the soil element. The consolidation curves of cemented clays are characterized by a well-defined knee at pressures corresponding to Pc; the better the sample quality, the more pronounced the knee. Hence, any appreciable change in the shape of the consolidation curve and in the value of Pc would be an indication of alteration or disturbance of the soil sample. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

summer

Saint-Jean-Vianney

oo ', kPa 69 69 83 83 103 103 103 103 345 345 483 483

Date of Test 1969 1974 1969 1974 1969 1974 1971 1974 1971 1974 1971 1974

1.1 1.3 1.1 2.6 1.4 3.4

0.3 0.7 -2.3 1.7 2.6

AVJV, %

0.7 0.8 1.0 0.8 0.6 0.7

1.3 1.0 1.2

112 129 116 703 554 831 712 931 777

1.1

0.9 1.5

eI, ~

131

132 112

(ol - oa)j, kPa

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

1971

summer 1968

Date of Sampling

Saint-Louis

Site

TABLE 5--1nfluence o f storage on CIU tests.

101 85 258 255 310 331

32 60 56

56

48 35

u/, kPa

4~

...g

fi)

T~

"-4 t'tl 0~ --4

--4 O -<

O

8-n

z

6

--I

2o

"10 20 m "1o

m z

o_

Go -o m

r'-

o

Oo

PO

LA ROCHELLE ET AL ON STORAGE A N D RECONSOLIDATION

200

143

I

ST-LOUIS 180

I 160 140 n~ 12o

-~

g

moo

_L ;I ',F

4t'

b-

I

=L~..~

60

IJ--; h / ~ -

40

g

20

(C~_O-3)Au

,

lq~,c~ClU (block) ' v-'~CIU(tube) 1974 CIU(block)

0

I

0

. . . . .

_..-

I

o

9

o =

9 ,L

I

2 3 4 e, deformotlon,%

5

6

FIG. 5--Comparison between CIU tests on block sample after storage with C I U tests on block and tube samples made after sampling by Lefebvre [19].

Consolidation tests were made on the block samples of Saint-Louis and of Saint-Jean-Vianney after prolonged periods of storage, and the results were compared with those obtained from initial tests made within a few weeks or months after sampling. As illustrated by the data given in Table 6, the values of pc have not changed during storage on either site. The consolidation curves remain essentially of the same shape as may be observed from the comparison of curves given in Fig. 6 for Saint-JeanVianney. Discussion--It thus seems that storage affects the strength characteristics of the cemented clays but has no influence on the consolidation characteristics. This paradoxical behavior may be explained by an adequate clay structure model and is not inconsistent with previous observations [6], in that limited sampling disturbance of cemented clays decreases the strength of the samples but has no influence on the value of pc. The observation that Pc is not affected by storage is contrary to previous findings by Bozozuk [17], who has found a decrease of 4.8 percent on samples which have been stored for 17 months; it should be noted, however, that tube samples were used by Bozozuk, and, again, this may show the importance of obtaining good quality samples for long periods of testing. Obviously, these results imply that the samples should be tested as soon after sampling as possible, especially if shear strength parameters are to be determined. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

144

SOIL

SPECIMEN

PREPARATION

FOR

LABORATORY

TESTING

TABLE 6--Influence of storage on the preconsolidation pressure. Preconsolidation Pressure-pc,

Date of Sampling

Date of Test

No. of Tests

kPa

Saint-Louis

1969 1969

1969 1974

3 2

164 164

Saint-Jean-Vianney

1971 1971

1971 1974

6 2

900 900

Site

1.4

1.5

1.2

I.I o

>o I0

09

0.8

0'75 6 789

I0

2

3

4 5 6 789

I00

pressure,

2

3

4

5 6 789 I000

2

3

kPo

FIG. 6----Comparison of consolidation curves from block samples before and after storage

period.

Conclusion The present paper is a report on a study o f the influence o f reconsolidation and storage time on the strength and consolidation characteristics o f sensitive cemented clays from eastern Canada. From the data presented, the following tentative conclusions are suggested. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

LA ROCHELLE ET AL ON STORAGE AND RECONSOLIDATION 145

1. In the case of tube samples of soft and weakly cemented clays, a decrease of undrained strength on the order of 15 percent takes place during the first "few days after sampling, under conditions of complete stress release. 2. The decrease of strength may possibly be attributed to water migration from the disturbed outer zone of the sample towards the central part. From the water content measurements across samples, it is reasoned that larger diameter samplers would attenuate this detrimental effect. It is also suggested, pending further studies, that, in more strongly cemented clays, the effect of water migration might not be as pronounced. 3. The reconsolidation of samples under the field stresses does restore at least part of the lost strength and rigidity. However, it is also shown that, on good quality samples, the effect of reconsolidation is negligible. 4. During long periods of storage in humid rooms, block samples of medium and strongly-cemented clays have suffered a reduction of undrained shear strength on the order of 10 to 20 percent. However, the preconsolidation pressure and the general shape of the consolidation curve have not been affected by storage. Many of the observations presented in this paper stress the detrimental effect of sampling disturbance and the importance of obtaining good quality samples for elaborate programs of testing.

Acknowledgments The field and laboratory work reported in this paper were carried out by J. Sarrailh, graduate student. The help of J. P. Dussault, J. Y. Julien, S. Par6, and M. Pouliot, technicians, is appreciated greatly. This investigation was carried out with the financial support of the Ministry of Education of the Province of Quebec and the National Research Council of Canada. References [1] Hvorslev, J., "Subsurface Exploration and Sampling of Soils for Civil Engineering Purposes," Waterways Experiment Station, Vicksburg, Miss., 1949, pp. 163-164. [2] Bjerrum, L. "Problems of Soil Mechanics and Construction on Soft Clays," Stateof-the-Art Report to Session IV, 8th International Conference on Soil Mech~ics and Foundation Engineering, Moscow, Vol. 3, 1973, pp. 1l 1-159. [3] Crawford, C. B., Geotechnique, Vol. 13, No. 2, 1963, pp. 132-146. [4] Conlon, R. J., Canadian Geotechnical Journal, Vol. 3, No. 3, 1966, pp. 113-144. [5] Mitchell, R. J., Canadian Geotechnical Journal, Vol. 7, No. 3, 1970, pp. 297-312. [6] La Rochelle, P. and Lefebvre, G. in Sampling of Soil and Rock, ASTM STP 483, American Society for Testing and Materials, 1971, pp. 143-163. [7] Sangrey, D. A., Geotechnique, Vol. 22, No. 1, pp. 139-152. [8] Tavenas, F. A., Chapeau, C., La Rochelle, P., and Roy, M., Canadian Geotechnical Journal, Vol. 11, No. 1, 1974, pp. 109-141. [9] La Rochelle, P., Trak, B., Travenas, F. A., and Roy, M., Canadian Geotechnical Journal, Vol. 11, No. 1, 1974, pp. 142-164.

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146

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

[10] Bjerrum, L., Geotechnique, Vol. 17, No. 2, 1967, pp. 83-119. [11] Lefebvre, G. and La Rochelle, P., Canadian Geotechnical Journal, Vol. 11, No. 1, 1974, pp. 89-108. [12] Tavenas, F. A., Chagnon, J. Y., and La Rochelle, P., Canadian Geotechnical Journal, Vol. 8, No. 3, 1971, pp. 463-478. [13] La Rochelle, P., discussion on the State-of-the-Art Report to Session IV, 8th International Conference on Soil Mechanics and Foundation Engineering, Vol. 4.2, Moscow, 1973, pp. 102-108. [14] Tavenas, F. A., Blanchette, G., Leroueil, S., Roy, M., and La Rochefie P., "Difficulties in the In Situ Determination of Ko in Soft Sensitive Clays," Specialty Conference on In Situ Measurement of Soil Properties, American Society of Civil Engineers, Raleigh, N.C., June 1975, Vol. 1, pp. 450--476. [15] Raymond, G. P., Townsend, D. L., and Lojkasek, M. J., Canadian Geotechnical Journal, Vol. 8, No. 4, 1971, pp. 546-557. [16] Brooker, E. W. and Ireland, H. O., Canadian Geotechnical Journal, Vol. 2, No. 1, 1965, pp. 1-15. [17] Bozozuk, M. in Sampling of Soil and Rock, A S T M STP 483, American Society for Testing and Materials, 1971, pp. 121-131. [18] Lo, K. Y. and Morin, J. P., Canadian Geotechnical Journal, Vol. 9, No. 3, 1972, pp. 261-277. [19] Lefebvre, G., "Contribution h l'~tude de la stabilit6 des pentes dans les argiles ciment6es," Ph.D. thesis, Universitd Laval, Qu6bec, Canada, 1970.

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J. K. Torrance'

Pore Water Extraction and the Effect of Sample Storage on the Pore Water Chemistry of Leda Clay

REFERENCE: Torrance, J. K., "Pore Water Extraction and the Effect of Sample Storage on the Pore Water Chemistry of Leda Clay," Soil Specimen Preparation for Laboratory Testing, ASTM STP 599, American Society for Testing and Materials, 1976, pp. 147-157. ABSTRACT: In recent years, the potential significance of chemical and mineralogical influences on the behavior of the post-glacial marine clays has been increasingly recognized by the soils engineering community. The precise relationships are not known, but it seems probable that, in some cases, small differences in chemical factors may explain the differences in behavior between otherwise similar samples. In connection with an investigation of these relationships, the experiments reported in this paper were undertaken to examine the magnitude of chemical change which may occur in low-salinity Leda clay during periods o f storage. Pore water extraction devices are described, and the effects on the pore water chemistry of three months storage, under a variety of standard and modified storage procedures, are reported. It is concluded that none of the storage procedures tested is entirely satisfactory, in that potentially significant changes in the pore water chemistry occurred. Finally, it is recommended that pore water chemistry be assessed more often than is the present practice when soils engineering tests are performed on Leda clay and that this be done as soon as possible after the sample is obtained from the field. KEY WORDS: soils, tests, clays, water chemistry, moisture content, storage procedures

In most soils engineering studies, it is considered sufficient, in addition to the standard engineering measurements, to describe the geological origin of the material, certain physical properties such as texture and density, and the dominant minerals present. Rarely is any description of the chemical state of the soil system included. This is acceptable for many soil materials, but there are instances where the lack of chemical information may lead to incomplete understanding of results or to misinterpretation. An example of the latter possibility occurs when one is working with the 'Associate professor, Department of Geography, Carleton University, Ottawa, Ont. Canada.

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148

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

post-glacial marine clays. The influence of chemical factors on marine clay behavior has been examined in Norway by various investigators working at the Norwegian Geotechnical Institute [1-3]. 2 In Canada, the main work to date has been published by Penner [4], Sangrey and Paul [5], and Torrance [6]. Bjerrum [1] found that, for the Norwegian marine clays, a relationship exists between the sensitivity and the salinity; namely, the sensitivity increases as the salinity decreases. In these materials, it also has been found [2,3] that relatively small differences in the concentrations of certain ions, at low but essentially constant pore water salinity, can explain differences in behavior between otherwise similar soils. The investigation of the role of chemical factors in the Canadian Leda clay has been less extensive. Penner [4] observed a relationship between electrokinetic potential and sensitivity for the Leda clays and noted that certain chemicals added to the soil affected its behavior. Sangrey and Paul [5] investigated the in situ pore water chemistry at sites in the Ottawa area and suggested that the sodium/calcium ratio in the pore water was an index of the depositional origin of the material and its susceptibility to a certain type of landslide. Further work at the Geotechnical Science Laboratories at Carleton University in Ottawa has extended these investigations and has shown, for individual samples of Leda clay, that a salinity-sensitivity relationship exists, similar to that observed in Norway [6]. The relationship, however, may vary greatly from one sample to another. These differences are thought to be related to other differences, that is, physical, mineralogical, and chemical, between samples. It was noted that, as in Norway, the various cations have different effectiveness in altering the soil behavior. It also has been found that the soduim/calcium ratio is inadequate as an index of origin since it depends on both the depositional origin and the degree of leaching and weathering of the Leda clay [7]. The problem of determining the importance of chemical factors in Leda clay is Complicated by the extreme variability of other factors known to influence its behavior. Experiments to date indicate that the relationships to be expected, at least for certain variants of Leda clay, will follow a pattern similar to that observed in Scandinavia. Among the most interesting of the Norwegian findings is the effect of low concentrations of various ions on the soil behavior [2,3,8]. If similar effects are important in Canada, then, in view of the nonuniformity of Leda clay, care must be taken to obtain the most reliable chemical information possible. In many engineering investigations there is a period of storage, usually under controlled conditions, before the laboratory investigation is undertaken or before all tests are completed. This storage period represents a time during which chemical changes may occur within the sample. The present investigation was under2Theitalic numbers in brackets refer to the list of referencesappended to this paper. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

TORRANCE ON PORE WATER CHEMISTRY OF LEDA CLAY

149

taken to determine the possible magnitude of changes in the pore water chemistry of a low-salinity Leda clay during storage under a variety of conditions.

Soil Material The soil material used in this investigation was taken in September 1974 from the site of the landslide of May 1973, north of Chelsea, Quebec. The Shelby tube samples were taken by hand from the base of the scarp at the side of the landslide scar. All samples were obtained from 10 to 12 ft below the surface and from an area measuring approximately 1 by 3 ft. The natural water content of the soil was normally between 55 and 65 percent (although higher and lower values were observed), the liquid limit ranged from 37 to 41 percent, and the plastic limit from 21 to 25 percent. Sensitivity, determined by the fall cone method, was between 10 and 20. This low value for the sensitivity was the result of the soil material coming from a near-surface location, in which it had been mildly affected by weathering. This weathering has caused an increase in the remolded shear strength.

Storage Procedures Standard storage techniques and some modifications of standard methods were tested to assess the amount of change which occurred in the pore water chemistry. The following storage procedures were used: 1. Left in the original Shelby tube. 2. Aluminum foil wrapped and waxed. 3. Plastic film wrapped and waxed. 4. Waxed only. 5. Extruded and placed in sealed plastic container with a nitrogen atmosphere. 6. Extruded, wrapped in plastic film, and placed in a sealed plastic container with a nitrogen atmosphere. 7. Extruded, and placed in a sealed plastic container with an air atmosphere. For all treatments except Treatment 1, the soil was extruded from the sample tube Within four days of sampling. Four inch samples of soil were used in each test. Before being prepared for storage, a slice of soil was taken along the full length of each segment. The surface of this slice, which had been in contact with the tube, was removed by scraping, and the remainder was subdivided lengthwise to allow water content determination on one portion and pore water extraction from the other. This procedure allowed the initial water content and pore water chemistry of the sample to be determined for comparison with the results obtained after storage. Samples prepared in the manner described were stored under conditions Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

150

SOIL SPECIMEN PREPARATION

FOR LABORATORY TESTING

o f room temperature (20~ and under conditions of high humidity at a temperature approximating the mean annual temperature of the sampling site (7 ~ Treatment 1 was carried out only under the latter conditions. The storage period reported in this paper was three months. While longer duration storage periods are being studied, three months is considered applicable to most commercial situations and is, therefore, of practical interest. Pore Water Extraction

The quality o f a pore water sample depends on the method by which it is obtained. Pore water extraction apparatus should meet certain basic requirements as to the amount of pressure that can be applied, and, at the same time, evaporation of water from the soil during extraction and from the extract while it is being collected should be minimal. Two different devices which are used in our laboratories to extract pore water from Leda clay will be described. The choice o f device depends mainly on the remolded shear strength of the soil. When samples with a remolded shear strength below approximately 0.5 t o n / m ~ are encountered, an air-activated pore water press has proven satisfactory (Fig. 1). This device consists o f a plastic sample chamber into which a disturbed sample of the soil is placed. At one end, there is a 1 mm pressure

applied

f r o m air c y l i n d e r

tcm I

I

/ I/ -metal

top plate

-rubber

gasket

- dental

dam

- s a m p l e chamber -connecting

. . . . . . . . . . . . . . . .

bolt

-protective

filter paper

-fine-pored

filter paper

plastic

outflow tube

T to

test

tube

FIG. l--Air-activated pore water press (schematic).

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TORRANCE ON PORE WATER CHEMISTRY OF LEDA CLAY

151

diameter opening which, during operation, is covered by a fine-pored filter paper. This, in turn, is protected physically by one or more standard filter papers. A fine plastic tube (as used with atomic absorption photometers) is connected to the opening and leads to the collecting test tube. The other end of the chamber is covered by a thin rubber membrane (dental dam) and a rigid top plate (which is fitted with connections to allow application of air pressure). A rubber gasket between the dental dam and the top plate has been found to lengthen the membrane life. The apparatus is assembled and pressure applied (slowly at first to prevent puncture of the filter paper) from a cylinder of compressed air. The thin rubber membrane applies pressure to the sample, while preventing the air from passing through the sample and evaporating an unknown quantity of pore water. The present apparatus resists air leakage for pressures up to 6 to 7 atm. If the sample chamber which measures 1 in. depp by 2 in. diameter is half full or more, 1 to 5 ml of pore water can be usually obtained in less than 1 h. This apparatus in inexpensive and fast, and it is possible to use the same air cylinder to apply pressure to a number of sample chambers simultaneously. When pore water is required from stiffer samples, a mechanical pore water press is used (Fig. 2). The design is a minor modification of a pore water press that the author first saw in operation at the Norwegian Geotechnical Institute. The soil chamber is constructed of steel, the 1-mmdiameter exit is protected by layers of filter paper, and pressure is applied to the disturbed soil sample through a steel piston. This device is slow to operate but is necessary with stiff samples and in cases when only a small amount of soil is available. The concentrations of sodium, calcium, magnesium, and potassium in the extracted pore water were determined using a Jarrell-Ash atomic absorption spectrophotometer. Results and Discussions

The concentrations of sodium, calcium, magnesium, and potassium found in the pore water and the water contents at the start of the experiments and those found after three months storage for each treatment are presented in Table 1. The results show that considerable change in the pore water chemistry can occur over this time period. The differences in water contents before and after storage create a problem in making comparisons with the aluminum foil wrapped and waxed samples at 7 ~ and with the waxed only samples at both temperatures. In these cases, the data suggest that significant water loss occurred during storage. The samples, however, showed no visual evidence of major water loss, and it seems probable that the differing water contents before and after storage represent, in large measure, natural water content variations. The substantial increase in water content for one of the plastic wrapped and waxed samples and the

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152

i 1 cm

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

I

~

~. . . . . . . . . . . . .

plunger

[

~'~::'J" . . . .

O-rings

.........

sample chamber

..........

protective filter paper

~----i~------fone'iP;~ ed f'lter paper

...............

FIG.

plastic

outflow tube

2--Mechanical pore water press--all materials are steel unless otherwise indicated.

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7 20

7 20

7 20

7 20

7 2O

Plastic wrapwaxed(duplicate samples)

Waxed

Container N~

Container, plastic wrap I',I2

Container, air

48.4 45.0 51.3 61.4

50.3 50.9 56.0

44.0 50.4

58.7 65.5

57.1 53.5

49.4 49.9

55.5 55.1 59.3

50.7 58.5

58.0 64.2

58.7 56.4

51.9 55.8

49.6 48.7 70.5 64.2

57.5 57.1 53.0 58.4

Initial

3 Months

34 24

22 26

18 20

28 30

24 29 34

24 16 23 16

16 to 34

Initial, ppm

32 61

20 30

27 34

39 20

45 21 29

20 49 35 56

18.8 19.5 22.5 16.5

3 Months, ppm

Na

-6 150

-9 15

50 70

40 - 33

88 - 27 - 15

-17 200 52 250

Change, ~

18 22

25 19

23 17

18 20

18 18 20

23 21 18 14

14 to 25

Initial, ppm

65 107

77 77

129 107

31 31

30 26 26

28 72 25 21

48 57 55 36

3 Months, ppm

Ca

260 390

210 300

460 530

72 55

67 44 30

22 240 39 50

Change, 07o

7.9 11.5

11 8.2

11.5 8.2

7.4 8.2

7.2 7.8 8.1

9.7 10 8.2 7.9

7 to 12

Initial, ppm

40 68

43 45

93 67

16.5 16

12 13 13

13 41 12 13

26 31 27 15

3 Months, ppm

Mg

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7 20

7

Storage Temperature

Water Content, o7o

400 490

290 450

7(30 720

110 95

67 40 38

34 310 45 65

Change, ~

21 21

26 20

20 19

20 22

20 Ig 20

19 17 19 17

17 to 22

Initial, ppm

42 42

34 36

49 48

24 29

32 24 22

20 33 17 18

40 33 31 24

Months, ppm

3

K

100 100

30 80

145 150

20 32

60 33 lO

5 95 - lO 6

Change, ~

analysis of pore waterfor Leda clay samples from Chelsea, Quebec at beginning of experiments and after three months storage.

Aluminum foil, waxed (duplicate samples)

Shelby tube, ends waxed (4 samples from tube)

Treatment

TABLE l--Chemical

ol

..k

r-

0

"11 rm o

0

.<

0 I m

m

m

~o

0

"o

0 z

ITI

2: O

:~ "In

154

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

initial water content differences between samples support this interpretation. In any case, the water content differences are insufficient to explain the magnitude of the variations in pore water chemistry. The chemical changes which occurred are not encouraging with regard to the possibility of storing samples for anything but short periods without the danger of unknown changes in total concentration and the concentration of various elements. In most cases, the concentrations of all elements in the pore water increased, and it should be noted that the percentage increase was different for each element. It would appear that reactions occur which attack the soil minerals to varying degrees. Any cation released from the structure of the minerals would engage in cation exchange such that the relative strengths of adsorption of these ions and those satisfying exchange sites also influence the pore water concentrations that result. It is the author's opinion that in no instances are the measured changes small enough to ignore. Furthermore, the duplicates for the aluminum foil wrapped samples and for storage in the Shelby tube indicate that the degree of change is not necessarily consistent for identical or similar storage procedures. With the paucity of information relating to the effect of chemical factors on Leda clay behavior, it is not possible to state what degree of change during storage should be considered an acceptable maximum. The two most common methods of storing samples are (1) in the original sample tube and (2) wrapped in aluminum foil and waxed. In Table l, the pore water chemistry at four positions in the Shelby tube after storage for three months are compared with the lower and upper limits of concentration found initially in all other samples. With the exception of sodium, all concentrations observed after storage in the tube were considerably higher than the maximum observed in any sample at the beginning of the experiment. With the second common storage method, the samples wrapped in aluminum foil, evidence of reaction (probably biologically aided) between the foil and the soil sample was present in all samples, as was indicated by discoloration of the foil. Additionally, in those aluminum foil wrapped samples stored at 20~ there was a buildup of gas pressure within the sample and a smell of hydrogen sulfide when the samples were opened. In this case, the magnitude of the pore water chemistry changes varied considerably for the duplicate samples at 7~ but the changes, with the exception of sodium, were relatively small and consistent at 20~ Redox potential measurements on the 20~ samples indicated that extreme reducing conditions had developed. Presumably, the reducing conditions enhanced the stability of the minerals, thus decreasing the magnitude of change. Despite the small chemical changes observed, it does not seem reasonable to recommend a storage procedure during which the redox potential undergoes major change. It was hoped that the use of plastic wrap in place of aluminum foil would eliminate these chemical reactions and decrease the magnitude of

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TORRANCE ON PORE WATER CHEMISTRY OF LEDA CLAY

155

chemical change during storage. The improvement was marginal, but the degree of change appears to be somewhat more consistent. Likewise, the samples which were only waxed exhibited considerable change, some of which may be attributable to water loss. The other storage procedures tested all involved storage of the samples in air-tight plastic containers. This types of storage might be used with sample trimmings or with material after other tests have been performed. In all cases, evaporation-condensation processes resulted in loss of water from the soil (as evidenced by free water in the container), even though water was not lost from the container. The wrapping of the sample in plastic to inhibit water transfer and the presence of the nitrogen atmosphere to inhibit oxidation reactions gave no improvement over storage with air. With all the samples stored in the containers there was a dramatic increase in the amount of calcium and magnesium in the pore water. The change cannot be explained by the water transfer within the container and is probably related to attack on the carbonates which are present in the soil. Long term storage in plastic containers cannot be recommended. It is not known whether storage in sealed glass jars would be superior. Conclusions and Recommendations

The conclusion drawn from these experiments is that, regardless of the method of storage, chemical changes which will increase the concentrations of most cations in the pore water can be expected to occur during storage. The results of pore water analysis carried out after extended periods of storage should not be expected to accurately represent the in situ chemistry of that material. The best storage procedures, as indicated by least overall change, are to extrude the sample and to wrap and wax it. The type of wrap, aluminum, or plastic made little difference. Likewise, the temperature of storage was not particularly important, although it does not seem reasonable to recommend any procedure where there is clear evidence of biological activity. This last consideration means that aluminum-wrapped samples should preferably be refrigerated. A temperature of 7 ~ which represents the mean annual ground temperature at Ottawa, is not necessarily the optimum storage temperature for inhibiting chemical change. Presumably, lower temperatures would be better. It has been found that the stability of sodium-montmorillonite increases as the salinity of the surrounding water increases [9]. If this observation can be applied to the marine clays, then it would be expected that the lower the electrolyte concentration of the pore water, the greater would be the degree of chemical change during storage. Thus, pore water chemistry changes in high-salinity Leda clay during storage may be small, but the dilemma exists that the low-salinity clays, which are the most susceptible to change in behavior when the pore water chemistry changes, are the ones Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

156

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

that are most susceptible to chemical alteration during storage. Since the precise relationship between s0il behavior and pore water chemistry for the Leda clays has not been established, it must be assumed that these changes in pore water chemistry during storage may be of importance. Knowledge as to the role of chemical factors in Leda clay behavior is limited. The relationships and interactions that occur between the various chemical elements and the soil are complicated. Preliminary laboratory investigations with chemical additions to Leda clay and investigations of the in situ chemistry of Leda clay give promise of leading to a more complete understanding of the behavior of this soil [6, 7]. Much more work is needed on the relationships between chemical and physical factors and the soil behavior as observed in the field and laboratory. In this regard, the author recommends that more investigators assess the chemical, mineralogical, and physical nature of the specimens that are tested. Pore water chemistry investigations should take place as soon as possible after the sample is obtained if in situ chemical information is desired. If research is being performed, knowledge of the chemical state of the soil should be obtained. The chemical investigations should be carried out at the beginning of the experiment on a sample of soil immediately adjacent to the specimen tested (to obtain initial chemical data), and on the specimen itself after the test is completed (to determine if changes have occurred during testing). The results presented in this paper suggest that pore water chemistry should be assessed as soon as possible after the sample is obtained from the field and that, if storage is necessary, the sample should be wrapped with aluminum foil or plastic wrap and sealed with wax. It is preferable that the sample be stored under refrigerated conditions. Evaporation of pore water during and after extraction should be kept to a minimum. The unpredictable and nonreproduceable nature of the chemical change during storage must be emphasized. Preliminary experiments with certain Leda clays and the experience from Norway indicate that relatively small differences in pore water chemistry can lead to differences in the remolded behavior of this type of soil. Only with good chemical and engineering data on a large number of samples can the actual relationships be determined. A ckn o wledgm en ts

The author wishes to acknowledge the support of the National Research Council of Canada and the assistance of L. Cowe in the laboratory portion of this investigation. References [1] Bjerrum, L., Geotechnique, Vol. 4, 1954, pp. 49-69. [2] Mourn, J., L6ken, T., and Torrance, J. K., Geotechnique, VoL 21, 1971, pp. 329-340. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

TORRANCE ON PORE WATER CHEMISTRY OF LEDA CLAY

157

[3] Moum, J, L6ken, T., and Torrance, J. K., Geotechnique, Vol. 22, 1972, pp. 675-676. [4] Penner, E., Canadian Journal of Earth Science, Vol. 2, 1965, pp. 425-441. [5] Sangrey, D. A. and Paul, M. J., Canadian Geotechnical Journal, Vol. 8, 1971, pp. 315-335. [6] Torrance, J. K., Canadian Geotechnical Journal, Vol. 12, 1975, pp. 326-335. [7] Torrance, J. K., "Leaching, Weathering, and Origin of Leda Clay in the Ottawa Area," Guelph Symposium on Geomorpholoty, in press. [8] Torrance, J. K., Geotechnique, Vol. 24, 1974, pp. 155-173. [9] Shainberg, I., Proceedings, Soil Science Society of America, Vol. 38, 1973, pp. 689-694.

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D. A . Sangrey, ~ D. K. N o o n a n , 2 and G. S. W e b b 2

Variation in Atterberg Limits of Soils Due to Hydration History and Specimen Preparation

REFERENCE: Sangrey, D. A., Noonan, D. K., and Webb, G. S., "Variation in Atterberg Limits of Soils Due to Hydration History and Specimen Preparation," Soil Specimen Preparation for Laboratory Testing, ASTM STP 599, American Society for Testing and Materials, 1976, pp. 158-168. ABSTRACT: Drying and rewetting of soils, as presently required in preparing material for Atterberg limit tests, causes significant variation in the liquid limit. Once dried, the rehydration of fine grained soils is time-dependent, requiring typically several weeks rather than the one day period often used. More important are the irreversible effects of drying many soils. Because so many applications of the Atterberg limit values are related to engineering problems of naturally moist soil, the limits determined for nondried soil are more appropriate. It is argued that ASTM standards should reflect this and recognize the ~vere changes brought about by drying. An alternative method allowing testing of nondried soils should be added to the present standards. KEY WORDS: soils, tests, Atterberg limits, liquid limit, soil hydration, swelling, clay minerals

"The liquid limit of a soil is the water content expressed as a percentage of the weight of oven-dried soil, at the boundary between the liquid and plastic states" (ASTM Test for Liquid Limit of Soils (D 423-66)). ASTM specifications require that the soil to be used in determining the liquid limit be a prepared dry sample (ASTM Dry Preparation of Soil Samples for Particle-Size Analysis and Determination of Soil Constants (D 421-58 (1965)), although no specific time for rehydration of the air-dried soil is defined except for check or referee tests. In these, 24 h of "seasoning" is JAssociate professor, School of Civil and Environmental Engineering, Cornell University, Ithaca, N.Y. 14853. 2Engineers, H. Q. Golder and Associates, Mississauga, Ont., Canada. 158 Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 9 Downloaded/printed by Copyright 1976byASTMIntemational www.astm.org University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

SANGREY ET AL ON SPECIMEN PREPARATION

159

specified, but no consideration is given allowing for variation in the liquid limit due to differing times for rehydration.3 In contrast to this standard, there is ample evidence, [1-5] 4 both theoretical and observational, that soil consistency, particularly the Atterberg limits, is significantly influenced by the degree of hydration of clay minerals. When air dried, these clay minerals will dehydrate to some degree, and, when more water becomes available upon rewetting, the clays rehydrate. The dehydration-rehydration sequence is time dependent, and achieving an asymptote to complete hydration may require more than one month [4] for some clay minerals. Other effects of the standardization procedure, such as chemical alteration of the soil constituents and irreversible cementing by precipitates, can also be noted [6-8]. This problem is particularly acute in tropical soils containing compounds of iron, aluminum, and other metals which form natural cements. The same factors influencing the formation and stability of laterite will affect the preparation of these soils for classification tests. It is reasonable to expect, and it has been demonstrated in a limited amount of testing [6, 7], 5 that soil plasticity will be irrevisibly changed if the amount or characteristics of cementing change. This factor may be as significant in magnitude as dehydration but, in contrast, is a permanent effect. The specific mechanism of plasticity change due to cementing is unknown. It is probably due to the aggregation of small particles which act as larger particles and the water holding capacity of cemented micro peds. Methods of wet preparation of soil samples have been specified by ASTM Wet Preparation of Soil Samples for Grain-Size Analysis and Determination of Soil Constants (D 2217-66), although this specification is not referenced in other ASTM classification test procedures. Two alternative methods of wet preparation are described, one from air-dried soil, the second from naturally moist soil. Oven drying procedures are also used in some soil preparation. However, these are not part of ASTM procedures for specimen preparation. Changes in soil properties might be acceptable if a standard for identification were the only objective. For soil, however, the plasticity characteristics 3The association of water to a clay soil is very complicated. Some water is quite free and mobile while other water is attracted, in varying degrees, to the clay particle. Still other water is strongly a part of the clay crystal structure. As the water content of a soil changes, especially as it is dried, all of the modes of associated water are affected, each to differing degrees, depending on the conditions of temperature, water content, time, etc. Obviously the detailed explanation of the results of this test program would involve this complicated problem. However, the main point of this work is to describe the effects of drying. Emphasis on explanations for these effects, especially hypotheses about the reasons, is unnecessary. For this reason, in the paper, the terms dehydration and rehydration will be used to cover the entire spectrum of water content changes. *The italic numbers in brackets refer to the list of references appended to this paper. Communication of test results by S. G. Samuels, Building Research Station, England and J. Pruntel, Cornell University, Ithaca, N.Y.

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160

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

are recognized as not only a means of soil classification, but also a relatively inexpensive method for estimating engineering characteristics of strength and compressibility [9]. Since the engineering application usually involves a naturally moist soil, it is reasonable to determine the liquid limit working from natural water content rather than from air-dried soil, as specified by ASTM. Certainly many engineering organizations and laboratories are currently testing from the natural water content in spite of the standard. It is also noteworthy that the British Standards Institution modified its original specification for liquid limit testing, which had been similar to ASTM's, to one permitting either naturally moist or rehydrated soils [10]. Two separate questions are at issue here, each with its own implications in terms of the present standards. The first is how long it takes for an airdried soil to rehydrate to some reasonably constant condition as far as the liquid limit test is concerned. I f dehydration-rehydration is simply a reversible, but time dependent, phenomenon, and if it is desirable to test a soil when more or less at equilibrium within this process, then it is sufficient to specify a rehydration time long enough to reach this condition. Such a rational is implicit in arguments like those of Winslow and Gates [2] and the current check test procedure of ASTM Method D 423-66 where a specific 24-h rehydration time is specified. A second question of perhaps even more significance is: to what degree will soils respond irreversibly to dehydration; in which case, any length of time for rehydration would not reproduce the natural soil behavior.

Test Program In an attempt to answer these questions, series of tests were run in which several different soils were subjected to drying and rewetting. The liquid limits were measured over a period of up to two months time for rehydration. A summary of the characteristics of these soils is presented in Table 1. Specimens 1 and 3 occurred naturally in deposits of marine or estuarine silty clay in the Ottawa and St. Lawrence River systems of Eastern Canada, respectively. They are representatives of the sensitive, naturally cemented, Leda-type clays. Specimens 2 and 5 were from deposits of normally consolidated, post-glacial, varved fresh water lake clays; both were taken in Eastern Ontario near to Kingston and New Liskeard, respectively. Specimen 4 was a lightly overconsolidated material from the Great Lakes basin near Lake St. Clair, while the final specimen was of Cretaceous age Bearpaw shale from Saskatchewan. The liquid and plastic limits listed in Table 1 were obtained by testing soils that had not been air dried but were wetted directly from the natural water content. Results of mineral analysis using X-ray diffraction are listed in Table 2. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

69.5 30.3 70.5 72 21 7 0.54

Specimen No.

Liquid limit, ~ Plastic limit, 07o Natural water content, 070 Clay (< 2 gin), 07o Silt (2/~m to 0.06 mm), 070 070> 0.06 mm Activity

34.2 18.8 44.4 55 43 2 0.28

New Liskeard 2 74.0 27.5 82.4 80 12 8 0.59

Louis ville 3

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Parkway I 24.5 16.9 22.4 27 63 10 0.27

Red Clay 4

TABLE 1--Identification characteristics of soils tested.

42.4 22.3 43.5 55 39 6 0.37

Kingston 5

95.0 27.6 22.0 40 46 14 1.68

Bearpaw Shale 6

z

zlJ m "10

m z

_o

"13 m

0 Z

r"

m ..<

Z

{D

162

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

TABLE 2--Mineralogy of soils tested. Specimen Abundant Clay Minerals

MinorClay Minerals

1

mica group, vermiculite

montmorillonite

2 3 4 5 6

mica group mica group, chlorite mica group, chlorite mica group montmoriUonite,mica group

vermiculite, chlorite vermiculite

Other Minerals

chlorite, vermiculite

quartz, feldspar, amphibole hornblende, cristobalite same as 1 same as 1 quartz same as 1

chlorite

quartz

The soils were subjected to the same testing procedure. A portion of each was air dried in a laboratory at average relative humidity of 55 percent and temperature o f 22~ for a period o f seven days. Similar portions of each were oven dried for a period of 20 h at 108~ The specimens were then mixed throughly with enough water to bring them to approximately the liquid limit determined from the original tests and listed in Table 1. At time intervals beginning at 15 min after mixing and continuing at roughly doubling increments thereafter, the liquid limits were determined for each soil. The equipment and method o f testing used conformed to the standards o f ASTM Method D 423-66 except for the method o f specimen preparation and the use of the Casagrande-type grooving tool. Between test periods, the specimens were stored in a cabinet at a relative humidity of >95 percent and a temperature of approximately 22~ The results of these tests are presented in Figs. 1-6. The liquid and plastic limits, defined by testing o f nondried specimens, are indicated on the left margin. Subsequent liquid limits for air and oven dried soils rehydrated for various periods of time are also shown. These results clearly indicate that the effects of rehydration time and irreversibility were significant for the soils tested. Previous work on the Atterberg limits, particularly that of Winslow and Gates, has concluded that most rehydrating soils reach a practical assymptote after 24 h, but that more active clay minerals, such as montmorillonite, may require several days. The results of this study demonstrate that, even for very inactive soils, 24 h is insufficient and that liquid limit changes due to rehydration may be significant for at least several weeks. It does appear that the time required increases with activity, or at least that the changes are more significant with the more active soils (for example, Bearpaw shale--No. 6.). A second significant observation is that the drying of soils, either in air or oven, does not necessarily lower the liquid limit, as is usually assumed [1] and frequently referenced in textbooks. This test program showed that, for three out of six air-dried and one of six oven-dried ones, the initial liquid limit was higher than that determined from the nondried speciCopyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

SANGREY

ET A L

ON

SPECIMEN

163

PREPARATION

70 wL

60

z LU FZ 0

I

50

&

PARKWAY

(LEDA

CLAY)-AIR

9

PARKWAY

(LEDA

CLAY)-OVEN

+--

9

9

-

I I|

?

§

I

o

r~ hi I--

"1"

40

-

30

=Wp

<

20t5

L

I

I

I

I0

I00

REHYDRATION

I000

TIME-HOURS

FIG. 1--Liquid limits measured for rehydrating Specimen 1, Parkway soil (note changes with additional cycle o f drying of the oven dried soil after 60 h).

70

60

NEW

LISKEARD

CLAY-AIR

9

NEW

LISKEARD

CLAY-OVEN

+

I

I.z hi I'-Z 0 0 (Z: ILl I-'<

50

40

NL

20 15

,, .I,. 4-

30

4,

Ilk

'1"

'@

i

6

9 4-

v

4.

;_

9Wp

I

I

I

I

I0

I00

REHYDRATION

I000

TIME-HOURS

FIG. 2--Liquid limits measured for rehydrating Specimen 2, New Liskeard clay.

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authori

164

SOIL SPECIMEN

PREPARATION

FOR

LABORATORY

TESTING

80 LOUISEVILLE

(LEDA

CLAY)-

AIR

LOUISEVILLE

(LEDA

CLAY)-OVEN

9

-w L 70

+-

60 IZ MJ I.z 0 tt,I.LI I,,<

50

4-0

30 wp

20 15

I

I

I

1

I

IO

I00

I000

REHYDRATION

FIG.

TIME-HOURS

3--Liquid fimits measured for Specimen 3, Louiseville soil.

80

70

I I,Z WJ l-Z 0 0 Iv I,LI I.--

RED

CLAY-

AIR

RED

CLAY-OVEN

9 +

60

50

40

30

:

+"

9

:

$

$

;

:

.

wL

20

m --Wp

15

1

I

I

I

IO

IOO

REHYDRATION FIG.

I000

TIME-HOURS

4---Liquid limits measuredfor Specimen 4, red clay.

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SANGREY

ET

AL

ON

165

SPECIMEN

PREPARATION

KINGSTON

CLAY

- AIR

9

KINGSTON

CLAY

-OVEN

§

80

70

60 I tz LLJ D-Z 0 0

50

r~

40

WL §

+

w

,~

9

9

+

4-

I

I

-

I

+

p,<

+

30

Wp 20 I I

15

I I0 REHYDRATION

J I00

I I000

TIME-HOURS

FIG. 5--Liquid limits measured for Specimen 5, Kingston clay.

150

130

I10

r z IJJ I,Z 0 0

wL

90

70 bJ t.< 50

30

I0

BEARPAW

SHALE-AIR

e"

BEARPAW

SHALE-OVEN

+

wp

-

-

I I

I I0 REHYDRATION

I I00

I I000

TIME-HOURS

FIG. 6--Liquid limits measured for Specimen 6, Bearpaw shale.

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166

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

men. A similar observation was noted by Torrance [1] for a Norwegian clay soil. The question of irreversibility is the most significant one with respect to drying of specimens for plasticity tests. If the effects of drying are not reversible, then the soil has certainly experienced major structural change and may be very different from the original material. The results of this testing program clearly show that irreversibility is very common, perhaps more common than not. Explanations for this probably involve several phenomena. The single effect most likely in the tests reported herein is irreversible precipitation of natural cements from the soil or soil water [2]. Because four of the six soils in this test program (No. 1, 2, 3, and 5) were of this type, the results may not be representative of the percentages of all soils which behave in this way. However, naturally cemented soils are extremely common in some parts of the world, and organic material in almost any soil could produce the same effect [3]. Low permeability is a second reason for irreversibility, or apparent irreversibility, as demonstrated by the montmorillonitic Bearpaw shale (No. 6). This soil had a natural water content (22 percent) below its plastic limit and was extremely overconsolidated. The liquid limit determined by wetting of this soil directly from the natural water content was 95 percent; however, with time, even this value changed, assuming an asymptote to 109 percent only after several weeks of soaking and apparent rehydration, as shown in Table 3. That a soil can be dehydrated by large consoli-

TABLE 3--Changes in liquid limit for Bearpaw Shale, Specimen 6, when hydrated from natural water content of 22 percent.

HydrationTime

Liquid Limit, %

24 h 2 days 3 days 5 days 8 days 21 days 34 days

95.0 100.2 107.5 107.9 111.6 109.0 108.6

dation stresses even though remaining saturated seems obvious from these data, and a rehydration time is necessary in order to ensure that some consistent measurement of the liquid limit is possible. Because of the low permeability of the Bearpaw shale, the time effects, whether rehydrating

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SANGREY ET AL ON SPECIMEN PREPARATION

167

air-dried, oven-dried, or nondried specimens, are so long that asymptotes may well be impractical to define. As Fig. 6 shows, even after 55 days, the liquid limit was still varying significantly for the Bearpaw shale. Conclusions There are important implications for the present standard of ASTM Test for Liquid Limit of Soils (D 423-66) in light of the variabilities reviewed in this paper. Without question, it is important that a standard method be as simple as possible and not require excessive time for specimen preparation or the test itself. If long time periods were prescribed, they would, no doubt, be ignored by most testing groups. This would seem to preclude a change in the present standard to allow for the hydration times of several weeks that appear to be necessary for most air-dried soils. If correlations between engineering behavior of natural soil and the Atterberg limits are to be meaningful, then the irreversible changes brought about by drying, as illustrated in most of the specimens tested, are very significant. As reflected in the British Standards [10], the logical solution to this problem is to prescribe testing of nondried specimens. On the other hand, specimen handling and preparation is greatly simplified if the soil is dry. There may also be instances when it is desirable to reduce soils to some common state by eliminating extraneous factors inherent in the natural material (although the author cannot imagine where this would really be beneficial). In either case, a method of dry preparation such as ASTM D 421 is warranted. In summary, it is necessary that the present standard for ASTM Test for Liquid Limit of Soils (D 423-66) be changed to provide the option of testing nondried soil specimens. Such a standard would require a description of the procedure of specimen preparation as part of the reporting of the liquid limit. A similar change for the plastic limit, ASTM Test for Plastic Limit and Plasticity Index of Soils, (D 424-59) (1965)) is probably also justified. Regardless of the method of test, it is important that engineers recognize the variability of soils as they are dehydrated and rehydrated, especially with respect to the Atterberg limits. This is particularly important when irreversible changes result.

Acknowledgments An ASTM Grant-in-Aid award to the senior author, while on the Staff of Queen's University at Kingston, supported part of the research reported herein. The assistance of Dr. R. H. M. van de Graaff in soil mineral analysis and interpretation is gratefully acknowledged. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

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SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

References [1] Casagrande, A., Public Roads, Vol. 13, 1932, p. 121. [2] Winslow, J. D. and Gates, G. R., Materials Research and Standards, Vol. 3, 1963, p. 205. [3] Muller-Von Moos, M. in Proceedings, International Conference on Soil Mechanics and Foundation Engineering, Vol. I, 1965, p. 17. [4] Grim, R. E., Applied Clay Mineralogy, McGraw-Hill, New York, 1962. [5] van Schuylenborgh, J., Journal of Agricultural Science, Vol. 2, 1954, p. 17. [6 "Soils of the Humin Tropics," Committee on Tropical Soils, Agricultural Board and National Research Council, National Academy of Sciences, Washington, D.C., 1972. [7] Frost, R. K. in Proceedings, First Southeast Asian Conference on Tropical Soils, Bangkok, 1967, p. 43. [8] Remillon, A., "Stabilization of Laterite Soils," Bulletin 108, Highway Research Board, 1955, p. 96. [9] Terzaghi, K. and Peak, R. B., Soil Mechanics in Engineering Practice, Wiley, New York, 1968. [lt)] Methods of Test for Soil Classification and Compaction, British Standards Institution, 1968. [11] Torrance, J. K., discussion in Engineering Geology, Vol. 4, p. 353. [12] Sangrey, D. A., Geotechnique, Vol. 22, No. l, p. 139.

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Arshud Mahmood,' J. K. Mitchell, 2 and Ulf Lh~dblom 3

Effect of Specimen Preparation Method on Grain Arrangement and Compressibility in Sand

REFERENCE: Mahmood, Arshud, Mitchell, J. K., and Lindblom, Ulf, "Effect of Specimen Preparation Method on Grain Arrangement and Compressibility in Sand," Soil Specimen Preparation for Laboratory Testing, ASTM STP 599, American Society for Testing and Materials, 1976, pp. 169-192. ABSTRACT: Effects of methods of specimenpreparation on fabric and compressibility were investigated in a medium grain-sized sand. The effect of vibratory compaction was compared with the effect of pluviation by characterizing the particle arrangements and measuring compressibility in a specially fabricated oedometer. The lateral stresses during compression were also measured. Specimens densified by vibration in layers had random grain orientation up to 100 percent relative density. When vibration was continued beyond the time interval needed for achieving 100 percent relative density, the grains acquired a preferred orientation. Pluviated specimens had randomly oriented grains at both low and high densities. The loose, pluviated specimens were much more compressible than the dense specimens prepared by the same procedure. The dense specimens when prepared by vibration were less compressible than the pluviated specimens. The lateral stresses were higher in the loose pluviated specimens than in the dense pluviated specimens during loading. On unloading, the same trend was present initially but reversed at lower axial loads. This trend was also measured in the specimens prepared by vibration. KEY WORDS: sands, soils, tests, specimen preparation, fabric, compressibility, vibration, laboratory studies, oedometers This study was carried o u t to investigate the possible effects o f the method of specimen preparation o n fabric a n d compressibility in a m e d i u m grain-sized sand. T h e objective was to d e t e r m i n e whether or n o t such a m a t e r i a l acquires different fabrics w h e n deposited in different ways; a n d , if so, whether different fabrics are associated with different compressibilities. I n s i m u l a t i n g the b e h a v i o r o f sands i n the l a b o r a t o r y , it has c o m m o n l y b e e n deemed sufficient to merely r e p r o d u c e the in situ relative density b y 'Project engineer, Woodward-Clyde Consultants, Orange, Calif. 92668. 2Professor of civil engineering, University of California, Berkeley, Calif. 94720. 3Head, Research Department, Hagconsult, Banergaten 37, 115 22 Stockholm, Sweden. 169 Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed byby ASTMInternational www.astm.org Copyright9 1976 University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

170

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

any means whatsoever. Engineered construction is being carried out to increasingly exacting specifications on more and more unsuitable sites, and it is becoming clear that a pertinent question is: How was this soil deposited? Significant aspects of such a question are: (1) what fabric did the soil acquire and (2) what is its stress history? In answering this question, it is possible to determine how to prepare the laboratory specimens to simulate the field density, the particle arrangement, and the in situ stress. This study represents a first step in answering these questions and deals with the effects of the method of specimen preparation and grain arrangement on one-dimensional compressibility.

Scope As prior tests have already shown that specimen preparation effects are significant in influencing the fabric and properties of a sand with elongate, angular particles [1,2], 4 a material that was likely to be far less susceptible to such behavior was used for the present study: Monterey No. 0 sand, a medium beach sand with rounded and only slightly elongate particles. The effect of vibratory compaction on fabric and properties was compared with the effects of specimen preparation by pluviation by characterizing the particle arrangements and measuring compressibility in a speciaUy fabricated oedometer. ,The lateral stresses during compression were also measured. Soil Fabric

The term fabric describes the basic framework or arrangement of individual constituents of an assemblage consisting of different components. A comprehensive description of the fabric of a soil sample [3,4,5] would involve reconstruction of a complex, three-dimensional spatial arrangement of individual grains, multigrained units, and pores. In biological and material sciences, the principles of stereology have long been applied in studying the internal structure of many objects by looking at thin slices or sections [6, 7]. Thin sections and broken, ground, or polished surfaces can be studied in several ways, depending upon the type and extent of information required and the nature and grain size of material being studied [8]. Observation by means of optical [9] or electron microscopes [10-14] has been employed. Impregnation with resins has been used for friable or cohesionless soils such as sands [2,15]. Arrangement and planar displacements can also be measured by X-radiography [16]. If the soil grains have shapes such that their geometrical axes coincide with the crystal axes, then some property of the mineral crystals can be used to define the particle arrangement, without having to define the 4The italic numbersin brackets refer to the list of referencesappendedto this paper. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

MAHMOOD ET AL ON PREPARATION METHODS FOR SAND

171

geometrical outlines of grains. For platy clay particles, the difference in refractive indices in the direction of long and short axes has been used in determining particle orientation [11]. X-ray diffraction has also been used for characterizing clay mineral particle orientations in soil specimens [17,18]. Other properties that have been used are magnetic anisotropy [19], dielectric anisotropy [20], dielectric dispersion [21,22], sonic and thermal anisotropies [23], permeability [24], and inductive conductivity [25].

One-Dimensional Compressibility Although compressibility characterized in the field by such means as the standard penetration test (through relationships with relative density) and static cone bearing capacity [26] may provide a basis for reasonable settlement predictions in specific cases, oedometer results obtained from laboratory prepared specimens are important in understanding factors controlling the compressibility behavior of sands. Laboratory measurements of one-dimensional compressibility of sands may also be useful for obtaining settlement estimates of foundations in situations where it can be assumed that no lateral strain occurs [27], provided correct density and fabric can be reproduced. Evaluation of relative density is susceptible to several errors [28], and fabric is even more difficult to evaluate. Measurements of one-dimensional compressibility can be used to characterize nonlinear stress-strain behavior [29,30] and to estimate footing settlements [31].

Measurement Many of the data on one-dimensional compressibility of sands have been obtained in conjunction with laboratory studies of at-rest radial earth pressures. Both types of studies necessitate equipment that permits little or no lateral deformation of the specimen [32]. Two types of equipment have generally been used: oedometers and triaxial cells. The oedometers can either be of a null design [33] or can be allowed to undergo small lateral deformations [34]. One method of characterizing the effect of lateral deformation on measured axial strains and radial stresses is by using a range of oedometer ring thicknesses [35]. The triaxial tests have mostly been an improvement upon and variation of the procedure suggested by Bishop and Henkel [36]. In some recent studies, Ko triaxial tests were used to study compressibility [37,38].

Experimental Investigation

Equipment It was desired to study the compressibility of sands in an apparatus that would: (a) offer minimum side friction, as large side friction affects measCopyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

172

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

surements of lateral stresses [32]; (b) permit a specimen size tall enough for mid-height measurements to be free of the end effects that cause errors in data from short samples [39]; and (c) possess enough lateral rigidity to permit horizontal stress measurements with very small radial deformations. An apparatus of the Swedish ring compressionmeter type was fabricated. These devices are oedometers that are easily compressible in the vertical direction but possess considerable lateral rigidity. Larger devices, based on similar principles, have been used in the study of crushed rock [40,41], and have provided excellent compressibility results. In these earlier investigations, the values of lateral stresses could not be recorded properly due to low sensitivity of the measurement techniques, and it is believed that an improvement has been effected in this respect in the present investigation. The ring device fabricated for the present study was 102 mm inside diameter and 136 mm high and is shown schematically in Fig. 1. It was

~Neoprene cork

3m

plan Elevation FIG. l--Schematic drawing of ring chamber. made of alternating rings of stainless steel and neoprene impregnated cork, glued together with an epoxy resin. Several materials were considered before selecting these two. The stainless steel provided lateral rigidity and was corrosion resistant. The neoprene rubber-impregnated cork combined the nearly elastic compression and rebound properties of rubber with the low Poisson's ratio of cork. The elastic recovery was important during rebound and for loading-unloading cycles. A low Poisson's ratio (0.21) ensured that the compressible rings did not disturb the soil specimen by squeezing inwards into the ring chamber. The high vertical compressibility and high lateral rigidity were similar in principle to that in devices used for cubical triaxial tests [42,43]. After connecting the eleven steel rings with ten cork-neoprene rings, Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

MAHMOOD ET AL ON PREPARATION METHODS FOR SAND

173

three of the steel rings were made into load cells by mounting strain gages around their perimeters. The strain gages consisted of 8 mutually perpendicular pairs (16 in all). These were connected together, as shown in Fig. 2, to compensate for temperature changes. A photograph of the apparatus is shown in Fig. 3. Horizontal

Horizontal (o)

~

Steel ring

Horizontal

(b)

FIG. 2--Arrangement o f strain gages on ring chamber: (a) circuit diagram; (b) location o f gages.

The chamber was calibrated, using air pressure applied from the inside through a thin rubber membrane. The changes in the height of specimen were measured using a linear variable differential transformer (LVDT). Axial load was measured, using a copper-beryllium load cell that had been calibrated with dead loads. The loads and height changes were read out on a digital voltmeter and teleprinter. The setup is shown in Fig. 4. Test Soil

The soil used in this investigation was a well-sorted beach sand known as Monterey No. 0 (zero). It is a uniform, medium grain size sand (ds0 = Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

174

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 3--Photograph of ring chamber.

0.43 ram; C, = 1.4), composed mostly of quartz and some feldspar. The grains are rounded and somewhat elongate (mean length/width ratio = 1.39), as shown in Fig. 5. The specific gravity of grains is 2.65, and the maximum and minimum densities, determined in accordance with ASTM Test for Relative Density of Cohesionless Soils (D 2049-69), are 1.70 and 1.45 g/cm 3, respectively [44].

Specimen Preparation Specimens were prepared in the ring chamber either by pluviation (pouring dry) or by vibration. Pouring from zero height was used for obtaining loose specimens. Dense specimens were produced either by slow pouring from a height or by vibratory compaction. Vibration was applied on top of the sand layers by means of a circular plate of the same diameter as the specimen vibrating vertically (frequency = 30 Hz, amplitude = approximately 0.5 ram). Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

MAHMOOD ET AL ON PREPARATION METHODS FOR SAND

175

FIG. 4--Photograph of experimentalsetup. As a check on the uniformity of specimens, resin-impregnated tube specimens were taken, as described in the next section. These cylinders o f resin-impregnated sand were sawed into V2-in. thick slices. The density o f these slices was measured by immersion in water, and the density profile o f specimens was determined. The differences in density of various slices were less than 1 percent. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

176

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 5--Grain shape distribution of Monterey No. 0 sand based on study of 227particles. Specimens f o r Fabric Study

Specimens were separately prepared for fabric study in a container identical to the ring chamber and by the same methods of preparation as used for making specimens for compressibility and radial stress measurements. A thin-walled acetate tube was pushed slowly into the specimen. In loose specimen, the tube could easily be pushed nearly to the bottom of the specimen. In dense specimens, however, tube penetration to a depth greater than one third of the specimen height caused disturbance. The fabric of all specimens, therefore, was determined by studying specimens from the upper third. Epoxy resin was poured into the specimen tube and permeated the specimen under a head of a few millimetres.The resin was allowed to harden overnight. The hardened specimens were then sawed to obtain vertical and horizontal faces, which were ground and finished on a diamond-cloth grinder. These faces were then photographed and enlarged by projecting them onto a sheet of paper and tracing the grain boundaries. Each complete tracing contained the outlines of 400 or more grains. The best estimates of the apparent long axes of elongated grains were drawn on the tracings and their orientations determined by assigning them to one of the eighteen 10-deg intervals between 0 and 180 deg. The number of particles in each of the 18 intervals was separately totalled and then computed as percentages of the total number of particles in a tracing. These percentage values were then plotted on polar coordinate graph paper as rose diagrams, as shown in Fig. 6. The portion of the diagram from 180 to 360 deg is a duplicate of the portion from zero to 180 deg. If all particles were randomly oriented, the plot would have been the circle shown by a broken line. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

MAHMOODETALONPREPARATIONMETHODSFORSAND 177 (o) /

5~176

~

~

(c)

FIG. 6--Grain orientation diagrams f o r specimens o f Monterey sand: (a) pluviated, density = 1.58 g / c m 3 Dr = 53 percent; (b)pluviated, density = 1,72 g/cm 3, Dr = 106 percent; (c) vibrated, density = 1.69 g/crn 3, DR = 97 percent; and (d) vibrated, density = 1.71 g / c m 3, DR = 104 percent. Testing Program

Frabric-compressibility relationships in sand were studied by testing specimens prepared at different densities by either vibration or pluviation. The as-compacted grain arrangements were determined for both methods of densification. The vertical compressibility and radial stress were measured on specimens prepared both ways. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

178

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

Fabric Results

Particle orientation diagrams, representing the initial or as-compacted fabrics of four specimens, are presented in Fig. 6. When poured to form a medium dense specimen, the sand particles acquired a grain orientation as shown in Fig. 6a. There is no strong preferred orientation of grain axes. Experiments with materials composed of elongate, angular grains had shown that pouring resulted in the grains acquiring an arrangement such that their long axes were preferentially oriented in the horizontal direct i o n - o r normal to the direction of fall [1]. This orientation of grains can be explained by considering the behavior of flat ellipsoids tumbling in free fall [45]. On a horizontal surface, more ellipsoids are likely to come to rest with their long axes normal to the direction of free fall. In the case of a soil such as the Monterey sand, which is composed of grains that are only slightly elongate and rounded, no strong orientation resulted on pouring. Evidently, at the end of fall when the nearly equant, rounded grains come to rest, they take up random positions. A very slow rate of pouring was used to obtain dense specimens, and the randomness of grains was still observed, as shown in Fig. 6b. Densification by vibration also produced a specimen with reasonably random grains, which can be seen in Fig. 6c. This grain arrangement resulting from dynamic densification is similar to that of soils composed of elongate, angular particles [1]. Specimens were prepared by vibrating each layer for a longer period of time than that required to attain 100 percent relative density, in order to develop some prestress in the specimen. The grain orientation diagram for one such overvibrated specimen is shown in Fig. 6d, where it may be seen that there is a recognizable preferred grain orientation, with the vector mean oriented at about 40 deg from the horizontal. It appears that, if the sand is kept in a fluidized state for an extended period by overvibration, then the grains get some freedom of mov~nent and gradually acquire a preferred orientation. That the grain axes become oriented at 40 deg could perhaps be due to the fact that densification to a high density involves frictional slippage at grain contacts, and the particles gradually become arranged to resist this change. This happens even though the vibratory stresses causing densification are cyclic and thus might not be expected to produce preferred orientation. The sustained application of vibratory compactive effort results in an incipient frictional failure. Such a failure in a mass should be evident at two complementary planes, but two planes might not be seen if the failure is initiated on one of them first. In Fig. 6d, all the grains are arranged as if in a stack, an arrangement known to develop when discrete plates are vibrated to a dense state. Figure 7 suggests how such an arrangement might develop in a sand with less elongate particles. In the ASTM Test for Relative Density of Cohesionless Soils, (D 2049-

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M A H M O O D ET AL ON PREPARATION METHODS FOR SAND

"Stack"

FIG.

179

Arrangement

7--Proposed stack arrangement of particles in an overvibrated specimen of sand.

69) vibratory compaction is specified for determination of maximum density. It is sometimes stated that any mechanical energy used for obtaining a densification in excess of the standard maximum density generally results in grain crushing. Since the vibration applied for preparing the overvibrated specimens did produce relative densities greater than 100 percent, it was suspected that grain crushing might have influenced the grain orientation results. A carefully controlled sieve analysis of a sand specimen before and after vibratory compaction did not reveal any measurable change in gradation, as shown in Fig. 8, and it can therefore be inferred that grain crushing was not significant enough to influence the grain orientation results obtained. 400

~

I

Sieve Sizes

27O ~ I

I00 50 I

I

302016

F

I

O Initiol $ievin 9

8O

g g

8

4

I

I

~, Sieving after 90

seconds vibration per layer

60

u

Monterey

Sand No. 0

9~ 4C ._o

J

O

2O

AI, = 5 1 1 0.05

0J

I 0.5

LO

I I I I 5

Groin Size, mm

FIG.

8--Effect of vibratory compaction on gradation of Monterey No. 0 sand.

The standard maximum density was also exceeded in specimens prepared by very slow pouring, a procedure in which there is no mechanical energy

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180

SOIL

SPECIMEN

PREPARATION

FOR

LABORATORY

TESTING

applied for compaction, and hence no grain crushing is likely. Other procedures have also been reported to result in higher densities [46] than obtained using the ASTM Method D 2049-69. C o m p r e s s i o n Test Results

Axial deformations and radial stresses measured on sand specimens in the ring chamber are presented in this section. The effect of relative density

on axial strain is shown in Figs. 9 and 10. AxioI Stress, ko,/cm z I Oj I I

I0 '

I

I

=

,

Density

ooozi9

I

F~i=~ ~ o

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la

,

,

,

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104

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0.006

-

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0 . 0 0 $ --

O.OlC

O.OIZ

Monterey S a n d

I FIG.

I

I

L

'

I

,

,

9--Effect of density on axial compression of pluviated specimens.

The effect of specimen density on the lateral stresses is presented in Figs. 11, 12, and 13. The variation of K, the ratio of radial stress to axial stress, is plotted versus the applied axial stress. This relationship for the loose and dense pluviated specimens is shown in Fig. 11. The lateral stresses were slightly higher in the loose specimen upon loading. This would be expected, as the dense specimen, being more rigid than the loose specimen resisted the applied vertical stresses better. The loose specimen was less rigid, and, being softer, it behaved more like fluid, transmitting vertical stress into the lateral direction. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

MAHMOOD ET AL ON PREPARATION METHODS FOR SAND

181

Axiol Stress,kg~mz I0 I

I

I

I

I

Density

0.00

0002

-

i

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i

%

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t~

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0.007

MontereySand

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- ~

I

I

I

I

i

lO--Effect of relative density on axial compression of specimens densified by vibration in six layers. FIG.

On unloading, the same trend was present initially, but below an axial stress of about 4 kg/cm 2, the radial stresses in the dense specimen became higher than in the loose sand, as seen in Fig. 11. The dense specimen was more rigid than the loose specimen, and hence the static loading resulted in relatively larger locked-in stresses during loading. During unloading, these locked-in lateral stresses resulted in higher K values in the dense specimen. This behavior was observed even when specimens were loaded only up to 1 kg/cm 2, as shown in Fig. 12. Similar trends showing a reversal in the comparative magnitudes of radial stresses on unloading were measured in the specimens prepared by vibration in six layers, as shown in Fig. 13, but the effect was not as pronounced.

Method of Specimen Preparation The dense specimens were prepared either by very slow pouring or by vibratory compaction. The stress-strain plots of dense specimens prepared by the two methods are compared in Fig. 14. The pluviated specimen was Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

182

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

1.4

1.2

I

I

I

| -

1.0

-

I

Density Q/cm 5

DR %

1.46 1.71

6 104

t

, o

I

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i iO

12

FIG. 11--Relationship between axial stress and K in specimens prepared by pluviation;

effect of density.

slightly denser than the specimen vibrated in six layers, yet the pluviated specimen showed greater deformation under the same load. The greater compressibility of the pluviated specimen is largely due to the axial deformation that occurs at low axial stress (less than 1 kg/cm 2) as may be seen from Fig. 15, where the data are replotted on a linear scale. The grain orientation diagrams of the pluviated and vibrated specimens were shown in Figs. 6b and c, respectively, indicating grain arrangements that were quite random, yet their axial deformations showed small differences. These small differences could have been due to a difference in the initial state of stress of the poured and the vibrated specimens. If any such difference existed, it was likely to show up in the initial value of measured lateral stresses. A plot of K (ratio of radial stress to axial stress) versus axial stress is shown in Fig. 16 for dense specimens prepared by pluviation and by vibration. The pluviated specimen had slightly higher lateral stresses. The vibrated specimen did not show any evidence of high locked-in stresses. It is intriguing that the vibrated specimen did not have the high lateral stresses that might be expected [47], and the behavior could not be explained on the basis of grain orientations. It may be that the vibrated specimen was less compressible because the particles had acquired an arrangement

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MAHMOOD ET AL ON PREPARATION METHODS FOR SAND

0.9

I

I

i

I

183

I

Monterey Sond

a ~ 0.8

\ xA

\

0.7

\ 0.6 U')

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0.2

0.4

0.6

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1.2

Axiol Stress, kQ/cml F I G . 12--Relationship between axial stress and K in pluviated specimens in the low axial stress range.

efficient in resisting one type of stress (due to vibratory compaction) that made it better able to resist a different type of stress also (due to static load). The pluviated specimen did not achieve its grain arrangement in order to resist any externally applied stresses and was, therefore, more compressible even under small axial stresses. Whatever changes take place in a specimen upon vibration should be amplified if the vibration is continued for longer periods. An average vibration time of 30 s per layer produced a specimen of 96 to 100 percent relative density, and vibration beyond 30 s per layer yielded relatively small increases in density. The stress-strain plots of three specimens densified by vibration for 30, 60 and 90 s on each layer are presented in Figs. 17 and 18 (a replot of Fig. 17 on linear axes). All three specimens had an equal amount of residual strain after complete unloading, as seen in Fig. 18, even though their peak axial strains (at 10 kg/cm 2) were different. This result points to the uniform efficiency of packing in the three speciCopyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

184

SOIL SPECIMEN PREPARATION

1.4

I

FOR LABORATORY TESTING

I

l

Density

9/~"

I,Z o

E

I

o

I.G

1.57 1.64 1.69

I

DR

~

-

50 / 80 ~,Vibrated 97 J -

o

(:16 l,J~loodinQ

0.6 o

^

-"~-~.~-'2~

_

0.4 "~:

- ~- --8:-~;

---

~-

- - - -o . . . .

-o

Loodinq Monterey Sond I

0

Z

I

I

4

6

Axiol S t r e s s ,

I

8 kg/cm 2

I

I0

12

FIG. 13--Relationship between axial stress and K in specimens densified by vibration in six layers.

mens, and shows that the overvibrated grain structure was relatively more resilient. The measured lateral stresses during loading of overvibrated specimens, as shown in Fig. 19, increased with decreasing density, as would be expected. Evidently, however, density differences were too small to cause a crossover of the curves on unloading, as was the case in Fig. 12. ' The compression test results shown in Figs. 10, 15, and 18 are replotted in Fig. 20, expressing compressibility as the ratio of strain to stress (1/modulus), obtained from the slopes o f stress-strain curves at axial stress values of 1, 4, and 10 kg/cm'. This figure shows that, at low axial stresses, the compressiblity is large and gradually decreases at high axial stresses of 4 and 10 kg/cmL The plots also show that, at each axial stress value, the compressibility is large at low relative densities, decreases with increasing relative density to a point, and then increases again. This optimum value of relative density, at which the lowest compressibility occurs, is about 85 percent for 1 kg/cm 2, 95 percent for 4 kg/cm 2, and approximately 100 percent for the 10 kg/cm 2 curve. Long duration of vibration could cause enough particle breakage in some sands to influence the compression behavior. However, no measurable change in grain size took place here, as already shown in Fig. 8. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

MAHMOOD ET AL ON PREPARATION METHODS FOR SAND

Axial

Stress,

185

kg/cm 2

IO I

I

I

I

I

1

I

I

Densit~E DR g/cm= %

O.OOI A a

t.69 1.71

97 104

Vibrated Ruvioted

0.00~

,a o.oo3 s

0.00, 0.005 0.006

Monterey Sond

I

0007

~o,~

I

I

I

l

i

i

,

FIG. 14--Effect of method of specimen preparation on the axial compression of dense sand.

Axial

0

Stress.

kg/cm 2

2

4

6

8

IO

I

!

I

l

I

12

Monterey Sond o.o~

9\ ~ b ~

Vibrated Phwioted

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o •

~ I. 71

"9"7 104

o 0.004 (/1

-~~ '

o K

~

,

,

~

~

0.006

0.008

I

I

I

I

I

FIG. 15--Effect o f method of specimen preparation on the axial compression of densesand. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

186

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

1.4

I

I

I

I

"1

M o n t e r e y Sand

Oensity D R ~l ~o

Vibroted

4~

1.69

97

o

1.71

104 Pluviofed

1.0

3

_

r ~ 0.4 Looding O.Z

O

I

I

I

I

I

2

4

6

El

10

12

AJtiol Stress, k g / c m 2 FIG. 16--Effect o f method o f specimen preparation on the relationship between axial stress and the ratio o f radial to axial stress.

Observation of long axes of grains showed that the overvibrated grains had a preferred orientation, as shown in Fig. 6. The vector mean of long axes of grains was inclined at about 40 deg, which is quite close to the probable planes of maximum shear under static load. This result points to the possibility that the larger compressibility of the dense, overvibrated specimens was related to their grain orientation. The fabric results, however, do not conclusively explain the changes in compressbility with overvibration, and it is believed that a better understanding of grain arrangement is needed, involving perhaps a study of grain contacts and the orientation of contact planes.

Values o f K The values of K, the ratio of radial stress to axial stress, are summarized in Table 1. For comparison, a few measured values of the coefficient of lateral pressure at rest reported in the literature are also included. The results from the various laboratory studies seem to be in the same general range of values. The values measured in the field by D'Appolonia et al [44] are substantially higher. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

MAHMOOD

ET A L O N

PREPARATION

METHODS

FOR S A N D

187

Axial Stress. i~/cm 2 01

10 Density g/cm5

0.001

o o

.~,,,,,,,,~

1.69 1.70 17',>

D R Vi brotion/Loyelr % seconds 95 I01 105

30 60 90

~ Loo d i ng

0O02

0.003

0004 ~

0O03

~"-,. ~,, ] QO06 Monterey Sond

I

0.007

I

I

I

I

J

I

I/

FIG. 17--Effect o f overvibration on the axial compression o f dense sand.

O 0

0.00~

Z I

4 I

Axiol Stress, kg/cm2 6 8 I0 ! I I Density DR Vibrotion//oyer o/cm3 % seconds

~'~-LoOdin~ ,, ~

0 ~ ~ 1.70

~" I01

60

~

io5

,o

17z

IZ

.~ 0.004 o3

o

.L UnloOding

--''.a . . . .

"'~",=..~..~

"~.,,,.~

Monterey Sond 0,008

J

I

I

i

t

FIG. 18--Effect o f overvibration on the axial compression o f dense sand, axial stress plotted to linear scale.

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

SOILSPECIMENPREPARATIONFORLABORATORYTESTING

188

1.4

I

I

I

Density g/01~3

-- I:~

I.Z

I

DR Vibration/Layer % seconds

',I

o~-

~ t704 OI

;; ~

60

~

o I.Tg

105

90

~

i i.O

I

-

-,,,,,

0.8

O.S

nm 0.4 Loadin9

0.2 Monterey Sond 0

0

I

I

Z

4 Axiol

I

I

I

6 8 Stress, kg/cm 2

I0

12

FIG.

19--Effect o f overvibration on the relationship between axial stress and K in specimens densified by vibration in six layers.

07

i

i

p .o,o

-I~

I

/

== .ooe Monterey Vibroted

,.r

==

.006

~

.004

~,

k

i

g

I

i

i

I

I

90

IO0

/

Sond Specimens

.o

4 kg/cm;

o

r

I0 k g / c m ~ .002 30

I

I

40

50

~"

~ I

I

I

60 70 80 Relative Density, D R, %

-

FIG.20--Relationship between compressibility and relative density. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

I10

.

...

"-'90

0.37 0.33 0.28

0.343

0.425

1.5

2.7

0.50 0.40

0.85

0.78

, .. 0.64 0.90 . . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . .

. . . . . . . . . . . .

. . . . . .

. . . . . .

0.58

0.53

0.82 0.80

0.74

4

1.20 1.30

1.02 1.12

. . . . . .

. . .

9

1.20

1.02

. . . . . .

0.95 1.12

10

0.85 1.02

8

vibration

vibration

Obrician

[24]coarse

to fine sand

A n d r a w e s a n d EI-Sboby, [46]; Silver sand; r o u n d e d particles

CO

Z CI

"11 0 "n

I 0

Z g

.-I

::0

"0 "11 rn "1o

O Z

2~ I-"

I"11 "--I

0 0 Cl

6.3 kip roller

D ' A p p o l o n i a et al [47]

Hendron, angular sands--from B r o o k e r a n d Ireland [48]

Hendrnn, rounded sands--from B r o o k e r a n d Ireland [48]

r o u n d e d sand grains

results f r o m this paper m a x i m u m vertical stress o n specimen = 10 k g / c m 2

Remarks

"1"

29.5 Hz"

27.5 Hz"

12.5 kip roller

pluviation

vibration

vibration

pluvialion vibration

Specimen P r e p a r a t i o n

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

~ D u r g u n o g l u a n d Mitchell [49], Fig. 5 . 8 b . bOCR = m a x i m u m vertical load/vertical load d u r i n g unloading. CAfter 40 passes; stresses perpendicular to roller path.

0.0 51.6 78.6

... ... ...

...

(n = 0.411) Dense

(n = 0.352)

...

Loose

.

30 36

.,. ...

.

34

...

.

0.38

30

9

.

0.35

0.40

47 ~

49 ~

97

104

.

0.42 0.39

38* 44 ~

0.33

0.60

0.46

32*

6

50 80

0.60 0.57

OCR b = 2

Loading

Initial DR, %

Unloading Method of

summary o f K (ralio o f radial to axial stress) values.

K = Radial Stress/Axial Stress

l--Comparative

~, deg

TABLE

190

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

Conclusions

A medium sand, composed of rounded, slightly elongate grains, when pluviated at both low and high densities, formed specimens with randomly oriented grains. When specimens were densified by vibration in layers, the grain orientation was random up to 100 percent relative density. When vibration was continued beyond the time interval needed for achieving 100 percent relative density, the grains acquired a preferred orientation with a vector mean at some inclination to the horizontal. The loose, pluviated specimens were much more compressible than the dense specimens prepared by the same procedure. On application of vertical load, the particles slide and overturn to reduce the void volume. The lateral stresses during loading were higher in the loose specimen; being less rigid, it had a greater tendency for sideward bulge than the dense specimens. On unloading also, the loose specimen had higher lateral stresses than the dense specimen, but this trend reversed itself towards the end of unloading. The same trend was also measured in the specimens prepared by vibration. The dense specimens, when prepared by vibration, were less compressible than the pluviated specimens. This difference in the behavior of specimens prepared by these two methods of preparation cannot be conclusively explained on the basis of either grain arrangement or locked-in stresses. The effects of stress history may also contribute to the observed behavior. The grains are randomly oriented in both pluviated and vibrated specimens, and the measured lateral stresses are slightly higher in the more compressible, pluviated specimen. It is believed that this study addressed two important issues bearing on the specimen preparation of cohesionless materials. First, how meaningful is a comparison of test results obtained from research efforts in the absence of fabric studies and, second, how much emphasis should be placed on fabric when in situ properties of granular materials are estimated by testing reconstituted specimens in the laboratory. The results of this study reaffirm that density as well as the method of densification should be specified in standardized testing procedures. Also, it points to the need for further research in testing of sands in order to obtain better understanding of how the field behavior of sands can be estimated from tests in the laboratory.

Acknowledgments The experimental work reported was carried out at the University of California, Berkeley, and the study was supported by The National Aeronautics and Space Administration through Grant NGR-05-003-406 and Contract NAS 9-11266. Joyce Blueford and Carl Bellamy helped with

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MAHMOOD ET AL ON PREPARATION METHODS FOR SAND

191

some portions of the fabric study. Addition support from Professional Development Committee of Woodward-Clyde Consultants is gratefully acknowledged. References [1] Mahmood, A. and Mitchell, J. K., Clays and Clay Minerals, Vol. 22, No. 5/6, pp. 397408. [2] Oda, M., Soils and Foundations, Vol. 12, No. 1, March 1972, pp. 17-35. [3] Casagrande, A. in Contributions to Soil Mechanics 1925-1940, Boston Society of Civil Engineers, Boston, Mass., 1940, pp. 72-106. [4] Lambe, T. W., "The Structure of Inorganic Soil," Proceedings, American Society of Civil Engineers, Separate No. 315, Oct. 1953. [5] Collins, K. and McGown, A., Geotechnique, Vol. 24, No. 2, June 1974, pp. 223-254. [6] Underwood, E. E., Quantitative Stereology, Addison-Wesley, Reading, Mass., t970. [7] Elias, H., Science, Vol. 174, No. 4013, 3 Dec. 1971, pp. 993-1000. [8] Brewer, R., Fabric and Mineral Analysis of Soils, Wiley, New York, 1964. [9] Mitchell, J. K., Proceedings, Highway Research Board, Vol. 35, 1956, pp. 693-713. llOl Smart, P., "Electron Microscope Methods in Soil Micromorphology," 4th International Working Meeting on Soil Micromorphology, Kingston, 1973. [I1] Smart, P. in Proceedings, International Symposium on Soil Structure, Gothenburg, 1973, Swedish Geotechnical Society, Stockholm, pp. 69-76. [12] Pusch, R., Engineering Geology, Vol. 3, 1966, pp. 433-443. [13] Smart, P. in Proceedings, International Conference on Structure, Solid Mechanical Engineering Design University of Southampton, 1964, pp. 21-1 to 21-7. [14] McConnachie, I., Geotechnique, Vol. 24, No. 2, June 1974, pp. 207-222. [15] Borowicka, H., Jr., Proceedings, Eighth International Conference on Soil Mechanics Foundation Engineering, Vol. 1.1, Moscow, 1973, pp. 71-77. [16] Andrawes, K. Z. and Butterfield, R., Geotechnique, Vol. 23, No. 4, Dec. 1973, pp. 571-576. [17] Martin, R. T., Clays and Clay Minerals, Vol. 26, pp. 271-287. [18] Moore, C. A., "Mineralogical and Pore Fluid Influences on Deformation Mechanisms in Clay Soils," Ph.D thesis, University of California, Berkeley, Calif., 1968. [19] Graham, J. W., "Significance of Magnetic Anisotropy in Appalachian Sedimentary Rocks," Geophysicalmonograph 10, American Geophysical Union, 1966, pp. 627-648. [20] Abrogast, J. L., Fay, C. H., and Kaufman, S., "Method and Apparatus for Determining Directional Dielectric Anisotropy in Solids," U.S. Patent Office, Patent 2,963,642, 1960. [21] Arulanandan, K. and Smith, S. S., Proceedings, American Society of Civil Engineers, Vol. 99, No. SM12, Dec. 1973, pp. 1113-1133. [22] Mitchell, J. K. and Aranandan, K., Proceedings, American Society of Civil Engineers, Journal of Soil Mechanics Foundation Division, Vol. 94, No. SM2, March 1968, pp. 447-472. [23] Nanz, R. H., "Exploration of Earth Formations Associated with Petroleum Deposits," U.S. Patent Office, Patent 2,963,641, 1960. [24] Olsen, H. W. in Clays and Clay Minerals, Proceedings of the 9th National Conference, 1962, Pergamon Press, New York, pp. 131-161. [25] Orr, W. R., "Method for Determining Directional Inductive Anisotropy of Materials by Measuring Q Factor," U.S. Patent Office, Patent 3,151,292, 1964. [26] Schmertmann, J. H., Proceedings, American Soceity of Civil Engineers, Vol. 96, No. SM3, May 1970, pp. 1011-1046. [27] Chaplin, T. K., Proceedings, 5th International Conference on Foundation Engineering, Vol. 2, 1961, pp. 33--41. [28] Selig, E. T. and Ladd, R. S. in Evaluation of Relative Density and Its Role in Geotechnical Projects Involving Cohesionless Soils, ASTM STP 523, American Society for Testing and Materials, 1973, pp. 487-504.

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192

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

[29] Janbu, N., "Soil Compressibility as Determined by Oedometer and Triaxial Tests," European Regional Conference on Soil Mechanics Foundation Engineering, Weisbaden, 1963, pp. 19-25. [30] Duncan, J. M. and Chang, C. Y., Proceedings, American Society of Civil Engineers, Vol. 96, No. SM5, Journal o f Soil Mechanics Foundation Division, Sept. 1970, pp. 1629-1953. [31] D'Appolonia, D. J., D'Appolonia, E., and Brissette, R. F., Proceedings, American Society of Civil Engineers, Vol. 94, No. SM3, Journal of Soil Mechanics Foundation Division, May 1968, pp. 735-759. [32] Schmidt, B. and Steen, O., "Compression of Sands," Bulletin No. 23, The Danish Geotechnical Institute, 1967. [33] Hendron, A. J., "The Behavior of Sand in One-dimensional Compression," Ph.D dissertation, Department of Civil Engineering, University of Illinois, Urbana, 1963. [34] Obrcian, V., "Determination of Lateral Pressures Associated with Consolidation of Granular Soils," Highway Research Record 284, Highway Research Board, 1969, pp. 13-24. [35] Calhoun, D. E. and Triandafilidis, G. E., Proceedings, 7th International Conference on Soil Mechanics and Foundation Engineering, Mexico City, 1969, Vol. 1, pp. 65-72. [36] Bishop, A. W. and Henkel, D. J. Measurement of Soil Properties in the Triaxial Test, Edward Arnold, London, 1962. [37] Moore, C. A., Proceedings, American Society of Civil Engineers, Vol. 97, No. SM9, Journal of SoiI Mechanics Foundation Division, Sept. 1971, pp. 1275-1292. [38] Campanella, R. G. and Vaid, Y. P., Canadian Geotechnical Journal, Vol. 9, Aug. 1972, pp. 249-260. [39] Andreasson, L., "Compressibility of Frictional Soils," Ph.D dissertation, Chalmers University of Technology, Gothenburg, Sweden (in Swedish with English summary). [40] Fumagalli, E., Proceedings, American Society of Civil Engineers, Vol. 95, No. SM1, Journal of Soil Mechanics Foundation Division, Jan. 1969, pp. 313-330. [41] Lindblom, U., "Compression Characteristics of Spread Fills from Blast Rock, with Special Reference to Settlements of Footings," Ph.D dissertation, Chalmers University of Technology, 1972, Gothenburg, Sweden (in Swedish with English summary). [42] Lade, P. V. and Duncan, J. M., Proceedings, American Society of Civil Engineers, Vol. 99, No. SM10, Oct. 1973, pp. 793-812. [43] Daniel, A. W. T., Harvey, R. C., and Burley, E., Proceedings, American Society of Civil Engineers, Vol. 101, No. GTS, Journal of Geotechnical Engineering Division, May 1975, pp. 508-512. [44] Durgunoglu, H. T., "Static Penetration Resistance of Soils," Ph.D dissertation, University of California, Berkeley, 1972. [45] Jizba, Z. V. in Procedures in Sedimentary Petrology, R. E. Carver, Ed., Wiley, New York, 1971. [46] Youd, T., Proceedings, American Society of Civil Engineers, Vol. 98, No. SM7, Journal o f Soil Mechanics Foundation Division, July 1972, pp. 709-726. [47] D'Appolonia, D., Whitman, R., and D'Appolonia, E., Proceedings, American Society of Civil Engineers, Vol. 95, No. SM1, Journal of Soil Mechanics Foundation Division, Jan. 1969, pp. 263-284. [48] Brooker, E. W. and Ireland, H. O., Canadian Geotechnical Journal, Vol. 2, No. 1, Feb. 1965, pp. 1-15. [49] Durgunoglu, H. T. and Mitchell, J. K., "Static Penetration Resistance of Soils," Geotechnical Engineering Report by Space Science Laboratory, University of California, Berkeley, April 1973.

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V. A . N a c c i ~ a n d R. A . D ' A n d r e a 2

A Technique for the Preparation of Specimens of Loose Layered Silts

REFERENCE: Nacci, V. A. and D'Andrea, R. A., "A Technique for the Preparation of Specimens of Loose Layered Silts," Soil Specimen Preparationfor Laboratory Testing, ASTM STP 599, American Society for Testing and Materials, 1976, pp. 193-201. ABSTRACT: A brief review of previously published results of shear strength testing of silty soils is presented. It is noted that, due to their highly porous structure and the resultant unfavorable effects derived from sampling, natural silts are among the least studied deposits in the world. When found in glaciated regions, these deposits are often laminated, usually contain small amounts of clay, and, when laminated, are invariably anisotropic. A typical naturally occurring silt, prevalent in the area of Providence, Rhode Island, is described in terms of its geologic origin and its in situ index properties. An experimental procedure which entails laboratory sedimentation, consolidation under small load increments, and laboratory freezing to obtain a soil specimen which may be handled without densification, is described. It is shown that these laboratory produced specimens duplicated the in situ varved structure and index properties of the natural silt. Although the paper reports in detail on the production of cylindrical triaxial specimens, the method may be readily adapted to produce specimens of virtually any size or shape. In conclusion, it is pointed up that, in utilizing this technique, specimens may be prepared with virtually any angular relationship between the inclination of the layers and the directions of the applied principal stresses, facilitating the investigation of anisotropic strength effects. KEY WORDS: soils, silts, sedimentation, freezing, anisotropy, shear strength, triaxial tests

As part of a comprehensive research project dealing with the determination of the mode of failure and the strength parameters of certain naturally occurring stratified silty deposits found in Rhode Island, a unique specimen preparation technique was devised and is described herein. Under natural conditions, the liquidity index of these silts is well above Professor of civil and ocean engineering, University of Rhode Island, Kingston, R.I. 02881. ~Assistant professor of Civil Engineering, Worcester Polytechnic Institute, Worcester, Mass. 01609.

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194

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

1.0, and, in many respects, they behave like loose sands, in that it is not uncommon for them to lose a high percentage of their undrained strength when the silt is subjected to limited remolding, which is inevitable in even the best undisturbed sampling techniques. A prime prerequisite to the aforementioned study was to obtain samples which preserved the in situ layering, structure, and porosity variations. However, conventional sampling techniques, including thin-walled and block sampling, produced inferior results, since the soil liquefied at the slightest disturbance. Furthermore, even when good samples were obtained, some densification took place in transit to the laboratory, and it was assumed that the resulting laboratory specimen would exhibit a higher strength than that present in situ, due to its decreased void ratio. A review of the relevant available literature reveals that the problem of obtaining adequate samples of silty soils seems almost universal. Some studies [1,2] 3 have employed remolded triaxial specimens of varying relative densities, and the large excess pore water pressures developed upon shearing the loose samples in these studies proved to be a good indicator of the expected behavior of the silts in question. However, as demonstrated in strength investigations on layered clays [3,4], it seemed likely that the shear strength would be dependent upon the shear plane orientation with regard to the strata, as well as soil structure and void ratio. Furthermore, Bjerrum [5] has pointed up the importance of anisotropic strength effects and their relevance to solutions of soil stability problems. Consequently, remolded samples (which would exhibit no anisotropy) were deemed inappropriate for this investigation. Investigations by Fagnoul [6] and Schultze and Odendahl [7] indicate that undisturbed silt samples were tested, but fail to indicate how these samples were obtained. Hirschfeld and Poulos [8] present drained shear test data from samples extracted from an undisturbed block. Fortunately, their silt was not saturated fully and had a high preconsolidation pressure. Both of these effects (which are absent in the Rhode Island silts under study) impart some rigidity to the soil, and thus probably facilitated sampling. In situ field strength tests, such as the vane shear, provide questionable results in silt, since drainage conditions are difficult to predict, due to the silt's relatively high permeability. Tests are likely to be drained partially and consequently would yield strengths greater than the undrained strength in the loose silt under consideration. Although Blight [9] presents a technique for performing drained field vane shear tests, undrained strength parameters for the Rhode Island silts were required for certain design purposes. Thus, field strength testing programs were discarded in favor of laboratory testing under monitored drainage conditions, and the decision was made 3The italicnumbersin brackets referto the list of referencesappendedto this paper. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

NACCI AND D'ANDREA ON LOOSE LAYERED SILTS

195

to attempt to duplicate the sensitivity and layering of the in situ material with laboratory sedimented specimens.

Soil Description In many parts of the world which have experienced glaciation, there exist clay, silt, and sand deposits with a definite banded or varved structure. The Rhode Island silts are illustrative of such sedimentary glacial lakebed deposits. Although the mode of sedimentation and the effects of electrolytes on deposition may be a matter of dispute, the typical Rhode Island silt is banded, almost devoid of clay particles, and in a fairly loose condition. Their characteristic light and dark bands imply an annual deposit, with the coarser silt and sand sizes having been deposited during periods when the waters of the parent glacial lake were more turbulent. The silts in the Providence, Rhode Island area are fairly consistent in lamina thickness, with natural water contents often well above the liquid limit. It is not uncommon for standard penetration test results to be two blows per foot or less. A typical deposit, which was sampled extensively for this investigation, was discovered at a location 1.5 km south of the Park Avenue overpass on Interstate Route 95 in Cranston, Rhode Island. The distinguishing geotechnical features of this deposit may be summarized as follows: 1. The successive layers are on the order of 0.9 cm in thickness and are composed mainly of a lightly colored coarser portion which grades into a thick dark band composed of rock flour and clay. No attempt was made to separate the materials comprising the layers, since the grading was often almost imperceptible. 2. Natural water contents varied between 30 and 38 percent. 3. The liquid and plastic limits for the material, as a whole, were 28 and 22 percent, respectively. 4. The grain size range and uniformity (again, of the entire mass) are depicted in Fig. 1. 5. The high natural water contents suggest a metastable soil structure, and the soil, when sampled, proved highly sensitive to disturbance.

Specimen Preparation Initially, bulk samples were procured from the site during the construction of Interstate Route 95. The soil was stored in barrels, and, as it was needed, it was mixed with sufficient distilled water to produce a mass which could be readily poured through a funnel. The resultant slurry was stored in sealed glass jars. Triaxial specimens were prepared by slow sedimentation of this slurry Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

196

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

100.

-.....

Z

a~ Z 40

,,I

~o 20

.3

.2

.06 SAND J

.02

I SILT SIZE L DIAMETER

.002

.01

.001

I CLAY SIZE

I

( rnm'~

FIG. l--Grain size distribution o f a varved Rhode Island silt.

through a solution of 0.1 normal sodium chloride in 3.48-cm-diameter solid perspex tubes (see Fig. 2). By varying the size of the tube in which

LOAD

POROUS STONE

VARVED SAMPLE

j PLASTIC

BEAKER

TUBE~

....-.----COARSE SAND

FIG. 2--Sedimentation apparatus.

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NACCI AND D'ANDREA ON LOOSE LAYERED SILTS

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the silt is sedimented, specimens of any size may be obtained. A small amount of the dilute silt suspension was poured through a funnel into the tube. The funnel was equipped with an upturned stem so that the soil would be initially directed upward through the solution inside the tube. Eventually, the silt began to settle at a rate governed by the prevailing grain size, thus producing layers graded in a fashion similar to those found in nature. The quantity of silt poured was determined via trial and error, until the sedimented varve formed was approximately equal in thickness and composition to the natural varves. Utilizing a sodium chloride solution accelerated the settling of the clay sized fraction, and additional charges could be poured at the rate of one per day until the cylinder was full. In order to prepare specimens of varied strata inclinations, Doyle [10] made modifications to the apparatus shown in Fig. 2 (which produces specimens with horizontally oriented layers). This apparatus was hinged at the base so that layers could be deposited at any angle up to 60 deg. Tilting the tube rack beyond 60 deg, however, caused the soil particles to fall along the side of the tube, rather than in a vertical direction through the solution. To obtain specimens whose strata were oriented between 60 and 90 deg with respect to the horizontal, a slotted sediment tube was used. The silt slurry was poured along the longitudinal axis, rather than into the end of the tube. Once filled, a plastic insert was pushed across the tube to act as a seal. At this point, in order to impart some rigidity to the essentially liquid sediment, the silt was consolidated under a pressure of slightly less than 89 kg/cm 2. This was accomplished by applying a direct load, as shown in Fig. 2, for the specimens with strata inclined from 0.0 to 60 deg with respect to the horizontal. The steeper strata orientations were consolidated, utilizing a hydraulic ram. In general, consolidation with the major principal stress perpendicular to the layering would be desired, in order to assess the effects of rotation of principal stresses on strength. Unfortunately, during consolidation for rigidity in either sedimentation apparatus, the major principal stress is perpendicular to the longitudinal axis of the tube. However, it was felt that the subsequent freezing of the specimen, described later in this article, separated the grains, and ensuing consolidation under the desired stress system could be performed in the appropriate test apparatus. After consolidation was completed, the cylinder with sedimented soil intact could be lifted out of the chamber and extruded into a split mold for triaxial testing. Unfortunately, the soil densified visibily, even though vacuum was applied, presumably altering the strength properties. This development prompted an additional step in specimen preparation technique, in order that the sample might be set up without undergoing drastic changes in void ratio. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

198

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

It was hypothesized that, if the silt exhibited little electrochemical bonding (that is, its behavior was influenced primarily by gravitational forces), a freeze-thaw cycle would not alter the particle arrangement significantly, and densification due to handling could be minimized. Lambe [11] has shown that differences in axial shrinkage of compacted samples may be utilized as an indicator of structural differences of samples of equal dry densities but compacted at different molding water contents. In order to demonstrate that the silt in question did not display intermolecular bonding and the corresponding structural differences at differing electrolyte concentrations, silt samples were compacted, wet and dry, of optimum water content, via a kneading type compaction apparatus, and their axial shrinkage upon drying was measured. Since equal shrinkages were recorded for equal dry densities, regardless of water content, it was concluded that the small percentage of clay sizes had little effect, and the silt's packing arrangement was not influenced by electrochemical effects. This conclusion led to the following method of placing samples in the desired shear apparatus without altering their structure. After the samples were consolidated under the aforementioned surcharge loading, the plastic tubes were removed carefully from the sedimentation beaker; excess fluid was drained; and rubber caps were applied to both ends of the tube. The samples were then placed in a conventional household freezer, wherein the pore water froze, imparting sufficient rigidity to the sample to allow handling without densification. Although the freezing mechanism expanded the silt, it was felt that, upon subsequent thawing, the silt particles would revert to their sedimented configurations. As a further check, X-ray photographs were taken of the frozen samples. The photos revealed that little ice lensing due to pore water migration occurred, and that the layering remained intact. The water content of a frozen sample was in the range of 34 to 35 percent, comparing favorably with the in situ condition. The freezing procedure would not have been feasible if compaction results had shown that freezing and thawing would have broken down intermolecular bonds within the soil. Since there were no such bonds to begin with, it was assumed that freezing the sample to facilitate installation, coupled with subsequent thawing before testing, would not change the structure of the silt significantly. Although many researchers, Fahlquist [12] included, report field sampling methods which involve freezing the soil while it is being sampled, to the best of the authors' knowledge, laboratory freezing as described herein has not been practiced extensively. Typical Testing Procedure and Result Triaxial specimens were prepared, as described previously, and trimmed to proper lengths while still frozen. Experience showed that the samples Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

NACCI A N D D'ANDREA ON LOOSE LAYERED SILTS

199

should be frozen no more than 24 h to maintain 100 percent saturation, and that, by setting up the sample in a chilled triaxial chamber, thawing could be prevented until a small confining pressure could be applied. A period of 24 h at room temperature was sufficient for complete thawing and consolidation under the seating consolidation pressure. The confining pressure required for the test was then applied and volume change versus time recorded in the normal manner. As an illustration, the following shear test will be described. The example specimen had varves perpendicular to its longitudinal axis and was consolidated isotropically under a pressure of 1 kg/cm 2 (yielding a postconsolidation water content of 32.46 percent). After consolidation, the normal procedure of back pressuring and testing for pore pressure response [13] was followed. Shearing loads were applied via undrained triaxial compression, using Geonor equipment [14], at a constant strain rate of 2.8 percent per hour. This rate was slow enough to ensure that the pore pressure measured at the specimen base identified an equalized pore pressure throughout the material. Figure 3, which is a plot of principal stress difference (o~ - 03) versus strain, and Fig. 4, depicting principal stress difference versus average principal effective stress (o~' + 02' + 03 ')/3, portray graphically the behavior of the example specimen under loading. The behavior is similar to that observed by Schultze and Odendahl [71, who shear tested a similar soil. Again, it must be emphasized that, based on the described research and 25 years of experience with this material, the authors believe that a truly undisturbed field sample cannot be obtained and tested successfully. The silt's behavior under load is essentially that of a slowly draining 80

70

~,

i

60

z so

~i 4o

0// 3O

10

0

1

2

3

4

5

6

STRaiN C%')

FIG. 3-- Typical stress-strain behavior. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

200

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

/

80

E

/

60

Z

i

40

20

0

o

2o

4o

6o

80

loo

12o

3

FIG. 4--Stress t~th for typical test.

granular material: granular, in that the silt's void ratio and stress history solely govern its strength, and slow draining, in that significant time is required to dissipate excess pore pressures developed upon application of shear stresses. Although a complete discussion of application of the parameters derived from the laboratory sedimented specimens to field cases is beyond the scope of this paper, use of results from laboratory specimens whose water contents equaled those of the in situ silt proved to yield successful solutions (in that no failures were observed) to problems concerning stability, structural foundations, and slopes of cuts, as well as reasonable predictions for settlement under preloads.

Conclusions Due to the highly unstable nature of many sedimentary silt deposits, even ratively undisturbed samples are difficult to obtain. As a result, most investigations of such silty sites make use of remolded products or use natural specimens, on which the effects of sampling and handling disturbance cannot be quantified. According to this study, silt samples may be sedimented artificially in the laboratory and frozen to provide sufficient rigidity to prelude densification during handling and test setup. The resultant specimens have the same ~soil solid constituents and similar water content and layering characteristics as the original in situ deposits. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

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Acknowledgments This study forms part of an investigation of the strength characteristics of saturated varved silts being conducted under the sponsorship of the Rhode Island Department of Public Works, in cooperation with the U.S. Department of Transportation Federal Highway Administration, Bureau of Public Roads. The opinions, findings, and conclusions are those of the authors and not necessarily those of the Bureau of Public Roads. References [1] Penman, A. D. M., Geotechnique, Vol. III, No. 8, Dec. 1953, pp. 312-238. [2] Schultze, E. and Norn, A. in Proceedings, Sixth International Conference on Soil Mechanics and Foundation Engineering, Vol. I, Montreal, 1965, pp. 350-353. [3] Lo, K. Y. and Milligan, V., Journal of the Soil Mechanics and Foundation Division, American Society of Civil Engineers, Vol. 93, No. SM1, Paper 5056, Jan. 1967, pp. 1-15. [4] Milligan, V., Soderman, L. and Rutka, A., Journal of the Soil Mechanics and Foundation Division, American Society of Civil Engineers, Vol. 88, No. SM4, Paper 3324, Aug. 1962, pp. 31-67. [5] Bjerrum, L. in Proceedings, Eighth International Conference on Soil Mechanics and Foundation Engineering, Vol. 3, Moscow, 1973, pp. 111-159. [6] Fagnoul, A. in Proceedings, Sixth International Conference on Soil Mechanics and Foundation Engineering, Vol. 1, Montreal, 1965, pp. 213-216. [7] Schultze, E. and Odendaltl, R. in Proceedings of the Geotechnical Conference, Vol. I, Oslo, 1967, pp. 239-242. [8] Hirschfeld, R. C. and Poulos, S. J. in Laboratory Shear Testing of Soils, ASTM STP 351, American Society for Testing and Materials, pp. 329-341. [9] Blight, G. E., Canadian Geotechnical Journal, Vol. 5, No. 3, Aug. 1968, pp. 142-149. [10] Doyle, E. H., "Influence of Strata Orientation on the Shear Strength of Varved Silt," unpublished M.S. thesis, University of Rhode Island, Kingston, R.I., 1968. [11] Lambe, T. W., Journal of the Soil Mechanics and Foundation Division, American Society for Civil Engineers, Vol. 84, No. SM2, Paper 1654, May 1958, pp. 1-34. [12] Fahlquist, F. E. in SoilMechanics 1941-1953, Boston Society of Civil Engineers, Boston, 1953, pp. 20-44. [13] Lowe, J., III, and Johnson, T. C. in Proceedings, American Society of Civil Engineers Research Conference on Shear Strength of Cohesive Soils, Boulder, Color., 1960, pp. 819-836. [14] Andersen, A. and Simons, N. in Proceedings, American Society of Civil Engineers Research Conference on Shear Strength of Cohesive Soils, Boulder, Colo., 1960, pp. 695-709.

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T. F. Z i m m i e ' a n d L. J. A l m a l e h ~

Shrinkage of Soil Specimens During Preparation for Porosimetry Tests

REFERENCE: Zimmie, T. F. and Almaleh, L. J., "Shrinkage of Soil Specimens During Preparation for Porosimetry Tests," Soil Specimen Preparation for Laboratory Testing, ASTM STP 599, American Society for Testing and Materials, 1976, pp. 202-215. ABSTRACT: Various methods of removing moisture from small soil samples were investigated. The samples were to be tested by mercury porosimetry, and one requirement is a constant volume drying process. Air-drying, oven-drying and freezedrying procedures were used. Three different clay types were used: kaolinite, montmorillonite, and an illite. The clays were fully saturated, and compaction was achieved by using static pressure. Freeze drying is the best practical method of drying the soil samples, as it produces the least amount of shrinkage in the soil. In certain instances, air drying and oven drying may produce satisfactory results. Total drying time for freeze drying can be accomplished in 5 to 10 h. A simple laboratory freeze-drying system is described and operating procedures and principles discussed. The results are applicable to the preparation of small size soil samples where it is desired to remove soil moisture and, at the same time, minimize disturbance to the soil structure.

KEY WORDS: soils, samples, clays, shrinkage, drying, freezing, moisture content, vacuum, mercury porosimetry, tests

In order to obtain pore size distribution measurements on soil by the use o f mercury porosimetry, almost all moisture must be removed from the soil. During the conduct o f a porosimetry test, the soil sample is subject to a v a c u u m prior to the intrusion of mercury into the soil pores. At typical natural water content, the vacuum will cause the water to vaporize, often disintegrating the sample but, in any case, rendering the pore size measurements useless. At lower water contents, on the order o f a few percent, the soil moisture can cause errors in the pore size measurements since water is essentially incompressible even at the pressures utilized for mercury porosimetry. The water content must be reduced to 1Assistant professor and graduate student, respectively, Rensselaer Polytechnic Institute, Troy, N.Y. 12181.

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ZIMMIE AND ALMALEH ON POROSIMETRY TEST SPECIMENS

203

a low value, less than a percent, prior to the performance of a porosimetry test. Although this research was conducted for the purpose of determining the most practical method of sample preparation for mercury porosimetry tests on clays, the results and methods reported herein would be applicable to other areas of study where soil moisture must be removed without disturbing the soil structure and interparticle arrangement, for example, electron microscopy. Various methods of soil dehydration are available. Oven drying and air drying are two of the most common methods, but, in general, they are not very satisfactory as constant volume dehydration procedures. Surface tension forces produced by the air-water menisci cause large amounts of shrinkage, and this structural disturbance effects the pore size arrangement also. Critical-region dehydration has been reported to yield successful results relative to volume change [1-2]. 2 With this method, the soil sample is subjected to slowly increasing temperature and pressure, regulated to keep the water in the pores liquid until the critical region is reached. Then careful regulation of temperature allows vapor transfer to an adjacent vessel. Since critical conditions occur at about 245 atm and 380 ~ there is some question about the possibility of modifying the clay minerals. In most cases, critical-region dehydration is not a practical method of moisture removal because of the apparatus and procedures required. Freeze-drying techniques have been used to dehydrate soil samples in an attempt to obtain a constant volume drying process and retain the original soil structure [2,3,4]. Vacuum freeze drying consists of rapidly freezing a specimen and then subjecting it to a vacuum so that the moisture, in the form of ice, is removed by the process of sublimation. The triple point of water occurs at a pressure of 4.6 mm Hg and about 0~ Only at the triple point can solid, liquid, and vapor phases exist simultaneously. At pressures less than 4.6 mm Hg and temperatures below 0~ water can only exist in the solid or vapor form. This sublimation region is of interest in freeze drying, since water can be directly removed by changing from the solid state to the vapor phase, that is, sublimation. Reference can be made to numerous general thermodynamic texts and chemistry texts for further details on phase diagrams and changes of state. The success of freeze drying is attributed to the fact that, after freezing, no shrinkage takes place, at least under ideal conditions, since surface tension forces caused by the air-water menisci have been eliminated. As a result of this research, it is concluded that vacuum freeze drying is the best practical method of dehydrating soil samples in preparation for mercury porosimetry testing. Therefore, the results and procedures used 2The italic numbers in brackets to refer to the list of references appended to this paper. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

204

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

will be discussed hereinafter as they apply to the preparation of small size (volumes of a few cubic centimetres) soil samples. It is beyond the scope of this paper to cover the subject of freeze drying in detail, and the reader is referred to other general articles for a more thorough discussion of freeze-drying techniques [5, 6].

Equipment General Considerations The basic elements required for a vacuum freeze-drying system consist of a specimen container to hold the sample, a condenser or cold trap to condense the moisture removed from the sample, and a vacuum pump. A vacuum gage in the system can be considered a convenient addition in order to monitor the operation. The freeze-drying apparatus used in this project is shown in Fig. 1, and the basic dements just mentioned

FIG. 1--Freeze-drying apparatus; A--Specimen container, B--Vacuum gage, C--Condenser, D m Vacuum pump.

can be identified. This simple and economical laboratory freeze-drying system is very adequate for small size soil samples. Commercial freeze-drying units are available, and they will contain the same basic elements as those just mentioned. In addition, convenience features may be obtained, such as automation and monitoring devices. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

ZIMMIE AND ALMALEH ON POROSIMETRY TEST SPECIMENS

205

The commercial units are designed normally for handling large volumes of sample on the order of litres, and, as a result, both initial and operating costs o f a commercial unit will be higher than the costs for a simple laboratory system. To the best o f the authors' knowledge, there is no commercial freeze-drying apparatus available that is specifically tailored to soils work, and, in most instances, a system similar to the one used in this project will be satisfactory for freeze drying small volumes o f soil. Relative to performance, the final freeze-dried product will be identical whether a commercial unit or apparatus similar to that shown in Fig. 1 is used, as long as the system characteristics are comparable. That is, if comparable samples, vacuums, temperatures, refrigerants, and times are used, the results will be comparable.

Apparatus The specimen container consisted of a 250-ml glass jar, able to withstand a vacuum (Fig. 2). In order to minimize handling, the soil samples were placed on a cage, immersed in liquid nitrogen until frozen, and then placed in the jar along with the cage. The cage consisted of stainless steel wire and aluminum screen, since the metals must be resistant to cryogenic temperatures. Normally, three soil samples were dried at a time. However, the volume of the jar and the capacity of the system are sufficient to produce 10 or 20 samples at a time. After soil samples are frozen, the freeze-drying process must begin rapidly so that melting does not occur. The 250-ml jar could be evacuated in a matter o f seconds, and there was no danger of the samples melting. A larger volume jar might necessitate a larger vacuum pump or a vacuum ballast or both in order to produce a vacuum quickly. The system should be evacuated in about 15 s to be certain that no sample melting occurs. The vacuum gage utilized was a diaphragm type gage, accurate to about 2 nun Hg. This type o f gage is not precise enough to read the actual vacuum in the system. Where precise vacuum readings are desired, special purpose vacuum gages must be used [6]. However, the diaphram gage in the system can be calibrated against a precision vacuum gage to establish a zero point, that is, the operating pressure of the system. Then loss o f vacuum caused by slight leaks or similar problems can be detected easily. This was the purpose of the gage shown in Fig. 1. The condenser consisted of a Dewar flask, suitable for cryogenic liquids, which held the refrigerant, and a commercial vacuum pump protector. The vacuum pump protector consisted of two compartments, the first, a condenser compartment which is immersed in the refrigerant to freeze out the sample moisture, and, the second, a compartment filled with a chemical dessicating agent to prevent water vapor from entering the vacuum pump. In order to maintain a proper vacuum, the dessicant Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

206

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 2--Specimo7jar for freeze drying. must be maintained or renewed periodically. Some water vapor will still pass through to the vacuum pump, and the pump oil must be checked and changed periodically to remove water from the oil in order to maintain the proper vacuum. The use of different condenser refrigerants will be discussed. However, if liquefied gases are used as a refrigerant, a Dewar flask, resistant to cryogenic temperatures, is required to hold the liquid gas. If dry ice and a solvent such as acetone or alcohol are used as a refrigerant, laboratory beakers o f the Pyrex type are sufficient. During operation, the condenser is surrounded with a thick layer o f foam insulation. This minimizes heat leak and loss of refrigerant. The vacuum pump was a c o m m o n laboratory Cenco Pressovac 4 pump available from the Central Scientific Co., Chicago, Illinois. The rated pumping capacity is 10 litres per minute free air displacement at normal operating speed, and the nameplate rating states that the pump will Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

ZIMMIE AND ALMALEH ON POROSIMETRY TEST SPECIMENS

207

produce a vacuum of 0.015 mm Hg (0.015 torr) or better. In practice, pumps of this type are capable o f producing much better vacuums, as low as 0.001 torr in leak-free systems. In an ideal system, water in the liquid state will not form if the pressure remains below 4.6 mm Hg. However, above 0~ the ice will turn to vapor, rupturing the sample. Even at temperatures below 0 ~ the subliming water vapor can become trapped in pores, causing a local increase in pressure, and the vapor can condense to the liquid phase. This becomes more probable as the system pressure approaches 4.6 mm Hg. Therefore, in practice, a vacuum lower than about 0.015 m m Hg should be maintained. This can be easily attained with the system and equipment shown in Fig. 1. The device shown in Fig. 3 was used for volume measurements on the

FIG. 3--Volume measuring device for soil samples.

soil samples. The dial gage can be read to 0.002 ram. The gage force is only about 50 g, and this force can be reduced by applying a slight squeezing force to the top extension o f the gage arm as the gage arm is brought into contact with the sample. As a result, even the softest soil could be measured without disturbance. The performance o f the volume measuring device was quite satisfactory, and the measurements were consistently reproducible. George [7], in his shrinkage studies on soil-cement mixtures, obtained satisfactory results using a dial gage measuring device similar in concept to the one used for this project. A precision micrometer Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

208

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

accurate to 0.002 m m and a caliper accurate to 0.002 m m were also tried as volume measuring devices. However, the use o f micrometer devices were abandoned, as the measurements could not be reproduced very precisely. A h m e d et al [4] have also reported difficulties in obtaining volume measurements using micrometers. Specimens Most of the tests were performed on two clays, a kaolinite and a sodium montmorillonite. These clays were selected because they have very different properties. In fact, they represent opposite ends of the spectrum normally encountered. One series of tests were performed using a local illitic clay o f glacial origin in order to ascertain that results that were similar to those for the kaolinite and montmorillonite samples could be obtained. The properties o f the clays are presented in Table 1, and the hydrometer grain size analysis for the kaolinite is shown in Fig. 4. TABLE l----Characteristicsof soils used. Property

Kaolinite

Sodium Montmorillonite

The Georgia Kaolin Co., Dry AmericanColloid Supplier Branch, Ga. Co., Chicago, Ill. WL, o70 39 "~500 We, % 26 85 Specific gravity 2.65 2.70 Percent finer than #200 sieve 98 95 Percent finer than 20 t~m 96 90 Percent finer than 0.5 lam 67 85

Illitic Clay In situ sample eastern New York State 43 29 2.80 99.6 98 80

Specimen Preparation The clay samples were remolded with sufficient water to assure complete saturation. The clays were compacted by using a static procedure. Different pressures were utilized to produce various water contents. The clays were placed in a standard Proctor mold (volume approximately 1000 cm3), and static pressure was applied by jacking. The compaction was carried out in stages until the final pressure was reached. The final static pressure w a s a b o u t 700 k N / m 2 for the highest water contents and about 2800 k N / m 2 for the lowest water contents. Consistent procedures were used in the mixing and compaction processes, and, for each clay type, the final water contents and void ratios could be satisfactorily duplicated to within a few percent. After compaction, the load was removed f r o m the soil, and the soil was allowed to rebound and come to equilibrium. The length o f the compaction period ranged f r o m one to three days. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

209

ZIMMIE AND ALMELEH ON POROSIMETRY TEST SPECIMENS

I00

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F I G . 4--Grain size distribution for kaolinite.

Samples for testing were taken from the center of the soil mass, where they were of uniform water content and void ratio. The outer portions of the soil mass tended to be more nonuniform than the center. Cube samples about 7 mm on a side were formed by trimming with a thin wire cutter. Procedure The oven-dried samples were dried at l l0~ and the air-dried samples at room temperature (20 ~ The samples to be freeze dried were placed on a cage (Fig. 2) and immersed directly into liquid nitrogen, where they froze in about 5 to 15 s. It is important that the samples be frozen rapidly and at temperatures below - 130~ to avoid the formation of ice crystals which can damage the soil structure [2]. To attain the low temperatures, liquefied gases must be used, usually liquid nitrogen. When a sample is placed in a liquefied gas, bubbling occurs from the heat transfer, and the sample becomes surrounded by a thermally insulating layer of gas which tends to retard the freezing process. In order to avoid this relative delay in freezing, the samples can be immersed directly into intermediate cooling liquids, for example iso-pentane cooled by liquid nitrogen [6]. For the small size samples used for mercury porosimetry tests, this refinement is not required, as freezing occurs rapidly. However, if large soil samples are to be frozen for other applications, the use of intermediate cooling liquids may be required. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

210

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

About 30 percent of the samples exhibited large cracks, heaves, and similar disruptions after freezing in liquid nitrogen (Fig. 5). There did not appear to be any predictable pattern to the cracking, and it occurred equally at all water contents and with all the clay types. Accurate volume

FIG. 5--Cracked samples after freezing in liquid nitrogen," 0eft) montmorillonite, (right) kaolinite; 7-ram cubes. measurements could not be obtained on cracked samples, and although porosimetry tests could be run on cracked samples, the results would be questionable. Therefore, the easiest solution was to allow for a number of cracked samples and discard them when cracking occurred. After freezing, the samples were quickly placed in the specimen jar and the vacuum applied. This transfer operation and the subsequent application of vacuum must occur rapidly to prevent the sample from melting. As mentioned in an earlier section, the system could be evacuated in a matter of seconds, and there was no danger of melting. The specimen jar remained exposed to room temperature as shown in Fig. 1. Sublimation is an evaporative process accompanied by cooling, so, although the specimen container is exposed to room temperature, the sample temperature will remain well below freezing. The soil samples were placed on the cage inside the specimen jar, never in direct contact with the glass. Samples should not make contact with the glass if the specimen container is exposed to temperatures above freezing, as heat conducted through the glass could cause sample melting. These observations were confirmed by using copper-constantan thermocouples inserted into a number of soil cubes. The refrigerant used in the condenser was a mixture of acetone and dry ice ( - 79 ~ After the freeze-drying operation was completed, volume measurements were made and the samples were oven dried to determine the amount of Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

ZIMMIE AND ALMALEH ON POROSIMETRY TEST SPECIMENS

211

moisture remaining after freeze-drying. The above procedure was followed after drying because this study was conducted to investigate sample preparation techniques. However, samples to be used for porosimetry tests are transferred to a glass vacuum dessicator where they can be stored for lengthly periods. Freeze-dried samples should be exposed to ambient room conditions for minimal times, since the soil will absorb moisture rapidly. Montmorillonite can absorb large amounts of moisture, negating the freeze-drying operation. Kaolinite will absorb much less moisture, and this can be removed by conventional oven drying without any further volume change. Nevertheless, as a general rule, freeze-dried samples should be stored in a moisture-free environment. Experimental W o r k R a t e o f Freeze Drying

Freeze-drying times of 5 to 6 h were required for the kaolinite clay, and 9 to 10 h for the montmorillonite. Typical results showing the rate of moisture removal from kaolinite are presented in Fig. 6. The shape of the curve is characteristic of the freeze-drying process [8], and, for the kaolinite, an initial constant-drying rate occured for about the first 3 h, followed by a falling-rate period from 3 to 6 h.

o 50

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FIG. 6---Typicalrate of freeze-drying curve showing soil water content versus time. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

212

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

Kearsey et al [9] have developed an equation for predicting the rate of ~ublimation of a slab of ice contained in such a manner that evaporation can occur from one face only. The rate of sublimation is given by G

3600yV(M/2nRT)P

=

(1)

where G y M R P T

= = = -= =

rate of evaporation in grams per square cm of surface per hour, evaporation coefficient, molecular weight, gas constant in dyne cm per mole per ~ vapor pressure of pure ice at the temperature of evaporation, and temperature in K.

Assuming y to be unity and expressing P in mm of mercury Eq 1 reduces to G

=

544(PA/--T)

(2)

Under ideal conditions, the maximum rate of sublimation will occur at 0~ and a vapor pressure of 4.6 mm Hg (4.6"t:orr). From Eq 2 this rate is found to be about 150 g/cm2/h. Assuming a sample temperature of - 3 0 ~ with the corresponding vapor pressure of 0.29 torr, the rate of evaporation is found to be about 10 g/cm2/h. If one assumes that evaporation occurred on all faces of the 7-mm cube sample of Fig. 6, the rate of moisture removal was approximately 0.01 g/cm2/h, several orders of magnitude less than the ideal. Strictly speaking, the above equations are not applicable to porous media; however, they do show that the rate of evaporation is dependent on the vapor pressure and temperature of the sample. In practice, operating temperatures and pressures will be well below 0~ and 4.6 torr. Operating pressures have been discussed previously in the apparatus section, and a margin of safety must also be observed with temperatures to prevent sample melting. However, if the sample temperature is too low, the rate of sublimation will also be low. In the freeze-drying experiments of Ahmed et al [4], the specimen container was immersed in a refrigerant ( - 4 0 ~ to prevent sample melting, and liquid nitrogen was used as a condenser refrigerant. Equations 1 and 2 indicate a low rate of evaporation using this method; however, a series of experiments were performed to determine the amount of sample shrinkage incurred. The sample jar was placed in an acetone-dry ice bath (-79~ and the condenser fluid was liquid nitrogen. The procedures used were the same as outlined by Ahmed et al [4], and total drying times of about 17 h were required for kaolinite and montmorillonite. Ahmed et al [4] reported drying times of 12 to 18 h on a Grundite clay. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

ZIMMIE AND ALMALEH ON POROSIMETRY TEST SPECIMENS

213

The shrinkage values obtained were slightly less when the specimen jar was immersed in a refrigerant; however, no special significance should be attached to the slight differences in shrinkage values. There appears to be no advantage to immersing the specimen jar in a refrigerant. On the other hand, the drying time is extended considerably. As pointed up previously, there is little danger of small soil samples melting if the specimen container is exposed to room temperature. Results

The results of drying tests on approximately 100 soil samples (cubes 7 mm on a side) are presented in Fig. 7. Each data point represents the AIR

DRIED 9

OVEN

DRIED

FREEZE 30

9

DRIED

9

KAOLINITE

MONTMORILL ONTE

T

2s

I ~ 2o

NOTE * AIR

f

z 15 .Iu

SAMPLES

DRIED I

OVEN

DRIED

DISINTEGRATED.

IO

o

11/

5

~yl

9

I1~'

0

9

I

I

I

I

I

I

IO

20

30

40

50

60

WATER FIG.

9

CONTENT:

%

DRY

I

& 9 I

I00

WEIGHT

I

I

150

I 200

OF

I

I 250

I

I 300

SOIL

7--Volume change versus water content f o r various drying methods.

mean of a test series. In addition, 15 to 20 pilot tests were performed to develop techniques, establish drying rates, and check procedures. Freeze drying produced the best results, that is, the least amount of shrinkage at all water contents. However, results comparable to freeze drying could be obtained by oven drying and air drying kaolinite samples with very low water contents. Montmorillonite samples must be freeze dried. Virtually all air-dried and oven-dried samples of montmorillonite disintegrated upon drying due to the large amount of shrinkage that occurred (Fig. 8). Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

214

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 8--Typical montmorillonite samples after drying (left)freeze dried, 7 mm cube; (center) air dried; (right)oven dried. A series of the illitic clay samples were freeze dried for comparative purposes. The water contents were 38 percent, and the mean final shrinkage value was about 5 percent, a value similar to that obtained for kaolinite. The shrinkage values for the clays can be considered to represent the absolute volume change due to drying procedures. Volume measurements were made on a number of samples after freezing in liquid nitrogen, and the amount of expansion measured was negligible. Rosenqvist [3] has reported a value as high as 3 percent expansion when an undisturbed soft Norwegian marine clay was frozen, and this is probably the highest value that Would be normally encountered. For freeze-dried samples, a final water content of 0.5 percent or less was considered a dry sample. Freeze-dried samples absorb moisture rapidly; rates of 2 percent water content per minute have been reported [6]. Thus, the sample absorbs moisture even during the weighing process. Practically speaking, there is no necessity for removing the last 0.5 percent moisture, since the sample will be exposed to the atmosphere while being transferred to the mercury porosimeter chamber.

Conclusions Vacuum freeze drying is the best practical method of preparation of soil samples for mercury porosimetry tests. Adequate drying can be accomplished in 5 to 10 h. In certain instances, air drying and oven drying may produce satisfactory samples for use in mercury porosimeter tests. This research was primarily intended to study soil sample drying methods for porosimetry testing. However, the results and methods are applicable to other areas where it is necessary to produce dry soil samples with minimal structural disturbance. Acknowledgments

The authors wish to thank T. C. Engel and D. E. Moellman, graduate Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

ZIMMIE AND ALMALEH ON POROSlMETRY TEST SPECIMENS

215

students at Rensselaer Polytechnic Institute, for their valuable assistance during this project. References [1] Sridharan, Asuri, Altschaeffl, A. G., and Diamond, Sidney, Journal of the Soil Mechanics and Foundations Division, American Society of Civil Engineers, Vol. 97, No. SM5, May 1971, pp. 771-787. [2] Gillott, J. E., Journal of Sedimentary Petrology, Vol. 39, 1969, pp. 90-105. [3] Rosenqvist, I. T., Journal of the Soil Mechanics and Foundations Division, American Society of Civil Engineers, Vol. 85, No. SM2, April 1959, pp. 31-53. [4] Ahmed, Syed, Lovell, C. W. Jr., and Diamond, Sidney, Journal of the Geotechnical Engineering Division, American Society of Civil Engineers, Vol. 100, No. GT4, April 1974, pp. 407-425. [5] Meryman, H. T., New York Academy of Science Annals, Vol. 85, March-May 1960, pp. 630-640. [6] Rowe, T. W. G., New York Academy o f Science Annals, Vol. 85, March-May, 1960, pp. 641-679. [7] George, K. P., Highway Research Record, No. 255, 1968, pp. 42-58. [8] Broughton, Geoffrey in Technique of Organic Chemistry, Vol. III, 2nd Ed., Arnold Weissberger, Ed., Interscience Publishers, New York, 1956, Chapter VI, pp. 787-839. [9] Kearsey, H. S., Strickland-Constable, R. F., and Bruce, E. W. in The Institute of Refrigeration, Imperial College of Science and Technology, Session 1953-1954, pp. 5-15.

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

A. R. Booth'

Compaction and Preparation of Soil Specimens for Oedometer Testing

REFERENCES: Booth, A. R., "Compaction and Preparation of Soil Specimens for Oedometer Testing," Soil Specimen Preparation for Laboratory Testing, ASTM STP 599, American Society for Testing and Materials, 1976, pp. 216-228. ABSTRACT: The paper describes the development of a method for the compaction of groups of identical pedometer specimens. Groups were compacted from several soils at various densities and moisture contents. The specimens in each group were tested later in the pedometer over the complete range of initial saturations, and it was important that they should be as near as possible to identical after compaction. The paper considers, by means of a literature review, the various methods of compaction available. The conclusion reached is that the static method is most suitable for compaction of relatively thin specimens. Soil was compacted directly into the pedometer rings under static loading. The advantages and disadvantages of this approach are discussed. A special mold developed for this purpose is described in detail. This enabled a known mass of soil to be compacted into a 76.2 by 19.05-mm-thick specimen. The procedure used in the laboratory is outlined. It was necessary to allow most specimens to either increase or decrease in moisture content after compaction so that pedometer tests could be carried out over the complete range of saturation. The procedures followed are described, and it is stressed that these model as closely as possible what would happen in a road embankment. Finally, the paper considers the dry densities of groups of specimens immediately prior to being placed in the pedometer. It is shown that variations within each group are lower than those found in undisturbed samples, and it is concluded that the standardized method produced groups of specimens which were closer to identical than would have been obtained by other methods. This, in turn, enabled the influence of other variables, principally the compaction moisture content and initial dry density, to be studied with more confidence.

KEY WORDS: soils, compaction, embankments, collapse settlement, consolidation, pedometers, density, partial saturation, roads

T h e r e h a v e b e e n several failures o f r o a d e m b a n k m e n t s in s o u t h e r n A f r i c a in r e c e n t y e a r s w h i c h c a n b e a t t r i b u t e d t o c o l l a p s e s e t t l e m e n t w i t h i n t h e c o m p a c t e d f i l l i n g , f o l l o w i n g w e t t i n g [1]. 2A r e s e a r c h p r o j e c t w a s i n i t i a t e d t o d e t e r mFormerly, chief research officer, National Institute for Road Research, Pretoria, South Africa; presently, senior engineer, Duff and Geddes, Edinburgh, Scotland. The italic numbers in brackets refer to the list of referencesappended to this paper.

216

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BOOTH ON SOIL SPECIMENS FOR OEDOMETER TESTING

217

mine how variations in the conditions imposed during construction affect the subsequent settlement of the filling, particularly following wetting. The results of this investigation are being reported elsewhere [1,2]. The experimental work was carried out in the laboratory, using standard 76.2 by 19.05-mm oedometer specimens. These were loaded, partly saturated, and soaked, at a particular pressure, in the same way as the partly saturated half of the double oedometer test [3,4]. Several soils were tested, and variations were made in the initial dry density, the compaction moisture content, and the maximum applied pressure. For each combination of imposed conditions, a group (usually between 6 and 24 in number) of specimens was tested over the full range of saturation, including one or two under water to represent full saturation. Since this work related to road embankments, the oedometer tests were carried out on compacted soils. In order to make valid comparisons between the results, it was essential to devise a method of preparation that produced a group of specimens that were as close to identical as possible after compaction. Most specimens were loaded at a moisture content other than that used for compaction, and a conditioning procedure was necessary to allow increases or reductions in moisture content in a way that modelled, as closely as possible, such changes in the field. This paper describes the methods that were adopted for the compaction and conditioning of the oedometer specimens. It also considers the uniformity of specimens and groups of specimens in the light of the results obtained.

Choice of Compaction Method Several investigations into the merits of different compaction methods have been reported. Leonards [5], quoting Mayo [7], used only static compaction to prepare his specimens. A Proctor penetration needle was used to detect variations in unit weight in a 2.5-in. (63.5-mm) high specimen, compacted statically. It was found that a diameter of 10 in. (254 mm) was necessary in order to reduce side friction to the point where the variation in unit weight within a specimen was less than 0.5 percent. Dawson [7] measured variation in density within specimens compacted in layers by dynamic methods in standard 4 and 6-in. (101.6 and 152.4-mm, respectively) diameter molds. The average density in the bottom third of the mold exceeded that in the top third, by over 2 percent in some cases. Dawson cut his specimens carefully into a number of pieces to measure these variations. Whitman et al [8] found that static compaction methods gave substantial density variations within specimens, with a height to diameter ratio of unity. When this ratio was reduced to 1"2, the density variations ceased to be significant. Shackel [9] devised a nondestructive method of measuring density, using Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

218

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

gamma rays, and could thus measure densities every 0.4 in. (10.2 mm) along the axis of 4-in. (101.6-mm) diameter specimens compacted statically. He found significant variations in the density, which increased considerably when the height to diameter ratio rose above unity. Gau and Olson [10] compared dynamic, static, and kneading compaction by means of a miniature penetrometer and concluded that the static method gave the most uniform specimens, though kneading may be more representative of field conditions. They agreed with Leonards [5] that greatest uniformity is achieved by static compaction in one thin layer. The general conclusion reached is that static compaction gives the most uniform specimens, and it as decided therefore to use this to prepare the oedometer specimens. It is recognized nevertheless, that there may be differences in the fabric and engineering properties of specimens compacted by other methods [11]. Construction of Mold

The soils used for the oedometer tests were essentially sandy with a relatively small proportion of finer particles (Fig. 1). When cutting oedometer specimens from blocks of these soils, it was found that a good deal of lack of fit occurred at the edges. It was decided therefore to compact the soil directly into the oedometer ring and accept any slight variation in density due to side friction. There is also the risk of built-in stresses within the soil and, more particularly, between the soil and the ring, but silicone grease was used in an attempt to reduce these. The mold used for compaction of the specimens is shown schematically in Fig. 2. It consists of a cylinder of 76.2 mm internal diameter, into which a standard oedometer ring fits so that the inside faces are flush. Pistons can be inserted at each end of the mold and are provided with flanges to prevent them entering the oedometer ring. The mold splits on both vertical and horizontal planes to facilitate the removal of the ring. The vertical joint is necessary because otherwise even slight distortion of the ring during compaction could lock it in the mold. The horizontal joint is arranged so that the ring is still supported when the top half of the mold is removed, thus preventing the ring sliding off the specimen. Figure 3 shows the mold with one side removed to demonstrate the position of the ring. No drains were provided in this mold since all specimens were compacted well below the saturation moisture content. Air escapes between the piston and the mold, provided the former is not inserted too rapidly. Both the mold and the pistons were made of brass, chosen because of its low cost and ease of machining during the development period. A large number of identical oedometer rings were also made from brass. The brass has not been marked badly by the soil during compaction, but a harder surface would be advisable if high densities were being used. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

219

BOOTH ON SOIL SPECIMENS FOR OEDOMETER TESTING

SIEVE NUMBERS

I00 90 8O 7o

60 z

u_ z

50

30 zo

200

I00

I

I I I i!

60

I

I

2O

40

I

IO

.of SOIL Z - - ~

x

// //

/ /

j,1

0-002

0-006

0"02

PARTICLE

SIZE

/

/ ~S'OIL

I~ 1/ 7 6

0-06

0-2

Y

0"6

2

mm (log scole)

SOIL X

DECOMPOSED GRANITE

SOIL Y

DECOMPOSED SANDSTONE

FROM NORTH-EASTERN TRANSVAAL FROM

SOIL Z

DECOMPOSED

MELSETTER, RHODESIA

QUARTZITE

DURBAN

FIG. l--Particle size distribution o f soils tested in the oedometer.

Method of Compaction The mold was simple to use, and a group o f six specimens took about 2 h to make. The same sequence was used throughout. The soil was stored dry, having first been wet sieved through a 2 mm screen. Rather more dry soil than was actually necessary for the number o f specimens being made was weighed in a bowl o f known weight to an accuracy of 0.1 g. Water was added to give the required moisture content and worked in by hand until the operator was satisfied that it was distributed evenly. This was not difficult with the types of soil being used. The wet soil was reweighed to determine the exact moisture content and stored in an air-tight bag while the amount required for each specimen was calculated. The assembled mold was placed on the bench with the oedometer ring and bottom piston in place, the inside faces having been greased sparingly. The necessary amount o f soil, weighed to an accuracy of 0.01 g, was put into the mold as shown in Fig. 4. This was probably the step at which the greatest lack of uniformity occurred, and considerable care was essential to ensure an even Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

220

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

1--,o..o.o-1 2 - ~ i

'~L~.

-tl

",,

i

'

MOLD

~

~

MOLD SPL,'rs

\OEOOMETER R,NG IN PLACE

_L~

i .~,..~../BOTTOM

I

PtSTON

I

CROSS SECTION THROUGH MOLD AND PISTONS

MOLD SPLITS ON THIS LINE

PLAN ON MOLD All dimensions in mm

FIG. 2--Arrangement of mold for compaction of 76.2-ram-diameter by 19.05-ram-thick oedometer specimens.

distribution of soil. The top piston was inserted and pressed home to compact the soil into the ring, as in Fig. 5. At higher densities, it was helpful to maintain the compacting pressure for some time to reduce rebound, and probably this would be essential if less permeable clayey soils were being compacted. Removal of the ring and specimen from the mold was relatively easy. The top piston and top part of the mold were removed (as shown in Fig. 6) and a porous disk placed over the end of the ring. The whole assembly was inverted, so that the disk supported both the ring and specimen. The other piston and the rest of the mold were removed, leaving the specimen in its ring standing on the porous disk. (Each piston was given a slight twist before being pulled away from the soil to release fine particles adhering to the face.) Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BOOTH ON SOIL SPECIMENS FOR OEDOMETER TESTING

221

FIG. 3--Mold partly assembled, showing position of ring.

Adjustment of the Degree of Saturation The testing program required that a group of specimens, identical immediately after compaction, should be tested in the oedometers over the range of saturations. (It was found to be more convenient to work in terms of the initial degree of saturation, rather than in terms of moisture contents.) Specimens to be tested at 100 percent saturation were soaked throughout the test as in the standard consolidation test. All other specimens were loaded, partly saturated, at constant moisture content, as usual for the partly saturated half o f the double oedometer test [4,12]. Loads were applied at intervals in logarithmic steps up to the chosen maximum, 440 k N / m 2, for most groups of specimens. At the end of compression under this load, the oedometers were flooded and the collapse settlement measured. For those specimens where the selected degree of saturation was higher than at compaction, it was necessary to introduce additional moisture into the soil. Specimens were allowed to stand in a humidity room until they had a moisture content slightly above that required, and, with a little experience, it was possible to judge the length o f time needed to achieve this. The specimen was then transferred to a dry porous disk and treated in exactly the same way as those to to be dried immediately after compaction. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

222

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG.4--Mold assembled and containing loose soil readyfor compaction. The behavior of the specimens in the humidity room differed from soil to soil. In some soils, there was a slow increase in moisture content, suggesting that it was rising simply by condensation on the exposed surface. Other soils gained moisture rapidly and, if left in the humidity room, swelled quite considerably within 89 h. The top surface always took a domed shape, presumably because swelling radially was giving rise to frictional stresses between the soil and the ring. Specimens which swelled in this way were rejected without being tested. The drying part of the process took place in the air-conditioned laboratory. The specimen, on a porous disk, was allowed to stand in the open and dry slowly. The process was easy to control by weighing the specimen, including ring and porous disk, until the mass fell to that required. Possible shrinkage during drying was checked in many cases by measuring the thickness before and after the drying stage, but differences were never found in excess of 0.5 percent and were usually much less. When the required saturation had been reached, the specimen, still on its disk, was placed in a double polythene bag and stored in the humidity room. Where possible, this storage extended for at least four days to allow time for the moisture to distribute more equally within the specimen. Adjusting the moisture content means following different stress paths, which may affect the test results to some extent [13]. There is, however, no other way of bringing the specimens to different initial conditions, and the methods do, in fact, model changes of moisture content in the field. These are likely to occur most readily shortly after compaction, before the addition of more filling. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BOOTH ON SOIL SPECIMENS FOR OEDOMETER TESTING

223

FIG. 5 - - Top piston being inserted into mold to compact soil.

Comparison of Specimens The success of the method of compaction and conditioning can be considered in terms of the dry densities of groups of specimens, The actual dry density of each specimen immediately prior to testing was calculated, based on the thickness measured at the time and the mass of the dry soil at the end of the test. The mean dry density of each group was then determined, though this differed from that intended by up to 3 percent in extreme cases. As a generalization, the mean dry density achieved exceeded that intended when this was relatively low but, at higher densities, the reverse applied. Increases in the compaction moisture content gave only marginally higher dry densities. Specimens were assumed to be sufficiently close to identical if the dry density was within 1 percent of the mean for that group. A total of 506 specimens were made and conditioned, as described in this paper (a number of others were subjected to a cyclic conditioning procedure [1]). These were divided into 37 groups of between 5 and 72 specimens, though only 6 groups had more Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

224

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 6--Mold partly dismantled showing soil compacted into oedometer ring. than 12. Of these specimens, only 39 fell outside the 1 percent limits. Only 5 groups had more than 15 percent unsatisfactory specimens, and more than half the groups had none at all [1]. Specimens were also prepared from undisturbed samples taken in the field, both of natural residual soils and of embankment filling. Sixty-six specimens were cut from two natural soils and three embankment fills, of which 37 fell outside the I percent limits. The uniformity within individual specimens cannot be quantified, but, in purely subjective terms, the laboratory compacted specimens appeared to be much more homogeneous than those obtained from the field. Effect on Results

The test results are not discussed in any detail here, as this has been done elsewhere [1,2]. Sufficient data will, however, be presented to demonstrate that the results obtained were adequate for the study being carried out. The basic analysis was considered in terms of the collapse occurring on wetting as a function of the moisture content during loading, and it was found that curves could be plotted. Figure 7 shows such a plot for specimens cut from a naturally occurring collapsing sand from Durban. The scatter of points is considerable, though not such as to make the drawing of a curve impossible. Figures 8 and 9 are typical examples of the plots for groups of specimens compacted in the mold. Figure 8 shows a case where the scatter of points is at its worst, whereas Fig. 9 is one of the better plots obtained. In neither case does the scatter of results cause any problems in interpretation. It might be thought that the scatter of results in Fig. 8 was due to variations Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

BOOTH ON SOIL SPECIMENS FOR OEDOMETER TESTING

225

0"14 --

%

• 0"12 --

Z

COLLAPSING SAND FROM DURBAN

0

MEAN DRY DENSITY 1611kg/m 3

~< 0'~0- 13-

== ~

~J 0 ' 0 8 -

• X

c~

0"06-

X

~KK

~0

"04-

_Z

~

~

0"02

X•

--

• I

5

I

10

MOISTURE

x-x~-x-x-• 15

I

20

• I

25

x-x-x-I

30

CONTENT DURING LOADING (%)

FIG. 7-- Typical results obtained from specimens cutfrom undisturbed sample of residual soil. in density. In fact, none of the 24 specimens in this group fell outside the 1 percent limits. In Fig. 10, the initial dry density has been plotted against moisture content, with the collapse marked against each point. There is no pattern to relate particularly the variations in collapse to those in dry density. It was, nevertheless, found [1,2] that the initial dry density was a major factor in determining the amount of collapse on wetting. The range of dry densities existing in any group of specimens, however, does seem to have been small enough to have prevented excessive variations in the test results. Such variations as did occur can be probably ascribed to local fluctuations in density or particle size within the specimen. These fluctuations would have been caused during the filling of the mold, immediately prior to compaction. It is doubtful whether any method of compaction can avoid this, since all require some kind of mold to be filled. Variations in moisture content within the specimen may have also contributed.

Conclusions The paper has described a method for compacting and conditioning specimens prior to testing, partly saturated, in the oedometer. The static method of

C opyri ght by ASTM Int ' l ( a l l r i g h t s r e s e r v e d ) ; W e d De c 2 2 1 4 : 0 0 : 0 0 E S T 2 0 1 0 D ownloaded/printed by U ni vers ity of Britis h Co l u m b i a Li b r a r y p u r s u a n t t o Li c e n se A g r eem en t . N o f u r t h er r ep r o d u ct i o n s au

226

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

O-16 - -

0.14 x

•x X ~

RECOMPACTED DECOMPOSED GRANITE FILLING FROM NORTH-EASTERN TRANSVAAL X ~

u~ 0- 0-I0

_

MEAN DRY DENSITY 1575 kg/m 3

x ,XXxx

0.08 D 0 I--

z

0.06 - -

X

~o4

~ 0"02 Ilg

1

5

1

10

~--x-x-xl

15

20

~

I ~ l

25

30

MOISTURE CONTENT DURING LOADING (%)

FIG. 8--Relatively poor results obtainedfrom specimens made in the mold.

compaction has been shown, in the past, to give the most uniform specimens. The conditioning procedure for adjusting the moisture content models, as closely as practicable, the changes occurring in compacted earthworks. Measurements of the dry density of the individual specimens immediately prior to testing have been used to consider the uniformity of each group of specimens. The variations within a group of laboratory compacted specimens is considerably less than in those cut from natural or compacted soils in the field. The homogeneity of individual specimens is also, by visual observation, markedly better than in the in situ soils. The results obtained from testing these specimens are adequate to enable conclusions to be drawn. Such variations as there were in the collapse settlements measured cannot be related to the initial dry densities of the specimens. They are probably caused by unavoidable local variation in dry density or particle Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

0-04

z

-

~

%

MOISTURE

I

I0

I

5

~

.x-

,520

20

I

,~

kg/m 3

C O N T E N T DURING LOADING (%)

15

MEAN DRY DENSITY

RECOMPACTEO DECOMPOSED QUARTZITE MELSETTER, RHODESIA

25

I

FROM

50

~_~

o

154C

0

~ 5 5 0 i_

1560

xO'100

~o.io2

I

.

.

9

x0

MEAN

xO'O62

xO.046-

~o.o67

x0

~0 x0

I % ABOVE MEAN

x0.055

.

I 15

I

20 MOISTURE CONTENT DURING LOADING (%}

I

I0

~8

I 25

x0

xO

I 50

FIG. 10---Poor resultsreplotted in terms of the dry density of individual

5

.

~O'OB2 xO'050 0.080 x0"097 l % BELOW MEAN

.

COLLAPSE SHOWN AS REDUCTION IN VOIDS RATIO

x0.12.80ill x0"095 --x' "-~xO.O85

xO.I21

1570 - ).141

_

specimens.

o=

I-

g

1880

~890

1600 - -

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

FIG. 9--Consistent resultsobtainedfrom specimens compacted in the mold.

I-

g

0,02

O.OB

~

o

0.08

0.10

O.12 - -

~ Z

a

J

3

~ m

-,~ 0,r ,r

z

0'14

bO ",4

m

m -,L m

8

0 m

m z Go

o_

co "1o m

r-

0 Z

O~ 0 0 "1-

228

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

size occurring during compaction. It is doubtful if any other method can eliminate this problem.

Acknowledgments The work described in this paper was carried out at the National Institute for Road Research in Pretoria, South Africa and is published by permission of the director. The author would also like to thank Professor K. Knight of the University of Natal in Durban, South Africa, for his interest throughout this work.

References [1] Booth, A. R., "Collapse Settlement in Compacted Soils," Council for Scientific and Industrial Research, Research Report 324, National Institute for Road Research, Bulletin 13, Pretoria, South Africa, to be published 1976. [2] Booth, A. R. in Proceedings, 6th Regional Conference for Africa on Soil Mechanics and Foundation Engineering, Durban, 1975, Vol. I, pp. 57-63. [3] Jennings, J. E. B. and Knight, K. in Proceedings, 4th International Conference on Soil Mechanics and Foundation Engineering, London, 1957, Vol. I, pp. 316-319. [4] Knight, K., "The Collapse of Structure of Sandy Subsoils on Wetting," Ph.D. thesis, University of the Witwatersrand, Johannesburg, South Africa, 1961. [5] Leonards, G. A. "Strength Characteristics of Compacted Clays," Proceedings, American Society of Civil Engineers, Vol. 79, Paper 360, 1953. [6] Mayo, R. I. "Compression Tests on Stabilized Soil Mixtures," M.Sc. thesis, Purdue University, Lafayette, Ind., 1939. [7] Dawson, R. F. in Papers on 1959 Meetings Soils, ASTM STP 254, American Society for Testing and Materials, 1959, pp. 308-317. [8] Whitman, R. V., Roberts, J. E. and Man, S., "One Dimensional Compression and Wave Velocity Tests, and Responses of Soil to Dynamic Loads," Report 4, Publ. 106, Soil Engineering Division, Department of Civil and Sanitary Engineering, Massachusetts Institute of of Technology, Cambridge, Mass., 1960. [9] Shackel, B., Australian Road Research, Vol. 4, No. 5, 1970, pp. 12-31. [10] Gau, F. L. and Olson, R. E., Journal of Materials, Vol. 6, 1971, pp. 874--888. [11] Kirkpatrick, W. M. and Rennie, I. A. in Proceedings, International Symposium on Soil Structure, Gothenburg, 1973, pp. 103-111. [12] Jennings, J. E. B. and Knight, K., Transactions, South African Institute of Civil Engineers, Vol. 6, 1956, pp. 255-256. [13] Barden, L., Madedor, A. O. and Sides, G. R., Journal of Soil Mechanics and Foundations Division, American Society of Civil Engineers, Vol. 95, 1969, pp. 33-51.

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T. Y. Chu' and S. N. Chen 2

Laboratory Preparation of Specimens for Simulating Field Moisture Conditions of Partially Saturated Soils

REFERENCE: Chu, T. Y. and Chen, S. N., "Laboratory Preparation of Specimens for Simulating Field Moisture Conditions of Partially Saturated Soils," Soil Specimen Preparation for Laboratory Testing, ASTM STP 599, American Society for Testing and Materials, 1976, pp. 229-247. ABSTRACT: Due to the significant effect of soil moisture on the behavior characteristics of a soil, one of the primary requirements in the preparation of specimens of a partially saturated soil is to simulate the suction in the soil under prevalent field conditons. This objective may be achieved by the conditioning or pretesting treatment of a soil specimen, so that a desired suction in the specimen is attained prior to testing. Relatively simple equipment such as that described in this paper, is suitable for the required treatment of specimens representing partially saturated soils. This paper presents results of undrained triaxial tests and repeated loading tests on soil specimens treated by using the equipment mentioned previously. Test data from specimens without any suction treatment are presented for comparison purposes. Information obtained from these experiments indicates that the simulation of the field moisture condition of a soil cannot be achieved simply by reproducing the field moisture content of the soil. The conditioning or pretesting treatment of soil specimens by using apparatus such as that presented in this paper is believed to be a preferable method for the purpose of simulating field moisture conditions.

KEY WORDS: soils, subgrades, unsaturation, specimens, treatment, soil, water, suction, triaxial tests

In consideration of the significant effect of soil moisture on the behavioral characteristics o f a soil, it is evident that a primary requirement in the preparation o f specimens representing the soil mass at a particular site is to simulate the field moisture conditions of the soil. The term moisture condition, as used in this paper, refers to all factors related to the water in a soil mass, such as the positive or negative pore water pressure, the ] Professor of engineering, University of South Carolina, Columbia, S.C. 29208. 2 Soil engineer, Tippetts-Abbett-McCarthy-Stratton Engineers and Architects, New York, N.Y. 10022. 229 Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 9 Downloaded/printed by by ASTM International www.astm.org Copyright 1976 University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

230

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

water content, and the degree of saturation. In the case of saturated soils, the objective of simulating field moisture conditions is often achieved by saturating specimens of desired density and soil structure with water, as well as by the application of a back pressure to the soil water, if necessary. On the other hand, laboratory preparations of specimens for simulating the field moisture conditions of unsaturated or partially saturated soils are sometimes rather complicated, due to the need for elaborate equipment and time-consuming procedures. This paper presents a relatively simple apparatus which has been developed primarily for the aforementioned objective in the case of partially saturated soils. The procedures involved are also comparatively simple. While the test results presented in this paper are based on the use of remolded specimens only, the developed equipment and procedures may be applied equally well in the preparation of undisturbed soil specimens prior to laboratory testing. The developed equipment and procedure for specimen treatment prior to laboratory testing may be applied for determining the behavior characteristics of partially saturated soils, as required in the design of pavements, foundations, and different types of retaining and earth structures. For instance, in the design of pavement structures, it is desirable to determine the subgrade characteristics at the anticipated field moisture conditions. This objective may be achieved by using the developed method for specimen treatment prior to subgrade evaluation tests. At the present time, the commonly used methods for specimen treatment in connection with subgrade evaluation tests include the saturation of test specimens by evacuation or complete immersion in water and the capillary absorption by the Texas procedure, as discussed elsewhere in this paper. In previous studies [1--4],3 however, subgrade soils below pavements were seldom found to be saturated or close to saturation, as represented by specimens treated by the methods mentioned previously. The findings from these studies indicate that, in areas of relatively high ground-water table, subgrade moisture variations may be predicted, according to the equilibrium suctions of the subgrade soils, provided that frost actions are not expected to occur in the subgrade. For this reason, the developed method for specimen treatment presented in this paper appears to be a preferable procedure in comparison with other methods for treating subgrade specimens prior to testing. Review of Current Methods for Pretesting Treatment In the laboratory testing of remolded or compacted soil specimens, the molding moisture conditions are usually different from those during ~The italic numbers in brackets refer to the list o f references appended to this paper.

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CHU AND CHEN ON PARTIALLY SATURATED SOILS

231

testing. Pretesting treatments of such specimens are, therefore, an essential step in the preparation of specimens for laboratory testing. The commonly used methods for pretesting treatment are described hereinafter. Many of these methods were developed in connection with the evaluation of subgrade soils, as required in the design of pavement structures.

Kansas Method In the Kansas method [5] for laboratory preparation prior to testing, soil specimens are saturated with water by evacuation. In view of the fact that subgrade soils below pavements usually are not saturated to the same extent as that of the laboratory specimens prepared according to the procedure just mentioned, correction factors, such as those formulated in the Kansas method for pavement design, are to be used in the application of test results for pavement design purposes.

California Bearing Ratio Test Method The California Bearing Ratio (CBR) test described in the ASTM Test for Bearing Ratio of Laboratory-Compacted Soils (D 1883-67, 1967) is used by the Corps of Engineers and many state highway departments for subgrade evaluation in connection with pavement design. In this test method, remolded soil specimens are soaked in water, under a simulated surcharge pressure, for four days prior to testing. Due to the application of a surcharge and the confinement of the soil specimen in the mold during soaking, the four-day soaking normally does not provide full saturation of the soil. Nevertheless, the degree of saturation of the soaked specimen might be substantially higher than that of the subgrade soil under the proposed pavement.

Texas Method The basic procedure for pretesting treatment in the Texas method [6] is to allow water to move upward into a remolded soil specimen by capillary absorption. The capillary flow of water is accomplished by keeping the soil specimen (resting on a porous stone) at 0.5 in. above the water level in a pan provided in the treatment assembly. While the Texas method may be considered as an improvement of the other methods described previously, insofar as the simulation of field moisture conditions of subgrade soils is concerned, it appears that the particular setup might be satisfactory in simulating the subgrade moisture conditions at certain sites and might not be satisfactory for the same purpose at other locations. In other words, some flexibility in the treatment assembly is Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

232

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

believed to be necessary in order to make it possible to achieve the objective of close simulation of the field moisture conditions of the soils.

Other Methods Seed et al [7,8] developed a method for preparing and conditioning soil specimens to simulate closely the field condition of a soil with respect to its density, soil structure, and degree of saturation. Using their method, specimens are prepared either by kneading compaction at a moisture content "dry of optimum" or by static compaction "wet of optimum." In either case, the compacted specimens are soaked to the desired degree of saturation. It was reported that the procedure of using kneading compaction dry of optimum and subsequent soaking would require considerable time and effort in achieving the desired degree of saturation. The soil water suction of a partially saturated soil specimen is not controlled or measured in this method for specimen preparation and conditioning. If it is desirable to control the soil water suction of a partially saturated soil specimen during pretesting treatment, apparatus such as those developed by Escario [9] and Alpan [10] for conducting swelling tests of expansive clays might be suitable, provided that necessary modifications are made in regard to the equipment and the related procedures. The use of these elaborate apparatus, however, is expected to require considerable time and effort in carrying out the desired treatment, prior to testing. For this reason, one of the considerations in developing the new method for pretesting treatment, as described herein, is to make the equipment and procedures relatively simple, so that this method may be applicable for routine tests in connection with subgrade evaluation or for other purposes.

Development of Equipment and Procedures for Pretesting Treatment The effect of soil water suction on the behavior characteristics of a partially saturated soil has been emphasized by many investigators. Recognizing the importance of this factor in laboratory testing, Burland [11] and Aitchson [12] suggested that one of the primary requirements in the preparation of specimens of any partially saturated soil should be the simulation of the soil water suction under field conditions. This is also the basic concept in developing the equipment and procedures described in this paper. It is to be noted that, in order to make the equipment as simple as possible, a suction plate device is included in the treatment assembly. The limitation of this device is the relatively low suction level (not exceeding a suction of 3.0 pF) that can be applied during treatment. The use of a complex equipment, such as an apparatus incorporating a pressure membrane device, is necessary for the application of relatively Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

CHU AND CHEN ON PARTIALLY SATURATED SOILS

233

high suction levels during treatment. As mentioned previously, it is expected that pretesting treatment using a rather complicated equipment requires considerable time and effort. The discussions presented herein, will refer only to the simple equipment and procedures developed from this study. A schematic diagram of the developed equipment is presented in Fig. 1. As shown in this figure, a soil specimen, which is 2 in. in diameter and 4 in. high, is placed between the upper and lower cylinder units (denoted as No. 7 in the figure). These units are connected to the tubing system and the graduated glass tube. Figure 2 shows the base chamber, including a ceramic plate at the top of the chamber. The device for providing a desired lateral pressure on the soil specimen under treatment is similar to the axial cell used in a capillary wetting assembly suggested by Hunt [13]. The surcharge load on the specimen is provided by placing proper weights on the upper cylinder units. This unit is free to move in a vertical direction. While the lateral pressure and the surcharge on the soil specimen may be adjusted to provide any desired value, the value of 1.0 psi was used as both the lateral and the surcharge pressure in treating all soil specimens prior to the tests reported in this paper. The desired suction in the soil water for specimen treatment is applied by adjusting the water level in the graduated glass tube shown at the lower right corner of Fig. 1. In other words, the depth of the water level below the center of the specimen is a variable which may be determined in accordance with the desired suction during the treatment. In this respect, one will note that other expressions such as tension, capillary potential, and negative pore water pressure, together with soil water suction or suction, have been used by various investigators in relating the stress levels of the water in partially saturated soils. These terms are sometimes defined in a slightly different manner. Discussions of these expressions and methods for the conversion from one expression to another are presented in publications by Croney and others [14,15]. It is not the intention of this paper to suggest specific definitions of these terms and to discuss other details. In accordance with the information given in the references indicated previously, it may be stated that, in the case of nonplastic soils, the depth of the water level (in the graduated tube) below the center of the specimen represents the applied tension or suction during treatment. For plastic soils, Croney's method [14] may be used to compute the required depth of the water level for a desired suction, if the recommended definitions of the various terms given in the aforementioned references are to be followed. An alternative [16] for providing the desired suction is to use mercury instead of water in the plastic tubing and the glass tube, as mentioned previously. This method for providing the desired suction would be required if it is substantially higher than 3 ft of water. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

234

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

arent c tubing

~ / - - - Graduated >~ _ _ ~ g ~ lass tube

Legend: i. 2-in. dia, 4-in. high soil specimen. 2. Ceramic plates. 3. Filter papers. 4. Rubber membrane. 5. Cylinder for providing lateral pressure on the soil specimen. 6. Rubber covers. 7. Chambers filled with de-aired water. 8. Transparent plastic cylinder. 9. Guide to keep the upper unit in proper position. i0. Cover to prevent evaporation. ii. Ring to provide a pool of water during preparation for specimen treatment. 12. Connection for applying desired lateral pressure.

p l a s t i c tubing

FIG. l--Setup for pretesting treatment of a soil specimen to simulate field moisture conditions.

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CHU AND CHEN ON PARTIALLY SATURATED SOILS

235

FIG. 2--Photograph showing the base chamber of the apparatus for treating specimens of partially saturated soils.

As shown in Fig. 1, the movement of water into or from the soil specimen is through the ceramic plates and the two pieces of filter paper in contact with the top and bottom of the specimen. The purpose of allowing flow to occur through both the top and bottom of the specimen is to reduce the time required for completing the treatment. The rubber covers (denoted as No. 6 in the figure) near the top and bottom of the specimen are provided to minimize the loss of moisture from the specimen due to evaporation. The treatment assembly illustrated in Fig. 1. is suitable for either undisturbed or remolded soil specimens. While remolded soil specimens may be prepared by any desired method of compaction, all specimens reported in this paper were compacted by using a drop hammer, except for a limited number of specimens that were prepared by kneading compaction. The procedure for the pretesting treatment of partially saturated soil specimens by the developed equipment is outlined in the following paragraphs. 1. Prepare soil specimens at the desired density and moisture content. After a designated waiting period (24 h used in this study), the specimen is ready for treatment. 2. Prepare the treatment assembly by saturating the ceramic plates and filter papers and by filling the upper and lower chambers with deaired water. The valves, which are marked as V-l, V-2, and V-3 in Fig. 1, are provided for the purpose just mentioned. In addition, a pool of water Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

236

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

above the lower ceramic plate may be created by pouring water into the area within the ring (marked as No. 11 in Fig. 1). The purpose of providing this pool of water is to assure that good contact exists among the specimen, the filter paper, and the ceramic plate. Any air entrapped in the contact areas may interfere with the successful completion of the intended treatment. 3. Place the specimen in the treatment assembly and apply the surcharge, as well as the confining pressure (lateral pressure), on the specimen. At the same time, apply the desired suction by adjusting the water level in the graduated glass tube. If mercury is used instead of water, as mentioned previously, adjust the mercury column to provide the desired suction. 4. When a moisture equilibrium condition is reached (as indicated by a nearly constant water level in the graduated glass tube over a period of time), remove the specimen from the treatment assembly. The height, diameter, and weight of a specimen are to be determined before and after the treatment to obtain the necessary data for computing the change in density, moisture content, and the degree of saturation. The treated specimens may be used for any desired laboratory test, such as the undrained triaxial compression test and the repeated loading test. Both types of test were performed in this study. Test Results and Discussion The physical properties of the soils used in this study are summarized in Table 1. Due to the fact that this study is related to part of a research project concerning the investigation of subgrade moisture conditions below pavements in South Carolina, the sampling locations are identified according to a numbering system used in that project. General information regarding the subgrade moisture investigation project was reported by Chu et al in a separate paper [3]. The samples from Sites 57 and 58 are micaceous soils obtained from the Piedmont region; the one from Site 80 is a silty sand obtained from the coastal plain area of South Carolina. Table 2 presents all data concerning pretesting treatment, as well as the dry density, moisture content, and the degree of saturation for both the as-molded condition and the condition before testing. The 3 and 1 ft suction levels for specimen treatment, as indicated in this table, were provided by using the equipment illustrated in Fig. 1. On the other hand, the 2.5 in. suction level for treatment was attained by using a device somewhat similar to the assembly for the capillary wetting of the soil specimens, as employed in the Texas method [6]. In other words, the specimen was placed on a filter paper above a porous stone, and the water level in a pan (holding the porous stone) was maintained at 0.5 in. below the bottom of

C opyri ght by ASTM Int ' l ( a l l r i g h t s r e s e r v e d ) ; W e d De c 2 2 1 4 : 0 0 : 0 0 E S T 2 0 1 0 D ownloaded/printed by U ni vers ity of Britis h Co l u m b i a Li b r a r y p u r s u a n t t o Li c e n se A g r eem en t . N o f u r t h er r ep r o d u ct i o n s au

98 ... 100

57 58 80

75 95 58

40

liquid limit. plastic limit. plasticity index. nonplastic. 157 N / m L

95 100 97

10 24 63 18

200 ...... 43 ......

LL

37

PL

Atterberg Limits

NP 10 NP

PI A-2-4 A-5(6) A-2-4

AASHTO Classification 2.74 2.74 2.65

Specific Gravity

101.2 97.7 120.5

Max Dry Density, lb/ft 3

17.5 23.5 9.3

Optimum Moisture Content, %

Laboratory Compaction Test a

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

aASTM Tests for Moisture-Density Relations of Soils, Using 5.5-1b Rammer and 12-in. Drop (D 698-70).

NOTES-- LL = PL = PI = NP = I lb/ft 3 =

4

Site No.

Percent Passing Sieve No.

TABLE 1--Physical properties o f soils.

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o

m E~

60

I'I" -<

-4

"o

0 Z

0 -1111 z

z E~

0 I Cl

i ft 2.5 in. 3 ft 1 ft 2.5 in.

3 ft 1 ft 2.5 in. 3 ft

2.5 in. no treatment

1 ft

no treatment 3 ft

96.6 95.5 95.5 95.0 95.4 95.0 94.5 93.6 93.2 93.3 93.2 98.0 98.0 98.0 98.0 121.0 121.0 121.0

Dry Density, lb/ft 3 17.6 17.8 17.8 17,8 17.8 17.8 23.6 23.6 23.5 24.0 23.5 24.0 24.0 24.0 24.0 8.8 8.8 8.8

Moisture Content, % 62 62 62 61 62 61 80 79 77 78 78 89 89 89 89 65 65 65

Degree o f Saturation, 07o 96.6 91.3 91.5 89.2 90.8 86.0 94.5 93.6 91.8 90.7 87.3 94.6 94.5 94.0 93.5 120.8 120.5 120.0

Dry Density, lb/ft 3 17.6 22.8 22.7 26.5 24.5 31.5 23.6 23.6 26.9 28.5 31.0 27.2 26.8 28.4 28.5 9.0 9.4 11.2

Moisture Content, % 62 71 72 80 76 88 80 79 86 89 88 92 91 95 95 66 67 79

Degree o f Saturation, 07o

Condition before Testing

1100 670 700 490 520 310 2490 2780 1300 830 670 1500 1800 1100 600 2280 1600 930

Modulus of Elasticity, psi

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I lb/ft 3 = 157 N / m 3, 1 psi = 6900 N / m 2, l ft = 0.3048 m. 1 in. = 0.0254 m, Specimens 58-1 through 58-5 have relatively low densities. Specimens 58-6 through 58-9 have relatively high densities.

16.5 15.3 13.0 9.5 11.2 8.0 29.5 28.0 15.8 8.4 6.2 24.0 28.2 12.2 10.0 22.8 18.5 11.8

Maximum Deviator Stress, psi

N O T E S - - T h e specimens are numbered in such a way that the first two n u m b e r s are referring to the site where the soil samples were obtained.

57-1 57-2 57-3 57-4 57-5 57-6 58-1 58-2 58-3 58-4 58-5 58-6 58-7 58-8 58-9 80-1 80-2 80-3

Specimen No.

Suction Level in Pretesting Treatment

As-molded Condition

T A B L E 2--Summary of data from undrained triaxial tests with 10 psi confining pressure.

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CHU AND CHEN ON PARTIALLY SATURATED SOILS

239

the specimen. For specimens 4 in. in height, as used in this study, the average tension in the specimen is 2.5 in. For the low plasticity or nonplastic soils used in this study, there is little or no difference between the tension or the suction of these soils. The time required for completing the treatment of all specimens employed in this study was less than one week. As indicated in Table 2, four groups of specimens having similar initial dry densities and moisture contents (see the columns under "Asmolded Condition" in this table) were prepared in this series of experiments. According to the density and moisture data in the columns under "Condition before Testing," substantial swelling of the specimens prepared by using soils from Sites 57 and 58 occurred during the treatment. The appreciable volume change during treatment is believed to be primarily due to the micaceous composition of the soils. If it is desirable to minimize the volume change of a specimen during the treatment, extremely high surcharge and lateral pressures would have to be applied on the specimen. The use of extremely high pressures, however, will result in an increase in the time needed to complete the treatment. Furthermore, the high degree of confinement of a specimen during treatment is likely to cause a less uniform condition with respect to the density and moisture content within the specimen (after the treatment) than that found when a relatively low pressure level is maintained during the treatment. The modulus of elasticity and the maximum deviator stress determined by undrained triaxial tests are also presented in Table 2. As expected, the test data indicate that, in the case of soil specimens subjected to pretesting treatment, an increase in the suction level maintained during treatment results in a corresponding increase in the modulus of elasticity and the maximum deviator stress of the specimen. This general trend of variation is true for all the samples tested. It is to be noted that, for soils from Sites 57 and 58, the triaxial test data obtained from specimens without treatment are included in Table 2. On the basis of the data in regard to the degree of saturation of the soil specimens before testing and the information from supplemental experiments, including tensiometer tests [17] and pressure plate extractor tests [3], the suction in any of the specimens without treatment was found to be higher than those in the treated specimens. For this reason, the modulus of elasticity and the maximum deviator stress of the specimens tested without treatment were found to be higher than those of the treated specimens. A comparison of stress-strain curves of both treated specimens and the one without treatment, prepared by using the soil from Site 58, is presented in Fig. 3. In addition to the undrained triaxial compression tests described previously, specimens with or without treatment were employed in repeated loading triaxial tests. Figure 4 illustrates the sinusoidal loading pattern

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240

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

35

I

I

I

!

s

30

I

l

Specimen No. 58-2 ( no treatment )

/."- O"b'-.,O,," !

25 i I I

? e~

!

20

| I I

if}

X----Specimen No. 58-3 ~ ( 3-ft suction )

9 Specimen No. 58-4 ( l-ft suction )

~

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i0

/~ ~/

/

~

Specimen No. 58-5 ( 2.5-in. suction )

..O~'"_O------O----~-...43

/

0

i

2

3

4

5

6

7

Axial Strain, percent

FIG. 3--Effect o f pretesting treatment on the stress-strain characteristics o f soil specimens determined by undrained triaxial tests.

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CHU AND CHEN ON PARTIALLY SATURATED SOILS

241

I I I I I I I I I I I I I I 'g

5

"~~~0.550"~60

oso

I

Resilient strain

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Time(onesecondpergrid) FIG, 4--Illustration o f the pattern in stress and strain variations in a repeated loading triaxial test (Specimen 58-17, l lO-th through l l4-th load applications).

with a frequency of one cycle in every 3 s, as used in the repeated loading tests conducted in this study. The applied maximum deviator stress during each test was 8 to 9 psi. The variations in the resilient and residual strains of a specimen, due to repeated load applications, are also illustrated in Fig. 4. Table 3 presents a summary of all data obtained from the repeated loading tests on specimens prepared by using the soils from Sites 57 and 58. The resilient modulus of a test specimen, as indicated in this table, was determined by dividing the difference between maximum and minimum deviator stresses by the corresponding resilient strain. Similar to the trend of variation observed from the data presented in Table 2, the test results of Specimens 57-7 through 57-9 and 58-10 through 58-13, as given in Table 3, show that the resilient modulus of the specimen tested without treatment is higher than those of treated specimens of the same soil. On the other hand, the residual strains of the specimens tested without treatment are lower than those of the treated specimens. In a comparison of the data of Specimens 58-11 through 58-13 (compacted by a drop hatmner) with those of Specimens 58-14 through 58-16 (compacted by a kneading compactor), it is noted that while all the specimens have similar as-molded conditions with respect to dry density and moisture content, Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

drop

hammer

99.0 99.0 99.0 98.1 97.1 97.2 97.0 97.5 97.3 97.0 93.5 93.2 94.0 93.2

Dry Density, lb/ft j 17.0 17.8 17.6 24.5 24.8 24.8 24.8 24.8 24.8 24.8 27.5 27.5 27.5 27.5

Moisture Content, 070 99.0 95.0 95.0 98.1 94.0 93.7 93.3 96.5 96.5 96.3 93.5 93.2 94.0 93.2

Dry Density, lb/ft ~ 17.0 23.4 23.4 24.5 27.4 27.8 27.5 26.3 26.2 26.2 27.5 27.5 27.5 27.5

Moisture Content, % 65 76 76 91 92 93 91 94 93 93 91 91 92 91

Degree of Saturation, 070

Condition before Testing

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65 68 67 91 90 90 90 91 91 90 91 91 92 91

Degree of Saturation, 070

As-molded Condition

157 N/m 3, 1 ksi = 6.9 x 106 N / m L

no treat n~ent

N O T E - - I ft = 0.305 m, I lb/ft 3 :

58-19 58-20

compactor

3 ft

kneading

58-17 58-18

3 ft

no treatment

3ft

no treatment

Suction Level in Pretesting Treatment

drop hammer

drop hammer

57-7 57-8 57-9 58-10 58-11 58-12 58-13 58-14 58-15 58-16

Equipment for Specimen No. Compaction

4.80 5.15 0.35 1.55 1.70 1.90 0.90 0.75 1.20 0.55 0.60 0.80 0.63

1.33

Residual Strain, 070

11.0 10.6 24.3 12.1 12.9 16.0 17.8 16.6 17.4 20.0 21.0 20.5 19.1

12.9

Resilient Modulus, ksi

NLA = 100

5.05 5.28 0.38 1.70 1.80 2.15 1.10 0.85 1.26 0.59 0.64 0.93 0.65

1.48

Residual Strain, 070

13.7 11.7 30.6 21.6 20.5 21.0 22.8 24.3 24.3 27.4 27.6 25.8 25.0

15.7

Resilient Modulus, ksi

NLA = 10000

Test Date at Number of Load Applications (NLA) Shown

T A B L E 3--Summary of data from repeated loading triaxial tests with 10 psi confining pressure.

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--4 0 -rl "< --4 m

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CHU AND CHEN ON PARTIALLY SATURATED SOILS

243

the difference in the method of compaction in preparing these two groups of specimens results in some difference in the dry densities and moisture contents after the pretesting treatment. The repeated loading test data are also different between these two groups of specimens. While it is not the intention of this paper to cover the effect on soil behavior due to the differences in the method of compaction, the test data of Specimens 58-11 through 58-16 are presented primarily to illustrate the fact that the use of the same suction level in pretesting treatment might result in different dry densities and moisture contents of treated specimens if different methods of compaction were used in specimen preparation. The discrepancies just mentioned are evidently related to the difference in the type of soil structure in the test specimens, due to the deviation in the method of compaction. Specimens 58-17 through 58-20 were prepared in such a manner that the dry densities and moisture contents under the as-molded condition are similar to those of Specimens 58-11 through 58-13 after the treatment with 3 ft suction. In other words, the dry densities and moisture contents of these two groups of specimens before testing are very close to each other. In view of the fact that Specimens 58-17 through 58-20 were tested without treatment, it is obvious that any difference in the repeated loading test data of these two groups of specimens is due primarily to the discrepancy with respect to the suction in the soil specimens. A comparison of the test data of these specimens may be made by referring to Table 3 as well as Figs. 5 and 6. The relatively high resilient moduli and comparatively low residual strains of Specimens 58-17 through 58-20, as shown in the table and illustrated by the figures, are again due to the fact that the specimens tested without treatment have a higher suction than that of the treated specimens of similar density and moisture content. As mentioned previously, the high suction values of the specimens tested without treatment were determined by supplemental experiments, including tensiometer and pressure plate extractor tests. Furthermore, it is recognized that remolded specimens of partially saturated soils normally would have suction levels much higher than those under the actual field conditions. Consequently, the simulation of the field moisture condition of a soil cannot be achieved simply by reproducing the field moisture content of the soil. For this reason, the conditioning or pretesting treatment of soil specimens by applying the desired suction is believed to be a preferable method for the purpose of simulating field moisture conditions. Information concerning the application of the developed method for the conditioning of subgrade soil specimens prior to laboratory testing for pavement design purposes was presented in other reports [3,4]. General Conclusion Results from undrained triaxial tests and repeated loading tests on Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authoriz

2.2 i0

2.0

1.8

1.6

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I 10,000

Specimen No. 58-13

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~

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5--Effect o f the pretesting treatment on the residual strain o f soil specimens.

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il

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""" "'"" .......

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Note:

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No.

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Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

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246

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

partially saturated soils presented in this paper indicate that the soil water suction in a test specimen is an important factor affecting the stressstrain characteristics of the soil. Consequently, one of the primary requirements in the preparation of specimens of a partially saturated soil is to simulate the suction in the soil under prevalent field conditions. To satisfy this requirement, it is necessary to include the step of conditioning or pretesting treatment during the preparation of soil specimens for laboratory testing. The method of treatment as described in this paper is believed to be suitable for the aforementioned purpose, provided that the desired suction level during treatment is relatively low. If extremely high suction levels are to be maintained during treatment, apparatus and procedures much more complicated than those reported in this paper would be required.

Acknowledgments The investigations in this study were conducted at the University of South Carolina and were mostly in connection with a subgrade moisture research project sponsored by the South Carolina State Highway Department and the Federal Highway Administration. The authors wish to express their appreciation to all who have assisted in the research project and to some of the staff members in the soils department, TippettsAbbett-McCarthy-Stratton Engineers and Architects, New York, for their assistance during the preparation of this paper. References [1] Aitchison, G. D. and Richards, B. G. in Moisture Equilibria and Moisture Changes in Soils Beneath Covered Areas, Butterworth, Australia, 1965. [2] Russam, K., "Subgrade Moisture Studies by the British Road Research Laboratory," Highway Research Record No. 301, 1970, pp. 5-17. [3] Chu, T. Y., Humphries, W. K., and Chen, S. N. in Proceedings, 3rd International Conference on the Structural Design of Asphalt Pavements, 1972, pp. 53-66. [4] Chu, T. Y. et al, "Soil Moisture as a Factor in Subgrade Evaluation," to be published in Conference Proceedings, American Society of Civil Engineers Pavement Design Specialty Conference, 1975. [5] Kansas State Highway Commission, "Design of Flexible Pavements Using the Triaxial Compression Test," Bulletin 8, Highway Research Board, 1947. [6] McDowell, C., "Road Test Findings Utilized in Analysis of Texas Triaxial Method of Pavement Design," The AASHO Road Test, Proceedings of a Conference held May 1962, St. Louis, Mo., Special Report 73, Highway Research Board, 1962. [7] Seed, H. B., Chan, C. K., and Lee, C. E. in Proceedings, 1962 International Conference on Structural Design of Asphalt Pavements, University of Michigan, Ann Arbor, Mich., 1963, pp. 611-636. [8] Monismith, C. L., Seed, H. B., Mitry, F. G., and C'han, C. K. in Proceedings, Second International Conference on the Structural Design of Asphalt Pavements, 1968, pp. 109-140. [9] Escario, V. in Proceedings, Second International Research and Engineering Conference on Expansive Clay Soils, 1969, pp. 207-217.

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CHU AND CHEN ON PARTIALLY SATURATED SOILS

247

[10| Alpah, I. in Proceedings, 4th Imernational Conference Soil Mechanics and Foundations Engineering, Vol. I, 1957, p. 3. [11] Burland, J. B., Moisture Equilibria and Moisture Changes in Soils Beneath Covered Areas, Butterworths, Australia, 1965, p. 270. [12] Aitchison, G. D. and Richards, B. D. in Proceedings, Second International Research and Engineering Conference on Expansive Clay Soils, 1969, pp. 66-84. [13] Hunt, J. E. in Special Procedures for Testing Soil and Rock for Engineering Purposes, A S T M STP 479, American Society for Testing and Materials, 1970, pp. 192-197. [14] Croney, D., Coleman, J. D., and Black, W. P. M., "Movement and Distribution of Water in Relation to Highway Design and Performance," Special Report 40, Highway Research Board, 1958, pp. 226-252. [15] Aitchison, G. D. et al in Moisture Equilibria and Moisture Changes in Soils Beneath Covered Area, Butterworths, Australia, 1965, p. 7. [16] Chu, T. Y. and Mou, C. H. in Proceedings, Third International Conference on Expansive Soils, 1973, pp. 177-185. [17] Taylor, S. A., Evans, D. D., and Kemper, W. D., "Evaluating Soil Water," Bulletin 426, Agricultural Experiment Station, Utah State University, Logan, 1961, pp. 29-33.

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R. T. Donaghe' and F. C. Townsend'

Scalping and Replacement Effects on the Compaction Characteristics of Earth-Rock Mixtures

REFERENCE: Donaghe, R. T. and Townsend, F. C., "Scalping and Replacement Effects on the Compaction Characteristics of Earth-Rock Mixtures," Soil Specimen

Preparation for Laboratory Testing, ASTM STP 599, American Society for Testing and Materials, 1976, pp. 248-277. ABSTRACT: This investigation examines the validity of a scalping and replacement procedure used by many laboratories to determine compaction characteristics of earth-rock mixtures containing oversized particles. Two commonly used methods of computing densities of full-sized specimens, based upon results from tests performed on minus No. 4 fractions of the actual total sample, were examined. Compaction tests were performed on full-scale and scalped and replaced specimens, in which both gravel and fines content were varied. The test results indicate that the scalping and replacement procedure results in significantly lower maximum dry unit weights and higher optimum water content than are obtained for full-scale specimens. The use of the theoretical methods provided better approximations of experimental results on full-scale specimens having gravel contents up to 70 percent than did the relationships developed using the scalping and replacement procedure. For gravel contents above 70 percent, better approximations were obtained using the scalping and replacement procedure. KEY WORDS: soils, compaction, density, moisture content, earth-rock mixtures L a b o r a t o r y tests t o d e t e r m i n e c o m p a c t i o n characteristics o f e a r t h - r o c k m i x t u r e s for use in field c o n t r o l h a v e b e e n subject t o q u e s t i o n for m a n y years. D u e t o l i m i t a t i o n s o f e q u i p m e n t size, l a b o r a t o r y tests a r e g e n e r a l l y p e r f o r m e d o n s m a l l s p e c i m e n s , thus p l a c i n g a limit o n t h e m a x i m u m d i a m e t e r o f t h e particles w h i c h c a n b e used in the specimens. M a n y l a b o r a t o r i e s , i n c l u d i n g t h o s e o f t h e C o r p s o f Engineers, scalp o v e r s i z e d particles o f full-scale s p e c i m e n s a n d r e p l a c e t h e particles with an equal p e r c e n t a g e , b y weight, o f smaller p a n i c l e s , a s s u m i n g t h a t results f r o m tests p e r f o r m e d in small m o l d s o n such m a t e r i a l are c o m p a r a b l e to t h o s e U.S. Army civil engineering technician and research engineer, respectively, Soils Research Facility, Waterways Experiment Station, Vicksburg, Miss. 39180. 248 Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by by ASTM International www.astm.org Copyright9 1976 University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

DONAGHE AND TOWNSEND ON SCALPING AND REPLACEMENT

249

obtained from tests performed on full-scale specimens in large molds) Other laboratories perform compaction tests on the minus No. 4 fraction of soils in small molds and then apply theoretical correction factors, based on the influence of gravel on compaction characteristics, to compute densities of the full-scale specimens containing gravel. In both cases, however, various investigators have found that additional modifications have to be made in the small-scale test results in order to obtain agreement with the results of tests on full-scale specimens. The objectives of this investigation were to determine the validity of the Corps of Engineers scalping and replacement procedure and to evaluate theoretical methods of computing dry unit weights of full-scale specimens, using results from tests performed on minus No. 4 fractions of the total specimen. These objectives were achieved by comparing compaction curves for full-scale specimens (3 in. maximum particle size) and scalped and replaced specimens ( 90 in. maximum particle size), determined by using a mechanical compactor with 18 and 6-in.-diameter molds, respectively. Additional comparisons involved compaction curves for scalped and replaced specimens ( 90 in. maximum particle size), determined by using a hand-held hammer with the 6-in.-diameter mold. Companion compaction curves on the minus No. 4 fraction of the full-scale material were determined by using a hand-held hammer and 4-in.-diameter mold. Effects investigated are given in Table 1.

Procedure

Equipment A mechanical compactor, manufactured by Howard Company and equipped with 5.5 and 24.7-1b rammers, having face diameters of 2.0 and 6.0 in., respectively, was used to perform the testing. Large-scale tests were performed using an 18-in.-diameter mold and 24.7Ab rammer, while small-scale tests were performed using a 6-in.-diameter mold and 5.5-1b rammer. A special harness for suspending the 18-in.-diameter mold and specimen from a forklift was rigged with an electronic load cell, sensitive to within 0.1 lb, to obtain the specimen-plus-mold weights. A photograph of the compactor, the 24.7-1b rammer, the 18-in.-diameter mold, and the weighing harness is shown in Fig. 1.3 All other weights ZThe Corps of Engineers scalping and replacement procedure for materials containing particles larger than 2 in. for compactingin a 12-in.-diametermoldis givenin the appendix. 3The 12-in.-diameter mold shown with the weighing harness was not used in this investigation. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

18

6

mechanical

mechanical

Gravel content (large-scale tests)

Gravel content (small-scale tests)

No. 4 sieve 3b

0 10 20 30 40 50 60 100

0 10 20 30 40 50 60 100

40

40

90

90 3b

0

Gravel Content, 070

No. 4 sieve

Max Particle Size

No. 4 sieve a/~a

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18

mechanical

Removal and replacement o f coarse particles

6 18 6 18

mechanical

Mold Dia, in.

Equipment size

Effect Investigated

Type o f Compactor Used

T A B L E l--Summary o f compaction data.

25

25

25

25

25

Fines Content, 070

130.9 133.5 132.5 132.3 131.1 131.9 129.5 103.6

133.9 135.0 136.1 137.2 138.0 137.1 134.9 112.0

134.1 138.0

130.9 133.9 131.1 134.1

M a x Dry Unit Weight ),a, lb/ft 3

C

8.6 7.7 8.1 8.1 7.9 7.8 9.5

...C

6.9 6.7 6.1 5.7 5.9 5.8 5.2

7.3 5.9

8.6 6.9 7.9 7.3

Optimum Water w, 07o

-4 m .--t

0

0

i--

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---i

t'"u 11

ill z

rn

I-"

co O

ol 0

6

mechanical

mechanical

Fines content (large-scale tests)

Fines content (small-scale tests)

40

40

3b

90

0 10 20 30 40 50 60 100

No. 4 sieve 3Aa

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

a Scalped and replaced material. b Full-scale material. cSingle-point test performed on dry material.

18

hand-held

Gravel content (small-scale tests)

15 25 35

15 25 35

25

134.8 131.1 126.6

141.8 138.0 133.3

132.6 132.5 131.8 132.0 132.0 129.3 128.5 101.2

7.4 7.9 9.4

4.9 5.9 7.5

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32 m

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252

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIO. l--Howard mechanical compactor with 12 and 18-in.-diameter molds and load cell harness f o r weighing.

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DONAGHE AND TOWNSEND ON SCALPING AND REPLACEMENT

253

were obtained by using a scale, having a sensitivity of 0.01 lb. Calibration data for the molds were: Nominal Mold Inside Diameter, in.

Area, ft2

Height, in.

Volume, ft 3

Weight, lb

6 18

0.196 1.767

4.582 18.005

0.075 2.651

6.21 321.40

Equipment used for tests with the hand-held hammer conformed to that used for standard compaction tests performed by the Corps of Engineers [1]4 and is similar to that specified in ASTM Tests for Moisture-Density Relations of Soils, Using 5.5-1b Rammer and 12-in. Drop (D 698-70), except the rammer is equipped with a sliding weight instead of a guidesleeve.

Material The materials tested in this investigation consisted of a subrounded to subangular washed gravel, having a maximum particle size of 3 in., a subrounded to subangular concrete mortar sand, and a clay (CL), combined according to the gradations given in Figs. 2 and 3. Classification data for the clay and sand are given in Fig. 4.

Specimen Preparation Batches for specimens were prepared by thoroughly mixing a predetermined amount of sand and CL material mixture (minus No. 4 material) with a measured quantity of water, using either a commercial kitchen mixer or a pugmill (specimens tested in 4 and 6-in.-diameter molds were mixed with the kitchen mixer). The resulting minus No. 4 material was then stored in airtight containers and allowed to cure for a period of at least 16 h. The plus No. 4 material for each batch was prepared by combining the air-dry portion (by weight) of material required for each sieve and then storing the resulting material in containers filled with water. Immediately prior to compaction, the cured minus No. 4 fraction was mixed with the saturated surface-dry aggregate. Each layer was hatched separately to prevent any variations in grading between layers. 4The italic numbers in brackets refer to the list of references appended to this paper. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

254

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

I

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DONAGHE AND TOWNSEND ON SCALPING AND REPLACEMENT

255

.O

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FIG. 4 - - G r a i n size distribution c u r v e s a n d classification data, C L material a n d sand.

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DONAGHE AND TOWNSEND ON SCALPING AND REPLACEMENT

257

Testing Procedure Table 2 lists pertinent data concerning the rammer sizes and the compaction procedure. All specimens were compacted in three layers, using compactive efforts made approximately equal to the standard effort (12 300 ft. lb/ft 3) by adjusting the number of blows. Test Results and Discussion

Results of the compaction tests are summarized in Table 1 and presented graphically in Figs. 5 through 15.

Effects of Equipment Figure 5 presents results of tests performed, using both large- and small-scale equipment, on scalped and replaced specimens and minus No. 4 sieve specimens, having gravel contents of 40 and 0 percent, with 25 percent fines. The data indicate that, for both gradations, maximum dry unit weight and optimtun water content varied with the different sizes of equipment used. Optimum water contents of the 0 and 40 percent gravel specimens were decreased by 1.7 and 0.6 percent, respectively, when the 18-in.-diameter mold and 24.7-1b rammer were used in place of the 6-in.-diameter mold and 5.5-1b rammer. The maximum dry unit weight of both materials was increased by 3.0 lb/fP for the same change in sizes of equipment. This difference in maximum dry unit weight falls within a range comparable to that determined by previous investigators [2-4], who indicated that the variation in densities determined using various mold sizes was limited to not more than 4 lb/ft 3. Since only limited testing was performed to determine the effects of variation in equipment size, no attempt was made to correct other test results for these effects. When comparisons which involve both large- and smallscale test results are made, however, it appears safe to assume that differences in densities due to varying the size of equipment used are limitied to not more than 4 lb/ft 3.

Effect of Removal and Replacement of Oversize Particles Figure 6 shows compaction curves and data for the tests performed on the scalped and replaced specimen having 40 percent gravel and 25 percent fines and on the corresponding full-scale specimen, both of which were tested using large-scale equipment. The curves indicate that, in this case, scalping and replacing oversize particles results in a lower maximum dry unit weight and a higher optimum water content. The 3.9 lb/fP decrease in maximum dry unit weight due to scalping and replacement of

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions autho

12 420

12 299

6

18

24.7

5.5

lb

Weight,

6

2

Dia of Circular Face, in.

24

12

Drop, in.

3

3

No. of Layers

220

56

Blows per Layer

7

2

24 b

24

Complete Coverage Along of CircumferMold Area enceof Mold

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

6c

4

At Center of Specimen

Blows per Coverage

aCompaction equipment used: hand and mechanical compaction: 6-in.-dia mold, mechanical compaction: 18-in.-dia mold. bPius 6 on last coverage. Cplus 4 on last coverage.

Compactive Effort, ft'lb/ft ~

Mold Dia, in.

Rammer a

TABLE 2--Compaction tests using hand-heM sliding weight rammer and Howard mechanical compactor.

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DONAGHE

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TOWNSEND

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SCALPING

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259

REPLACEMENT

T E S T GRADATIONS 4SIEVE 3

0PENINGS=I = 3/4 4

10

SIEVE NUMBERS 40

200

0

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CONTENT,

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0

40

6" M O L D

18" M O L D

6" M O L D

18" M O L D

8.6 130.9

6.9 133.9

7.9 131.1

7.3 134.1

I

I

I

I

.I

4

6

8

10

12

WATER CONTENT p ~

FIG. 5--Compaction curves f o r tests to determine effect o f varying mold diameter.

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260

SOIL SPECIMEN

PREPARATION

TEST

I00

80 Z k,IZ

60

.

:

I0 I

SIEVE NUMBERS 40 200

P 0

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ANDREPLACEDSPECIMEN) NOTE:

T E S T S P E R F O R M E O USING 18*IN.-O~AM M O L D .

)Z

125

Q

MAX. P A R T I C L E SIZE, IN,

120

::]/4 OPT MAX. 115

I 4

W, % ~Xd. P C F

3

7.3 134,1

5.9 138.0

I I 6 8 WATER C O N T E N T t ='7o

I l0

I )2

FIG. 6--Compaction curves for tests to determine effect of removal and replacement of oversize particles.

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DONAGHE AND TOWNSEND ON SCALPING AND REPLACEMENT

261

oversize particles compares to a difference of 2.2 lb/ft 3 obtained for a material from DeGray Dam (48 percent gravel, 26 percent fines) tested in a pervious Corps of Engineers investigation [5] and a difference of 4 lb/ft 3 obtained for a material containing 59 percent gravel and 14 percent fines, tested in an investigation by the South Atlantic Division Laboratory of the Corps of Engineers [4]. It should be pointed up that results of comparable tests on specimens having gravel contents other than 40 percent would probably not indicate the same differences due to scalping and replacement.

Effects of Gravel Content Full-Scale Specimens--Figure 7 presents compaction curves and data for large-scale tests performed on specimens having gravel contents ranging from 0 to 100 percent and a maximum particle size of 3 in. (except, of course, for the 0-gravel material). The fines content (minus No. 200 sieve material) of each specimen, except for that of the 100-percent gravel material, was maintained at 25 percent. The maximum dry unit weight versus gravel content curve for the tests on full-scale samples (Fig. 8) shows that the maximum dry unit weight increased as the gravel content varied from 0 to 40 percent and then decreased with increasing gravel content above 40 percent. The increase in density between gravel content of 0 and 40 percent was 4.1 lb/ft 3. It is common knowledge that a threshold or optimum gravel content exists, past which, particle interference produces lower densities. However, other investigators have not shown optimum gravel content as low as 40 percent or such small increases in density with increasing gravel content in the range from zero to optimum. Holtz and Lowitz [6], for example, showed a 17 lb/ft 3 increase in maximum dry unit weight for a comparable increase in gravel content (0 to 40 percent) and an optimum gravel content of between 60 and 70 percent in tests conducted on a clayey gravel (liquid limit = 49, plasticity index = 28). However, in their tests, as well as in those of other previous investigators [2,3] the fines content varied significantly because the gradation of the minus No. 4 fraction remained unchanged with the addition of various amounts of gravel. It is thought that this may account for the difference in results between those obtained for this investigation and those obtained previously. As will be shown in the following paragraphs, fines content has a greater effect on maximum dry unit weight than does gravel content. The optimum water content versus gravel content curve for full-scale specimens (Fig. 8) shows that, except for a small increase in optimum water content occurring for gravel contents slightly above the optimum gravel content, optimum water content decreases with increasing gravel content. Scalped and Replaced Specimens--Figures 9 and 10 present compaction curves and test data for the small-scale tests, using the hand-held rammer

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262

SOIL SPECIMEN

PREPARATION

TEST

100

~SIEVE 3

FOR LABORATORY

GRADATIONS

0PENINGS~i = 3/4 4

-

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~ ,

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10 ~

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or

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I

c,-' ~ \ \

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20

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60

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50% Gr ~

135

40% Gr

U 20~7o Gr

~'0130

60% Gr

I'0

t.o I'Z t25

NOTE:

TESTS PERFORMED USING 18-1N. MECHANICAL COMPACTOR,

DIAM MOLDI

AND

a

GRAVELCONTENT,% 120

0 OPT MAX. *

W, % ~d' PCF

SINGLE POINT

I

115 2

4

10

20

30

40

50

60

6.9

6.7

6,1

5.7

5.9

5.8

5.2

133.9

135.0

136.1

137.2

138.0

137.1

134.9

TEST

PERFORMED

-* 112.0

ON DRY MATERIAL.

I 6 WATER

1O0

CONTENT~

I

[

8

i0

I 12

07o

FIG. 7--Compaction curves for tests performed on full-scale specimens having variable

gravel contents.

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

P

"1

Z

O

O

0-I ~0

(31O~ ~0

1-~

m z_

OO

O

:

M A X I M U M DRY U N I T

O

rm~

WEIGHT

F,

O

)'dMAx~PCF

OPTIMUM

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

5'

o,.2

/

r-

WATER C O N T E N T ~ aTo

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264

SOIL

SPECIMEN

PREPARATION

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140 NOTE:

TESTS

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135

% a_

;~130

-

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115 2

10

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40

8.1

7.9

132,3 131.1

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ON D R Y M A T E R I A L .

i 6 WATER

CONTENT,

30

CONTENT;

I

I

I

8

I0

I?_..

~

FIG. 9--Compaction curves f o r small-scale tests performed on specimens having scalped and replaced coarse particles with variable gravel contents by mechanical compactor.

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

DONAGHE

AND

TOWNSEND

ON

SCALPING

AND

265

REPLACEMENT

TEST GRADATIONS "SIEVE I00

8O

3

t

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--

NOTE:

135

TESTS

PERFORMED

USING 6-1N.-DIAM MOLD.

--

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z

20% Gr

125

o,~,i~ /

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7.9

7.9

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132.6

132.5

131.8

132.0

132.0

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128.5

POINT

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I

115

10

4

PERFORMED

100 --*

101.2

ON D R Y M A T E R I A L .

I 6 WATER CONTENT/

I

I

8

I0

I 12

07o

FIG. l O--Compaction curves for small-scale tests performed on specimens having scalped and replaced coarse particles with variable gravel contents by hand-held compactor.

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266

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

and mechanical compactor, performed on scalped and replaced specimens having a maximum particle size of 3/4 in. (except, of course, for the 0-gravel material) and gravel content ranging from 0 to 100 percent. The fines content of each specimen (except for the 100-percent gravel material) was maintained at 25 percent. Figure 8 shows no significant difference in optimum water content and maximum dry unit weight between tests performed using mechanical and hand-held rammers. Figure 8 also shows that the threshold gravel content at which particle interference begins to produce lower maximum dry unit weights is between 10 and 20 percent for the scalped and replaced specimens, as compared to the value of 40 percent for the full-scale specimens. This finding, that the more poorly graded gravel fraction resulting from the scalping and replacement procedure significantly lowers the optimum gravel content, is contrary to that of Holtz and Lowitz [6], who indicated that the optimum gravel content is only slightly decreased for a poorly graded gravel fraction in small-scale tests. However, as mentioned previously, their results reflect effects due to both gravel and fines variation, and it may be possible that the increase in maximum dry unit weight values with increasing gravel content for their tests was due to the decrease in fines content, rather than to increased gravel content. When comparing the compaction results in Fig. 8, it should be remembered that differences in maximum dry unit weight between the scalped and replaced and the full-scale specimens reflect effects due to equipment size, in addition to the effects of the scalping and replacement procedure. The optimum water content versus gravel content curves for the scalped and replaced specimens (Fig. 8) show that, unlike that for the full-scale specimens, there was no significant change in optimum water content with increasing gravel content up to 50 percent. In this range (0 to 50 percent), optimum water content values for the full-scale specimens decreased from 6.9 to 5.8 percent, while those of the scalped and replaced specimens averaged 7.9 percent. The higher optimum water content values for the scalped and replaced specimens were apparently caused by the interference of the gravel on the compaction of the fines.

Effect of Fines Content Full-Scale Specimens--Compaction curves and data for full-scale specimens having fines contents (percent by weight passing the No. 200 sieve) of 15, 25, and 35 percent and a maximum particle size of 3 in. are given in Fig. 11. The gravel content of each sample was 40 percent. The data show that, as the fines content increased from 15 to 35 percent, the maximum dry unit weight decreased, and the optimum water content increased. The influence of fines on compaction characteristics was much greater than that of gravel. A change in gravel content from 0 to 40 percent re-

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DONAGHE

AND

TOWNSEND

ON

SCALPING

AND

267

REPLACEMENT

suited in an increase in maximum dry unit weight o f 4.1 lb/ft 3 and a decrease in o p t i m u m water content o f 1.2 percent, whereas a change in fines content from 15 to 35 percent resulted in a 8.5 lb/ft ~ decrease in maxiTEST

|00

~SIEVE 3 \

OPENINGS'I 9 3/4 4 I ,

\

8O

GRADATIONS

I

I

I '

~

L

SIEVE NUMBERS-40 200

10 I

I

I

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I

i

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140

TESTS PERFORMED

USING 18-1N.-DIAM MOLD.

--

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0-

0% T W I--

~ 13e_ o

125

FINES CONTENT, ( G r : 40%)

--

15'7,

OPT MAX.

I

J20

2

4

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35%

4.9

5.9

7.5

141.8

138.0

133.3

I

I

I

I

6

8

I0

12

WATER CONTE NT,

eTo

FIG. l l--Compaction curves for large-scale tests performed on full-scale specimens having variable fines contents.

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268

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

mum dry unit weight and a 2.6 percent increase in optimtnn water content. Scalped and Replaced Specimens--Compaction curves and test data for small-scale tests on scalped and replaced specimens having 3A in. maximum particle sizes with fines contents of 15, 25, and 35 percent are given in Fig. 12. The gravel content of each specimen was 40 percent. These results, summarized in Fig. 13, show that the effect of fines content in the small-scale tests is similar to that in the large-scale tests. In the small-scale tests, an increase in fines content from 15" to 35 percent decreased the maximum dry unit weight 8.2 lb/fP and increased the optimum water content 2.0 percentage points. By comparison, for the same variation in fines content for the large-scale tests, the maximum dry unit weight decreased 8.5 lb/ft 3, and the optimum water content increased 2.6 percentage points. As in the case of tests performed to determine effects of varying gravel contents, the higher optimum water content values obtained for scalped and replaced specimens was probably caused by a lower compactive effort being applied to the fines as a result of interference of the gravel. The differences in maximum dry unit weight between the scalped and replaced and the full-scale specimens, indicated by the curves in Fig. 13, are due to the scalping and replacement procedure and the effects of variation in equipment size.

Applicability of Theoretical Equations and the Scalping and Replacement Procedure Various theoretical equations have been derived to compute the total density of a material containing oversize particles, based on densities obtained from small-scale tests without replacement. (a) The most commonly used equation, presented by several investigators, including Shockley [7], is as follows y~ =

yfOmyw X YsP~ + G.,ywP~

100

(i)

where ya = calculated dry unit weight of total sample, Yi = dry unit weight of finer fraction (from compaction tests on scalped material), G m = bulk specific gravity of coarser fraction (oversize material), yw -- unit weight of water, and Pi and Pc -- percent of fine and coarse material by weight of the sample.

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DONAGHE

AND

TOWNSEND

ON

TEST

I00

*SIEVE 3

td Z

AND

~ I0

SIEVE NUMBERS 40

!

200

,

I

I

i

0

'

I

i

I I

I !

2 0 ~c cc

t,. 60

4o o~

jz

I

G.

2O

269

REPLACEMENT

GRADATIONS

OPENINGS~'{ " 3/4 4

n- 8O

SCALPING

I

I

U

I

,,

',

: i "1 I

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~

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I

I.-

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25% F i

so

1 5%

J

I

I IO0 = I ~ FINES~ L

[,4

GRAVEL

-I

:

SAND

140 NOTE:

TESTS

PERFORMED

USrNG 6-1N.-DIAM

MOLD.

135

n

~"

%

130

t~

I-

Z

12~ o

o

120

FINES CONTENT, ( G r = 40%)

--

15% OPT MAX. 115 2

W, % "Xd, P C F

25%

% 36%

7.4

7.9

9.4

134.8

131.1

126.6

I

I

I

I

I

4

6

B

I0

IZ

WATER

CONTENT

I '='7o

FIG. 12--Compaction curves f o r small-scale tests performed on specimens having scalped and replaced coarse particles with variable fines contents.

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270

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

12

~

/

I0

I-Z

MOLD)~

SCALPED AND REPLACED SPECIMENS (6-1N,-DIAM ~

8

/ /

/ o

J "~

I0

FULL-SC~LE SPECIMENS

20

30

40

50

145

t,_ 1 4 0 Q.

\

\ N

\.

-....

I- 1 3 5

\

IZ

4

D

X

~[ 1 2 5

1,20

t0 FINES

20 CONTENT~

30 070 ( G r = 4 0 ~

40

50

FIG. 13--Optimum water content and maximum dry unit weight versus fines content for full-scale and scalped and replaced specimens.

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

DONAGHE AND TOWNSEND ON SCALPING AND REPLACEMENT 271

The equation is based on the assumption that the coarser particles simply replace the finer material, with the density of the finer material remaining constant. Generally, Yi is considered to be the density of minus No. 4 material. However, its value can be based upon other sizes. For example, calculations can be made for plus 90 material, if the density of the minus 90 material is known. In this case, P~ amd Pc are the percentages of material finer and coarser than 90 in., respectively. The equation makes no allowances for the interference of the coarser particles to the compaction of the finer fraction at higher gravel contents. The gravel content at which interference is initiated is referred to as the limiting gravel content and is the gravel content at which a theoretical curve, based on Eq 1, corrected for equipment effects, and the experimental curve, begin to diverge. Tests performed by the U.S. Bureau of Reclamation [8] have shown that, for poorly graded fractions, such as may result from the scalping and replacement procedure, the limiting gravel content may be less than 10 percent, while for well-graded coarse fractions, such as might occur when the maximum particle size is increased to that of a full-scale sample, the limiting gravel content may be as high as 35 to 40 percent. (b) The FAA [9] uses the following equation to account for oversized particles

yd=

PyYs + 0.9 PcGmyw 100

(2)

where the terms are the same as those defined in Eq 1. The equation is simply the weighted average of the densities of the finer and coarser fractions, based on the percent by weight of each in the total sample. A factor of 0.9 is used to account for the coarser particles interfering with the compaction of the fine fraction. Table 3 lists test data for small-scale tests conducted on minus No. 4 fractions of the full-scale samples, using a hand-held rammer and 4-in.diameter mold. These tests were conducted to provide input data for the preceding equations. Figure 14 shows relationships between dry unit weight and gravel content, developed using results of these tests with Eqs 1 and 2, along with experimental values obtained for the full-scale and the scalped and replaced samples. As may be seen, the curves show that, for gravel contents in the range from 0 to 50 percent, the use of Eq 1, with results of tests performed on finer fractions, provides a better approximation of experimental results than does Eq 2 or the relationship developed from tests performed on scalped and replaced samples. The curve based on Eq 1 almost coincides with the experimental curve up to the limiting gravel content of approximately 35 percent. The curve based on Eq 2, on the other hand, indicates significantly lower maximum dry

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272

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

TABLE3--Compaction tests performed on minus No. 4 fractions of full-scale specimens. MinusNo. 4 Fractionsof Full-ScaleSpecimens Having VariableGravelContents Gravel content, 070 Optimum w, 070 Max Ya,lb/ft~

10 8.2 132.0

20 8.6 13L0

30 9.3 128.5

40 9.8 127.0

50 10.5 124.8

60 11.9 119.3

MinusNo. 4 Fractionsof Full-ScaleSpecimens Having VariableFinesContents Fines content, % Optimum w, ~ Max )'a, lb/ft~

15 8.1 132.1

25 9.8 127.0

35 11.7 121.5

unit weight values than those indicated by the experimental curve for gravel content in this same range (from 0 to 50 percent). At a gravel content of 40 percent, for example, the value computed using Eq 2 is 3.8 lb/fP less than the corresponding experimental value. Thus, it appears that the use of results from tests performed on minus No. 4 fractions of full-scale samples and Eq 1 would provide the most suitable means of predicting densities of full-scale samples having gravel contents below about 50 percent. By comparison, results of the previous Corps of Engineers study [5] on full-scale and scalped and replaced samples of a clayey sandy gravel from DeGray Dam, having a gravel content of 54 percent, showed that the scalping and replacing procedure gave slightly better estimates of the full-scale maximum dry unit weights than did Eq 1. The maximum dry unit weight of the full-size sample was 127.9 lb/fP, and the maximum dry unit weight of the small-scale test on scalped and replaced material underestimated this density by 4.3 lb/fP. On the other hand, Eq 1 overestimated the full-size maximum dry unit weight by 4.9 lb/fP. Most of the error associated with Eq 1 is attributed to the increased particle interference, which occurs at higher gravel content. In the case of Eq 2, which considers particle interference through a correction factor of 0.9, excellent agreement was obtained between the calculated maximum dry unit weight, 128.1 lb/fP, and the observed value, 127.9 lb/fP. This agreement by Eq 2, as opposed to the overestimation of Eq 1, indicates that particle interference must be accounted for at higher gravel contents. Relationships between fines content and maximum dry unit weight, developed using Eqs 1 and 2, with results from tests performed on minus No. 4 fractions of the full-scale samples, are given in Fig. 15. Also included are experimental relationships developed from tests performed on the full-scale and the scalped and replaced samples. These curves show that, for tests in which the fines content is varied while maintaining the

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273

DONAGHE AND TOWNSEND ON SCALPING AND REPLACEMENT

170

1

T

EQUATIONS APPLIED TO DATA GIVEN IN FIG. T A B L E GM = 2.58

160

/ THEORETICAL CURVE BASED ON SHOCKLEY'IS EQUATION (EQ I ) . . . . ~

b

150

THEORETICAL CURVE BASED ON EAA EQUATION (EQ 2)~..~

/

/ /

4/"

/ . ~ 140 0

t--

q

1:30 o

g 120

--

- -

EXPERINENTAL CURVE N SCALPED AND REPLACED SPECIMENS~'~ (6-1N,-DIAM M O L D ) ~ ~ ~

N

~

\

I I I

IlO

I00

0

20

40 ~0 GRAVEL CONTENT.=, ~

80

I O0

FIG. 14--Experimental and theoretical relationships between maximum dry unit weight and gravel content.

gravel content constant (40 percent), Eq 1 once again provides a better approximation of experimental results than does Eq 2 or the relationship developed from results of tests performed on scalped and replaced samples. It is of interest to note that, since Eq 1 provides the best agreement with the full-scale experimental curve, the threshold gravel content at which particle interference produces lower maximum dry unit weights (optimum gravel content) may not have been affected by varying the fines content.

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274

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

145

"%,

b r X ,,{

140

I3: 0

f

~----THEORETICAL CURVE BASED ON ~~ .~ /= ~ r / SHOCKLEY'SEQUATION (EQ l)

~'X~

~

\~EXPERIMENTAL CURVE FULL-SCALE \ P E CIMEN5 (78.1N-DIAM MOLD) 1:35

x~

Z

~

e~

~b.~---THEORETICAL CURVE BASED ON FAA EQUATION (EQ 2)

EXPERIMENTAL CURVE, SCALPED AND EPLACED SPECIMENS (6.1N.-DIAM MOLs

~z5 6

I0

20

FINES

30 CONTENT,

40

50

60

'~o

FIG. 15--Experimental and theoretical relationships between maximum dry unit weight and fines content.

Conclusions The results of this program of tests performed on blended earth-rock material indicate the following conclusions. (a) Maximum dry unit weight and optimum water content vary with sizes of equipment used. When the mold diameter is increased from 6 to 18 in. and the rammer weight is increased from 5.5 to 24.7 lb, the maximum dry unit weight of specimens containing 0 and 40 percent gravel with 25 percent fines is increased by 3.0 lb/ft 3. For the same increase in equipment size, the optimum water content for the specimen containing no gravel is decreased by 1.7 percent, while that of the specimen containing 40 percent gravel is decreased by 0.6 percent. (b) The scalping and replacement procedure results in a lower maximum dry unit weight and a higher optimum water content than obtained when testing the full-scale specimen. In the case of tests performed using an 18in.-diameter mold on a full-scale and a scalped and replaced sample having 40 percent gravel and 25 percent fines, the maximum dry unit weight and optimum water content of the scalped and replaced sample is 3.9 lb/ft 3 lower and 1.4 percent higher, respectively, than the corresponding values for the full-scale sample. (c) For tests in which the gravel content is varied while maintaining the fines content constant, the optimum gravel content, that is, gravel Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

DONAGHE AND TOWNSEND ON SCALPING AND REPLACEMENT

275

content producing the highest density, is decreased from 40 percent to between 10 and 20 percent by the scalping and replacement procedure. Thus, in the range of gravel contents from between 10 and 20 percent to 40 percent, the difference between maximum dry unit weights determined for fuU-scale and for scalped and replaced specimens increases with increasing gravel contents. (d) When the fines content is varied from 15 to 35 percent while maintaining the gravel content constant, maximum dry unit weights and optimum water contents of the full-scale and the scalped and replaced samples vary similarly. (e) The influence of fines content on compaction characteristics is much greater Shan that of gravel content. A change in gravel content from 0 to 40 percent resulted in a difference in maximum dry unit weight of 4.1 lb/ft 3 and a difference in optimum water content of 1.0 percent, whereas a change in fines content from 15 to 35 percent resuked in an 8.5 lb/ft 3 difference in maximum dry unit weight and a 2.6 percent difference in optimum water content. (10 For gravel contents ranging up to 50 percent, the Eqs 1 and 2 provide better approximations of experimental results than do the relationships developed using the Corps of Engineers scalping and replacement procedure. In this range, Eq 1 provides a better estimate of actual results than does Eq 2. For gravel contents above 50 percent, Eq 2 produces the best agreement for gravel content in the range from 50 to 60 percent.

Acknowledgments The tests described and the resulting data presented herein, unless otherwise noted, were obtained from research conducted by the Waterways Experiment Station for the Civil Works Investigation Studies Program sponsored by the Office, Chief of Engineers. Permission to publish this information was granted by the Chief of Engineers. W. J. Hughes was the principal engineering technical assisting in the laboratory program. Additional personnel assisting in performing tests were Isaac Smith, Jr., Mose Ellis, and R. L. January.

APPENDIX The scalping and replacement procedure as outlined in Ref 1 follows: (a) Spread the material to be tested in flat pans and air dry the entire sample. Other means, such as ovens and heat lamps, may be used to accelerate drying if the maximum drying temperature is 60 ~ (b) Reduce all aggregates, or lumps formed during drying, of fine-grained material to particles finer than the No. 4 sieve. With a wire brush or other means, remove all fine-grained material that may be clinging to rock sizes, taking care not to lose the fine-grained material. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

276

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

(c) Separate all the material, using a set of sieves ranging from the largest particle size in the sample to the No. 4 sieve. The total sample must be processed to determine the as-received gradation. (d) Place the material retained on each sieve and that passing the No. 4 sieve in separate containers, weigh the contents of each, and compute the percent of the total sample retained on each sieve as follows: 070 retained = dry weight of material retained on sieve x 10007o dry weight of total sample (e) If 10 percent or less of a field sample is retained on the 2-in. sieve, the partides larger than this size should be discarded, and replacement is not necessary. (]) If more than 10 percent of a field sample is retained on the 2-in. sieve, it will be necessary to remove the plus 2-in. sizes, and replace them with an equal weight of material between the 2-in. and No. 4 sieve sizes. The gradation of the replacement material must be the same relative gradation as that of the total sample between the 2-in. and the No. 4 sieve sizes. The percent passing the No. 4 sieve remains constant and is equal to the percent passing the No. 4 sieve for the total as-received sample. For each sieve between the 2-in. and the No. 4 sizes, the percent required to replace the plus 2-in. sizes is computed as follows replacement O7o = total 070 of __2-in. sizes x 070 retained on one sieve s total ~ between 2-in. and No. 4 sieve For each sieve, add the "replacement 070" to the "~ retained" on that sieve initially. This gives the percent by weight of a test specimen required for each sieve size in order to reconstitute a specimen with the +2-in. sizes replaced with sizes ranging from the 2-in. to the No. 4 sizes.

References [1] "Engineering and Design: Laboratory Soils Testing," Engineer Manual EM 1110-2-1906, Office, Chief of Engineers, Department of the Army, Washington, D.C., Nov. 1970. [2] Ziegler, E. J., "Effect of Material Retained on the Number 4 Sieve on the Compaction Test of Soil," Proceedings, Highway Research Board, Vol. 28, 1948. [3] Cunny, R. W. and Strohm, W. E., Jr., "Compaction Tests on Gravelly Soils with Cohesive Soil Matrix," Miscellaneous Paper No. 3-676, U.S. Army Engineer Waterways Experiment Station, Vicksburg, Miss., Oct. 1964. [4] "Procedures and Equipment for Determining the Maximum Standard Compaction Density of Granular Material (6 and 12 in. Diameter Molds)," U.S. Army Engineer Division Laboratory, South Atlantic, March 1968. [5] Donaghe, R. T. and Townsend, F. C., "Compaction Characteristics of Earth-Rock Mixtures, Report 1, Vicksburg Silty Clay and DeGray Dam Clayey Sandy Gravel," Miscellaneous Paper No. S-73-25, U.S. Army Engineer Waterways Experiment Station, Vicksburg, Miss., May 1973. [6] Holtz, W. G. and Lowitz, C. A., "Compaction Characteristics of Gravelly Soils," Earth Laboratory Report No. 509, U.S. Bureau of Reclamation, Sept. 1957. [71 Shockley, W. G., "Correction of Unit Weight and Moisture Content for Soils Containing Gravel Sizes," Technical Data Sheet No. 2, U.S. Army Engineer Waterways Experiment Station, June 1948.. [8] U.S. Bureau of Reclamation, "Research on Compaction Control Testing of Gravelly Soils," Earth Research Program, EM 662, 8 Aug. 1963. 5Any sieve between 2 in. and No. 4 sieve sizes.

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DONAGHE AND TOWNSEND ON SCALPING AND REPLACEMENT

277

[9] Mainfort, R. C. and Lawton, W. L., "Laboratory Compaction Tests of Coarse-Graded Pavement and Embankment Materials," Proceedings, Highway Research Board, Vol. 32, 1953.

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P. Y. L e e I

Study of Irregular Compaction Curves

REFERENCE: Lee, P. Y., "Study of Irregular Compaction Curves," Soil Specimen Preparation for Laboratory Testing, ASTM STP 599, American Society for Testing and Materials, 1976, pp. 278-288. ABSTRACT: Curves of dry density versus moisture content, as yielded by the standard ASTM compaction tests, usually show a single maximum, but some soils exhibit compaction curves with irregular shapes. This study examines some of the influences of soil preparation on these irregular compaction curves. When a surface tension reducing agent, such as aerosol-AY, is added to the water in soil specimen preparation, the shape of the irregular compaction curve will be altered. This effect is regarded as a verification of the effects of negative pore water pressure on the irregular compaction curve. In repeated compaction tests, it is economical and convenient to reuse the soil specimens. The results of liquid and plastic limit tests reveal approximatdy the same values in new and reused specimens. However, the reused soil specimens reveal a predominantly higher density in the zone of lower water content in an irregular compaction curve.

KEY WORDS: soils, tests, compaction, clays, surface tension, soil mechanics, density, clay minerals, water content

Many soils are too weak and unstable to perform adequately as good engineering materials. Despite their inadequacies, many of these materials must be used in engineering construction because it is not economically feasible to bypass or replace them. To correct their deficiencies, such soils must be stabilized. Various methods of stabilization are currently used, but the most common and most useful method is that of soil compaction. The compaction process involves optimizing the water content so that maximum density is reached, thereby providing the greatest stability. For most soils commonly encountered in earth construction, there is a single peak in the dry density versus moisture content relationship (Fig. 1). In contrast to this, some soils yield compaction curves with two peaks or a curve that is oddly shaped (Fig. 2). An acceptable and l Associate professor, Department of Civil Engineering, Tulane University, New Orleans, La. 70118.

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279

LEE ON IRREGULAR COMPACTION CURVES

m

.

I

I

I

I

I

I

|

!

i

!

CONTENT

- %

-

r~

~z

MOISTURE

I

I

CONTENT

- %

FIG. 1--Typical single peak compaction curve.

MOISTURE

FIG. 2--Irregular compaction curves.

experimentally demonstrable explanation for these irregularly shaped curves has not been given; attempts have been made to rationalize the phenomena. Because of the limitied research on the subject, the existing theories of compaction have not been extended to explain these irregular curves. The compaction process itself is only partially understood; it has been known for some time, however, that water plays an extremely important role, especially with fine-grained cohesive soils. The effect of water on the common single peak compaction curve is included in some of the basic theories of compaction; however, the scope of this discussion indudes only a limited explanation of the effect of water on irregular compaction curves. Therefore, a laboratory investigation was designed to analyze the irregularly shaped compaction curves in detail. Of the four compaction theories proposed previously, Proctor's capillarity and lubrication theory [1], 2 Hogentogler's viscous water theory [2], Lambe's physico-chemical theory [3], and Olson's effective stress theory [4], none has been extended to explain the complete irregular compaction curve; all have been developed for a single peak compaction curve. Several shapes have since been observed to which the preceding theories have not been applied [4-7]. Olson noted this deficiency when he stated that the effective stress theory can only be applied to the peak at the higher water content. Olson presented a tentative explanation for his having double peaked curves in limited experiments on the kneading compaction of an ill!tic clay. For the first peak at the lower water content in the double peak compaction curve, O]son reasoned that, when the soil is dry, high surface friction is 2The italic numbers in brackets refer to the list o f references appended to this paper.

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280

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

present, resulting in high shear strength and a low dry density. As water is added, the effect is one of lubrication, thereby increasing the dry density. At this point, water menisci begin to form, tending to increase the soil strength by providing more particle contact area. The balance point between lubrication and the formation of water menisci then occurs at the low-water-content peak point. Additional water results in more menisci, causing an increase in effective stress and a decrease in dry density because less particle interference is required to resist the applied pressure. When all the possible menisci have developed, the minimum point between the two compaction peaks is reached. Olson then reasoned that additional water reduces the pressure differential across the menisci, at which point, the effective stress considerations should begin to apply. With only limited data available, Olson proposed that the double peak curve could only form in soils containing a dominant percentage of plateshaped collodal particles. Additional research in this area included work of Lee and Suedkamp [5] on characteristics of irregularly shaped compaction curves. They investigated 34 different soils possessing a wide range of characteristics and identified the compaction curve for each. By mixing different portions of commercially available soils, they were able to obtain a large variety of soil characteristics in their specimens. The compaction curve for each specimen was then produced by ASTM Tests for Moisture-Density Relations of Soils, Using 5.5-1b Rammer and 12-in. Drop (D 698-70). They found that specimens with a liquid limit between 30 and 70 usually yielded the typical single peak compaction curve. Specimens with a liquid limit of less than 30 and greater than 70 usually produced irregularly shaped curves. There are only a few exceptions to this rule which was probably caused by the mineral constituents of the specific specimens. The classification breakdown is shown in Fig. 3. Olson's hypothesis is logically appealing; hence, the first objective of this investigation was to examine the influence of negative pore pressure on the irregularly shaped compaction curve by using a surface tension reducing agent, together with water. It has long been suspected that there is a difference on moisturedensity curves between values obtained by reusing the same portion of soil repeatedly and values obtained by using a new batch of soil for each point on the compaction curve. Concerning this viewpoint, Nelson and Sowers [8] concluded from a series of tests that the single peak compaction curve produced by reusing the same specimen of soil appears to have a slightly greater maximum dry density than the single peak compaction curve produced by using separate batches of soil for each test. It should be noted that their investigations were limited to examination of the single peak compaction curve. Therefore, an investigation of the characteristics of irregularly shaped compaction curves was deemed desirable.

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LEE ON IRREGULAR COMPACTION CURVES 281 16

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REGION OF TYPICAL SINGLE PEAK CURVES /

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FIG. 3--A classification of compaction curves [5].

Laboratory Investigation Soil Specimens

To adequately investigate the influence of the soil preparation on the irregularly shaped curves, it was necessary to compare several discrete types of curves. For this reason, a particular selection of soil specimens was necessary in order to include soils with a wide variance of characteristics, especially in compaction curves. The approximate compaction curves were needed as a starting point; therefore, full use was made of the paper but Lee and Suedkamp on irregularly shaped curves. Noting their classification scheme of soils according to compaction curve characteristics, and knowing that the compaction curve for each soil had been identified, it was possible to make a good selection of appropriate soils for investigation. A few of these were soils found in nature, while others were laboratory combinations of kaolinite, montmorillonite, illite, quartz, and felspar, so that the percentages of each mineral could be known. Selection of minerals provided a good analogy to natural soils and provided maximum information about the constitution of the soil specimens. Several specimens were selected, both natural and combined, for this investigation, and their composition is given in Table 1. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

282

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

TABLE 1--Selected soil specimens.

Specimen No. 1 2 3 4 5 6 7

Sand, a

Montmorillonite, b %

100 0 75 0 0 0 75 0 25 50 Pierre shale (natural soil) Pierre bentonite (natural soil)

Illinite, c %

Kaolinite, d %

Liquid Limit

Plasticity Index

0 0 100 25 25

0 25 0 0 0

0 13 51 17 172 110 310

NP NP 21 NP 142 66 244

apure sand obtainedlocally. bBentoniteobtained from Baroid Divisionof Natural Lead Co., Houston, Tex. CGrundite obtained from GreenRefractioneryCo., Morris, Ill. d Clay Hydrite--121obtainedfrom Thompson-HaywardChemicalCo., Kansas City, Kans.

Testing Procedure and Test Apparatus In the compaction process, a variety of factors influence test results to various degree. These parameters include the type and magnitude of compaction effort, size and shape of the mold, type of support for the mold, depth o f compacted soil, and soil preparation methods. In an effort to provide a good degree of comparison and of conlxol, standard procedures were specified for soil compaction, namely Standard American Association o f State Highway Officials (AASHO) Designation T99, also known as ASTM Tests for Moisture-Density Relations of Soils, Using 5.5-1b Rammer and 12-in. Drop (D 698-70). In this method, soil passing a No. 4 sieve (4.76 mm) is compacted in a 4-in. (10.16-cm) diameter mold by dropping a 5.5-1b (2.5-kg) hammer from a height of 12 in. (30.5 cm), 25 times per layer, for 3 layers in the 6-in. (15.2-cm) high mold. In preparation for each test, the soils were air dried and pulverized to pass a No. 4 sieve, mixed with water in increments of two or three percent. Each sample was stored in a leakproof plastic bag. A batch of samples was sealed in a large plastic bag and stored in a moisture control room for at least 24 h. Aerosol-AY (100 percent) was used to reduce the surface tension of the water by 55 percent. This was accomplished by mixing two percent Aerosol-AY, by weight, with water.

Discussion of Test Results

Reduction o f Surface Tension o f Water by Aerosol Pore pressure is decreased as the water content is increased. This phenomenon is directly related to the surface tension of the water, since Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

LEE O N I R R E G U L A R

COMPACTION

CURVES

283

surface tension causes water menisci to form and they act as a bond between the soil particles. This tends to cause more interparticle friction and a lower dry density, thus causing the depression noted between the two peaks in the double peak compaction curve. However, if Aerosol-AY, a commercial surface tension reducing agent, is introduced in the water, the shape of the double peak curve should be altered. This was found to be the case. Aerosol-AY was introduced into the water, causing a 55 percent reduction in surface tension and notable changes in the shapes of the double peak compaction curves (Figs. 4-7). In each test, the double-peak curve was replaced by a single peak compaction curve or one closely resembling it. Since capillary pressure is directly related to surface tension, the results of these tests verify the fact that pore pressure, or negative pore pressure caused by surface tension, is the dominant factor in the formulation of the double-peak compaction curve.

l-] - A E R O S O L

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A 115

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MOISTURE CONTENT, PER CENT

FIG. 4--Double peak curve showing effects of aerosol, Specimen 2.

Influence of Reusing Soil Specimens It is economical and convenient to reuse soil specimens; however, it is impossible to thoroughly break up the air-dried aggregations of soil in the reused specimen into individual particles showing the same grain-size distribution as a new specimen, particularly in cohesive soils. In the cornCopyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

284

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

[] - A E R O S O L O-

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t2S

120

A

o

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MOISTURE CONTENT, PER CENT

FIG. 5--Double peak curve showing effects of aerosol, Specimen 4.

85

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O

- STANDARD

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20

30

40

50

60

MOISTURE

CONTENT,

PER C E N T

FIG. 6--Double peak curve showing effects of aerosol, Specimen 6.

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LEE ON IRREGULAR COMPACTION CURVES

- AEROSOL O

- STANDARD

285

TREATED TEST

85

80 m

75

73

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PER C E N T

FIG. 7--1rregular curve showing effects of aerosol. paction tests, reused specimens usually gave results different from that of new specimens. For the cohesive soils (Figs. 8,9), the significant differences in the shape of compaction curves between the new and reused specimens are located on the zone of low-water content. The results of liquid-limit and plasticlimit tests reveal approximately the same values in new and reused specimens, except that the liquid limit of Specimen 5 was found to be slightly greater for the reused specimen than the new one. As indicated in Table 1, these new specimens were combinations of known minerals obtained from commercial sources. These commercial soil minerals were very uniform products. According to the results of sieve analysis, however, the grainsize distribution of new specimens appeared to be much more uniform, containing many more fine particles than the reused specimens. As Michaels [9] has pointed up, dried and pulverized clay consists of packets of many tightly bonded clay particles. The packets do not tend to stick to each other, and the dry soil lacks cohesion, behaving more like granular particles. If this theory is true, the higher density of reused clay at lower water contents is predominantly due to the reduction of the shear strength caused by the decrease of cohesion between the soil particles and the nonuniform grain-size distribution. In addition, air-dried cohesive soil, having been exposed to the air for a long time, usually retains a small portion of water which is distributed evenly around the Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

286

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

STANDARD

TEST

TEMPERATURE - 80 ~

105 " - - ~ ' ~ ~ J

A

[]

- NEW

SPECIMEN SPECIMEN

100 9

95

Z

90

0

5

10

15 20 25 M O I S T U R E CONTENT, PERCENT

30

FIG. 8--Shape ofcompaction curves for test using new specimen compared with test reusing the soil, Specimen 5.

STANDARD

TEST

TEMPERATURE ~] - N E W

- 80 ~

SPECIMEN

110 O

- REUSED

SPECIMEN

% tO0

C3 90

0

5

10

15 20 25 MOISTURE CONTENT, PERCENT

30

FIG. 9--Shape o f compaction curves for test using new specimen compared with test reusing the soil, Specimen 3.

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LEE ON IRREGULAR COMPACTION CURVES

287

soil particles. It seems difficult for the new specimen, on the other hand, to have an even distribution of water if mixed with a small amount of water. The intraparticle lubrication effect, therefore, is greater in the reused specimen corresponding to a higher dry density. As more water is added, the effects of packets tend to disappear, and both specimens should have a more even distribution of water content. As a result, the dry density attains the same normal value. In pure sand, shown in Fig. 10, the reused specimen appeared to have a higher dry density in the portion between the zero water content and STANDARD

TEST

TEMPERATURE

- 80~

[]

- NE~V

SPECIMEN

O

- REUSED

110

v~

z

SPECIMEN

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95

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.,,

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I

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FIG. lO--Shape o f compaction curves f o r test using n e w specimen compared with test reusing the soil.

optimum point (water content from 0 to 8 percent), but there was no significant variation close to and beyond the optimum point. It was found that the particle distribution curve from the sieve analysis exhibited identical shapes for both specimens. However, the percentage passing was slightly higher on the distribution curve of sieve analysis for the reused specimen. Therefore, it could be theorized that the difference of dry density between reused sand and new sand is attributable to the different grain-size distribution. Conclusions On the basis o f this series of laboratory tests, the following conclusions are suggested: Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

288

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

1. Reused specimens of cohesive clay obtain predominantly higher densities, in the region of lower water content, as compared to new specimens. The existence of packets of many tightly bonded clay particles in reused clay and the uneven distribution of water in new clay may have affected this phenomenon. 2. When a surface tension reducing agent, such as Aerosol-AY, is added to the water in a soil, the shape of the double peak compaction curve will be altered. Surface tension has been shown to cause reduction in pore pressure due to capillarity, thus causing the depression between the two peaks. However, when the surface tension of the water is reduced considerably, the double peak curves are replaced by single peak curves or by curves closely resembling single peak curves. This is verification of the direct relationship between surface tension and pore pressure, or, more exactly, negative pore pressure.

Ackno wledgment The financial support provided by the National Science Foundation is gratefully acknowledged. References [1] Proctor, R. R. in Engineering New Record, McGraw-Hill, New York, Vol. III, 1933, p. 245. [2l Hogentogler, C. A. in Proceedings, Highway Research Board, National Research Council, 1936, pp. 309-316. [3] Lambe, T. W., Transactions, American Society of Civil Engineers, Vol. 125, 1960, pp. 682-706. [4 ] Olson, T. E., Journal of Soil Mechanics and Foundation Division, Proceedings, American Sodety of Civil Engineers, Vol. 89, No. SM2, 1963, pp. 27-45. [5] Lee, P. Y. and Suedkamp, R. J., "Characteristics of Irregularly Shaped Compaction Curves," Highway Research Record 381, 1972, pp. 1-9. [6] Lee, P. Y. and Hsu, D. C., "Temperature Effect on Irregularly Shaped Compaction Curves of Soils," Highway Research Record 438, 1973, pp. 16-20. [7] Johnson, A. W. and Sallberg, J. R., "Factors Influencing Compaction Test Results," Bulletin No. 319, Highway Research Board, National Research Council, 1962, pp. 1-146. [8] Nelson, G. H. and Sowers, G. F., "Effect of Re-using Soil on Moisture-Density Curves," Proceedings, No. 29, Highway Research Board, 1949, pp. 482--487. [9] Michaels, A. S., Discussion of "Physico-t3aemical Properties of Soils: Soil-Water System," by I. Th. Rosenqvist, Proceedings, Paper 2010, American Society of Civil Engineers, April 1959.

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J. E. Gillott'

Importance of Specimen Preparation in Microscopy

REFERENCE: Gillott, J. E., "Importance of Specimen Preparation in Microscopy," Soil Specimen Preparation for Laboratory Testing, ASTM STP 599, American Society for Testing and Materials, 1976, pp. 289-307.

ABSTRACT: Samples must be representative and specimen preparation must introduce ,'is few changes as possible if microscopic information is to be significant in characterization and classification of soils and rocks and in interpretation of their engineering behavior. Soils and recent sediments are damaged most easily, and specimen preparation is most complex for these materials. Air drying may cause shrinkage and distortion; freeze drying and drying by the critical point method provide partial solutions. Even so, differences in the procedure followed in the removal or replacement of water can cause marked differences in appearance of specimens examined on the electron and optical microscopes. Sectioning and surface exposure may also introduce artifacts. Soil specimens for fabric study commonly are fractured when frozen, since cutting may not be used generally. The use of ion bombardment in metal coating and as an etch method is discussed, and the possibility of improving specimen preparation procedures by combining an etch technique with freeze drying is outlined. KEY WORDS: soils, rocks, microscopy, water replacement, freeze drying, ion beams ion bombardment

Engineers examine soils and rocks microscopically to understand their behavior and properties better and to place classifications on a more scientific foundation. The most commonly used instruments are the optical microscope, the transmission electron microscope, and the scanning electron microscope. X-ray diffraction procedures are used widely in all areas of microstructural analysis but will not be described in this paper dealing with microscopic methods. Laboratory preparation varies with the information sought and the technique employed. Fabric studies may involve drying or water replacement, fracture, or sectioning and replication, or coating. Information to ~Professor, Department of Civil Engineering, University of Calgary, Calgary, Alta., Canada.

289

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290

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

do with size distribution and morphology of minerals often involves specimen disaggregation and dispersion. Users of the optical microscope had less success in interpreting soil structure than was achieved in the related field of petrology because clay, often the most important constituent in soil, is too fine grained to be resolved satisfactorily by light optics. Nonetheless, useful information to do with both overall fabric relations and composition of silt- and sand-sized minerals can be obtained. Light will pass through a standard thin section (',,30 /am) of most soils, and slides are made by a modification of the procedure used for the rocks. Specimens examined on the transmission electron microscope may be individual particles, replicas, or ultrathin sections. The microtome has been employed to make sections from soils, supported by impregnating agents, but practical difficulties limit the method to a few select clays, and replicas or dispersed particles are examined, more commonly. The specimen is deposited on a metal disk containing a fine grid, which may range from 200 to 1000 meshes to the inch, depending on the application. Special supports are available also or can be made for particular uses. The grid is coated with collodion, formvar, carbon, or other material, evaporated under vacuum. In general, evaporated films give the best resuits. In the scanning electron microscope, there is a reflection geometry between the beam incident on the specimen and the image-forming beam, so the electrons do not have to pass through the material, and this simplifies specimen preparation. Dried materials are needed for replication and for direct viewing on the electron microscope because the specimen has to be placed in the high vacuum system where water molecules would evaporate and collide with the electrons and cause a loss of resolution. Air drying may be inadequate because many soils shrink markedly, and fabric relations may be changed; it is also possible that the morphology of thin clay minerals may be affected. Drying shrinkage results from an increase in surface free energy of the material and a rise in the capillary potential or soil moisture suction; moisture migration is a common accompaniment. Sometimes, water is replaced before optical thin sections are made, and this is also sometimes done in connection with studies involving the electron microscope. The method by which the surface of the soil or rock is exposed requires consideration when a replica is to be prepared or when the specimen is to be viewed on the scanning electron microscope. In the latter case, even though the reflection geometry makes possible the examination of thick materials, the information is derived from only a very thin layer because of the low penetrating power of the electron beam. Indeed this fact may sometimes be used to an advantage. If the accelerating potential is reduced, the depth of penetration of the electron beam is reduced also, so the

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GILLO'I-I" ON SPECIMEN PREPARATION IN MICROSCOPY 291

image-forming electrons originate near the surface. When there are thin, platy, or elongate particles projecting into voids or above the surface of the specimen, the image formed at low kilovoltage reveals greater surface detail. At higher kilovoltage, electrons which form the image may originate from both surfaces of platy minerals if the primary beam passes through the thin particle (Fig. la and b). Most soils deform more readily than rocks at low stress levels and dilate when unloaded. Relief of pressure may be accompanied also by exsolution of gas from pore liquids. Such changes may distort the fabric, and the collection of specimens for fabric study should be carried out so as to reduce these effects to a minimum. Even though special core samplers are used, it is still probable that they sometimes cause fabric damage, particularly near the edges. Bulk specimens are more satisfactory, but collection is not always possible. The field orientation should be recorded, and the specimen coated or packaged to prevent drying.

Microscopic Methods The polarizing microscope is the standard version of the light (optical) microscope used for examination of materials such as rocks and soils. By about 1880, inherent limitations of glass lenses, which gave rise to chromatic aberration, spherical aberration, coma, astigmatism, distortion, etc., had been corrected almost completely, and, by 1931, when the electron microscope was invented, the optical microscope was not very inferior to the modem instrument. Today, as in 1931, performance of the light microscope is limited by resolution, which prevents formation of images which show detail in the range less than 0.2 ~an. Spacings which are smaller than about one third the wavelength of the wave motion do not cause diffraction. Since diffraction is involved in image formation, it follows that detail less than about 2000 A may not be observed, even theoretically, when light optics are used, since the wavelength range of the visible spectrum is 4000 to 7600 A. Practical limitations commonly make for unsatisfactory viewing of mineralogical materials in the micrometer size range. In practice, little more than general orientation can be deduced from clay soil sections of standard thickness (~30/am). Many other techniques, such as phase contrast and ultraviolet microscopy, are now available but have received scant attention in this field, and details of the morphology of minerals in the clay size range and fabric relations in fine grained rocks and clay soils are obtained generally by observations on transmission or scanning electron microscopes. When it was realized that electrons have the properties of waves as well as particles, and electron diffraction was established, the production of an electron microscope became a possibility. The wavelength of the electron beam is related to the accelerating potential, and, at 60 kV, it is about

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292

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 1--Scanning electron micrographs of rocks and soils. 0.05 ~,. As the resolution of a microscope is directly proportional to the wavelength of the radiation used to form the image, it is evident that it Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

GILLOTI" ON SPECIMEN PREPARATION IN MICROSCOPY

293

should be possible to observe much f'mer detail with an electron microscope than with a light microscope. The correctness of these ideas was demonstrated with the first electron microscopes. Electron lenses suffer from similar defects to glass lenses, but electron optics is a much younger science than light optics, and electron lenses are not nearly so well corrected as are glass lenses. Even so, detail as fine as about 1.5 A can be resolved with transmission electron microscopy and lattice spacings of about 2 A observed, mainly because of the very short wavelength of the electron beam. The approximate resolving power of the unaided eye, optical, and electron microscopes, in relation to the wavelength of the electromagnetic spectrum and particle size range of fine sediments, is shown in Fig. 2. At the present time, there are two major classes of electron microscope--the transmission instrument and the scanning dectron microscope. In the transmission microscope, the beam passes through the specimen or a replica, whereas a reflection arrangement is employed in the scanning microscope. It is also possible to employ reflection methods with the transmission microscope. Recently, scanning transmission electron microscopes, which combine the features of both instruments, have become available commercially. High voltage microscopes, which may be operated at a potential of the order of one million volts, are also available; among other advantages, the greater penetrating power of the beam makes it possible to examine thicker specimens. The versatility of the electron microscope has been increased greatly by a variety of analytical attachments. The facility to record general polycrystalline or selected area electron diffraction pattern was one of the first. Mineralogical and structural analysis is thus possible from areas as small as 250 A with some transmission instruments. Attachments to the scanning electron microscope now make possible elemental analysis by energy dispersive and wavelength dispersive X-ray spectrometric techniques from areas of 1/am or less. Light elements below about atomic number eight are difficult to detect because of the low X-ray yield. These elements may be picked up by Auger electron analysis [1]2 and by measurement of energy loss of electrons transmitted through very thin specimens. Observation in the cathodoluminescence mode and use of auxiliary devices, such as the hot-cold stage, deformation stage, etc., give additional information (Fig. 3).

Specimen Preparation for Fabric Analysis Fabric refers to the orientation and arrangement of particles--individual crystals or more or less cemented agglomerations of crystalline and noncrystalline material--and voids. The word microstructure is sometimes The italic numbers in brackets refer to the list of references appended to this paper. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

294

)'-RAYS

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING VISIBLE ULTRAVIOLET

X-RAYS

I

I

I0-10

INFRARED

I

I0-8

I0- 6

(1~)

MICROWAVES

I

I

I0" 4

I0-2

(I/J.m)

[_ ELECTRON I " OPTICAL i~ MICROSCOPE MICROSCOPE CLAY ---

SILT

UNAIDED EYE

SAND

I

1

2/J.m

RADIO

I I

/ELECTROMAGNETIC

-----L, WAVELENGTH (METRES) ~

(

SPECTRUM

LIMIT OF L RESOLUTION S~)IL CLASSIFICATION

GRAVEL I

60Fro

CLAYS AND SILTS

M. I . T .

2 rnm

SANDS

GRAVELS A.S.T.M.

75p.m ( 5 0 % OR MORE PASSES No. 2 0 0 SIEVE )

4.75 mm

FIG. 2--Resolving power relative to soil particle size range and wavelength of electromagnetic spectrum.

ELECTRON

SECONDARY ELECTRON IMAGE LUMINESCENCE

BEAM

BACK- SCATTERED ELECTRON IMAGE

~

ENERGY

TENSILE/COMPRESSIVE IS/SHE, c IME N'/ A HOT/COLD STAGE

9 ~r

r t

t

~ , ' ," J

STAGE

r

DISPERSIVE

LWAVE - LENGTH DISPERSIVE

SCANNING T P,ANSMISSION

FIG. 3--1maging and accessory techniques in scanning electron microscopy. used similarly, but, in soil engineering, this term also implies consideration of the bond between the particles and their interactions in the soilwater system, as well as fabric relations. Optical and electron microscopes have been the instruments used most commonly in fabric investigations, though X-ray diffraction and radiography have been employed also. In some studies, more than one technique has been used to analyze the fabric of similar specimens [2 ]. Fabric has also been analyzed by measurement of sound propagation [3 ], thermal conductivity [4], and dielectric properties [5 ]. In these cases, the anisotropic or isotropic nature of the specimen, with respect to the physical property measured, reveals indirectly the presence or absence of preferred orientation among the mineral particles. Specimen preparation techniques differ, depending on the method of fabric analysis, but moisture removal or replacement from poorly lithified materials and exposure of the surface for examination present problems common to most microscopic methods of fabric study. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

GILLOTT ON SPECIMEN PREPARATION IN MICROSCOPY

295

Drying Air Drying--Spedmens may be air dried if shrinkage on drying is not regarded as detrimental to the fabric information to be obtained. It is evident that drying shrinkage will effect the greatest changes in fabric when the specimen is weak and the environment in which it is found is not subject to wetting and drying cycles. For example, air-dried sensitive soils often show a relatively dense fabric when compared with the more open fabric of the same area when the specimen is frozen on a cold stage (Fig. 1c and d). Displacement of minerals is also readily apparent if. a comparison is made between these two photographs; this is not to imply that the frozen soil is undisturbed, since freezing is accompanied by volume change, which may well cause fabric disturbance. Replacement of the water (surface tension 72.75 x 10- 3 N/m at 20~ by another more volatile liquid, such as ethyl alcohol (22.75 x 10 -3 N/m) or acetone (23.70 x 10-3 N/m), should reduce drying shrinkage. As with other replacement techniques, fabric damage due to fluid migration is a possibility. Freeze DryingmFreeze drying reduces shrinkage and distortion of the specimen because ice is removed by sublimation. The surface energy of the solid-vapor interface is commonly higher than that of the liquid-vapor interface, but no meniscus forms in pores and capillary spaces. Since ice is much less mobile than liquid water, fluid migration into finer pores is largely prevented as moisture is removed. The method, however, has certain drawbacks. When the specimen is frozen, the conversion of liquid water to ice one in larger void spaces will be accompanied by an increase in specific volume of about 9 percent. So specimen dilation at the outset may induce fabric change. Moisture movements within the specimen, on freezing, have been attributed to at least three causes. First, the unfrozen solutions will become more concentrated, and differences in concentration may arise because ice already formed will be composed of fresh water. So moisture will migrate to reestablish equilibrium in solution concentration and also to feed growing ice crystals. Vapor pressure gradients are thought to be another cause of moisture movements. Water held close to surfaces remains unfrozen because its freezing point is depressed. As it is metastable, however, its vapor pressure is high, relative to that of ice elsewhere in the system. So desorption takes place, and the water migrates towards the ice in response to the gradient [6, 7]. Fabric changes have also been attributed to the tendency of crystals to expel impurities to grain boundaries [8] and to grain growth of ice crystals during freezing [9, 10]. Small crystals are probably less damaging in these regards, so the use of ice nucleators, such as amino acids [11], and clathrate hydrate formers, such as chloroform and ether [12], should be considered. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

296

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

Theoretical considerations show that the possibilities of fabric damage on freeze drying are reduced if ice #ass, rather than crystalline ice, is formed. For example, there is no tendency for material to be concentrated at ice grain boundaries, since they do not exist in glass, and the volume increase on solidification of the water is reduced. Glass formation is favored by rapid cooling, by high viscosity near the freezing point, and by depression of the freezing point. Direct immersion in liquid nitrogen (N2) ( - 196~ appears to cause rapid freezing, but the rate of cooling is less than required because of the formation of an insulating envelope of gas surrounding the specimen. This pitfall can be avoided if the specimen is immersed in propane, isopentane, or Freon 22, supercooled in liquid N2. These compounds have a low freezing point, so they are liquids at very low temperatures. In addition, they have a relatively high boiling point, so they are not vaporized as readily by heat from the specimen as is N2; therefore, cooling is more efficient. That cooling rate and drying procedure can affect fabric is underlined by the remarkable differences in appearance of flocculated Wyoming bentonite when air dried and when freeze dried, following freezing by direct immersion in liquid N2 and by immersion in supercooled Freon 22 (Fig. 4a, b, and c). An increase in viscosity to favor ice glass formation has been used experimentally by biologists. They have introduced glycerol, dimethyl sulfoxide, ammonium acetate, polyvinyl pyrrolidone, and sucrose into tissue fluids with some success. The tendency to moisture migration, due to vapor pressure gradients, should be reduced also because of the smaller difference in vapor pressure between the liquid and solid phases. The vapor pressure of the solution is reduced by the low vapor pressure solutes, while that of the solid is raised, since ice glass has a higher vapor pressure than crystalline ice. Even when ice glass is formed successfully in the specimen, there is a considerable possibility that it will devitrify and recrystallize during sublimation. It has been reported that this occurs at about - 130~ [13], which is close to the recrystallization temperature--a temperature of about half of the melting point, in degrees Kelvin, at which many materials recrystallize. However, the rate of sublimination is extremely slow, owing to the low vapor pressure of ice below this temperature. Replacement of water by a liquid such as amyl acetate, which solidifies in the vitreous state more readily than water, and which has a relatively high vapor pressure below the freezing point, may be advantageous. The possibility of fabric damage, due to flow of fluids during water replacement, is again a consideration. Another partial solution is to freeze-etch the specimen. A surface layer of ice is sublimated at a very low temperature (~,, - 1 0 0 ~ by passing a metal surface at an even lower temperature (~ - 196~ the temperature of liquid N2) close to the specimen. The temperature gradient leads to rapid sublimation of the ice from the surface layer of the specimen. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

GILLO'I-I" ON SPECIMEN PREPARATION IN MICROSCOPY

297

FIG. 4--Scanning electron and optical micrographs showing effects of specimen pretreatment.

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298

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

Thus the surface, which is replicated for examination on the transmission electron microscope or examined directly on the scanning electron microscope, is dried under very favorable conditions. The bulk of the ice from the remainder of the specimen is removed generally by sublimation at a higher temperature. If a hot-cold stage is available, the specimen may be examined on the scanning electron microscope in the frozen state. Reasonably good micrographs may be taken up to about x 2000 magnification without the need to metallize the specimen. Frost sometimes forms on the surface and obscures detail. It may be removed by sublimation in the microscope if the temperature is raised slightly for a short time.

Critical Point Drying--Critical point drying involves raising the temperature and pressure of the liquid phase to values above those of the critical point at which the physical properties of a liquid and its vapor become the same. At this point, the interface vanishes, so surface tension forces cease to exist. The critical point of water is at a temperature of 374~ and at a pressure of 2.25 • 107 N/m 2, so the specimen has to be placed in a strong container to withstand the pressure. The critical point occurs at lower values of temperature and pressure for other fluids. In the case of liquid carbon dioxide, it is at 31 ~ and 73.97 x 105 N/m s, and for ethyl alcohol, it is at 243~ and 65.26 x lff N / m s. Various authors have used the method, either evaporating the water itself or replacing this by another liquid and removing the new fluid subsequently, under supercritical conditions [14-17]. Supercritical water is known to be chemically reactive, but water replacement involves the same problems associated with fluid flow, as discussed previously. Exsolution of gas from pore fluids, osmotic effects, and pressure buildup, due to inadequate permeability of the specimen during fluid expansion, are other possible causes of fabric damage. Surface interactions between pore fluids and soil minerals will cause the critical point to deviate from that of the bulk liquid, so drying by this method should be carried out at temperatures and pressures above those of the published values for the critical point for the water or replacement liquid. Surface Exposure Surface exposure requires care in the preparation of specimens for either scanning electron microscopy or transmission electron microscopy. Cutting should not be used generally, since it is a shearing action which may orient platy or elongate minerals, though it may be possible to reduce or overcome this problem by impregnation. Specimens which are Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

GILLOTT ON SPECIMEN PREPARATION IN MICROSCOPY

299

to be freeze dried are generally fractured when frozen, the technique being termed freeze fracturing. Air dried soils and specimens dried by the critical point method are fractured generally after drying. Air dried soil is often hard and brittle, but critical point dried material may be very fragile, and loose particles are liable to fall on the surface and obscure the true fabric. Some particles of this sort can be removed by a jet of compressed clean air or gas. If the specimen is sufficiently strong, it may be peeled with adhesive tape [18]. A plane of failure may be predetermined to some extent by notching the specimen, but the fracture will propagate along a surface of weakness, so it may not be completely representative. Conversely, careful study of fracture surfaces [19] can give information regarding crystal orientation, defects and the distribution of impurities, particularly when these are concentrated at grain boundaries. Electron fractography has given useful information in the study of metals and other materials.

Preparation of Optical Thin Sections When the optical microscope is used to study fabric, thin sections of soils are made by a modification of the method employed routinely for making thin sections of rocks. The water in the soil is either removed or replaced before the thin section is made. If the water is removed, it is common practice to support the specimen by impregnation with a suitable resin before sectioning. Often, the water is replaced by Carbowax 6000 [20]. This replacement is often accompanied by volumetric change, the amount of which depends on the void ratio, but which is nonetheless much smaller than that caused by air drying of a water-containing specimen [21]. Carbowax crystallizes on slow cooling, and the spherulites may sometimes be seen superimposed on the original fabric of the soil. Rate of cooling affects the size of the spherulites markedly (Fig. 4d, e and ./) and ultrarapid cooling in Freon 22 surrounded by liquid N2 largely overcomes the problem of spherulite formation in soil specimens [22]. Since Carbowax 6000 is water-soluble, the thin section is made by dry grinding or by grinding in an organic liquid such as kerosene. Specimen Preparation for Analysis of Particle Size and Shape

Individual clay mineral particles may be examined on the electron microscope. On the transmission instrument, agglomerations show few distinguishing features because the electron beam fails to pass through the specimen. Also, overheating is more probable, due to absorption of the electron beam and accompanying interactions in the specimen. Phase changes, fusion, and evaporation or sublimation of the specimen, and destruction of the support film may result. On the other hand, when Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

300

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

clay minerals are too thin ( •50 A), contrast becomes inadequate for the clear observation of detail; shadow casting generally solves this problem. Disaggregation is carried out in water by hand grinding, by use of a homogenizer, ultrasonic vibrator, or other methods. The simplest procedure is to allow a drop of the suspension to evaporate on the specimen grid, which has been previously coated with a support film. This technique often gives unsatisfactory results because the particles agglomerate during drying, though this tendency can be reduced by use of a dispersing agent. As with bulk clays, droplets containing suspended clay may be freeze dried or dried by the critical point method. The droplet is usually placed on a support grid, prior to freezing and drying. Association of smectites freeze dried from droplet suspension has been described [9,23], and marked differences in fabric underline heavily the importance of specimen preparation (Fig. 4a, b, and c). Various methods of spray drying have been employed also. An atomizer or nebulizer is used to produce a mist of clay particles in water droplets, which are collected on the support grid. The suspension placed in the nebulizer may contain a dispersing agent, or the solution may be designed to gel during drying to prevent reaggregation of the clay minerals [24]. Other modifications have included use of an electrostatic precipitator [25] and paper extraction thimble [26] to improve the efficiency with which the clay is recovered. The mist may be dried by passing it down an electrically heated tube. Another technique is to disaggregate the specimen in amyl acetate and make a smear between two glass slides. When dry, this may be shadowed and backed by collodion or evaporated carbon before being floated off on distilled water, scooped up, and placed on the specimen grid. Electrodeposition [27] and replica techniques for study of dispersions have been employed also. Particle shape and size distribution information can, of course, be obtained from bulk specimens, and this method has some advantages, since disaggregation is sometimes accompanied by breakup of particles, so that shape and size distribution are affected. Ion Bombardment

Etch techniques have increased the information obtained by microscopic examination of rocks, metals, and other materials, but have been little used in the study of soils because, until recently, there were no dry techniques available. It has been known for over a hundred years that ion bombardment of solid surfaces is accompanied by physical and chemical interactions (Fig. 5). The ejection of atoms from the solid is termed sputtering and is thought to result from a transfer of momentum, due to a series of collisions between the incoming ion and the atoms of the target. Recently, the effect has been used as a means of etching. The Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

GILLOTT ON SPECIMEN PREPARATION IN MICROSCOPY

301

ION BOMBARDMENT t

NEUTRAL ATOMS r REFLECTEDNNEU~I

ION IMPLANTS - - ~ t ~ S

,

RADIATION ~f~

(~ //1~~ p ECi M EN " ~

ECONDARY ELECTRONS

HEAT

I

ARTIFACTS ETCH PATTERN DUETO LOW YIELD DUE TO GRAINBOUNDARIES, COMPONENTS,CONTAMINATION,COMPOSITIONDIFFERENCES, DEFECTS, ETC. CRYSTALORIENTATION-SELECTIVE EROSIONOF DENSEATOMICPLANES

,

SPUTTERCOATING METAL DEPOSITED FROM NUMEROUS DIRECTIONSAND IMPLANTED

FIG. 5--Results and uses of ion bombardment. rate at which the surface is eroded depends upon the nature of the solid, the crystal orientation, and structure, and on the energy, angle of incidence, and nature of the incident ions. Various methods have been used to neutralize the positive charge which builds up on the surface of insulators. These have included an electron beam [28], a Langmuir probe [29], and imposition of a high frequency field [30]. Much recent work has been stimulated by interest in ion pumping, surface reactions, the nature of interatomic forces, ion implantation of semiconductors, and film preparation in microelectronic circuitry. There are extensive reviews of the principles [31-34]. Ceramics, fibers, biological materials, and metals have been sputter etched, and thin foils for observation on the transmission electron microscope have been prepared by this technique. Limitations arise because of differences in the rate at which the surface is eroded, so that artifacts results. Conical shapes are often produced, due to the presence of variable yield components and to effects of original topography which determine the local angle of incidence of the ion beam and so influence rate of sputtering. Such problems are reduced by rotation of the specimen. The effects of topography may also be reduced by polishing rocks before they are etched. In some limestones, grain boundaries between dolomite (rhombic crystals ~30 /am) and matrix (calcite as irregular crystals r~2 to 10 gin and clay) were etched selectively, and circular pits were visible in the polished surface after bombardment with argon ions in a r-f field. As a method of revealing fabric detail, the overall result Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

302

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

of ion etching has, so far, been somewhat disappointing in comparison with acid etching. Nonetheless, in some areas the disturbed polished layer was removed, and the structure of the f'me matrix was revealed. The detail made visible by ion etching a polished limestone may be compared with that seen in the same polished limestone after acid etching (Fig 1 e and f ) . Contrary to present experience, other workers have reported that, in some metals, hillocks form at grain boundaries and also in other areas where atoms are relatively protected from collision chains, as at the point of emergence of dislocations extending perpendicularly below the sputtered surface. Equipment for ion etching is available as an attachment to the scanning electron microscope, so that the effects of sputtering can be observed on a before and after basis. The metal coating is, however, removed by the ion beam, so a loss of resolution is common in the observation of nonconductors. Other commercial equipment is available, which operates as an independent unit.

Replication, Shadowing, and Coating A replica is a thin film, relatively transparent to electrons, which reproduces the surface features of the specimen reasonably faithfully. Replicas are made because the electron beam has a very low penetrating power which depends upon the accelerating potential and the atomic number of the atoms in the specimen. At 50 to 100 kV, the beam will generally only pass through specimens which are between 1000 and 2000 A in thickness. Thus, replicas provide a relatively simple alternative to ultrathin sectioning of bulk specimens and, in the study of disaggregated material, avoid many problems caused by agglomeration of particles. The best replicas are made by vacuum evaporation. Carbon commonly is deposited to a thickness of about 100 A, from a direction normal to the surface (Fig. 6). The carbon replica may be backed by polystyrene, and the clay specimen removed in water and by dissolution with hydrofluoric acid. The polystyrene is dissolved in ethylene dichloride, the replica washed by being floated on several dishes of distilled water, and mounted on the specimen support grid. Replicas may also be made of plastics, formvar, or collodion applied as a liquid or plastic. This procedure is generally not well suited to clays, as the liquid flows into pore spaces, and true surface features become hard to recognize. Also, plastic often has to be pressed against the surface to obtain the required intimacy of contact for good replication, and pressure is obviously undesirable for weak or friable specimens. Replicas made by vacuum evaporation are more suitable. Both one and two stage replicas are used, and positive or negative impressions of the surface may be made. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

GILLOTT ON SPECIMEN PREPARATION IN MICROSCOPY

303

APPARATUS CHAMBEREVACUATEII"~ HEAVY META "~'~"~ SHADOWING- [ ~ " ~

~ECTRIC

ARC

Ii' L INDICATOR dl ~ CARBONREPLICA "~SAMPLE |

|

I I I (~) REPLICATION WITH EVAPORATED CARBON PRE-SRADOWING/; ,~ W,TH HEAVY ~ METAL J ~/ ~ ~~ ~~ ' ~ CARBON REPLICA

I

I

REPLICATION WITH (~) EVAPORATED CARBON POST-SHADOWING) ~. l ! WITH H E A V Y / ~ ~. ~ ,~ METAL / ~ ~ ~ r CARBON J'~ ~ -~. ~ REPLICA

FIG. 6--Shadow casting and replication for examination on transmission electron microscope.

Contrast in the electron image results largely from differences in scattering power of different parts of the specimen or replica. This may result from surface roughness in the object, which causes different parts of the surface to present orientations to the beam, which range from normal to near parallel; hence, the thickness which the electrons must traverse varies. However shadowing at 20 to 45 deg by a heavy metal such as platinum or gold/palladium of either the specimen or replica is a more reliable method of ensuring that contrast is adequate (Fig. 6). The metal is deposited generally to a thickness of about 10 A. In addition, the height or shape of surface features may be derived if the shadowing angle is known and shadow lengths are measured. Details of the procedures for shadow casting and replication have been published [35-37]. In the scanning electron microscope, nonconductors such as rocks and clay soils rapidly acquire a charge which causes a serious loss of resolution. Charge buildup is prevented by the deposition of a conductive layer on the specimen surface. Metal coating is carried out commonly in a vacuum chamber by evaporation of a suitable metal from an electrically-heated filament (Fig. 7). Sputtering may be used also (Fig. 5). This method often gives a superior coating to that obtained by the more conventional thermal evaporation technique. Problems due to charging are reduced greatly, and this advantage is particularly noticeable when the specimens are porous or have an irregular topography, as is the case in fabric studies of many clay soils. The better coating results in part, probably, from

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304

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

APPARATUS ~

~

PLANETARY ROTATION OF SAMPLES EVACUATED CHAMBER EVAPORATED METAL COATING

STEPS HIGHER THAN COATING THICKNESS AND RE-ENTRANT ANGLES TEND TO BREAK CONTINUITY ANO IMPAIR EFFICIENCY OF CONDUCTIVE LAYER BREAK IN COATING ED ATING

EVAPORATED METAL COATING

FIG. 7--Metalizing specimens for examination on scanning electron microscope.

the higher energies with which the sputtered atoms reach the specimen, where they form a better bond to the surface being coated. Another factor is thought to arise from the relatively poor vacuum in the system. This leads to collisions between the gas and metal atoms being deposited, which consequently arrive at the specimen from numerous angles so surfaces not directly in line of sight with the source receive a metal coating. Discussion

It is now accepted widely that constituents in the micrometre and submicrometre size range and crystal imperfections play an important and often decisive role in the engineering behavior of soils and rocks. The detail accessible to microscopic observation has been extended almost to the angstrom range. Attachments available to existing microscopes have made them almost as much a miniature laboratory as an instrument for observing fine detail. There is every indication that this trend will continue. If the information is to be meaningful, the specimen must be representative, and specimen preparation must introduce as few artifacts as possible. The scanning electron microscope makes it possible to resolve detail well beyond that accessible to observation by means of the optical microCopyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

GILLOTT ON SPECIMEN PREPARATION IN MICROSCOPY

305

scope; its introduction has also simplified specimen preparation for robust specimens. The preparation of fragile soils and sediments is, however, still relatively complex, though the problems are well understood. All present techniques have limitations, but procedures most likely to give satisfactory results are established. It is best to examine specimens for fabric analysis on both the optical and electron microscopes. The optical instrument often gives an overview of general fabric relations, and the presence of such features as preferred orientation are generally more easy to detect than on the electron microscope. Relative disposition of minerals, contact relations, authigenic crystal growth, morphology of minerals, and surface texture of sediment grains are observed best on the electron microscope. The preparation of optical thin sections is a well established technique and is normally no problem, unless weak soils or sediments are to be sectioned. Moisture removal or replacement and impregnation are then required. Specimens for observation on the electron microscope have to be dried. In fabric studies of soils not thought to have dried in nature, this is accomplished best by the critical point method or by freeze drying; freeze etching is desirable. It is probable that ice~glass forms only very close to the surface of soils, even when cooling is ultrarapid. A fracture most probably passes through parts of a frozen specimen where there is crystalline ice; so, it is improbable that the surface exposed by freeze fracture ever contained ice glass. For this reason, a procedure such as ion beam etching should make it possible to improve specimen preparation technique. Observation on the scanning electron microscope of the corner of a rectangular specimen discloses fabric relations on three orthogonal surfaces. Ice glass could be formed close to the three surfaces of such a specimen and dried by freeze etching. A specimen of soil is readily cut to this shape, but the surfaces would not be representative because of the disturbance of the minerals caused by the shearing action of cutting. If the disturbed layer can be removed by ion bombardment or other etch technique and without production of new artifacts, three representative surfaces of a specimen, which has been dried under very favorable conditions, could be viewed. When the higher resolution of the transmission electron microscope is required, replica techniques offer a more convenient procedure than ultrathin sectioning (Fig. 8). Specimen preparation for analysis of particle shape or size, or single crystal electron diffraction, may be carried out by one of the numerous techniques of dispersion (Fig. 9). Conclusions

The preparation of robust specimens for microscopic examination presents few problems generally and may be carried out by routine proCopyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

306

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

SAMPLE

FREEZE / FRACTURE/DRY OR CRITICAL po INT DRy / F RACTURE~,,,,,,,,,,,~ I

REPLACE WATER (CARBOWAX6000)

IMPREGNATE~

I SECTION

r

I

METAL COAT SEM

I

I REPLICA

I

I

]

OPT,CAL MICROSCOPE

IMPREGNATE

'

ULTRA THIN SECTION I

TEM

TEM

FIG. 8--Specimen treatment prior to microscopic examination of soil fabric.

SAMPLE

I

DISPERSE

I I

DRY

J

OPTICAL MICROSCOPE

i

METALIZE I SEM

I I REPLICA I TEM

I TEM

SEM" SCANNING ELECTRON MICROSCOPE TEM: TRANSMISSION ELECTRON MICROSCOPE

FIG. 9--Specimen treatment prior to microscopic examination of particle shape and size of soil minerals. cedures. When the specimen may be distorted by drying shrinkage, more elaborate techniques of specimen preparation are required. While the problems are fairly well identified, present methods provide only a partial solution. Technical developments in the next few years may lead to improvements. Significant improvements in microscopic methods are to be expected from new and more versatile auxiliary stages. Acknowledgments

I wish to express my sincere thanks for technical assistance to Howard Johnson, Lorna Moffat, Peter Gifford, and Bob Fitch. Financial support from the National Research Council of Canada and the Defence Research Board of Canada is acknowledged. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

GILLOTT ON SPECIMEN PREPARATION IN MICROSCOPY

307

References [1] MacDonald, N. C., Marcus, H. L., and Palmburg, P. W., Proceedings, O. Johari, Ed., 3rd Annual Scanning Electron Microscopy Symposium, liT Research Institute, Chicago, Ill., 1970, p. 25. [2] Gillott, J. E., Journal o f Engineering Geology, Vol. 4, No. 2, April 1970, pp. 133-153. [3] Kaarsberg, E. A., Journal of Geology, Vol. 67, No. 4, 1959, pp. 447-472. [4] Penner, E. in International Clay Conference, Vol. 1, I. T. Rosenquist and P. GraftPeterson, Eds., Pergamon, London, 1963, pp. 365-376. [5] Winkelmolen, A. M., American Association Petroleum Geol~,ists Bulletin, Vol. 56, No. 11, 1972, pp. 2150-2159. [6] Weiser, C. J., Science, Vol. 169, 1970, pp. 1269-1278. [7] Litvan, G. G., Cryobiology, Vol. 9, 1972, pp. 182-191. [8] Lincoln, J. and Tettenhorst, R., Clays and Clay Minerals, Vol. 19, 1971, pp. 103-107. [9] Hoffman, U., Scharrer, E., Czerch, W., Friihauf, K., and Burck, W., Berichte der Deutscher Keramischen Gesellschaft, Vol. 39, 1962, pp. 125-130. [10] Borst, R. L., Journal o f Petroleum Technology, Oct. 1971, pp. 1193-1201. [11] Garten, V. A. and Head, R. B., Nature, Vol. 205, No. 4967, 1965, pp. 160-162. [12] Tracey, M. V., Proceedings, Royal Society, Vol. 171, 1968, pp. 59-65. [13] Meryman, H. T. and Kafig, E., "The Study of Frozen Specimens, Ice Crystals and Ice Crystal Growth by Electron Microscopy," Research Report NM 000 018.01.09, Naval Medical Research Institute, Bethesda, Md., 1955, pp. 529-544. [14] Anderson, T. F., Proceedings, 2nd International Conference on Electron Microscopy, 1950, pp. 567-576. [15] Horridge, G. A. and Tamm, S. L., Science, Vol. 163, 1969, pp. 817-818. [16] Gillott, J. E., Journal of Sedimentary Petrology, Vol. 39, No. 1, 1969, pp. 90-105. [17] Gillott, J. E., Clay in Engineering Geology, Elsevier, New York, 1968, pp. 296. [18] Sides, G. and Barden, L., Canadian Geotechnical Journal, Vol. 8, 1971, pp. 392-399. [19] Zapffe, C. A., Worden, C. O., and Zapffe, C., American Mineralogist, Vol. 36, 1951, pp. 202-232. [20] Mitchell, J. K., Proceedings, Highway Research Board, Vol. 35, 1956, pp. 693-713. [21] Quigley, R. M. and Thompson, C. D., Canadian Geotechnical Journal, Vol. III, 1966, pp. 61-73. [22] Gillott, J. E., Clays and Clay Minerals, Vol. 22, 1974, pp. 199-200. [23] Bates, T. F., "Selected Electron Micrographs of Clays," Circular No. 51, College of Mineral Industries, Pennsylvania State University, University Park, Pa. [24] Schofield, F., British Journal o f Applied Physics, Vol. 2, 1951, pp. 18-19. [25] Jonas, E. C. and Kuykendall, J. R., Clay Minerals, Vol. 6, 1966, pp. 232-235. [26] Hughes, R. and Bohor, B., American Mineralogist, Vol. 55, Sept.-Oct. 1970, pp. 17801786. [27] Mering, J., Oberlin, A., and Villiere, J., Bulletin Societie Francaise Mineralogie et Cristallographie, Vol. 79, 1956, pp. 515-522. [28] Akishin, A. I., Vasil'ev, S. S., and Isaev, L. N., Bulletin o f the Academy of Sciences o f the U.S.S.R., Physical Series, Vol. 26, 1963, p. 1379. [29] Jorgenson, G. V. and Wehner, G. K., Journal of Applied Physics, Vol. 36, 1965, p. 2672. [30] Anderson, G. S., Mayer, W. N., and Wehner, G. K., Journal of Applied Physics, Vol. 33, 1962, p. 2991. [31] Tsong, I. S. T. and Barber, D. J., Journal o f Materials Science, Vol. 8, 1973, pp. 123135. [32] Holland, L. and Priestland, C. R. D., Vacuum, Vol. 22, No. 4, 1972, pp. 133-141. [33] Carter, G. and Colligon, J. S. in Ion Bombardment o f Solids, Heinemann, London, 1968, p. 446. [34] Davidse, P. D., Vacuum, Vol. 17, No. 3, 1966, pp. 139-145. [35] Comer, J. J. and Turley, J. W., Journal o f Applied Physics, Vol. 26, 1955, pp. 346350. [36] Techniques for Electron Microscopy, Kay, D., Ed., BlackweU, Oxford, 1965. [37] The Electron-Optical Investigation of Clays, Mineralogical Society, Gard, J. A., Ed., London, 1971. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

A . L Johnson' a n d R. P. M o s t o n 2

Use of Ultrasonic Energy for Disaggregation of Soil Samples

REFERENCE: Johnson, A. I. and Moston, R. P., "Use of Ultrasonic Energy for

Disaggregation of Soil Samples," Soil Specimen Preparation for Laboratory Testing, ASTM STP 599, American Society for Testing and Materials, 1976, pp. 308-319. ABSTRACT: A method for faster and more nearly complete dispersion of soil samples for particle size analysis now appears feasible. Using agitation by ultrasonic energy, disaggregation and dispersion appeared to be as thorough in 10 min as by mechanical preparation through a sieve with 0.0625-mm openings. The ultrasonic agitation appeared to cause no deleterious effect, on the primary particle sizes. The time and effort required to obtain high quality dispersion is much less for ultrasonic than for mechanical techniques, especially for heavy soils. KEY WORDS: particle size, nondestructive tests, soil properties, ultrasonics, soils, tests A s o n e p h a s e o f r e s e a r c h o n l a n d subsidence in C a l i f o r n i a , a p p r o x i m a t e l y 500 u n d i s t u r b e d s a m p l e s were t a k e n f r o m d e p t h s as great as 2000 ft in t h e S a n J o a q u i n a n d S a n t a C l a r a Valley [1-3]. 3 T h e p a r t i c l e size d i s t r i b u t i o n f o r these s a m p l e s was a n a l y z e d in the G e o l o g i c a l S u r v e y ' s H y d r o l o g i c L a b o r a t o r y in D e n v e r , C o l o r a d o [4]. Because t h e m e c h a n i c a l d i s a g g r e g a t i o n o f these s a m p l e s was t i m e c o n s u m i n g , u l t r a s o n i c e n e r g y w a s s t u d i e d to f i n d a faster a n d s i m p l e r m e t h o d f o r t h e d i s a g g r e g a t i o n a n d s u b s e q u e n t d i s p e r s i o n o f t h e f i n e - t e x t u r e d samples. This p a p e r , b r i e f l y describing s o m e results o f this s t u d y , shows t h a t ultrasonic e n e r g y p r o v i d e s a m u c h faster a n d m o r e c o m p l e t e d i s a g g r e g a t i o n a n d d i s p e r s i o n o f the f i n e - t e x t u r e d soils. Ultrasonic Equipment

In t h e f o l l o w i n g discussion o f u l t r a s o n i c energy o r s o u n d , sonic s o u n d 1Assistant chief, Office of Water Data Coordination, U.S. Geological Survey, National Center, Reston, Va. 22092 2District materials engineer, Colorado Division of Highways, Grand Junction, Colo. 3The italic numbers in brackets refer to the list of references appended to this paper. 308

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JOHNSON AND MOSTON ON USE OF ULTRASONIC ENERGY

309

(range of sounds audible to the human ear) is considered to be greater than 18 to 20 kHz. All sound waves above the audible range are considered ultrasonic. The use of ultrasonic energy for the testing and processing of a wide variety of materials has invaded virtually every phase of science and industry [5,6]. One wide-spread use has been the ultrasonic cleaning of parts. Ultrasonic cleaning equipment was adapted to soil dispersion during this study. Energy required for ultrasonic cleaning is produced when mechanical vibrations in the cleaning liquid, induced by the high-intensity sound waves, cause "cavitation." Cavitation, or cold boiling, is established in liquids when the amplitude of an applied sound pressure is sufficient to cause continuous rupture of the entrapped air bubbles. Rod [7] states that the forces developed in the immediate locale of such collapsing bubbles are on the order of several hundred atraospheres of pressure, even though only a small fraction of this high pressure is required to initiate the process. If such cavitation is developed and aggregations of solids--such as clay particles, for examplemare immersed in the liquid, the individual solids (clay particles) are separated by the large forces associated with the cavitation. Because cavitation is induced in liquids under the action of a sufficiently intense sound source, the effectiveness is dependent upon the applied power. However, the cavitation does not increase linearly with applied power because of a scattering effect. Thus, there is a point beyond which an increase in power is no longer economical. For example, more than ten times the power needed at 10 kHz is required to cavitate water at 400 kHz. The number of ultrasonic transducers and the power used for any particular ultrasonic cleaning application will be determined, at least in part, by the operating frequency proposed for use in the equipment. Because cavitation is less effective as the frequency of sound rises, low frequencies are generally superior for cleaning purposes. However, slight changes in frequency (20 to 40 kHz, for example) have little effect, and frequencies of 40 kHz or more are usually used because of the human discomfort caused by sound of less frequency. Two main types of acoustic transducers are used to transform electricdriving energy into sound energy--the magnostrictive and the piezoelectric types. The magnostrictive-type transducer, in simple form, is nothing more than a bar magnet, usually in the form of a stack of thin sheets of nickel or a nickel alloy, which has a large change in dimensions per unit change in applied current. If the bar is wound with a few turns of wire and electric current is allowed to flow through this wire, the bar is magnetized and becomes shorter, and, when the current ceases to flow, the bar becomes longer again. Thus, if the current through the coil is varied at the resonant frequency of the bar, it can be made to oscillate regularly, and cavitation is developed at the liquid-transducer interface. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

310

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

For well-designed magnostrictive-type transducers operating at about 30 kHz, approximately 70 percent o f the electrical driving force results in effective sound energy, and the remainder is lost as heat. With piezoelectric-type transducers, the oscillations required to create cavitation are obtained by impressing opposite and alternating electric charges (positive and negative) on the top and b o t t o m faces o f slabs of crystalline materials such as quartz, barium titanate, and certain ferroceramics. H e n r y [5] states that, as often as several million times per second, the slab becomes thicker when the upper face is made positive and the lower face negative and becomes thinner when the charges are reversed. Behaving much as a piston, the upper face o f the crystal is driven at its resonant frequency and transmits its vibrations to the liquid in the cleaning chamber. Those piezoelectric materials which deliver the greatest acoustic output per unit of input voltage are generally the least stable electrically and mechanically; quartz is probably the most stable. Because o f lower internal losses, piezoelectric-type transducers (although more fragile) are probably more suitable than magnostrictive-type transducers for applications requiring frequencies above 100 kHz. Ultrasonic cleaning equipment, such as m a y be used for soil dispersion, consists of two main units: (1) an electronic signal generator, and (2) a fluid container with one or more attached acoustic transducers. A secondary fluid container, such as a beaker o f glass or metal, is used inside the primary fluid container to hold the soil or rock sample. Water is used in both the primary and secondary liquid container to provide the most efficient medium for transmission o f the ultrasonic energy. The ultrasonic cleaning apparatus used in the present investigation is described in Table I. This equipment was generously loaned to the study by m a n u facttlrers and by other offices o f the Geological Survey.

TABLE 1--Test data for ultrasonic equipment. Signal Generator

Transducers

Equipment Designation

Power Output, W

Operating Temperatures, ~

Power Input, W

A B C D E

125 to 500 35 to 140 60 to 240 50 to 200 40 to 120

125 unknown 83 130 to 150 110

125 to 500 20 to 80 60 to 240 50 to 200 40 to 120

F

50 to 200

150

Frequency, Type

barium titanate barium titanate barium titanate magnostrictive piezoelectric ceramic 50 to 200 barium titanate

kHz 38 90 40 20 40 40

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JOHNSON AND MOSTON ON USE OF ULTRASONIC ENERGY

311

Testing Methods The accuracy of any particle size analysis is dependent primarily on the effectiveness of the disaggregation and dispersion methods used to separate the soil sample into its fundamental particles. However, this analysis must be done without deleterious effects to the soil particles, such as the fracturing of any of the fundamental particles. The present study was initiated to determine the speed and effectiveness of disaggregation and dispersion of soil samples in water by the use of ultrasonic energy. With the standard procedure (ASTM Particle Size Analysis of Soils (D 422-72)) for determining particle size distribution, a soil sample is disaggregated and dispersed by: (1) mechanical manipulation, (2) agitation by shaking or stirring, and (3) chemical deflocculation. The mechanical manipulation is normally done by a mortar and rubber-covered pestle-taking as long as several hours to work a sample of suitable size down through a sieve with 0.0625-mm openings. For the study reported in this paper, the mechanical manipulation was followed with dispersion of the sample by at least 8 h soaking in distilled water and 8 h agitation by shaking in a reciprocating shaker. A chemical deflocculating agent, sodium hexametaphosphate, was added prior to shaking to assist in separation of the individual particles. To evaluate characteristics of different ultrasonic equipment, a number of 50-g samples of soil, passing through a sieve with 2-mm openings, were obtained by splitting a large soil sample. Each 50-g sample was placed in a 500-ml beaker containing 190 ml of water and 10 ml of sodium hexametaphosphate and was stirred with a stirring rod. The beaker was placed immediately in the liquid container of the ultrasonic cleaning equipment being tested and was subjected to ultrasonic agitation for a period of 10 to 30 min. Following the ultrasonic agitation, the particle size distribution of the soil was determined by the standard methods for sieve analysis and hydrometer analysis. Sample splits were tested in a number of different ultrasonic cleaners ranging in frequency and power, as shown in Table I. To compare the results of the ultrasonic method of disaggregation and dispersion with those of the more orthodox methods, particle size analyses were made also on the same soils but prepared by the conventional procedure of mechanical disaggregation and chemical deflocculation. Some sample splits were also analyzed without use of deflocculent. The mechanical disaggregation with the mortar and pestle was varied somewhat by preparing one sample split to pass through a sieve with 2.0-mm openings (No. 10), a second sample split to pass through a sieve with 0.420-mm openings (No. 40), and a third sample split to pass through a sieve with 0.0625-mm openings (No. 230). These sample splits were analyzed by the ASTM standard sieve and hydrometer procedures, Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

312

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

mentioned previously, to obtain their particle size distribution. Duplicate sample splits were also treated to ultrasonic agitation and analyzed similarly (Table 2). The results of some of these analyses are shown in Figs. 1, 2, and 3, for one of the samples tested.

P A R T I C L E * S I Z E DIAMETER IN ~,AILLIMETERS CLAY S I Z E S

SILT SIZES

SAND S I Z E S V. line

''"

79.2

20.0

0.6

83.5

16.3

0.2

Fbne

~%d bum

Coarse

V.Co~rse

0.2

F I G . l--Graph showing distribution of particles after mechanical and ultrasonic disaggregation o f clay samples passing No. 10 sieve.

Summary The disaggregation of the silt- and clay-size particles appeared to be essentially complete after only 10 rain of ultrasonic agitation, even though the samples had been prepared only down to 2-mm size. Microscopic examination of the soil suspensions demonstrated that many of the aggregations of smaller particles (Fig. 4) resulting from purely mechanical Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

JOHNSON AND MOSTON ON USE OF ULTRASONIC ENERGY

313

TABLE 2--Results of various mechanical and ultrasonic disaggregation on samples

of four different textures.

Disaggregation Treatment

Silt Content, ~

Clay Content," ~

Sonic Velocity, kHz

Bonny Loess P10, mechanical P40, mechanical P230, mechanical P10, ultrasonic, 5 min P10, ultrasonic, 10 min PI0, ultrasonic, 20 rain PI 0, ultrasonic, 30 min PI0, ultrasonic, 15 min P40, ultrasonic, 15 min P230, ultrasonic, 15 min

70.6 73.8 74.8 74.1 72.9 69.7 68.0 71.8 70.2 69.4

13.6 12.2 12.0 10.1 12.3 16.5 17.6 13.8 14.2 16.4

... ...

20.0 20.2 25.6 18.1 16.3 16.8 18.3 18.1 16.3 16.5

79.2 79.2 74.2 81.5 83.5 83.0 81.5 81.5 83.5 83.5

... ...

14.0 14.6 14.0 12.0 14.0 14.0 14.0 14.3 14.0 14.6 13.8

... ...

56" 56 56 58 55 54 54

Clay P10, mechanical P40, mechanical P230, mechanical P10, ultrasonic, 5 min P10, ultrasonic, 10 min PI0, ultrasonic, 20 min P10, ultrasonic, 30 min P10, ultrasonic, 15 min P40, ultrasonic, 15 min P230, ultrasonic, 15 min

'60" 6O 59 60 60 60 60

Sandy Silt PIG mechanical P40 mechanical P230 mechanical PI0 ultrasonic, 5 rain P10 ultrasonic, 10 rain P10 ultrasonic, 15 rain P I 0 ultrasonic, 20 rain P10 ultrasonic, 30 min P10. ultrasonic, 15 rain P40, ultrasonic, 15 min P230, ultrasonic, 15 min

38.9 41.0 40.5 41.9 41.0 41.4 40.5 41.3 41.4 41.0 41.6

60 58 60 60 60 60 62 62

Very Fine Sand PI0, mechanical P40, mechanical P230, mechanical P10, ultrasonic, 5 min P10, ultrasonic, 10 min P10, ultrasonic, 20 min PI0, ultrasonic, 30 min PI0, ultrasonic, 15 min P40, ultrasonic, 15 min P230, ultrasonic, 15 rain

17.8 19.4 20.4 16.8 20.6 18.0

3.2 3.2 3.2 3.2 3.2 3.2

19.2

3.2

18.8 20.4 18.4

3.2 3.2 3.2

56 56 59 59 56 58 58

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314

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

I00

I|

20 ~ Ln

II

~--~_

I

'16 m

5 mln

| 1

// 7C

b

/

ii

,

ta 60 n~

~ 5r,

~, 3G

I0 I u

o o.

o P A R T I C L E - S I Z E D I A M E T E R IN M I L L I M E T E R S

CLAY SIZES

< ~0~4 ~m -___

81.5 83.5 83.0

SAND S I Z E S

SILT SIZES 0.004 - 0.0~,25 ~

18.i 16.3 16.8

.0625-.125

0.2 0.2

.125-~25

.25-.5

.S-I

I-2

0.2

F I G . 2--Graph showing effect of ultrasonic agitation of various periods of clay samples passing No. 10 sieve.

disaggregation, were separated (Fig. 5) when treated with sodium hexametophosphate. Comparison of Figs. 5 and 6 shows that disaggregation by ultrasonic action for only a short period of time provides disaggregation equivalent to that received from a much longer mechanical preparation. Microscopic examination of the soil suspensions also indicated that good quality disaggregation was obtained by ultrasonic action, without any apparent deleterious effect on the fundamental particles. There is, however, some indication that ultrasonic vibration of distilled water will cause the formation of nitric acid, thus lowering the pH of the soil suspension [8]. A lowered pH could reduce the efficiency of the deflocculation process. Continuing research on the ultrasonic method, therefore, Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

JOHNSON AND MOSTON ON USE OF ULTRASONIC ENERGY

315

l-^

o

r

o

o

c~ ~ c~ c5

c~

PARTICLE-SIZE CLAY SIZES •

~r

0.004 mm

c~ D I A M E T E R IN M I L L I M E T E R S SAND SIZES

SILT SIZES 0.00~t - 0-0625 mm

16.4 14 o2

69.4 70.2

13.8

7l .8

V. line

Fine

.0625..125

.125-_25

4.6 5.0 5.2

2.2 2.0

2~0

Medlum

.25..5

4,5 4.8 4.4

Co0rse

.5-1

2.8 3.4 2.4

V.Coorse

1.2

0.2 004 004

FIG. 3--Graphs showing effects o f ultrasonic agitation o f 15 min on loess samples prepared through various sieves.

should include more study to determine if this factor is critical within the range of agitation time. Disaggregation by purely mechanical means and chemical deflocculation, even when the sample was prepared to pass a 0.0625-mm sieve, does not appear to be any more complete for most type soils than that done by ultrasonic energy. Analyses of samples prepared by this method provided as much as 20 percent fewer clay-size particles than did analyses using samples disaggregated and dispersed by ultrasonic energy. A clay sample prepared only to 2.0-ram size provided 4.3 percent fewer clay-size particles than obtained by the ultrasonic method (Fig. 1). Figure 2 shows that a clay sample provided maximum clay content after 10 min disagCopyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

316

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 4--Photomicrograph of clay samples following mechanical preparation, passing No. 40 sieve, with no deflocculating agent (X880).

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JOHNSON AND MOSTON ON USE OF ULTRASONIC ENERGY

317

FIG. 5--Photomicrograph of clay sample following mechanical preparation, passing No. 40 sieve, treated with sodium hexametaphosphate and shaken 8 h ~880 ).

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318

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

FIG. 6--Photomicrograph of clay sample following mechanical preparation, passing No. 40 sieve, treated with hexametaphosphate, and ultrasonically agitated for 30 min (X880).

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JOHNSON AND MOSTON ON USE OF ULTRASONIC ENERGY

319

gregation by ultrasonic energy. A silt (loess) sample gave 2.6 percent more clay-size partices, using ultrasonic treatment on material prepared through the No. 230 (0.0625-mm) sieve, than when using material prepared through the No. 10 (2.0-mm) sieve (Fig. 3). Although these may not seem to be greatly significant differences, the important point is that considerably less time and effort were spent preparing the sample by the ultrasonic technique. The authors believe ultrasonic disaggregation has considerable potential and further research on the method is recommended. References [1] Poland, J. F. and Davis, G. H., Transaction, American Geophysical Union, Vol. 37, No. 3, 1956, pp. 287-296. [2] "Progress Report on Land Subsidence Investigations in the San Joaquin Valley, California, Through 1957," Inter-Agency Committee on Land Subsidence in the San Joaquin Valley, Sacramento, Calif., 1958. [3] Poland, J. F. and Green, J. H., "Subsidence in the Santa Clara Valley, California--A Progress Report," U.S. Geological Survey Water-Supply Paper 1619-C, 1962. [4] Johnson, A. I., Moston, R. P., and Morris, D. A., "Physical and Hydrologic Properties of Water-Bearing Deposits in Subsiding Areas in Central California," Professional Paper 497-A, U.S. Geological Survey, 1968. [5] Henry, G. E., Scientific American, Vol. 190, No. 9, 1954, pp. 54--63. [6] Goldman, Richard, Ultrasonic Technology, Reinhold, New York, 1962. [7] Rod, R. L.,Research and Engineering, Vol. 1, No. 2, Sept. 1955, pp. 18-23. [8] Akiya, Schichiro and Okui, Seiichi, Journal, Pharmaceutical Society of Japan, Vol. 67, 1947, pp. 233-235.

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P. V. Lade' and H. Nejadi-Babadai I

Soil Drying by Microwave Oven

REFERENCE: Lade, P. V. and Nejadi-Babadai, H., "Soil Drying by Microwave Oven," Soil Specimen Preparation for Laboratory Testing, ASTM STP 599, American Society for Testing and Materials, 1976, pp. 320-340.

ABSTRACT: An investigation of the usefulness of the microwave oven for water content determination and for drying of soil before testing was performed. Soils ranging from highly plastic clays to clean sand were used. The influences of initial water content, amount of soil, and soil type on the drying time were investigated. Differences between water content determined using a microwave oven and a conventional oven decreased with decreasing plasticity of the soils. The microwave oven can be used for quick drying of soil, but accurate determinations of water content may only be achieved for nonplastic soils such as coarse silts, sands, and gravels. The change in classification according to the Atterberg limits was taken as a measure of the effects of heating by microwaves. The plasticity and swelling potential of the soils decreased after preheating in the microwave oven, and these effects are comparable to the effects of drying the soils in a conventional oven at high temperatures. KEY WORDS: Atterberg limits, clays, tests, moisture, soil mechanics, temperature, soils, plastic properties

The microwave oven presents definite potential advantages over the conventional oven when used for soil drying. Substantial time savings and low purchase price render the microwave oven very attractive for use in water content determination and for drying of soil before testing. However, due to the nature of the action of the microwaves and the sensitivity of clay minerals to drying temperature, the amount of water driven off in the microwave oven may not correspond to that evaporated in a conventional oven at a given temperature. Due to the complex heating process by microwaves, only practical experience can demonstrate the effects of microwave heating. Therefore, a systematic investigation of the usefulness of the microwave oven for soil drying and water content determination and the possible effect of microwave heating on the soil behavior was performed. ~Assistant professor of engineering and applied science and former graduate student, respectively, University of California, Los Angeles, Calif. 90024

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LADE AND NEJADI-BABADAI ON MICROWAVE OVEN DRYING

321

Heating with Microwaves Process oYHeating with Microwaves

In the process of heating with microwaves, electric field energy is converted into heat through frictional action between molecules of the material in the electric field [1].2 This process is best understood from consideration of the action of dipole molecules in an electric field. Some molecules such as water molecules, are natural dipoles, that is, the centers of the positive and the negative charges do not coincide but are separated by a small distance. In the absence of an electric field, the dipole molecules in a material are in thermal equilibrium, that is, they oscillate about their position of rest as a result of thermal agitation. However, if an external electric field is imposed, the dipole molecules will rotate to align themselves in the direction of the field. Due to frictional resistance of the material, the molecules will be impeded in their rotation, and a finite time is required for their alignment. Thus, the response of a dipole molecule in an electric field is of a viscous nature. If the direction of the electric field is alternated at sufficiently high frequency, the energy lost, due to molecular friction in the material, becomes significant. Thus, energy is converted to heat through the frictional action at high frequencies of the alternating electric field. The energy converted into heat increases in proportion to the frequency, f, and the square of the electric field strength, E Pheat = C X

f X E2

(1)

where C is a constant which depends on the dielectric constant of the material. The penetration depth of the electromagnetic waves decreases greatly with increasing frequency [1]. Limits are therefore set to the heat treatment with microwaves because of the diminished penetration depth experienced at the high frequencies which are most favorable for effective heating. Thus, it can happen that the waves do not reach the nucleus of large volumes of material. However, due to heat transfer from the surface to the interior of the material, heating with microwaves will still have an effect at distances greater than the microwave penetration depth. Further limitations to the effectiveness of microwave heating are posed by the reflection of a portion of the waves at the surface of the material and by damping of the waves on penetration into the material [2]. The dielectric constant of a soil, upon which the constant C in Eq 1 depends, decreases with increasing frequency, a phenomenon referred to as dielectric dispersion [3]. The dielectric constant also decreases with 2The italic n u m b e r s in brackets refer to the list o f references appended to this paper.

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322

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

decreasing water content; and the type o f clay, the clay content, and the temperature also affect the dielectric dispersion. Due to the influence of various factors on the dielectric constant, the effectiveness of heating with microwaves can only be explored through practical experience. It should be noted that only materials which are able to absorb the microwaves, as explained previously, can be heated by the microwaves. Metal objects reflect the microwaves, and materials such as glass, paper, and some plastic objects let the microwaves pass through them without affecting the molecules of which they are formed.

Microwave Oven The microwave oven used for this investigation was purchased for approximately $180 (March 1974). It had a rated power requirement of 0.97 k W and, like most microwave ovens, was set to operate at a frequency o f 2450 M H z .

Effects of Heating Clay Mineral Systems Because the temperatures generated within soil specimens in a microwave oven are difficult to control, an appreciation of the potential value o f a microwave oven for drying soil must include considerations of the possible harmful effects which uncontrolled temperatures m a y have on the soil. The effects of heating on the physico-chemical behavior o f clay mineral systems have been studied to great extent, and various techniques and measures of the changes in this behavior have been employed. The alteration o f the engineering properties of clays and clayey soils, when heated, has often been expressed in terms of the changes in the values of the liquid, plastic, and shrinkage limits. Grim [4] writes that when clays are heated to increasingly high temperatures, they first lose their pore water and then their adsorbed water. As the adsorbed water is lost, the flat plane surfaces tend to come together. Apparently the complete collapse of the particles makes it extremely difficult for water to again penetrate to me adsorbing surfaces. As a consequence, heating clays to temperatures of the order of 100 to 300~ tends to substantially reduce their plastic character when they are again mixed with water. When the clay minerals are heated to temperatures of the order of 500 ~C and above, they lose their hydroxyl water with a consequent change in the crystalline structure, and, as a further consequence, the shape of the particles and the character of the surface change. The result is likely to be development of a substantially nonplastic material. Grim [4] summarizes further: ' ' I n the case of the expanding clay minerals,

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LADE AND NEJADI-BABADAI ON MICROWAVE OVEN DRYING

323

temperatures in the order of 100 to 300~ cause the loss of the ability to swell again. The loss of swelling potential is related to the complete loss of interlayer water." The loss of water from clays, which occurs with increasing drying temperature, may be quite substantial and results in apparent increasing water content [5]. As the clays are heated at increasingly high temperatures, water is lost in irregular amounts and dehydration curves, typical for each type of clay, are obtained [4]. It has often been reported that heating prior to determination of the plasticity of a soil results in lower values of the liquid limit and the plastic limit [5-10]. The results of tests performed by Radhakrishnan et al [10] on black cotton soils showed that these soils became nonplastic, and the clay-size fraction was reduced in the 500 to 700~ range. At even higher temperatures, the soils became friable and sandy, and consistency limit tests could not be performed. The effect of partial or complete dehydration on the subsequent swelling of clay soils, when mixed with water, has also been studied to some extent [4,10--12]. It has been found that the shrinkage limit values generally increase with increasing preheating temperature, thus indicating decreases in the ability of the soils to swell.

Drying by Micro waves An investigation of the use of a microwave oven for quick determination of the water content of soils was performed by Ryley [13]. On the basis of the tests performed for this investigation, it was concluded that the microwave drying method results i n water content analyses accurate to within 0.4 percent for most soils. The specimens dried within 10 to 15 min, but it was concluded that the drying method is unsuitable for soils containing gypsum or materials with high carbon content.

Preliminary Investigations Preliminary investigations concerning the choice of soil container and the uniformity of the microwave radiation within the oven were performed prior to any testing of soils. Since microwaves are unable to penetrate metal objects such as aluminum soil drying cans used in the conventional oven, a number of container types were tested in the microwave oven in order to select a suitable soil container. The criteria used for selecting a suitable soil container were that it (1) does not impede the microwaves noticeably, (2) does not burn or deform, and (3) does not experience any loss in weight. Experiments were devised to test the ability of various types of containers to meet these three criteria [14]. As a result of this investigation, glass Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

324

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

beakers were selected as the most suitable containers to be used for soil drying in the microwave oven. Although microwave ovens are designed to produce uniform radiation, an investigation of the distribution of the microwave radiation was performed. Glass beakers were filled with 100 ml o f cold water and, in turn, placed at nine different points in the oven, and the time to reach boiling o f the water was noted for each point. The microwave radiation was found to be only slightly nonuniform. During all subsequent testing, the glass beaker with soil to be heated was placed in the center of the oven. Determination of Water Content

Soils Tested Soil types ranging from highly plastic clays to clean sand were used in the investigation o f the applicability of the microwave oven to soil drying and water content determinations. Some o f the characteristics o f all the soils tested are given in Table 1. Liquid limits, plastic limits, and shrinkage limits were determined according to the standard procedures [15,16]. A conventional forced draft oven, which was set at 105 ~ was used for this purpose. A comprehensive range o f plasticity was covered, and all soils tested were inorganic. Due to the already well-documented sensitivity of clay minerals to drying temperature [4-12], the main effort was concentrated on the clay types. Seven o f the clays were obtained in form of air-dry powder. The two marine clays were obtained in the form o f wet specimens.

Factors Influencing Drying Time in a Microwave Oven Due to the sensitivity of clay minerals to drying temperature, the water content of a soil has been defined as that corresponding to drying the soil at 105~ (or 110 _+ 5~ according to the ASTM Test for Laboratory Determination of Moisture Content of Soil (D 2216-71)) until no further weight loss occurs. Unfortunately, the temperature to which the soil is heated in the microwave oven is difficult to measure, but the heating process can be continued until a final constant weight is obtained. Therefore, only experiments will show whether the water content determined using the microwave oven corresponds to those determined using a conventional oven. The following factors emerge as being important in considerations regarding the usefulness of the microwave oven for water content determinations: 1. Influence of initial water content on drying time. 2. Influence o f amount of soil on drying time. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

.

.

.

.

98 44 54 12.7a

. . . . . . . . . 1(~ tO0

.

.

514 45 469 9.2 a

.

.

Dixie Bond Bentonite

.

.

.

. . 40-28 60-72

.

147 41 106 . .

NBC-1 Marine Clay

.

.

.

.

.

.

.

. . 40-28 60-72

.

151 48 103 . . .

NBC-23 Marine Clay

.

.

.

.

.

.

. .., 100

.

57 27 30

Haley Clay

. .

. .

. 48 52

.

55 24 31 11.7

.

Grundite Clay

. 5 51 44

.

.

48 24 24 12.7

H-Soil

.

1 - - - C h a r a c t e r i s t i c s o f soils tested.

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

aThese values are believed to be slightly too high.

Silt, ~ (0.002 to 0.06 mm) Clay, ~ (<2 ~m)

Gravel, % (>2 mm) Sand, 070 (0.06 to 2.0 ram)

Liquid limit Plastic limit Plasticity index Shrinkage limit

Characteristic

Black Hills Bentonite

TABLE

.

76 21

. 3

30 20 10 16.5

M-Soil

80 12

8

24 23 1 20.2

L-Soil

13 2

9 76

. . . .

. . . .

. . . .

Silty Sand . . . .

. . . .

. . . .

. . . .

. . . .

...

i~

Ottawa Sand

. . . . . .

2

25 73

. . . .

Antelope Valley Sand

r-

O'l

CO

0 .<

O < Fn Z

< m

~.

20 O

O Z

~, ~ "'0

~"

t._

z m

z o

o m

326

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

3. Influence of soil type on drying time. 4. Comparison of water content determined using microwave oven and conventional oven at 105 ~ The effects of each of these factors have been investigated, and the results are presented in the following paragraphs.

Effects of Initial Water Content and Soil Type Clays--Batches of clay powder were thoroughly mixed with water to produce wet soil with uniform but different water content. A specimen of 100 g of wet soil was placed in the microwave oven and dried. The glass beaker with the specimen was periodically taken out of the oven, covered with a lid, weighed, and put into the oven for further drying. This procedure was continued until no loss in weight was registered between two consecutive weighings. The water content was calculated after each weighing, assuming that the soil weight at that time was the correct dry weight. The change in apparent water content with drying time is shown in Fig. 1 for one of the seven clays and clayey silts tested. Data for the other soils were similar. The soil specimens were found to dry relatively slowly in the beginning, and the highest rate of drying occurred after 2 to 5 min of exposure to microwave radiation. It has been found that soil which is wet on the surface reflects the microwaves better and therefore absorbs less energy than dry soil [2]. Thus, the heating process is relatively slow until the surface of the initially wet soil specimen has been dried out to some extent. The soil specimens with the higher water content dried slower initially than those with the lower water content, which is in accord with the findings regarding microwave reflection from wet surfaces [2]. All soil specimens, which weighed 100 g, initially dried to constant weight in about 20 min. However, the soils with high plasticity and high initial water content tended to require slightly longer time for complete drying than the soils having low plasticity and low initial water content. Control specimens were dried for 24 h in a conventional oven, at 105 ~ All control specimens were taken from the same batches of soil as the specimens dried in the microwave oven. The water contents determined from the specimens dried at 105 ~ are also shown on Fig. 1 for Haley clay. It may be seen that the microwave heating, in all cases, resulted in removal of slightly more water from the soil than that evaporated by conventional heating at 105 ~ For Haley clay, the difference in water content varied from 1.3 to 2.4 percent, depending on the initial water content. Table 2 shows the water contents from the microwave oven and the conventional oven, and the differences between these water contents for different types of soil. It may be seen that the differences in water contents increase with decreasing initial water content for all the clays and clayey silts tested. This may Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions autho

327

LADE AND NEJADI-BABADAI ON MICROWAVE OVEN DRYING

200

I

I

I

I

I

I

I

q

MICROWAVE OVEN: 148.6 % CONVENTIONAL OVEN: 147.3%qi ~

160

V

121.2 o-~

~

120 I'Z bJ t,'oZ,._)

80

hi

4O

0

0

~

o

I

I

5

I0

86., a 5

A

zx-"---

- ' -

o

I

I

A

65.3 % 609% " -~ ~

I

15 20 25 DRYING TIME (rain)

I

I

50

55

1 o

40

FIG. l--Drying curves for lO0-g specimens of Haley clay with different initial water contents.

be explained as follows: For the same amount of wet soil, the amount of solid material increases with decreasing water content. Assuming that the amount of absorbed and hydroxyl water increases with the amount of solid material, the water available for removal by microwave heating (but not for removal by conventional heating at 105 ~ increases with decreasing water content. Therefore, the difference between water contents determined from the two different heating processes increases with decreasing water content. The data presented in Table 2 indicate that drying of clayey soils in the microwave oven corresponds to drying in a conventional oven at temperatures higher than 105 ~ Increasing the drying temperature results in removal of adsorbed water and some hydroxyl water. Highly plastic clays can adsorb more water to the surfaces of the particles than low plastic silts. The difference in water contents, as determined by microwave oven and conventional oven, is therefore greater for the highly plastic bentonites than for the low plastic L-soil.

Sands--The results of tests on Antelope Valley sand, with negligible amount of particles smaller than 2/~m and on silty sand with 2 percent clay (< 2/~m) are also shown in Table 2. It may be seen that the differences between the water contents determined by the microwave oven and by the conventional oven were only 0.5 and 0.2 percent, respectively. Thus, very little additional water was removed by the microwave heating, and this little water was most likely associated with the clay minerals Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

328

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

TABLE 2--Comparison o f water contents for lO0-g specimens with different initial water contents dried in the microwave oven and the conventional oven.

Type of Soil

Water Content Determined Using Microwave Oven, %

Difference Water Content Between Determined Using Water Contents, Conventional Oven, % %

Black Hills bentonite

926.1 692.7 659.0

906.5 660.8 603.0

19.6 31.9 56.0

Dixie Bond bentonite

138.4 130.3 84.1

127.3 116.6 70.1

11.1 13.7 14.0

Haley clay

148.6 121.2 88.3 63.3

147.3 119.8 86.1 60.9

1.3 1.4 2.2 2.4

Grundite clay

97.8 61.5 54.1 40.8

96.3 59.8 52.2 38.4

1.5 1.7 1.9 2.4

H-soil

56.2 50.4 41.6

55.1 49.0 40.1

1.1 1.4 1.5

M-soil

43.0 31.2 30.4

42.7 30.5 29.3

0.3 0.7 1.1

L-soil

41.6 33.5 27.9

40.7 32.4 26.5

0.9 1. I 1.4

Antelope Valley sand

11.6

11.1

0.5

Silty sand

15.3

15.1

0.2

8.0

8.0

0.0

Ottawa sand

present in the soils. Tests on clean Ottawa sand, consisting o f quartz grains, resulted in exactly the same water content determined by the two methods of heating, which also supports the hypothesis that only clay minerals are sensitive to the drying process in the microwave oven.

Effects of Amount of Soil and Soil Type Batches o f soil with uniform water content were prepared as follows. Air-dry powder was mixed with water to produce a uniform lean mix, with water content above the liquid limit. This liquid soil was then consolidated in a large double draining consolidometer to produce a cylinder of soil with uniform water content. Specimens ranging in weight from 10 to 500 g were cut from the soil cylinder and dried in the microwave oven, Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized

LADE AND NEJADI-BABADAI ON MICROWAVE OVEN DRYING

329

according to the procedure outlined previously. Control specimens were dried for 24 h in the conventional oven at 105 ~ for comparison of water contents. Typical drying curves are shown in Fig. 2 for soil specimens of different sizes. The same characteristic shape was observed in all cases, regardless of the weight of soil. The larger specimens required longer drying times. This increase in drying time with increasing amount of wet soil may be explained as follows: the initial microwave penetration is impeded because a portion of the waves is reflected from the wet surface of the soil, and the penetration depth of the microwaves is limited, as explained previously. The loss in weight from equally wet specimens is therefore initially proportional to the surface area of the specimens. For specimens with the same shape, the ratio between volume and surface area is proportional to the linear dimensions of the specimens. Therefore, the amount of water being driven off, relative to the volume of the specimen, is smaller for a large specimen than for a small specimen, and the initial slopes of the drying curves consequently decrease with increasing amount of wet soil. The time required for complete drying of the soil specimens in the microwave oven increased with amount of wet soil. Complete drying of the small specimens (10 g) was accomplished within 10 to 20 min. The highly plastic clays with high initial water content tended to require longer time for complete drying than less plastic soils with low water content. The difference is drying time between the high and low plastic soils was more pronounced for the large specimens (500 g), for which the drying times ranged from about 20 min to 1 h. The water contents obtained from the two different drying methods and the differences in water content are listed in Table 3 for the seven types of clay soil tested. The difference in water content for each type of soil is seen to decrease with increasing amount of wet soil. This may be explained by the limited penetration depth of the microwaves. Thus, for large volumes of soil, the microwaves may not reach the interior of the specimen, and the drying of the soil in the middle is accomplished through heat transfer from the surface. It was noticed that a specimen, which was first dried in the microwave oven, then cooled down in the desiccator, and again heated by microwaves, does not heat up to the same high temperature as the initially wet specimen. Therefore, after the outer shell of a specimen had dried, the heat generation by microwaves and the heat transfer to the interior of the specimen decreased. Some of the more strongly held water molecules, which could not be reached by the microwaves, may therefore not have evaporated from the large specimens. The difference between the two water contents therefore decreased with increasing specimen size. However, this effect is somewhat obscured by a tendency of the large specimens to crack, due to high vapor pressures generated inside the specimens by the rapid heating process in the microwave oven. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

330

SOIL

SPECIMEN

70 I

t

PREPARATION

1

oE/

I

FOR

I

LABORATORY

,

TESTING

r

g

f

L

/

g ~ 3o

~ 20

0 0

5

I

I

I

L

I

I

I0

15

20

25

30

35

40

DRYING TIME (mi n) FIG.

2--Drying curves for specimens o f Haley clay showing influence of amount o f soil.

Effects of Microwave Heating on Soil Characteristics Due to the nature of the microwave heating process, the temperature to which the soil is heated in the microwave oven is difficult to measure. However, the change in the soil classification according to the Atterberg limits may be taken as a measure of the effects of heating by microwaves. Liquid limit, plastic limit, and shrinkage limit tests were performed on clays and clayey silts which, in powder form, first had been either (1) dried in the microwave oven for 20 min, (2) dried in the conventional oven at 105 ~ for 24 h, or (3) left in initial air-dry condition before testing. All water content determinations, subsequent to these treatments, were made using the conventional oven. The results from the liquid limit and plastic limit tests are presented on plasticity charts in Fig. 3 for soils of low plasticity and in Fig. 4 for highly plastic clays. Liquid limits, plastic limits, and plasticity indices all decreased. Clay soils predried in the microwave oven gave the greatest changes, and those predried in the conventional oven gave intermediate changes, as compared to the values obtained from only air-dried soil. This sequence is referred to hereafter as M-C-A. The most extreme reductions in plasticity occurred for the highly plastic Black Hills bentonite, and the smallest reductions occurred for the low plastic L-soil. The general trend indicated on the plasticity charts is to change the soil classification in the order M-C-A along the A-line towards the silt region. The same trend has Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

LADE AND NEJADI-BABADAI ON MICROWAVE OVEN DRYING

331

TABLE 3--Comparison o f water contents f o r specimens with the s a m e initial water content and different wet weights dried in the m i c r o w a v e oven and the conventional oven.

Water Content Determined Using Micro-

Type of Soil

Initial Amount of Wet Soil, g

wave Oven, e/0

Water Content Determined Using Con-

Difference

ventional Oven, Between Water 07o

Contents, %

10

746.3

50 200 500

735.6 728.4 709.0

702.0

33.6 26.4 7.0

Dixie Bond bentonite

10 50 200 500

81.4 80.7 80.5 80.1

73.8

7.6 6.9 6.7 6.3

Haley clay

10 50 250 500

66.7 64.5 64.0 64.0

63.1

3.6 1.4 0.9 0.9

Black Hills bentonite

44.3

10

41.1

50 200 500

40.2 40.0 40.0

H-soil

10 50 200 500

35.4 34.8 34.3 34.0

33.7

1.7 1.1 0.6 0.3

M-soil

10 50 250 500

27.3 26.9 26.5 26.4

26.0

1.3 0.9 0.5 0.4

L-soil

10 50 200 500

27.7 27.4 27.5 26.9

26.5

1.2 0.9 1.0 0.4

Grundite clay

1.4 39.7

0.5 0.3 0.3

been observed for soils being heated conventionally to increasingly high temperatures, [4, 6-10]. These observations indicate that drying of clayey soils in a microwave oven corresponds to drying in a conventional oven at temperatures higher than 105 ~ However, the temperature corresponding to the microwave heating is most likely different for different types of clay mineral. The loss of plasticity has been attributed to irreversible loss of adsorbed water and to breakdown of the crystalline structure of the clay minerals, due to loss of hydroxyl water [4] and fusion of the flat surfaces of the clay mineral particles [10]. This produces larger soil particles, with properties closer to those of silt. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

332

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

[

i

I

I

I

I

I

l

I

50

60

70

o AIR-DRY SOIL z~ SOIL HEATED IN CONVENTIONAL OVEN a SOIL HEATED IN MICROWAVE OVEN

40

x 30 la.i t:3 >.-

20

o_ io

M-

o

t 0

/~

I0

L- SOIL

20

I

30 40 LIQUID L.IMIT

F I G . 3--Plasticity chart showing reduction in plasticity after heat treatment in conventional oven and microwave oven f o r soils o f low plasticity.

6O

x

t

40-

i

sO0

4oo

I

I

I BLACKHILLS BENTON,TE

I

/ /

zo |

E

/0 /

/-

/ /

[

I

I

I

I

o AIR-DRY SOIL //o A SOIL HEATED IN CONVENTIONAL / , , ~ OVEN ///A-, [] SOIL HEATED IN M I C R O W A V E / /

OVEN

_

BOND

~ GRUNDITE

///-

BENTONITE

I

I I

I I

~ 3oBoo

I0-

0

0

J

~

I0

20

I 30

I

40 50 60 LIQUID LIMIT

I

I

I

70

80

90

I O0

FIG. 4--Plasticity chart showing reduction in plasticity after heat treatment in conventional oven and microwave oven f o r highly plastic clays.

Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

LADE AND NEJADI-BABADAI ON MICROWAVE OVEN DRYING

333

The shrinkage limits were found to increase in the order A-C-M, thus indicating a reduction in swelling potential of the soils having initially been heated in the two ovens. Similar reductions in swelling potential have been observed for soils which have been heated to high temperatures

[4, 10--121. Liquid limit and plastic limit tests were performed on the two marine days, designated NBC-1 and NBC-23. These soils were only available in wet condition and were not predried. Half of each specimen tested was dried in the microwave oven, and the other half was dried in the conventional oven. The results of the Atterberg limit tests are listed in Table 4. Both liquid limits and plastic limits were found to be higher for the specimens dried in the microwave oven than those obtained from soil drying in the conventional oven. These results are in agreement with the other data described herein. Thus, it is concluded that, if the microwave oven is used for soil drying, values of liquid and plastic limits will be determined to be too high.

Summary and Conclusions This paper presents the results of an investigation of the usefulness of the microwave oven for water content determination and soil drying before testing. Twelve different soils, ranging from highly plastic clays to clean sand, were used in the investigation. All soils were inorganic, and the main effort was concentrated on drying of different clay types. The influences of initial water content, amount of soil, and soil type on the drying time were investigated. Typically, the time for complete drying varied from about 10 min for 10 g to about 45 min for 500 g of wet soil. Highly plastic clays with high water content tended to require longer time, whereas low plastic soils with low water content dried completely in shorter time. The water contents determined using the microwave oven were higher for all soils containing clay minerals than those determined for the same soils, using the conventional oven at 105~ The greatest difference in these water contents was observed for small volumes of highly plastic clays. The difference decreased with increasing volume, increasing initial water content, and decreasing plasticity. Typically, the difference in water content varied from 0.3 to 1.5 percent for low to medium plastic soils to 14 percent for very highly plastic clays. Negligible or no difference was observed for silty sand and clean sand. The change in soil classification, according to the Atterberg limits, was taken as a measure of the effects of heating by microwaves. The liquid limit, the plastic limit, and the plasticity index were found to decrease for all the soils when they had been preheated in the microwave oven. The largest reductions in these limits occurred for the highly plastic days, and the Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

334

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

TABLE 4--Liquid and plastic limits for marine clays determined using the microwave oven and the conventional oven.

Limit

Determined Using Microwave Oven

Determined Using Conventional Oven

NBC-I

LL PL

165 46

147 41

NBC-23

LL PL

173 55

151 48

Type of Soil

smallest reductions were observed for the low plastic soils. The shrinkage limits tended to increase after preheating in the microwave oven, thus indicating a reduction in swelling potential of the soils. The effects of preheating a plastic soil in the microwave oven are comparable to the effects of drying the soil in a conventional oven at high temperatures. It may be concluded that the microwave oven can be used for quick drying of soil, but the characteristics of plastic soils will change. Although accurate determinations of water content may only be achieved for nonplastic soils such as coarse silts, sands, and gravels, the error encountered for plastic soils may be acceptable for many purposes. Thus, the microwave oven may be useful for rapid determinations of approximate water content of clay soils and for research purposes, when control specimens show no difference in water content. However, it is not recommended to use this oven on a routine basis until the magnitude of the possible error has been established for the type of soil to be dried.

Acknowledgment The writers express their appreciation to the National Science Foundation which provided support for this study under Grant GK 37445. K. L. Lee of the University of California at Los Angeles provided valuable advice for the preparation of the manuscript.

References [1] Piischner, H., Heating with Microwaves, Centrex Publishing Co., Eindhoven, The Netherlands, 1966. [2] Waite, W. P., Cook, K. R., and Bryan, B. B., "Broad Spectrum Microwave Systems for Remotely Measuring Soil Moisture Content," Publication No. 18, Water Resources Research Center, University of Arkansas, Fayetteville, 1973. [3] Arulanandan, K., Basu, R., and Scharlin, R. J., Highway Research Record, No. 426, 1973, pp. 23-32. [4] Grim, R. E., Applied Clay Minerology, McGraw-Hill, New York, 1962.

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LADE AND NEJADI-BABADAI ON MICROWAVE OVEN DRYING

335

[5] Lambe, T. W., "How Dry is a 'Dry' Soil?", Proceedings, Highway Research Board, 1949, pp. 491-496. [6] Casagrande, A., Public Roads, Vol. 13, 1939, pp. 121-130. [7] Chandrasekharan, E. C., Boominathan, S., Sadayan, E., and Narayanaswamy Setty, K. R., Proceedings, International Conference on Effects of Temperature and Heat on Engineering Behavior of Soils, Special Report 103, Highway Research Board, 1969, pp. 161-172. [8] Laguros, J. G., Proceedings, International Conference on Effects of Temperature and Heat on Engineering Behavior of Soils, Special Report 103, Highway Research Board, 1969, pp. 186-193. [9] Lambe, T. W., and Martin, R. T., Proceedings, Highway Research Board, Vol. 34, 1955, pp. 566-582. [10] Radhakrishnan, N., Katti, R. K., and Hussain, M., Proceedings, Third Asian Regional Conference on Soil Mechanics and Foundation Engineering, Vol. I, Haifa, 1967, pp. 111-115. [11] Aylmore, L. A. G., Quirk, J. P., and Sills, I. D., Proceedings, International Conference on Effects of Temperature and Heat on Engineering Behavior of Soils, Special Report 103, Highway Research Board, 1969, pp. 31-38. [12] Leonards, G. A., Foundation Engineering, McGraw-Hill, New York, 1962. [13] Ryley, M. D., "The Use of a Microwave Oven for the Rapid Determination of Moisture Content of Soils," Report LR 280, Road Research Laboratory, Berkshire, England, 1969. [14] Lade, P. V., and Nejadi-Babadai, H., "Characteristics of Soil Drying by Microwave Oven," Report No. UCLA-ENG-7476, School of Engineering and Applied Science, University of California, Los Angeles, Oct. 1974. [15] Lambe, T. W., Soil Testing for Engineers, Wiley, New York, 1951. [16] "Laboratory Soils Testing," Engineer Manual EM 1110-2-1906, Office of the Chief of Engineers, Department of the Army, Washington, D. C., 1970.

DISCUSSION

P. A . Gilbert ~ a n d M. M . A l - H u s s a i n i I (written d i s c u s s i o n ) - - T h e aut h o r s have p r e s e n t e d v a l u a b l e a n d interesting i n f o r m a t i o n o n soil d r y i n g in a m i c r o w a v e oven. I n a r e c e n t study, 2 t h e W a t e r w a y s E x p e r i m e n t Station (WES) conducted a number of water content determinations on vario u s t y p e s o f soil, o f which t h e p e r t i n e n t characteristics a r e p r e s e n t e d in T a b l e 5. B o t h c o n v e n t i o n a l a n d m i c r o w a v e ovens were u s e d in the test p r o g r a m , a n d the results are p r e s e n t e d in T a b l e 6. T h e s t u d y shows t h a t , while m i c r o w a v e e n e r g y o f f e r s a r a p i d m e t h o d f o r d r y i n g soil, it s h o u l d b e used with c a u t i o n since m i c r o w a v e e x p o s u r e t i m e g r e a t l y influences t h e r e s u l t i n g water c o n t e n t . I n all tests c o n d u c t e d , the s t a n d a r d d e v i a t i o n in water c o n t e n t , as o b t a i n e d b y the t w o m e t h o d s , was less t h a n 0.5 p e r c e n t . T h e c o n s i s t e n c y a n d r e l i a b i l i t y o f the results d e p e n d o n a c c u r a t e k n o w ~Civil engineer and research civil engineer, respectively, Soils and Pavements Laboratory, U.S. Army Engineer Waterways Experiment Station, Vicksburg, Miss. 39180. 2Gilbert, P. A., "Feasibility Study--Microwave Oven Used for Rapid Determination of Soil Water Contents," WES MP 3-478, U.S. Army Engineer Waterways Experiment Station, Vicksburg, Miss. 39180.

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336

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

TABLE 5--Description o f materials tested. Atterberg Limits, % Identification

LL b

pL c

PI d

Description

Long Lake clay (CH)

71

23

48

fat dark brown, alluvial soil composed of silt and clay-size partides of montmorillonite with some quartz and illite; extremely sticky and plastic when wet

Vicksburg buckshot clay (CH)

56

22

34

similar to Long Lake clay

Vicksburg silty clay (CH)

34

22

12

fine-grained loessial deposits. Minus 2 fraction is about 9 percent of total specimen and is predominately montmorillonite

79

26

53

inorganic gray clay from Atchafalaya River Basin, La.

679

36

643

commercial bentonite drilling mud, approximately 95 percent montmor_illonite

Brazilian clay (CH)

96

38

58

dark red clay whitish clay, percent iron gravity G, =

DeGray Dam fines (GC)

37

14

23

clayey sandy gravel; only minus No. 4 material tested

NP e

NP

NP

fine, uniform quartz sand with some feldspar from Mississippi River near Vicksburz. Miss.

EABPL a gray clay

(CH) Aquagel(CH)

Reid Bedford sand (SP)

EABPL a peat (Pt)

with mottles of approximately 17 oxides, specific 3.12

fibrous organic material from Atchafalaya River Basin, La.

QEABPL = East Atchafalaya Basin Protection Levee. bLL = liquid limit. cpL = plastic limit. dpI = plasticity index. eNp = nonplastic.

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17.13 16.48 14.21 141.35 58.39 274.32 373.04 47.22 44.74 10.60 5.13 5.54

Vicksburg silty clay

EABPL a gray clay

Aquagel

Brazilian clay

DeGray Dam fines

Reid Bedford sand ignited

5.08 5.53

10.55

47.05 44.21

275.06 374.16

142.09 59.79

17.09 16.54 14.19

49.33

52.09

Microwave b (predicted)

. ..

0.05 0.01

0.05

0.17 0.53

- 0.74 - 1.12

- 0.74 - 1.40

0.04 - 0.06 0.02

- 0.14

0.11

Difference

1551

620 754

795

913 1225

1302 1451

980 1247

1054 682 600

966

1240

Time in Microwave Oven, s c

470.53

471.20 733.50

709.28

387.86 599.00

349.88 346.60

282.53 653.95

989.28 501.36 357.60

463.14

574.69

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con

-

Wet Weight of Microwave Oven Specimen, g

aEABPL = East Atchafalaya Basin Protection Levee. bWater content determined from Eq 11 using conventional oven water contents and experimental water tent-drying time data. c Time required for water loss equivalent to water loss in conventional oven.

220.25

49.19

Vicksburg buckshot clay

EABPL a peat

52.20

Long Lake clay

Material

Conventional (actual)

Water Content, Percent

oven drying times for various soils.

TABLE 6--Summary o f water contents measured by conventional and microwave ovens and microwave I'-

< m O < m z ID .-n ..<

O

O Z

o

z

z o

O m

338

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

ledge o f exposure time. The exposure time required in the microwave oven to produce water contents comparable to those obtained in a conventional oven can be expressed as follows T=

4.2Mw[(0.2/w + 1)(100 - to) + 539] P

where T Mw w to P

= = = = =

time in microwave oven, s, mass o f water present in soil-water mixture, g, water content of soil specimen, initial temperature o f soil-water mixture, and power output of oven, W.

This governing equation indicates that an estimate o f the specimen water content is required. This is not a serious limitation since a duplicate specimen can be incrementally dried in the microwave oven to obtain a water content estimate as input into the equation. The results o f the investigation indicate that microwave ovens are not suitable for drying soils with high organic content or soils containing gypsum. In addition, microwave energy must be used with care when drying soils with high metallic content, such as iron ore and bauxite. The WES study also pointed up the difficulties involved in the simultaneous drying o f multiple specimens. These difficulties were grave enough that this procedure is not recommended. The use of metallic tare cans or plates should be avoided, as they shield their contents from microwaves. Glass vessels should be used as specimen containers since they are transparent to microwaves. The study showed that drying time in a microwave oven is influenced by the amount of water present in a soil-water mixture and is not significantly affected by the amount, type, or plasticity of the soil. The WES study did not show any evidence o f the effect o f wet surface on drying time; therefore, it is doubtful that there is any significant reflection of microwave from wet surfaces. The authors have shown that the water content reached a constant value after about 20 min of drying time. On the other hand, tests at WES show that, while this is indeed the case for sand and silt (Fig. 5), it is not true with clay (Fig. 6), for which water content continued to increase with increasing drying time. The authors indicated correctly that, the larger the volume o f specimen, the closer the value o f water content obtained by the conventional and microwave ovens. For this reason, WES tests have been conducted on specimens o f not less than 250 g, which should be the lower limit for any successful water content determination in the microwave oven. A difference in water content of 14 to 56 percent for certain plastic soils, as indicated by the authors, is unusually high, which may indicate that the soils may have been overheated by the microwave oven. Copyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized

339

LADE AND NEJADI-BABADAI ON MICROWAVE OVEN DRYING

.50

~.o

....-.o

IZ 40 El O n" El n

30 Z El IZ 0 2.0 O nr" I.i.I

L EGEN D

OB R A Z I L I A N

CLAY ix V I C K S B U R G B U C K S H O T C L A Y I1~, 9 C O N V E N T I O N A L OVEN WATER C O N T E N T

I0

0

200

400

600

800

1000

1200

1400

l iO0

T I M E IN OVEN;, S

FIG. 5 - - T i m e in microwave oven versus water content o f clay.

z

6 --o

z

4

~ Z

LEGEND

#

8

2

~

o 0

.

200

400

OREIDBEDFORDS A N D [] REID BEDFORD S A N D 9 C O N V E N T I O N A L OVEN WATER C O N T E N T

600 800 I DO0 T I M E IN OVEN~ S

1200

1400

1600

FIG. 6 - - T i m e in microwave oven versus water content o f sand.

It is vital that normal safety precautions be observed when using microwave ovens, since microwaves are a form of radiation energy and should be treated as such; also, microwaves affect heart pacers and other sustaining devices at substantial distances. P. V. Lade and H. Nejadi-Babadai (authors" closure)--The authors appreciate the discussion and the additional information provided by Gilbert and AI-Hussaini. The authors agree that the exposure time required in the microwave oven is critical for correct determination of water content of plastic soils due to the sensitivity of clay minerals to drying ternCopyright by ASTM Int'l (all rights reserved); Wed Dec 22 14:00:00 EST 2010 Downloaded/printed by University of British Columbia Library pursuant to License Agreement. No further reproductions authorized.

340

SOIL SPECIMEN PREPARATION FOR LABORATORY TESTING

perature. Since the temperatures generated by the microwaves in the soil specimen are difficult if not impossible to control, a fairly precise estimate of microwave exposure time may substitute the condition used for the conventional oven that drying should continue until no further weight loss occurs. An estimate of exposure time is complicated by nonuniform temperature distribution in the specimen and, therefore, by influence of most of the factors mentioned in the paper. Gilbert and A1-Hussaini present a formula for estimating the microwave exposure time on the basis of the mass of water present in the soil water mixture, the initial water content, the initial temperature of the specimen, and the power output of the oven. The microwave oven used for the present study had a power output of 400 W. Whereas the trend indicated by this formula appears to be correct, the drying times calculated for the clays tested in the present study are 30 to 40 percent too short, resulting in water contents which are too small compared to those obtained from the conventional oven. However, it is possible that only part of the power output is effective in heating the soil specimen, and the formula for exposure time, therefore, may be better than indicated by the calculations. Microwave reflection from wet soil surfaces has been studied previously ([2] of the paper), and it is influenced by the complex dielectric constant and, therefore, the water content of the wet soil. The microwave reflection was offered as an explanation for the shape of the initial parts of the drying curves. However, the reflection may not have significant effects on the time for complete drying, as pointed up by Gilbert and AI-Hussaini. The ability of clays to adsorb water increases with increasing plasticity. Since the clays in the present study were dried until no further weight loss occurred, the adsorbed water and some of the hydroxyl water was removed due to temperatures higher than 105 ~ generated by the microwaves. Thus, the clays were overheated in the microwave oven, as suggested by Gilbert and AI-Hussaini, and this resulted in the rather high differences in water content for the highly plastic clays. The results of the studies performed at the U.S. Army Engineer Waterways Experiment Station and at the University of California at Los Angeles serve to indicate the problems and the complexity involved in using a microwave oven for water content determination.

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