Recommendations on Excavations

Recommendations on Excavations Published by the German Society for Geotechnics (Deutsche Gesellschaft fur Geotechnik e...

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Recommendations on Excavations Published by the German Society for Geotechnics (Deutsche Gesellschaft fur Geotechnik e.V.)

E-rnst & Sohn A Wiley Company

Working Group for Excavations of the German Society for Geotechnics Chairman: Univ.-Prof. Dr.-lng. habil. Dr.-lng. E.h. A. Weissenbach Am Geholz 14, 22844 Norderstedt, Germany Translator: Alan Johnson, Barsinghauser Strasse 32, 3 0 8 9 0 Barsinghausen, Germany

Cover figure: Earth pressure redistribution in the retreating stage with support from backfilling (Figure R 6 8 - l b ) Library of Congress Card No.: applied for British Library Cataloguing-in-Publication Data A Catalog record for this book is available from the British Library. Bibliographic information publishd by Die Deutsche Bibliothek Die Deutsche Bibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data is available in the Internet at ISBN 3-433-01712-3

0 2003 Ernst & Sohn Verlag fur Architektur und technische Wissenschaften GmbH & Co. KG, Berlin Ail rights reserved, especially those of translation into other languages. No part of this book shaii be reproduced in any form - i.e. by photocopying, microphotography, or any other process - or be rendered or translated into a language useable by machines, especially data processing machines, without the written permission of the publisher. Typesetting: Manueia Treindl, Laaber Printing: Strauss Offsetdruck GmbH, Morlenbach Binding: GroRbuchbinderei J. Schaffer GmbH & Co. KG, Grunstadt Printed in Germany

Preface In 1965 the former Deutsche Gesellschaftfur Erd- und Grundbau (German Society for Geotechnical and Foundation Engineering), now the Deuische Gesellschaft fur Geotechnik (German Society for Geotechnics), called the “Tunnel Engineering” Working Group into life. The wide-ranging tasks of the Working Group were divided into the three sub-groups “General”, “Open Cut Methods” and “Trenchless Technology”. The “Open Cut Methods” Working Group at first busied itself only with the urgent questions of analysis, design and construction of excavation enclosures. The preliminary results of these efforts were published in 1968 as the “Recommendations for Calculation of Braced or Anchored Soldier Pile Walls with Free Earth Support for Excavation Structures”’. During the course of work involving questions concerning analysis, design and construction of excavation enclosures, it was recognised that the subject matter was so comprehensive that the Deutsche Gesellschaft fur Erd- und Grundbau (German Society for Geotechnical and Foundation Engineering) decided to remove this area from the “Tunnel Engineering” Working Group and transfer it to a separate Working Group, that of “Excavations”; the personnel involved were almost completely identical with those of the previous “Open Cut Methods” Group. The first publication of the new Working Group appeared with the title “Recommendations of the Working Group for Excavations” in the journal “Die Bautechnik” (Construction Technology) in 1970. It was based on a thorough reworking, restructuring and enhancement of the proposals published in 1968 and consisted of 24 recommendations, continuously numbered and prefixed by the letters EB. The letters “EB” are an abbreviation of the German “Empfehlungen fur Baugruben” (“Recommendations for Excavations”). In this English translation “R’ is used to designate “Recommendation”. In the period following this, the Working Group for Excavations published new and reworked recommendations in two-year periods. In 1980, the 57 recommendations strewn throughout “Die Bautechnik”, volumes 1970, 1972, 1974, 1976, 1978 and 1980, were collected into one booklet and given the name Empfehlungen des Arbeitskreises “Baugruben”, EAB (Recommendations of the Working Group for Excavations, EAB). The “EAB” abbreviation results from the initial letters of the German title. In subsequent years, the Working Group for Excavations published new and reworked recommendations at irregular intervals. In 1988 the 2”d Edition was published and in 1996 the 3‘d Edition of the collected “Recommendations of the Working Group for Excavations, EAB”. The “Recommendations for Excavations, EAB”, published here, also include the Recommendations published during the last seven years. The recommendations R 90 to R 101 involve the subject of “Excavations in Soft Soils’’, recommendation R 102 concerns application of the modulus of subgrade reaction procedure and recommendation R 103 involves application of the Finite Element Method. VI1

The recommendations presented here are based on the global safety factor concept. Using this concept, all possible actions and impacts are computed, traced throughout the complete construction to the transmission into the ground, the maximum stresses occurring in all critical component sections and interfaces between the structure and the ground are determined, and adherence to the safety factors deemed necessary for defined limit states examined, e.g. against reaching failure stresses. This corresponds to the actual physical behaviour of the structure: the computed actions and impacts can occur at their full magnitude, the failure state strength is not utilised. In Germany, the “Recommendations of the Working Group for Excavations, EAB”, represent recognised best practice. They do not represent statutory regulations; however, persons involved in cases of regress or accidents must justify ignorance of or non-adherence to these regulations. All major analysis software applications are based on these recommendations. They -

simplify the design and analysis of excavation enclosures

- unify load approaches and analysis procedures - guarantee the stability of the excavation structure and its individual

components and - guarantee economical design of excavation structures.

Users of the “Recommendations on Excavations” should expect to enjoy a similar degree of utility from this volume. Users may be astonished at the confusing numbers of the individual recommendations. Similar to the recommendations of the Committee for Waterfront Structures (EAU), every recommendation of the Working Group for Excavations is numbered consecutively when starting a new subject. In order to achieve good communication between the users, the given number is kept as long as the recommendation exists. When in different editions the number of a chapter may change, each subject keeps its recommendation number. Anton Weissenbach

VI11

Contents Members of the Working Group for Excavations . . . . . . . . . . . . . . . . . . . V Notes for the user . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

VI

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

vi1

Terms and notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 1.2

General Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Engineering requirements for applying the Recommendations (R 1) Support of retaining walls (R 67) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xi11 1

.. 1 1

Analysis principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7

3 Analysis load cases and allowable stresses (R 24) . . . . . . . . . . . . . . . .3 4 Determination 6f soil properties (R 2) . . . . . . . . . . . . . . . . . . . . . . . . . Wall friction angle (R 89) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 General requirements for adopting live loads (R 3) . . . . . . . . . . . . . . .6 Live loads from road and rail traffic (R 55) ..................... 7 Live loads from site traffic and site operations (R 56) . . . . . . . . . . . . .9 Live loads from excavators and lifting equipment (R 57) . . . . . . . . . . 11

3 3.1

3.8

Magnitude and distribution of earth pressure . . . . . . . . . . . . . . . .14 Magnitude of earth pressure as a function of the selected construction method (R 8) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Magnitude of active earth pressure without surcharge loads (R 4) . . . 15 Distribution of active earth pressure without surcharge loads (R 5) . . 18 Magnitude of active earth pressure from live loads (R 6) . . . . . . . . . .20 Distribution of active earth pressure from live loads (R 7) . . . . . . . . .22 Superimposing earth pressure components for unsupported retaining walls (R 71) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Superimposing earth pressure components for supported retaining walls (R 72) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Earth pressure in retreating states (R 68) . . . . . . . . . . . . . . . . . . . . . . 27

4 4.1 4.2 4.3 4.4 4.5 4.6

General stipulations for analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Determination of internal forces (R 11) ....................... 29 Modulus of subgrade reaction method (R 102, Draft) . . . . . . . . . . . .30 Finite element method (R 103. Draft) . . . . . . . . . . . . . . . . . . . . . . . . . 34 Limit load design method (R 27) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 40 Equilibrium of vertical forces (R 9) . . . . . . . . . . . . . . . . . . . . . . . . . . Stability analysis in special cases (R 10) . . . . . . . . . . . . . . . . . . . . . . 42

3.2 3.3 3.4 3.5 3.6 3.7

IX

5 5.1 5.2 5.3 5.4 5.5 5.6

6 6.1 6.2 6.3 6.4 6.5

Analysis approaches for soldier pile walls . . . . . . . . . . . . . . . . . . . 44 Load model determination for soldier pile walls with active earth 44 pressure (R 12) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pressure diagrams for supported soldier pile walls (R 69) . . . . . . . . .45 Simplified earth pressure for braced soldier pile walls (R 13) . . . . . . 47 Passive earth pressure for soldier pile walls with free earth supports (R 14) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Toe restraint for soldier pile walls (R 25) . . . . . . . . . . . . . . . . . . . . . . 51 Equilibrium of horizontal forces for soldier pile walls (R 15) . . . . . . 53 Analysis of sheet pile walls and in-situ concrete walls . . . . . . . . . 57 Load model determination for sheet pile walls and in-situ concrete walls with active earth pressure (R 16) . . . . . . . . . . . . . . . . . 57 Pressure diagrams for supported sheet pile walls and in-situ 59 concrete walls (R 70) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Simplified earth pressure for braced sheet pile walls and in-situ concrete walls (R 17) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Passive earth pressure for sheet pile walls and in-situ concrete walls with free earth supports (R 19) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Toe restraint for sheet pile walls and in-situ concrete walls (R 26) . . 64

7.2 7.3 7.4 7.5

Anchored retaining walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Magnitude and distribution of earth pressure for anchored retaining walls (R 42) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Analysis of force transmission from anchors to the ground (R 43) . . 68 Analysis of stability at low failure plane (R 44) . . . . . . . . . . . . . . . . .69 71 Analysis of global stability (R 45) . . . . . . . . . . . . . . . . . . . . . . . . . . . Displacements in anchored retaining walls (R 46) . . . . . . . . . . . . . . .72

8 8.1 8.2 8.3

Excavations with special ground plans ...................... Excavations with circular plan (R 73) . . . . . . . . . . . . . . . . . . . . . . . . . Excavations with oval plan (R 74) . . . . . . . . . . . . . . . . . . . . . . . . . . . Excavations with rectangular plan (R 75) ......................

9 9.1 9.2

Excavations adjacent to structures . . . . . . . . . . . . . . . . . . . . . . . . . 89 Engineering measures for excavations adjacent to structures (R 20) . 89 Analysis of retaining walls with active earth pressure for excavations adjacent to structures (R 21) . . . . . . . . . . . . . . . . . . . 91 Active earth pressure for large distances to structures (R 28) . . . . . . . 92 Active earth pressure for small distances to structures (R 29) . . . . . . 94 Analysis of retaining walls with increased active earth pressure (R22) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Analysis of the retaining wall with at-rest earth pressure (R 23) . . . . 99 Mutual influence of opposing retaining walls for excavations 103 adjacent to structures (R 30) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7 7.1

9.3 9.4 9.5 9.6 9.7

X

74 74 79 84

10 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9

Excavations in water ................................... 107 General remarks on excavations in water (R 58) ...............107 Seepage pressure (R 59) .................................. 108 Dewatered excavations (R 60) .............................. 109 Analysis of hydraulic heave safety (R 61) .................... 111 Analysis of buoyancy safety (R 62) ......................... 114 Stability analysis of retaining walls in water (R 63) .............117 Design and construction of excavations in water (R 64) ..........120 Water management (R 65) ................................. 122 Monitoring excavations in water (R 66) ...................... 123

11

Excavations in unstable rock ............................. General Recommendations for excavations in unstable rock

11.1

124

(R38) ................................................ 124 11.2 Magnitude of rock support pressure (R 39) ...................126 11.3 Distribution of rock support pressure (R 40) ..................128 11.4 Bearing capacity of rock for support forces at the wall toe (R 41) . . 129

12.10 12.11 12.12

Excavations in soft soils ................................. 130 Scope of the Recommendationsfor excavations in soft soils (R90. Draft) ........................................... 130 Slopes in soft soils (R 9 1. Draft) ............................ 131 Lining systems in soft soils (R 92. Draft) ..................... 133 Construction procedure in soft soils (R 93. Draft) ..............136 Shear strength of soft soils (R 94. Draft) ..................... 139 Earth pressure on retaining walls in soft soils (R 95. Draft) ....... 144 Soil reactions in soft soils (R 96. Draft) ...................... 147 Water pressure in soft soils (R 97. Draft) ..................... 152 Determination of embedment depths and internal forces for excavations in soft soils (R 98. Draft) ..................... 157 Further stability analyses for excavations in soft soils (R 99. Draft) 160 Water management for excavations in soft soils (R 100. Draft) .... 163 Serviceability of excavation structures in soft soils (R 101. Draft) . 163

13 13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8 13.9

Dimensioning of structural elements ...................... 167 Dimensioning of soldier pile infilling (R 47) ..................167 Dimensioning of soldier piles (R 48) ........................ 168 Dimensioning of sheet piles (R 49) .......................... 169 Dimensioning of in-situ concrete walls (R 50) .................170 Dimensioning of walings (R 51) ............................ 170 Dimensioning of struts (R 52) .............................. 171 Dimensioning of grouted anchors (R 87) ..................... 173 Dimensioning of trench sheeting and bracing (R 53) . . . . . . . . . . . .174 Dimensioning of provisional bridges and excavation covers

12 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9

(R54)

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

175

XI

Measurements on excavations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 Purpose of measurements (R 31) . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 Preparation. implementation and evaluation measurements (R 32) . 178 179 Measured variables (R 33) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 Measurement methods (R 34) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Requirements on the measurement area (R 35) . . . . . . . . . . . . . . . . .183 Number of measurement areas and measurement sections (R36) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 185 14.7 Reading measurement values (R 37) .........................

14 14.1 14.2 14.3 14.4 14.5 14.6

Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 A 1: Unit conversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 A 2: Relative density of cohesionless soils . . . . . . . . . . . . . . . . . . . . . . . . 188 A 3: Consistency of cohesive soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 A 4: Soil properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 A 5: Guide values for the modulus of subgrade reaction ks.h for soil above the groundwater table ......................... 194 A 6: Allowable stresses for timber bracing . . . . . . . . . . . . . . . . . . . . . . . . 195 A 7: Maximum allowable stresses for steel bracing . . . . . . . . . . . . . . . . .197 199 A 8: Grouted anchor safety factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A 9: Allowable compressive loads for soldier piles . . . . . . . . . . . . . . . . .200 Furtherworkprogramme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

202

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

203

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

212

List of Recommendations in numerical order

XI1

1

General Recommendations

1.1

Engineering requirements for applying the Recommendations (R 1)

If no other stipulations are explicitly made in the individual Recommendations they shall apply under the following engineering preconditions: 1. The complete height of the retaining wall is lined.

2. The soldier piles of soldier pile walls are installed so that intimate contact with the ground is guaranteed. The infilling or lining can consist of wood, concrete, steel or stabilised soil. It must be installed so that distribution is as uniform as possible over the ground surface. Soil excavation should not advance considerably faster than 0.5 m in advance of lining in cohesionless soils or 1.00 m in cohesive soils. 3. Sheet pile walls and trench sheet piles are installed so that intimate contact with the ground is guaranteed. Toe reinforcement is permitted.

4. In-situ concrete walls are manufactured as diaphragm walls or as bored pile walls. See EN 1538 for manufacturing diaphragm walls. For bored pile walls proceed according to EN 1536. Accidental or planned spacing between the piles is generally lined according to Paragraph 2 .

5. In the horizontal projection, struts or anchors are arranged perpendicular to the retaining wall. They are wedged or prestressed so that frictional contact with the retaining wall is guaranteed. 6. Braced excavations are lined in the same manner on both sides with vertical soldier pile walls, sheet pile walls or in-situ concrete walls. The struts are arranged horizontally. The ground on both sides of the braced excavation displays approximately the same height, similar surface features and similar subsurface properties. If these preconditions are not fulfilled, or those in the individual Recommendations, and no Recommendations are available for such special cases, this does not exclude adoption of the remaining Recommendations. However, the consequences of any deviations must be investigated and taken into consideration.

1.2

Support of retaining walls (R 67)

1. Retaining walls are known as unsupported if they are neither braced nor anchored and their stability is based solely on their restraint in the ground. 2 . Retaining walls are known as yieldingly supported if the wall support points can yield with increasing load, e.g. in cases where the supports are heavily inclined and when using non-prestressed or only slightly prestressed anchors.

3. Retaining wall supports are known as only slightly yielding if a) struts are at least wedged for frictional contact, b) grouted anchors are tested to EN 1537 and are prestressed to at least 80 % of the computed force required for the next construction stage, c) frictional contact with driven piles, bored piles or grouted piles, which verifiably display only small head deflection under load, is developed. 4. Retaining wall supports are known as nearly inflexible if designed according to R 22, Paragraph 1 (Section 9 . 9 , utilising increased active earth pressure, and the struts and anchors are prestressed according to R 22, Paragraph 6.

5. Retaining wall supports are defined as being inflexible only if they are designed either for reduced or for the full at-rest earth pressure according to R 23 (Section 9.6) and the supports are prestressed accordingly. Furthermore, the anchors of anchored retaining walls must be socketed in non-yielding rock strata or be designed substantially longer than required by calculations.

2

2

Analysis principles

2.1

Analysis load cases and allowable stresses (R 24)

1, It is generally sufficient to analyse stability for Load Case H, which includes the following loads: a) self-weight of the excavation structure, if necessary taking provisional bridges and excavation covers into consideration, b) directly acting live loads as given in R 3, Paragraph 1 (Section 2.3), c) earth pressure due to soil self-weight, structural and live loads, if necessary taking cohesion into consideration, d) water pressure. 2. In special cases, it may be necessary to investigate for Load Case HZ, which considers the following loads in addition to those of Load Case H: a) brake and nosing forces, e.g. for excavations beside or below railway or tram lines, b) exceptional loads and improbable or rarely occurring combinations of loads or points of acting, e.g. unusual water levels, c) the influence of temperature on struts, e.g. steel H-section struts without buckling protection devices or struts in narrow excavations with frostsensitive ground. The impact of temperature changes on the remaining excavation structure need not be investigated for flexible walls. 3. Besides the loads already mentioned, it may be necessary in particularly unusual cases to consider exceptional loads, which do not occur under normal circumstances, e.g.: a) short-term exceptional loads, e.g. when testing, overstressing or unloading anchors or struts, b) impact of construction machinery against the supports of provisional bridges or excavation covers or against the intermediate supports of buckling protection devices, c) loads caused by the failure of operating or stabilising installations, if the effects cannot be countered by appropriate measures, d) loads caused by the failure of particularly susceptible bearing members, e.g. struts or anchors, e) loads due to scouring in front of the retaining wall.

4. The following allowable stresses apply for analysis of structural elements: a) The increased allowable stresses according to Appendix A 6 and A 7 apply for Load Cases H and HZ as well as for the case described in Paragraph 3a.

3

b) The allowable stresses for investigation of limit states according to Paragraphs 3b to 3e must be determined in cooperation with the appropriate authority.

5. When determining the allowable effective stress in conjunction with limit load design, the following steel yield stress values must be applied: a) b) c) d) e)

for S 235: for S 240: for S 270: for S 355: for S 390:

fy = 240 MN/m2 fy = 240 MN/mZ fy = 270 MN/mZ fy = 360 MN/m2 fy = 390 MN/mZ

6. If the stability analysis of excavation structures, provisional bridges and excavation covers is performed according to DIN 1054, the safety factor may be determined as follows: a) in Load Case H as for Load Case 2 according to EAU [2], b) in Load Case HZ, and for investigation of exceptional cases according to Paragraph 3, as for Load Case 3 according to EAU [2].

2.2

Determination of soil properties (R 2)

1. To determine earth pressure and passive earth pressure, the calculation values @, c‘, @u and c, for shear strength, and y and y’ for unit weight may be taken from Appendix A 4. 2. The cohesion of cohesive soils may be introduced into the analysis according to Appendix A 4, if this is not restricted by R 4, Paragraph 3 (Section 3.2).

3. The capillary cohesion of sandy soils may be taken into consideration if it cannot be lost by drying or flooding or due to rising groundwater or water ingress from above during construction work. However, higher values than c’ = 2.0 kN/mZmay only be applied if they are confirmed by local experience or can be checked by measurements on installed lining.

2.3

Wall friction angle (R 89)

1. The characteristic wall friction angle 6 is principally dependent on the: a) shear strength of the ground, b) surface roughness of the wall, c) relative movement between wall and ground. Furthermore, the selected slip surface also plays a role. 4

2. The following cases are differentiated with regard to wall roughness: a) A rear wall face is known as “keyed” if, due to its shape, it displays such a convolute surface that the wall friction acting immediately between the wall and the ground is not critical, but the friction in a planar failure surface in the ground, which only partially contacts the wall. This is always the case in pile walls. It is also true as an approximation for sheet pile walls. b) The untreated surfaces of steel, concrete and wood can generally be considered as “rough”, in particular the surfaces of soldier piles and infill walls. c) The surface of a diaphragm wall may be classified as “slightly rough” if filter cake development is low, e.g. for diaphragm walls in cohesive soil. Empiricism indicates that this is also the case for diaphragm walls in cohesionless soil if the standing time of the mud-supported trench is kept short according to the general stipulations for manufacture. d) A rear wall face should be classified as “smooth” if the soil displays “smeary” properties due to its clay content and consistency. The possible physical wall friction may only be adopted if the earth pressure or passive earth pressure analysis was based on curved or non-circular slip surfaces. If approximately planar slip surfaces are used, the wall friction angle must be reduced to compensate for the resulting error. The following wall friction angles, as a function of the friction angle $, are to be taken:

I Wall texture

I Curved slip surfaces I Planar slip surfaces Q

Keyed wall

ISl=Q

I 6 I =2/3.

Rough wall

30” 2 I 6 I IQ - 2.5”

I 6 I =’I3 Q

Slightly rough wall

I 6 I = ’I3. Q

I 6 I = ‘ I 3$.

Smooth wall

161=0

161=0

9

4. The sign of the wall friction angle is dependent on the relative displacement between the wall and the ground: a) For active earth pressure the wall friction angle is positive if the earth wedge moves downwards more than the wall as shown in Figure R 89-la. b) For active earth pressure the wall friction angle is negative if the wall moves downwards more than the ground as shown in Figure R 89-lb. The same applies in principle for determination of the passive earth pressure. See also Figure R 19-1 (Section 6.4). 5

ai Positive wall friction angle

61 Negative wall friction angle

Figure R 89-1. Wall friction angle for active earth pressure

General requirements for adopting live loads (R 3)

2.4

1. The following are described as live loads: a) loads from road and rail traffic according to R 55 (Section 2.4), b) loads from site traffic and site operations according to R 56 (Section 2.5), c) loads from excavators and lifting equipment according to R 57 (Section 2.6). See R 24 (Section 2.1) for classification of these loads into principal and additional loads.

2. If no precise investigations are carried out, the individual tyre contact widths of rubber-tyred vehicles and construction equipment are assumed as follows: a) b) c) d) e)

0.60 m for wheel loads of 100 kN (10 t), 0.46 m for wheel loads of 65 kN (6.5 t), 0.40 m for wheel loads of 50 kN (5.0 t), 0.30 m for wheel loads of 40 kN (4.0 t), 0.26 m for wheel loads of 30 kN (3.0 t).

Where necessary, these values may be linearly interpolated. The contact length in travel direction is always 0.20 m. 3. A load distribution in all directions within the upper road layers may be assumed as shown in Figure R 3- 1 as follows, independent of the properties and thickness d of the load distributing layers:

a) distribution with a = d for the wearing course and base courses of bituminous layers, concrete or tightly interlocked stone paving, b) distribution with a = 0.75 d for hydraulically stabilised gravel or crushed stone base courses, c) distribution with a = 0.50 d for non-stabilised gravel or crushed stone base courses. 6

Torior

m,lI;

1 d

I

I o

t ai Load distribution in section

bl Load distribution in plan

Figure R 3-1. Load distribution in the upper road layers

For base course quality requirements see the relevant technical regulations and guidelines for base courses in highway engineering. 4. If no road pavement is installed, the contact areas of rubber-tyred vehicles and construction equipment increase as a result of settlement into the surface. As an approximation, the contact area lengths and widths that apply to paved roads in Paragraph 2 may be increased by 15 cm, if no precise investigations are carried out.

5. In order to determine the earth pressure, a point load or a bounded distributed load as shown in Figure R 3-2a may be converted to an equivalent strip load and the load projection be assumed at approximately 45" to the horizontal. If the effects of neighbouring loads overlap, a simplified approach with a common contact area for both loads may be applied as shown in Figure R 3-2b.

r

7

L---

--1

Wtdth of equivalent strtp load

r---L---

Retaining wal al Single load

-

45 a

""

Retaining wall

bi Two loads

Figure R 3-2. Conversion of bounded distributed loads to strip loads

6. If, in braced excavations, only one wall is loaded by earth pressure from live loads, the opposite wall must be designed for the same internal forces unless, for elastic retaining structures, the resulting earth pressure on the support points is analysed. Reinforcement of the infilling of soldier pile walls on the opposite side is not necessary. 2.5

Live loads from road and rail traMic (R 55)

1. The axle spacing and axle loads generally adopted for the design of bridges are not critical for the excavation structure. When analysing the stability of 7

ai Single axle load I10 kN

bl Double axle load 2 x 80kN

cl Trple axle load 3 x 70 kN

Figure R 55-1. Critical axle loads

excavation structures it is sufficient to base the analysis on the actual axle spacing and the allowable axle loads of commonly licensed road vehicles and to investigate the following load combinations: a) Single axle loads of 115 kN (1 1.5 t) as shown in Figure R 55- la). b) Double axle loads of 160 kN (16.0 t) as shown in Figure R 55-lb). c) Triple axle loads of 210 kN (21.0 t) as shown in Figure R 55-lc). The axle loads may be evenly distributed across all wheels of one axle or an axle group. An impact surcharge need not be taken into consideration. 2. The following recommendations apply to determination of earth pressure acting on the retaining wall due to wheel loads according to Paragraph 1: a) R 3 , Paragraph 2 (Section 2.3), for the contact area. b) R 3 , Paragraph 3 , for load distribution in the upper road layers. c) R 3 , Paragraph 5 , for load distribution in the ground. The influence of vehicle wheels on the side of the vehicle away from the retaining wall, and the influence of vehicles in more distant lanes, need not be individually investigated. Instead, an unbounded distributed load of 10 kN/mZ is applied immediately adjacent to the wheel loads nearest to the retaining wall. 3. If it is certain that:

a) the loads according to Paragraph 1 will not be exceeded, b) the road pavement including the bituminous base course layers consists of concrete or tightly interlocked stone paving and is at least 15 cm thick, c) a distance of at least 1.O m remains between the wheel contact areas and the rear of the retaining wall, a more precise investigation according to Paragraph 2 may be dispensed with and an unbounded distributed load of p = 10 kN/mz be adopted as equivalent load. For lesser distances, the distributed load must be located in a strip 1.5 m wide immediately adjacent to the retaining wall and increased as follows: by p’ = 10 kN/mZ, if the contact areas remain at a distance of at least 0.60 m, by p’ = 40 kN/mZ, if no spacing is adhered to, e.g. in the area of provisional bridges. 8

Figure R 55-2. Equivalent load for road traffic at less than 1.0 m from the retaining wall

See also Figure R 55-2. The load distribution in the road pavement is already considered in these approaches.

4. If, when applying the equivalent loads, vehicles heavier than those given in Paragraph 1 must be taken into consideration, the equivalent loads given in Paragraph 3 may be converted in a ratio corresponding to the axle loads if the individual vehicles, tractors and trailers do not have more than three axles. Special investigations must be carried out for vehicles with more than three axles, e.g. wagon-carrying trailers. 5 . If a kerb is supported against the retaining wall, a horizontal nosing force must be applied. The magnitude is given in the relevant regulations for bridge engineering. The nosing force must be adopted as a principal load when designing the kerb and as a surcharge when designing the excavation structure.

6 . If the retaining wall lies within a rail vehicle load projection, the live loads or equivalent loads are adopted on the basis of the regulations of the transport service provider concerned. A dynamic coefficient need not be taken into consideration. It is sufficient to apply an unbounded distributed load of p = 10 kN/mz for tramlines if a minimum distance of 0.6 m between the ends of the sleepers and the retaining wall is adhered to. Centrifugal and nosing forces must be taken into consideration where necessary.

7. When designing provisional bridges and excavation covers the relevant regulations for bridges apply and those of the appropriate transport service provider for rail traffic. On multiple-lane provisional bridges and excavation covers where vehicles travel which require a special permit due to their axle loads or total gross weight, and which are therefore not commonly licensed, it is generally sufficient to provide one specially designed lane for this purpose.

2.6

Live loads from site traffic and site operations (R 56)

1. Construction materials normally stored in the open or in a site hut are generally taken into consideration by means of an unbounded distributed load of p = 10 kN/m2. If large earth masses or large quantities of steel, stones and similar materials are stored in the immediate vicinity of the excavation, more precise investigations must be carried out. The same applies to silo loads.

9

2 . When applying equivalent loads for vehicles licensed for general public roads, such as heavy goods vehicles, tractors and trailers, R 55, Paragraph 3 (Section 2.5) also applies when no road pavement is installed. If construction vehicles cannot be associated with the loads given in R 55, Paragraph 1, due to their axle loads or the number of axles, R 55, Paragraph 4 applies accordingly. The adoption of live loads from site traffic is not necessary if the influence of excavators and lifting equipment according to R 57, Paragraph 2 (Section 2.5) is already taken into consideration for the same area. Excavators and lifting equipment that only travel along the outside of the excavation must be taken into consideration as road vehicles.

3. If the earth pressure from construction vehicles is not determined with the help of equivalent loads according to Paragraph 2, the following recommendations apply: a) R 3, Paragraph 2 (Section 2.4), for the contact areas of rubber-tyred vehicles, b) R 3, Paragraph 3, for load distribution in the upper road layers, c) R 3, Paragraph 4, for the increase in contact area where no pavement is present, d) R 3, Paragraph 5, for load distribution in the ground. The influence of vehicle wheels on the side of the vehicle away from the retaining wall, and the influence of vehicles in more distant lanes, need not be individually investigated. Instead, an unbounded distributed load p = 10 kN/m2 is applied immediately adjacent to the wheel load nearest the retaining wall. 4. When designing excavation covers which will serve as working areas or storage areas for formwork, reinforced concrete and similar work, Paragraph 1 applies accordingly. The anticipated loads must be adopted for provisional bridges and excavation covers for site traffic. The same applies to non-rubber-tyred site traffic, e.g. road rollers or crawler excavators. The current regulations for bridges apply accordingly with regard to dynamic coefficients, surcharges and exceptional loads. If several loaded vehicles, e.g. ready-mixed concrete vehicles, can simultaneously travel successively or park in one lane, or beside each other in neighbouring lanes, this must be taken into consideration.

5. When designing struts, a vertical live load of at least p = 1.0 kN/m2 must be applied to consider unavoidable loads caused by site operations, light covers, gantries, bracing and similar loads where greater vertical loads are not envisaged, besides self-weight and the normal force. Horizontal loads, e.g. resulting from bracing or formwork supports, must be taken into consideration in strut design. Struts may not be loaded with live loads in utility trench construction with vertical or horizontal bracing or soldier pile walls lined with a plank curtain. Otherwise, see R 52, Paragraph 5 (Section 11.6). 10

6. If no structural protection against impact of construction machinery is installed, a point load P = 100 kN in all directions at a height of 1.20 m above the excavation level must be taken into consideration when designing the supports of provisional bridges or excavation covers and the intermediate supports of buckling protection devices.

2.7

Live loads from excavators and lifting equipment (R 57)

1. Excavators and lifting equipment operating at short distances from the excavation impose large stresses on the retaining wall structure. Separate investigation of the impact of earth pressure magnitude and distribution may only be dispensed with if the following distances to the retaining wall are adhered to:

1S O m for a gross weight of 10 t 2.50 m for a gross weight of 30 t 3.50 m for a gross weight of 50 t 4.50 m for a gross weight of 70 t Intermediate values may be linearly interpolated. If the distances given here are adhered to it is sufficient to apply an unbounded distributed load of p = 10 kN/m2. 2. If excavators or lifting equipment operate beside the retaining wall at distances smaller than those given in Paragraph 1, the resulting earth pressure magnitude and distribution must be determined. If this is based on the excavators or lifting equipment point loads, the following apply: a) The contact area of tracked equipment is taken from the manufacturer’s specifications. b) The contact area of rubber-tyred equipment is according to R 3, Paragraph 2 (Section 2.3). c) For information on load distribution in the upper road layers, see R 3, Paragraph 3. d) For information on the increase in contact area where no pavement is installed see R 3, Paragraph 4. e) For load distribution in the ground, see R 3, Paragraph 5. Where applicable, the effect of load distributing sub-bases such as excavator mattresses, timber packing or rails laid on sleepers may be taken into consideration. 3. When determining earth pressure according to Paragraph 2. all critical excavator and lifting equipment distances from the retaining wall and all critical positions of the crane chassis and the boom must be taken into consideration. As an approximation for Load Case H (principal loads) according to R 24 (Section 2.1), analysis may be based on the following load distribution:

11

a) with a boom in equipment travel direction: 40 % of the gross weight at each of the two more heavily loaded wheels or half of the length of both tracks on tracked vehicles, b) with the boom positioned diagonally: 50 % of the gross weight at the more heavily loaded wheel or half of the length of the more heavily loaded track on tracked vehicles, c) with the boom perpendicular to the travel direction: 40 % of the gross weight at the two more heavily loaded wheels or 80 % of the gross weight at the more heavily loaded track on tracked vehicles. The impact of loads acting on the respectively lower stressed wheels or tracks need not be individually investigated. Instead, an unbounded distributed load p = 10 kN/m2 is applied immediately adjacent to the wheel load nearest the retaining wall. 4. As an approximation, the point loads of excavators and lifting equipment can be substituted by an unbounded distributed load p = 10 kN/mz and an additional strip load p’, which begins immediately adjacent to the retaining wall as shown in Figure R 57-1 and covers the complete length travelled by the vehicle. For construction machinery on tracks, rubber-tyred construction machinery with not more than two axles, and construction machinery running on rails and sleepers, the magnitude and width may be assumed as follows for Load Case H (principal loads), according to R 24 (Section 2.1), as a function of the distance to the retaining wall:

Gross weight of equipment 10 t

30t 50 t 70t

Additional strip load p’ Adjacent to wall

0.60 m from wall

50 kN/mz 110 kN/mz 140 kN/mz 150 kN/m2

20 kN/mz 40 kN/mz 50 kN/m2 60 kN/m2

W/dth of

m I

+-6?PtJ/nlng

WJ[l

Figure R 57-1. Equivalent load for excavators and lifting equipment

12

Width of strip load p’ 1.50 m 2.00 m 2.50 m 3.00 m

Intermediate values may be inserted linearly; weights below 10 t may be linearly extrapolated. Otherwise, the following apply: a) Supporting devices (outriggers) must have a floor contact area of at least 0.25 mz or be placed on an appropriate load distributing structure. b) In principle, the distance between the retaining wall and the equipment refers to the floor contact area. However, if the equipment travels perpendicular to the side of the excavation, the vertical projection of the wheels or the tracks may not intersect the rear edge of the retaining wall. Where equipment travels on rails and sleepers, the distance to the sleeper ends represents the critical distance. c) If the road surface is metalled, load distribution at 45" from the rear edge of the equivalent load may be assumed. 5. The gross weight of excavators and lifting equipment consists of a) the operating weight of the equipment based on the manufacturer's specifications and b) the weight of the extracted earth or any coupled loads. 6. If, in exceptional cases, Load Case HZ (principal and additional loads) is investigated according to R 24 (Section 2. l), the values given in Paragraph 3 must be increased as follows: a) from 40 % to 50 % b) from 50 % to 70 % c) from 80 % to 100 % The equivalent loads given in Paragraph 4 must be increased by 30 %. 7. When designing provisional bridges and excavation covers which are also to serve as work areas for excavators or lifting equipment, the following apply:

a) The applicable loads are determined according to Paragraphs 3 , 5 and 6. b) The contact areas of tracked equipment are taken from the manufacturer's

specifications; R 3, Paragraph 2 (Section 2.4) applies for determination of the contact areas of rubber-tyred equipment. c) The dynamic coefficient is assumed at 41 = 1.20, independent of the span.

d) For braking action, acceleration loads and for nosing forces, a horizontal point load of '/?* of the vertical load given in Paragraph 5 must be adopted at the critical location and in the critical direction at the height of the contact area. Additional investigations may be required for ditchers. e) Further surcharges and exceptional loads are adopted according to the current regulations for bridges. f) Where appropriate, consideration must be given to loads from site traffic, which occurs simultaneously to loads from excavators and lifting equipment, according to R 56, Paragraph 4 (Section 2.6).

13

3

Magnitude and distribution of earth pressure

3.1

Magnitude of earth pressure as a function of the selected construction method (R 8)

1. The magnitude of the earth pressure is highly dependent on the degree of deflection and deformation of the retaining wall as a result of material excavation. The decisive factors here are: a) the flexibility of the support, see R 67 (Section 1.2), b) the flexibility of the earth support, see R 14 (Section 5.4) and R 19 (Section 6.4), c) the spacing of the support points and the flexural stiffness of the retaining wall. With regard to flexural stiffness, diaphragm walls and pile walls can generally be viewed as flexurally stiff and low-deformation walls, sheet pile walls and soldier pile walls as flexurally soft or elastic. 2. If a theoretical excavation case is considered in which any deflection or pressure relief of the ground is avoided when installing sheet pile walls or in-situ concrete walls, wall loading from at-rest earth pressure must be taken into consideration. However, because it is not possible in practice to keep retaining walls completely free of deformation and deflection, the effective earth pressure is generally smaller than the at-rest earth pressure E,.

3. For multiple-braced sheet pile walls with relatively small support point centres and slightly yielding supports, and for braced in-situ concrete walls in general, an earth pressure value must be assumed which lies between the at-rest earth pressure and the active earth pressure, if the struts are prestressed with a force greater than 30 % of that projected for the fully excavated stage. This also applies to multiple-braced soldier pile walls, if the struts are prestressed with a force more than 60 % of that projected for the fully excavated stage. 4. If the struts are prestressed with forces smaller than those given in Paragraph 3, it can be assumed that the wall will be deformed or displaced by a value corresponding to 1 % of the wall height in medium-dense to dense cohesionless soil or at least firm, cohesive soil. This generally suffices to reduce the earth pressure from the passive earth pressure value to the active earth pressure value. This is generally the case for unsupported retaining walls restrained in the ground, regardless of the types of soils present. 5. The magnitude of the anticipated earth pressure acting on anchored retaining walls is primarily dependent on the prestressing load of the anchors. See also R 42 (Section 7.1).

6. See R 68 (Section 3.8) for earth pressure during retreating states. 14

3.2

Magnitude of active earth pressure without surcharge loads (R 4)

1. The magnitude of the active earth pressure E, from soil self-weight and, where applicable, cohesion may be determined using planar slip surfaces based on classical earth pressure theory, if the self-weight force W, the earth pressure E, and the resultant of the stresses in the slip surface Q approximately intersect in one point. In individual cases, this is dependent on the selected combination of wall inclination, ground inclination and wall friction angle, see Figure R 4-1. This also applies to stratified soils. 2. The wall friction angle 6, is principally dependent on R 89 (Section 2.3). It may generally be adopted for soldier pile walls, sheet pile walls and in-situ concrete walls with a positive wall friction angle if the vertical forces can be completely transmitted into the ground (Figure R 89-la, Section 2.3). IfZV = 0 cannot otherwise be demonstrated, a smaller, or negative, wall friction angle must be introduced into the earth pressure analysis (Figure R 89-lb, Section 2.3). This may be necessary if large vertical forces are transmitted to the retaining wall, e.g. for provisional bridges or inclined anchors. See also R 9 (Section 4.1).

3. The earth pressure must be determined in two alternative ways for homogeneous cohesive soils and stratified soils: a) with the shear strengths according to R 2 (Section 2.2), in areas of both cohesionless and cohesive soils (Figure R 4-2b), b) with the shear strengths according to R 2 (Section 2.2), in areas of cohesionless soils and with a minimum earth pressure which follows from the equivalent friction angle (PEquiv with cEquiv= 0 in the cohesive layers (design earth pressure, Figure R 4-2 c).

a1 Farces intersect in one point

bl Forces do not intersect in one point

Figure R 4-1. Criteria for adopting planar slip surfaces

15

a1 Stratified ground

bl Earth pressure according to Paragraph 3a

cl Earth pressure according to Paragraph 3b

Figure R 4-2. Determination of active earth pressure for partially cohesive soils

Further analysis must be based on the more unfavourable load assumption. If the magnitude of the anticipated earth pressure is sufficiently well known from long-term measurements in similar conditions, and is checked in individual cases on the lining being installed, the equivalent friction angle may be reduced to @Equiv = 45". Determination of the earth pressure for supported retaining walls may also be based on classical earth pressure distribution as shown in Figure R 4-2 or Figure R 4-3. Computed tensile stresses occumng as the result of cohesion in cohesive soils may be taken into consideration in full for determination of the earth pressure magnitude according to Paragraph 3a, if earth pressure redistribution is anticipated for the given conditions. See also Figure R 4-3c. If redistribution is not anticipated, e.g. for unsupported retaining walls restrained in the ground, this is disregarded. See also Figure R 4-3d. In cohesive soils and rocky ground, local experience should be examined for indications that the earth pressure may increase with time due to the swelling capacity of the ground, by frost action, by thawing after a period of frost or for other reasons, over and above that determined for the respec-

ai Earth pressure from soil self- weqht

bl Farth pressure resulting from cohesion

ci Earth pressure for supported retaining walls

dl Earth pressure for unsupported retaining walls

Figure R 4-3. Determination of active earth pressure in cohesive soil

16

el Minimum earth pressure

tive soil properties. In addition, where rocky ground is involved, it should be established whether bedding or joints predetermine certain slip surfaces, which influences the magnitude of the earth pressure. See also R 38 (Section 11.1). 6. Based on theoretical considerations, a larger earth pressure than computed using classical earth pressure theory is anticipated for wall displacements other than rotation around the wall toe or a lower point. Despite this, it is not necessary, in fitting with measurements on previously executed excavations, to increase the earth pressure determined on the basis of Paragraphs 1 to 4. This only applies if the earth pressure is based, as is normally the case, on conservative ground properties, e.g. the values in Appendix A 4. Only when all soil mechanical investigation methods have been consciously applied to acquire realistic soil properties and thus the smallest realistic active earth pressure value, can a surcharge of 10 % be adopted for analysis of singlepropped walls and of 20 % for multiple-propped walls. 7. By applying model tests and taking measurements on previously executed excavations (see [69] and [73]) it has been demonstrated that under certain circumstances a portion of the earth pressure can be redistributed to below the excavation level when using flexible retaining walls, with the result that the earth pressure acting above the excavation level is smaller than the computed active earth pressure E,. This can be the case for example: a) for a yieldingly anchored, flexible retaining wall (Figure R 4-4a), b) when removing the lowest row of struts on a multiple-braced wall (Figure R 4-4b). However, this reduction may only be adopted at a maximum 20 % for stability analysis of soldier pile walls or 10 % for sheet pile walls, and only when confirmed by measurements in comparable conditions or if these approaches have been checked against measurements on previously installed bracing.

ai Flexibly anchored solder pile wall

bl Soldier pile wall strut removal

Figure R 4-4. Possible earth pressure redistribution in the region below the excavation level for flexible retaining walls

17

Distribution of active earth pressure without surcharge loads (R 5)

3.3

1. Unsupported retaining walls restrained in the ground rotate around a point at depth. Accordingly, classical earth pressure distribution must be anticipated in such cases. See also R 4, Paragraph 4 (Section 3.2) for cohesive soil.

2. Supported retaining walls rotate around higher, alternating pivots in the course of excavation progress, associated with parallel deflection and bending. Earth pressure distribution varies based on the precise interaction of these influences. Influencing factors include: a) b) c) d)

the type of retaining wall and the method of installation and/or infilling, the flexural stiffness of the retaining wall, the number and configuration of the struts and/or anchors, the size of the respective excavation stage before installation of the struts and/or anchors, e) the prestressing of the struts and/or anchors.

Furthermore, f) the site morphology [90] and g) the type and stratification of the ground may play a role. In contrast to classical earth pressure distribution, the earth pressure is generally concentrated at the wall supports. The regions between the support points are unloaded if the wall bends correspondingly. The previously recorded deformation at each respective construction stage is decisive here (see [5, 6, 321). Redistribution is generally smaller for flexible supports. In some circumstances no earth pressure redistribution takes place. Where large construction projects are involved, it is always advisable to investigate the earth pressure distribution by taking measurements on the lining. 3. For braced retaining walls in cohesionless soils and non-yielding supports according to R 67, Paragraph 3 (Section 1.2), the following rules can be assumed in principle, based on theoretical considerations and available measurements (see [3-9, 11-14,32,46, 52,67,73, 891): a) Earth pressure distribution always commences at ground level with the ordinate at zero and then increases much faster with depth than when based on classical earth pressure theory. b) The largest load ordinate can be found at the height of the support in single-propped walls. It is at the height of the upper support in doublepropped walls, if this is installed very low; it is at the height of the lower support on the other hand, if the upper support is near ground level. In multiple-propped walls it is generally located at a support level within the central third of the excavation depth. 18

c) The effective earth pressure generally ends with the ordinate at zero at the excavation level for soldier pile walls. As an approximation for sheet pile walls, diaphragm walls and pile walls, a reduction to half of the largest load ordinate can be assumed at the height of the point of zero stress if the classical earth pressure and the effective earth pressure are superimposed. d) The earth pressure resultant from soil self-weight and wide-area live loads is almost always higher than half of the excavation depth. The resultant of the computed earth pressure for sheet pile walls, diaphragm walls and pile walls is generally below half of the distance from ground level to the point of zero stress. e) This applies to medium-dense to dense soils. Loosely compacted, cohesionless soil is also subject to earth pressure redistribution, although only to a minor extent. The earth pressure resultant for sheet pile walls and in-situ concrete walls is lower than that for soldier pile walls, all else being equal.

4. Paragraph 3 applies accordingly for braced retaining walls in cohesive soils (see [IO, 15, 16,47,90]). However, considering the influence of soil consistency, the following must be observed: a) In semi-solid to stiff cohesive soils, earth pressure redistribution similar to medium-dense to densely compacted, cohesionless soils can be assumed. Nevertheless, in the case of stiff cohesive soils the preconditions for applying Recommendations R 38 to R 41 (Sections 11.1 to 11.4) should be examined. b) In individual cases in firm, cohesive soils, the earth pressure distribution may more resemble either that of a medium-dense or that of a loosely compacted, cohesionless soil. The clay content, lime content and sensitivity are critical in this respect. c) In soft, cohesive soils the earth pressure redistribution is at most equal to that of loosely compacted, cohesionless soils, but often lower. At least in some cases of sheet pile walls and in-situ concrete walls, classical earth pressure redistribution may occur.

5. Paragraphs 3 and 4 apply without restriction for anchored retaining walls, if the anchors are prestressed so that wall deflection is similar to that for bracing. However, as this is generally not the case, and because correspondingly larger or smaller prestressing compels different earth pressure distributions and, furthermore, because the ground not only acts as a load but also accepts anchor forces, different regulations and additional requirements may also apply to anchored retaining walls. See R 42 to R 46 (Sections 7.1 to 7.5). 6. Because of the numerous possible impacts, the actual earth pressure distribution can only be approximately determined. Determination of internal forces should therefore be based on as simple a pressure diagram as possible, bounded by straight lines, e.g. one of the pressure diagrams shown in Figure R 5-1. The bending points and load increments of the selected pressure diagrams may be located at the support points to simplify analysis. See R 13 (Sec19

-7

f bl

a)

dl

Cl

el

H

'7

^^

-T

4 fl

i ' g)

hl

il

kl

Figure R 5-1. Pressure diagrams for supported retaining walls (examples)

tion 5.3) and R 16 (Section 6.1) for information on the heights H and H' of the pressure diagrams. If the preconditions given there are fulfilled, the pressure diagrams can be adopted according to R 69 (Section 5.2) or R 70 (Section 6.2).

7. If the anticipated earth pressure distribution cannot be estimated with sufficient precision due to unusual circumstances, e.g. layers of soft ground, organic ground or the simultaneous use of struts and anchors, the selected approaches must be checked by measurements on the lining in order to allow initiation of special structural measures before a critical stage is reached. If this is not possible it may be necessary to analyse using two pressure diagrams, which restrict the possible earth pressure distribution. The most unfavourable internal forces are always decisive for the design of individual components.

3.4

Magnitude of active earth pressure from live loads (R 6)

1. Determination of the active earth pressure from live loads can generally be

based on the same wall friction angle 6, as that used for determination of the active earth pressure from soil self-weight. See also R 4, Paragraph 2 (Section 3.2).

2. The magnitude of the active earth pressure from wide-area live loads can generally be based on the same slip surface as that used for determination of the active earth pressure from soil self-weight. 20

ai Sllp surface at angle tp,

bi Forced sip surface at angle iu,

Figure R 6-1. Assumed slip surfaces for determination of active earth pressure from soil self-weight and live loads

3. The slip surfaces shown in Figure R 6-la, originating at the rear edge of the load area or at the line load and running parallel to the slip surface at an angle 6,, which is decisive for determination of the earth pressure from soil selfweight, may be used to approximately determine the active earth pressure from line or strip loads according to R 3, R 55, R 56 and R 57 (Sections 2.4 to 2.6). However, see also Paragraph 6.

4. If the effect of cohesion is considered for determination of the active earth pressure from line or strip loads based on the assumed slip surface shown in Figure R 6-la, the earth pressure from live loads is reduced correspondingly. However, since the effect of cohesion may not be introduced into the analysis twice, the earth pressure from soil self-weight and wide live loads must be increased by the same amount with which the earth pressure from live loads was reduced for cases in which supported retaining walls are based on earth pressure redistribution considering the effects of cohesion. See also R 72, Paragraph 3 (Section 3.7). For unsupported retaining walls restrained in the ground, the computed decrease in earth pressure from live loads specified above can be utilised to the same extent that computed tensile stresses are reduced. See also R 71 (Section 3.6). 5. If the magnitude of the earth pressure from soil self-weight is determined with the help of an equivalent friction angle according to R 4, Paragraph 3 (Section 3.2), this can also be used as the basis for determination of earth pressure from wide equivalent loads up to p = 10 kN/m2. In contrast, the associated earth pressure from line loads and strip loads must always be determined on the basis of Paragraphs 3 and 4. 6. To determine the active earth pressure from line or strip loads for unsupported retaining walls which are merely restrained in the ground a forced slip surface must also be investigated. This runs from the rear edge of the load area or from the line load to the intersection with the rear of the wall and the excavation level, or to the fulcrum or the actual or theoretical wall toe

21

(Figure R 6-lb). The same also applies to very flexibly supported retaining walls. The determining factor in each case is the more unfavourable cumulative earth pressure from soil self-weight and live loads. 7. For determination of earth pressure from building loads see R 21 to R 23 and R 28 to R 30 (Sections 9.2 to 9.7).

3.5

Distribution of active earth pressure from live loads (R 7)

1. For unsupported retaining walls, the earth pressure from unbounded distributed loads is adopted as a rectangle over the complete wall height based on classical earth pressure theory. It is integrated into the pressure diagram according to R 5 , Paragraph 6 (Section 3.3) for supported retaining walls. 2. The earth pressure from strip loads p’ or from line loads can be adopted as a simple pressure diagram, bounded at the top and bottom as follows: a) According to classical earth pressure theory, the pressure diagram for unsupported retaining walls begins at the height at which a straight line at an angle (I to the horizontal, originating at the front edge of the strip load or at the line load, intersects the rear of the wall. This can be higher for supported retaining walls. b) The pressure diagram generally ends at the height at which a straight line at an angle 6, to the horizontal, originating at the rear edge of the strip load or at the line load, intersects the rear of the wall. When the earth pressure is determined using forced slip surfaces according to R 6, Paragraph 6 (Section 3.4), the pressure diagram ends at the intersection of the forced slip surface with the rear of the wall. 3. The shape of the pressure diagram can be specified as follows for unsupported retaining walls: a) In the case of strip loads adjoining the wall, a rectangular pressure diagram based on classical earth pressure theory results as shown in Figure R 7-la). b) In the case of line loads and classical earth pressure theory, an earth pressure distribution results which can be substituted, as a conservative approximation, by a triangular pressure diagram as shown in Figure R 7- IC). c) The earth pressure distribution for strip loads not adjoining the wall must be determined using an appropriate approximation method investigation. Using a straight-line interpolation as a function of the distance-to-width ratio of the load, the result is a trapezoidal pressure diagram as shown in Figure R 7-lb. 4. To a large extent, the pressure diagram can be freely selected as shown in Figure R 7-2b for supported retaining walls. Adjustment of the pressure dia-

gram to the support points is permissible; however, the resultant may not be below the point at which a straight line originating at the rear edge of the strip

22

a1 Strip load a4oining the wall

bl Strip load at a distance from the wall

cl Line load

Figure R 7-1. Earth pressure from live loads for unsupported walls

al Retaining wall, live load and load distribution

61 Possible simple pressure diagrams (examples1

Figure R 7-2. Assumed earth pressure from live loads for supported walls

load or at the line load and running at an angle of 45" from the horizontal, meets the rear of the wall.

3.6

Superimposing earth pressure components for unsupported retaining walls (R 71)

1. Classical earth pressure redistribution is always decisive for unsupported walls restrained in the ground. The earth pressure components from soil self-weight, unbounded distributed load p, and locally acting strip load p' or line load p, as well as the negative earth pressure component resulting from cohesion, must be superimposed. Here, the principal differentiation is between a) earth pressure determination with slip surfaces at an angle as shown in Figure R 6-la (Section 3.4), b) earth pressure determination with forced slip surfaces at an angle 6, as shown in Figure R 6-lb (Section 3.4). 23

2. The following pressure diagrams result from superimposing individual earth pressure components in homogeneous, cohesionless soils, taking R 7, Paragraph 1 (Section 3.5) into consideration: a) the pressure diagram given in Figure R 7 1-1 assuming slip surfaces at an angle 6,, b) the pressure diagram given in Figure R 7 1-2 assuming slip surfaces at an angle 6,. 3. The following pressure diagrams result for homogeneous cohesive soils, taking R 4, Paragraph 3 (Section 3.2) and R 7, Paragraph 1 (Section 3.5) into consideration:

a) the pressure diagram given in Figure R 7 1-3 with shear strength according to R 2 (Section 2.2), assuming slip surfaces at an angle 6, a) the pressure diagram given in Figure R 7 1-4 with shear strength according to R 2 (Section 2.2), assuming slip surfaces at an angle 6,, c) the pressure diagram given in Figure R 71-5, assuming a minimum earth pressure according to R 4, Paragraph 3b (Section 3.2).

ai load

bi Soil self-welght

r) Distributed load p

di Sfrp load p’

e) Superimposed

Figure R 71-1. Earth pressure distribution for an unsupported retaining wall, restrained in cohesionless soil, assuming slip surfaces at an angle 6, (example)

ai load

bi Soil self- welght

0 Distributed load p

di Strp load p’

el Superimposed

Figure R 71-2. Earth pressure distribution for an unsupported retaining wall, restrained in cohesionless soil, assuming slip surfaces at an angle 6, (example)

24

al Load

bl Soil self- weght and cohesion

cl Distributed load p

dl Strip load p’

el Superimposed

Figure R 71-3.Earth pressure distribution for an unsupported retaining wall, restrained in cohesive soil, assuming slip surfaces at an angle 6, (example)

\yI..--\ \ \ \ \ \

\ \ \ \ \ \

\ \

\

a/ 1oad

bl Sod self- weght and cohesion

cl Distributed

dl Strip load p‘

e/ Superimposed

load p

Figure R 71-4.Earth pressure distribution for an unsupported retaining wall, restrained in cohesive soil, assuming slip surfaces at an angle 19, (example)

a) Load

bl Soil self-wetght and cohesion

cl Distributed load p

k dl Strip load p’

e/ Superimposed

Figure R 71-5.Earth pressure distribution for an unsupported retaining wall, restrained in cohesive soil, assuming a minimum earth pressure (example)

25

4. If the most critical load approach cannot be established, all possible pressure diagrams must be determined for individual cases, together with the corresponding internal forces and embedment depths. The design should be based on the cases with the largest bending moment and the largest embedment depth, even if these were not determined using the same approach.

3.7

Superimposing earth pressure components for supported retaining walls (R 72)

1. The active earth pressure from unbounded distributed loads is incorporated into a common load model according to R 5, Paragraph 6 (Section 3.3), together with the earth pressure from soil self-weight, if necessary considering cohesion or a minimum earth pressure factor according to R 4, Paragraph 3 (Section 3.2). 2. The earth pressure diagram from line or strip loads selected according to R 7, Paragraph 4 (Section 3.5) is superimposed on the basic diagram given in Paragraph 1. To facilitate simpler analysis it is permissible to base the determination of internal forces for the vertical supporting members on a different pressure diagram than that used for determination of the internal forces of the horizontal supporting members. 3. The following possibilities are applicable for determination of the pressure diagram for supported retaining walls in cohesive soil: a) If cohesion is taken fully into consideration for the earth pressure from soil self-weight, the earth pressure from live loads must be applied without consideration of cohesion (Figure R 72-lb). b) If cohesion is considered as shown in Figure R 72-la for determination of earth pressure from live loads, the earth pressure from soil self-weight must be increased correspondingly (Figure R 7-3c) according to R 6, Paragraph 4 (Section 3.4). Either of the two approaches may be selected. The total active earth pressure E, is the same in both cases. 4. The simplified approaches according to Paragraph 2 and Paragraph 3 only apply if the earth pressure from line or strip loads is smaller than the earth pressure from soil self-weight and unbounded distributed loads, taking cohesion into consideration where necessary or an equivalent friction angle according to R 4, Paragraph 3 (Section 3.2). If it is larger, further investigations must be carried out. It is generally sufficient to base the determination of internal forces on two opposite, extreme, but feasible earth pressure distributions. 5. With regard to the magnitude and distribution of earth pressure from the weight of road vehicles and heavy construction equipment, R 6 (Section 3.4) and 26

Earth pressure from live load p’ without consideration of cohesion

Earth pressure from live load p’ with consideration of cohesion

from soil self- weight with full consideration m

z al Excavation. live load and load dlstribution

bl Consideration of cohesion for earth pressure from soil self- weght only

I

of cohesion

cl Considerationof cohesion for earth pressure from live load

Figure R 72-1. Consideration of cohesion for earth pressure from live loads with supported retaining walls and rectangular pressure diagrams

R 7 (Section 3.5) apply, even when the earth pressure from soil self-weight was determined, due to cohesion influence, using an equivalent friction angle according to R 4, Paragraph 3 (Section 3.2).

6 . In the case of yieldingly supported retaining walls an earth pressure distribution according to R 71 results as a lower limit case for earth pressure redistribution (Section 3.6). 7. For the distribution of earth pressure from building loads see R 28 and R 29 (Sections 9.3 and 9.4).

3.8

Earth pressure in retreating states (R 68)

1. Retreating conditions arise in supported retaining wall systems when, after manufacturing parts of the building andor after partial bacMilling of the excavation or the work space, a row of struts is removed or a set of anchors is unloaded.

2. If no considerable deflections or deformations of the retaining wall are anticipated during removal of struts and/or unloading of anchors, the earth pressure diagram selected for the largest excavation depth must also be maintained in the retreating state. 3. If a deflection of more than 0.2 %O is associated with the new span when removing struts or unloading anchors in densely compacted, cohesionless soil or at least plastic, cohesive soil, earth pressure redistribution over the

27

remaining excavation depth must be anticipated corresponding to the new supporting conditions. The earth pressure in the region of the removed supports is reduced; it is partially redistributed to the supports above and partially to those below. With a more precise definition of the pressure diagram based on [89] and [90], substantially more favourable internal design forces can result as a function of deflection than in a case where the pressure diagram from the previous construction stage is retained or a new pressure diagram selected with the same total earth pressure. 4. If the span increases by at least 30 % or deflection is shown to be greater than 0.2 %O of the new span, triple- or multiple-propped sheet pile walls and soldier pile walls may be analysed using the following load approaches: a) If, after removal, the lowest row of struts or anchors is replaced by a support on the blinding concrete, the load ordinate ehsat the height of the new lowest support must be increased by 15 % as shown in Figure R 68-la and allowed to decrease to zero at the excavation level. b) If, after removal, the lowest set of struts or anchors is replaced by a support on part of the structure or on the backfill, the load ordinate ehsat the height of the new lowest support must be increased by 5 % as shown in Figure R 68-lb and allowed to decrease to ehu= $5 ehsat the level of the top of the backfill. If only one row of struts or anchors is present in the retreating state, then the pressure diagram must be selected on the basis of the regulations for singlepropped soldier pile walls, if more precise stipulations are not made in Paragraph 3. 5 . The wall deformations associated with the removal of the highest row of struts

andlor unloading of the highest row of anchors are usually sufficient to reduce the upwardly redistributed earth pressure to the classical active earth pressure.

<

After strut

ai Support on the blinding concrete

bi Support from the structure or backfill

Figure R 68-1. Pressure diagrams for soldier pile walls in retreating states

28

4

General stipulations for analysis

4.1

Determination of internal forces (R 11)

1. All advancing and retreating states for excavating and backfilling must be investigated. Advancing states refers to all construction stages until reaching the final excavation level; retreating states refers to all construction stages during backfilling of the excavation and during removal or repositioning of struts.

2. When analysing single- or multiple-propped retaining walls, the structural system may be based on a beam on inflexible supports. The ground reaction may be adopted as a point load if the associated errors for determination of internal forces and the deformations from the lowest support to the wall toe can be accepted. The deformations in the various construction stages and the effects on subsequent construction stages need not generally be investigated.

3. There is no obligatory analysis method. Besides methods based on elastic theory, the modulus of subgrade reaction method according to R 102 (Section 4.2) and the finite element method according to R 103 (Section 4.3) may be considered. A limit load design according to R 27 (Section 4.4) may be adopted for steel components, e.g. in soldier piles, sheet pile walls, girders and cable bridges. 4. If the results of the determination of internal forces based on elastic theory indicate numerical overloading of a soldier pile wall or sheet pile wall at a single support point, the components of the bending moment which are over and above the allowable amount may be redistributed (Figure R 11-1),if determination of internal forces is based on an appropriately accurate pressure diagram according to R 12, Paragraph 3 (Section 5.1) or R 16, Paragraph 3 (Section 6.1). The effects on the bending moments in the neighbouring fields and at the neighbouring support points must be demonstrated; however, the shear and support forces at the investigated support may not be reduced. After moment redistribution, the allowable stresses may not be exceeded at any point, taking normal forces into consideration. In addition, the minimum thicknesses for the flange and webs must be demonstrated according to R 27, Paragraph 7 (Section 4.4).

5. Moment redistribution may also be performed for in-situ concrete walls and bored piles under bending loads; however, support moments may not be reduced by more than 15 %. If an in-situ concrete wall is subsequently utilised as a load-bearing member in a permanent structure, it may prove expedient to forego reduction of the support moment for the construction stage.

29

AM

M >adm M

a1 Original moment dstribution

M'=M+ AMsadmM

bl Moment redstribution

cl Modified moment dstribution

Figure R 11-1. Redistribution of bending moments

4.2

Modulus of subgrade reaction method (R 102, Draft) The modulus of subgrade reaction method may be adopted for analysis of embedment depth, to determine the internal forces and in some cases for analysis of serviceability. This allows the interaction between the wall and the ground, the actual load-bearing behaviour and the anticipated displacements and deformations to be better assessed than for the assumption of a predetermined passive earth pressure distribution and a predetermined displacement of the wall toe. Adoption of the modulus of subgrade reaction method assumes strict separation of actions and reactions. Superimposing earth pressure and passive earth pressure below the excavation level is not permitted. There is therefore no point of zero stress as a reference point for the earth pressure redistribution in supported retaining walls. Instead, it is recommended to only integrate the active earth pressure from ground level to the excavation level in the redistribution as shown in Figure R 102-1 and to retain the classical earth pressure, linearly increasing with depth, unchanged below the excavation level. As an approximation, it may be assumed that the original at-rest earth pressure is more or less retained below the excavation level even after excavation

30

Earth pressure from site operations Earth pressure from soil self-k wght

Earth pressure from building loads

Figure R 102-1. Load model for continuous elastic support in cohesionless soil

is complete on the excavation side of the wall. In general cases, this follows from: eog= Y . K, (H + zp) However, once the excavation is complete, the limit value eph

= epgh

+ epch

of the passive earth pressure determined with & = 0 may apply in the region immediately below the excavation level, due to the reversal of the principal stresses as a result of unloading. The case epch= 0 is shown in Figure R 102-1.

4. The ground reaction below the intersection of eog and e p h in excess of the at-rest earth pressure may be adopted as a function of the local displacement sh as the soil stress Oh

= ‘s,h

’

sh

See also Figure R 102-1. See Paragraphs 5 to 9 for determination and adoption of the modulus of subgrade reaction ks,h. If the intersection of eog and eph lie beneath the base of the wall, analysis using the modulus of subgrade reaction procedure is not possible because the largest possible ground reaction is already available to take up support forces without substantial displacement.

5. The most reliable values for the modulus of subgrade reaction ks,hare acquired on the basis of a load - deflection relationship for the passive earth pressure as shown in Figure R 102-2 using:

31

0 Epgh

E, EO 0

0 %

S

sFallw~

Wall displacement

Figure R 102-2. Determination of modulus of subgrade reaction

Here, the following points apply:

Ebh must initially be estimated and then improved iteratively. - The value of the remaining at-rest earth pressure force E, ensues from eph und eogfor the given embedment depth t, as shown in Figure R 102-1. - The parameters s and s, follow from Figure R 102-2 as the displacements associated with ELh and E,. - The utilised passive earth pressure

The load-displacement relationship is described by the passive earth pressure mobilisation curve. See the information in [94] for cohesionless soil and in [95] for cohesive soil. 6. As an approximation, the modulus of subgrade reaction k ,, may be derived from the forced modulus Es,h: a) The following applies for in-situ concrete walls and sheet pile walls according to [96]:

Here, the embedment depth t, is decisive. b) The following applies for soldier piles based on DIN 1054 (2003): ‘s.h

ks,h = b The flange width b is decisive for driven soldier piles. For soldier piles installed in pre-drilled boreholes and concreted at the toe, the borehole diameter D replaces the flange width b. Otherwise, this approach assumes that a displacement of s = 0.03 .b or s = 0.03 .D or a maximum of 20 mm is not exceeded. 32

c) The constrained modulus Es,hmust be derived from the anticipated stress range. If the constrained modulus E, is only known for the vertical stress it must be converted, as an approximation, to a horizontal stress using a factor of 0.5 = f = 1.0.

7. Guide values as a function of the degree of utilisation of the passive earth pressure according to [ 1211 are given in the Appendix A5. They apply for wet soils. The given values must be halved for buoyant soils. 8. If the stiffness of the retaining wall and the ground allow an earth restraint, the determined modulus of subgrade reaction may be doubled below the point of rotation without further analysis provided the ground conditions do not deteriorate.

9. Generally, a constant modulus of subgrade reaction can be assumed. However, in cohesionless soils the modulus of subgrade reaction must be reduced far enough in the upper region to avoid the soil stresses o h becoming greater than the passive earth pressure eph.At the start point of the ground resistance, at eog= ephas shown in Figure R 102-1, o h = 0. It may be expedient to adopt a modulus of subgrade reaction increasing with depth for large embedment depths or to increase in stages with depth. If a conservative average value is not adopted, the modulus of subgrade reaction should be adjusted to the ground conditions where soil layers change. 10. Generally, a realistic average modulus of subgrade reaction value may be adopted for analysis. If in doubt, it may be necessary to analyse with upper and lower limit values in order to study the possible effects. 11. It must be demonstrated at all times that the passive earth pressure determined using 6, # 0 is not completely utilised by the ground reaction RB resulting from the integration of the soil stresses ohand the remaining at-rest earth pressure E,. The following must be observed: a) Generally, the safety factor rl=

2 1.50 R B -k

must be adhered to. If the modulus of subgrade reaction guide values given in Table 1 are adopted, the determined safety factors must be commensurate with the associated degree of mobilisation. b) For free earth support, this analysis may be based on the passive earth pressure %h,P for parallel displacement as shown in Figure R 102-3a. In the case of full or partial earth restraint, the passive earth pressure E,,,, for rotation around the toe as shown in Figure R 102-3b is decisive. As an approximation, the following applies for cohesionless soils according to [91]: 33

epgh.P

Oh

ai Parallel movement

epgh,P

bt Rotation around toe

Figure R 102-3. Utilisation of passive earth pressure in cohesionless soil

This relationship may also be applied as an approximation for cohesive soils. c) The E, component must also be taken into consideration for analysis if it is partially or completely tallied against the loads on the outside of the wall for practical analysis reasons or for reasons involving software analysis.

4.3

Finite element method (R 103, Draft)

1. In principle the finite-element-method is suitable for - determination of the characteristic stresses in critical sections through the

excavation structure and in the excavation structure-ground, - estimation of settlements and deflections of the retaining wall, and the

ground behind the retaining wall and below the excavation level, - analysis of safety against slope failure and general (global) failure. See

the following Paragraphs for details.

2. Numerical analysis of excavation structures using FEM can be particularly useful if the use of classical beam structural analysis, in association with simplified load approaches, leads to inadequate results or if special demands are placed on the analysis results due to geometrical boundary conditions or complex ground conditions. This can involve the following cases, e.g.: a) Retaining walls with support conditions which do not allow confident determination of the magnitude and distribution of earth pressure, e.g. with yielding anchors and a flexible wall. b) Excavations with complex geometrical dimensions, e.g. salient or reentrant comers and staggered retaining walls with a berm width which does not allow confident determination of the magnitude and distribution of earth pressure using classical assumptions.

34

c) Excavation structures in which a realistic assessment of the impacts from excavation, strut or anchor prestressing on the earth pressure redistribution and deflections of the retaining wall is required. d) Excavation structures in which a realistic assessment of the seepage and associated water pressures is required. e) Excavations adjacent to buildings, pipelines, other structures or traffic areas. The impact of negative porewater pressures, which can ensue in the course of excavation, cannot yet be reliably assessed by FEM. Notes on the application of FEM for analysis of excavation structures can be taken from the Recommendations of the Working group “Numerik in der Geotechnik” [ 1221 (Numerical Methods in Geotechnics). 3. The application of FEM and definition of the constitutive equations used therein require particular care and experience, as well as specialised knowledge of soil mechanics, in particular for determination of the necessary material parameters and variables. It is therefore generally assigned the Geotechnical Category 3 (GC 3, according to EC 7). Here, the following points are recommended: a) A geotechnical expert trained in the required field and in possession of the appropriate experience should be employed for planning the necessary investigations and monitoring the technically correct execution of exposures, as well as for field and laboratory testing. b) It is expected of the geotechnical expert that a constitutive equation is recommended, which allows realistic determination of the stress and displacement conditions, taking the problem proposition and the local ground conditions into consideration, e.g. consolidation conditions, granulometric properties and degree of compaction. c) When determining the material parameters and variables required for numerical analysis the Recommendations of the Working Group “Numerical Methods in Geotechnics” must be observed.

4. The following procedure should be adhered to for numerical analysis: a) A suitable constitutive equation, which allows consideration of excavation unloading processes, must be selected. b) The parameter calculation values required for the selected constitutive equation must be determined from laboratory and field tests, or by employing empirical values acquired in comparable ground conditions. In association with Geotechnical Category 3, triaxial tests should generally be carried out to determine the decisive stiffness parameters for the critical soil layers. Consolidation tests may suffice for cohesive soils, depending on the constitutive equation adopted. c) Failing the relevant experience, initial numerical calculations must be performed to examine and optimise the range of the analysis and the mesh subdivisions, as well as the required modelling steps for the excavation condition being investigated. 35

d) If possible, initial numerical calculations must be performed on measurement data from excavations with comparable ground conditions in order to calibrate and check the selected parameters for the constitutive equation. These investigations and initial calculations are necessary to achieve realistic results, taking the numerous possibilities for selection and editing of parameters into consideration. In order to make the analysis assumptions transparent, the numerical analysis should always be accompanied by appraisable documentation of the points a) to d).

5. Generally speaking, realistic upper and lower limit values of the respective soil parameters should be included: a) For analysis of bearing capacity, only the conservative calculation values are initially required. b) For serviceability analysis it is generally sufficient to adopt the mean value of the upper and lower calculation values as the soil property with the highest probability of occurring. In both cases it may be necessary to perform the analysis with both the upper and the lower value, e.g. if the impacts are in part favourable and in part unfavourable or if the possible result boundaries need to be determined. 6. If the dimensions of the components used in the numerical model cannot be defined with empirical values, e.g. thickness and length of the retaining wall, and the prestressing forces of struts and anchors in particular, initial calculation by means of classical rod structural analysis is recommended in order to reduce the extent of iterative component optimisation. 7. Suitable contact elements must be adopted to consider the component-ground interaction. See also R 4, Paragraph 2 for adopting the wall friction angle. 8. It must be demonstrated at all times that the limit value of the horizontal component of the passive earth pressure determined using 6 , # 0 is not completely utilised by the ground reaction R, resulting from the integration of the horizontal soil stresses o h in the embedment area of the wall. Generally, the safety factor

q=-

2 1.50 Bh

must be adhered to. For a free earth support, this may be based on the passive earth pressure E for parallel displacement as shown in Figure R 102-3a. In the case of fu1fpoh;Ppartialearth restraint, the passive earth pressure E,,,,, for rotation around the toe as shown in Figure R 102-3b is decisive. As an approximation, the following applies for continuous walls according to [9I]: = o.62 ’ Proceed accordingly for soldier pile walls.

36

9. The following apply for analysis of equilibrium of vertical forces: a) Because the corresponding equilibrium condition is already contained within the numerical analysis, the analysis according to R 9, Paragraph la, which guarantees the selected negative wall friction angle for passive earth pressure, may be dispensed with. b) On the basis of R 9, Paragraph l b it must be demonstrated at all times that the downward direqted vertical force

V = W + E,

+ A, + P,

can be accepted by the friction force R, in the embedment area of the wall and the point resistance Q, below the wall toe. Where: W the self-weight of the wall, E, the vertical component of the earth pressure resulting from the integration of vertical stresses on the rear face of the wall, A, the vertical component of an anchor force, P, any other vertical force acting immediately on the wall, e.g. from an excavation covering. A skin friction resistance Ar.qr or the friction force B,.tan @ may be adopted as the friction force R, on the inside of the wall. This gives - the skin friction resistance as the product of the developed surface A, and

the skin friction qr, - the friction force as the product of the horizontal support force B, and the

friction coefficient tan $. Generally, the safety factor

must be adhered to. See also R 9, Paragraph 5. 10.For homogeneous, cohesive soil and in cohesive soil layers, an additional

analysis based on R 4, Paragraph 3 must be performed with the minimum earth pressure coefficient Kagh= 0.20 or with the equivalent friction angle @E uiv = 40°, whereby all other parameters remain unchanged. The design of inlividual components must be based on the most unfavourable results. 11. Analysis of safety against slope failure and general failure can only be performed using FEM if based on the Fellenius circular-arc method and the shear strength in the soil and in the component - ground contact area is reduced in stages until no computed equilibrium state is possible or a computed failure state occurs. The limits of this method and notes on the definition of necessary convergence criteria can be taken from the Recommendations of the Working Group “Numerical Methods in Geotechnics”. The safety against base 37

heave can also be demonstrated in this manner, but not that of deep-seated stability. 12. The results of the numerical analysis must be transparent. This is particularly the case for the loading of the retaining wall and for the internal forces, e.g. bending moments, shear forces and support forces, as well as for displacements, e.g. deformations of the wall and the ground. The following are recommended: a) A graph of the horizontal earth pressure components and the water pressure for the height of the retaining wall in the individual excavation stages. b) A graph of the internal forces for the height of the retaining wall in the individual excavation stages. c) A graph of the internal forces at the critical sections as a function of the construction stage. d) A graph of the horizontal wall displacement at various points of the retaining wall as a function of the construction stage. e) A graph of the surface settlements at various points of the ground surface as a function of the construction stage. Furthermore, evaluations of plastic regions and vector displacements may be useful for result assessment.

4.4

Limit load design method (R 27)

1. When applying the limit load method, analysis of adherence to given allowable stresses under service load is replaced by demonstrating that the smallest limit load of the structure is larger or equal to vq times the service load. By limit load or plastic limit load, the load increment for a given load model is understood, under which the structure, after generation of a sufficient number of plastic hinges, becomes a kinematic chain, either locally or as a whole. A safety factor vq= 1.5 is required for Load Case H according to R 24, Paragraph 1 (Section 2.1) and vq= 1.3 for Load Case HZ according to R 24, Paragraph 2.

2. The following requirements apply for analysis of soldier piles, sheet pile walls, girders and cable bridges using limit load methods: a) the cross-section of individual sections must be at least simple symmetrical, b) the load only acts in planes of symmetry, c) the section is safeguarded against buckling and overturning at the locations of possible plastic hinges, see R 48, Paragraph 3 (Section 13.2). 3. When analysing using the limit load method, earth pressure distribution must be applied corresponding to wall deflection in a manner assumed to be compatible with the static and dynamic deformation states of the wall, taking plastic hinges into consideration. See [ 181for example. This may be deviated 38

from when applying approximation methods if it is obvious or, if necessary, demonstrated, that by simplification no smaller cross sections are calculated than with consideration of this demand. See also [I91 and [27]. The use of a continuous rectangle according to R 13 (Section 5.3) or R 17 (Section 6.3) is only permissible if no more realistic pressure diagram is applicable. See R 69 (Section 5.2) and R 70 (Section 6.2). 4. If the decisive design bending moments are determined with the help of compensation between span and support moments for rolled sections with a continuous cross section to [ 191 or [27], stress analysis can be performed in place of the strict analysis, as if the internal forces had been determined with the help of elastic theory. In this case the allowable bending, shear and effective stresses may not be exceeded. See also R 24 (Section 2.1). 5 . When determining internal forces according to the limit load method it must

also be assumed, in analogy to determining internal forces based on elastic theory, that the stresses in the serviceability state of the structure may not reach the yield point. However, the corresponding analysis can be dispensed with if a) for Load Case H, a safety factor vq = 1.7, and for Load Case HZ, a safety factor vq = 1.5 against reaching the plastic limit load is demonstrated for analysis using the strict limit load method, b) the edge stress CT is not decisive for analysis using the simplified limit load method according to Paragraph 4 but rather the effective stress 0". If the corresponding analysis must be performed it is sufficient to investigate the critical design construction stage. Here, non-yielding support points may be assumed according to R 11, Paragraph 2 (Section 4.1). 6. The continuity effect that occurs in the serviceability state must be taken into consideration when determining shear and support forces. If necessary, the shear forces and support forces determined for the full plastification condition in the plastic hinges must therefore be increased by appropriate surcharges. See also [19] or [521. Regardless of whether determination of internal forces and design were performed on the basis of Paragraph 1 or Paragraph 4, rolled sections for soldier piles, girders and cable bridges must be investigated for adherence to the required minimum tilting and buckling thicknesses for flanges and webs in the regions of possible plastic hinges [28]. The stipulations of EN 12 063 are critical for sheet pile walls. If the minimum thicknesses are not achieved at any one point, either a more precise stability investigation must be performed or it must be demonstrated that when applying elastic theory the allowable stresses are not exceeded for the critical construction stage.

39

4.5

Equilibrium of vertical forces (R 9)

1. Generally, it must be demonstrated that the vertical forces occurring within the system can be accepted or completely transmitted to the ground. In principle, two cases must be differentiated:

a) If the downward acting vertical forces are relatively small, it must be demonstrated that they ensure, with sufficient safety, the occurrence of the selected negative wall friction angle for passive earth pressure. See also Paragraph 2. b) If the downward acting vertical forces are greater than the vertical component of the passive earth pressure determined with a negative wall friction angle, it must be demonstrated that they can be transmitted to the subsurface with sufficient safety. See also Paragraphs 3 to 5. If conditions do not make it obvious that only one of the two cases is decisive, both must be analysed. Regardless of which case is decisive, the impacts on the earth side of the retaining wall and, where applicable, from above, must be dealt with separate to the impacts on the air side of the retaining wall and, where applicable, in the contact surface. Superimposing of earth pressure and passive earth pressure below the excavation level, which is usual when determining embedment depths and internal forces for sheet pile walls and in-situ concrete walls, is not permitted for these analyses. 2. A safety factor of 1.5 is generally required for analysis according to Paragraph la. If the load approach adopted in the restraint area for soldier pile walls restrained in the ground, sheet pile walls or in-situ concrete walls according to R 25 (Section 5.5) or R 26 (Section 6.5) is selected after BLUM [23], then it will suffice to select the wall friction angle for passive earth pressure so that the vertical component of the passive earth pressure is no greater than the sum of the downwardly acting forces. The counter-force C at the theoretical wall toe may be adopted with a maximum angle 8, = + 9. If the actual effective passive earth pressure force is taken as the basis for closer investigation, a factor of safety of 1.5 must be demonstrated for the existence of the necessary vertical force. The support force E’, determined on the basis of the load approach after BLUMmay be reduced \y half of the computed equivalent force c h for determination of the actual effective passive earth pressure force. See also Figure R 9- 1. 3. Only the actual effective forces may be adopted for analysis according to Paragraph lb:

a) Skin friction at the rear of the wall may only be applied if the active earth pressure was not determined with positive wall friction. b) The support force E’phdetermined on the basis of the load approach after BLUMmay be reduced by half of the computed equivalent force c h for walls restrained in the ground. The equivalent force C may only be applied at half value.

40

without redistribution

1 T'Ch.k

T h e o r e k toe

Figure 9-1. Effective component of passive earth pressure with earth restraint after BLUM

The remainder of the equivalent force C may be applied with a negative wall friction angle of up to 6, = - 4'. 4. The relevant regulations must be applied correspondingly for analysis according to Paragraph lb. For driven soldier piles, for bored piles and for soldier piles embedded in boreholes and grouted at the toe, the specifications detailed in Appendix A 9 may be adopted. The embedment depths given may be reduced to 1.5 m if sufficient bearing capacity is demonstrated for the selected embedment depth. Additionally, for driven or vibrated soldier piles or sheet piles, the necessary plug formation must be guaranteed when driving through load-bearing strata.

5. The following safety factors must be adhered to for analysis according to Paragraph lb: a) An embedment depth of 1.5 m is generally sufficient for excavation depths of up to 10 m and favourable ground conditions without further analysis, if only the self-weight of the wall and the vertical earth pressure component must be transmitted. b) For transmitting the vertical forces resulting from wall self-weight and earth pressure a safety factor of 1.3 is required for excavation depths greater than 10 m or for cohesionless soils below the excavation level which are not at least medium-dense, or cohesive soils which are not of at least firm consistency. c) In cases where further vertical loads must be transmitted, such as the support forces of provisional bridges or vertical forces from inclined anchoring, a safety factor of 1.5 is necessary for acceptance of vertical forces. d) If the settlement of the retaining wall must be kept small, e.g. for excavations close to other structures, a minimum factor of safety of 2.0 is required for acceptance of the vertical forces. 6. If equilibrium of the vertical impacts and resistances cannot be demonstrated with the selected approach, the following apply: 41

a) In the case of approach 1 a), the negative wall friction must be reduced by an appropriate amount when determining the passive earth pressure, b) in the case of approach 1 b), the positive wall friction must be reduced in the region above the excavation level, or negative wall friction adopted if load transmission with this configuration is possible. In both cases, the embedment depth and design internal forces must be redetermined with the new values. The reduction in the passive earth pressure coefficient or the increase in the earth pressure coefficient associated with the reduction in the wall friction angle must be taken into consideration.

4.6

Stability analysis in special cases (R 10)

1. It may be necessary to apply a safety factor of 1.5 for analysis of base heave for soils with a friction angle less than $' = 25", especially if soft, cohesive soils are present below the excavation level. See also [25], [26] and [52]. 2. Bearing capacity safety must be demonstrated independent of the ground conditions if a heavy foundation is present approximately at the excavation level and only a small distance from the outside of the retaining wall (Figure R 10-1).

Figure R 10-1. Heavily loaded deep foundation beside the retaining wall

a) Very heavy foundation

b) Very steep slope

Figure R 10-2. General failure investigation for braced excavations

42

3. Analysis of general stability may be necessary for braced excavations if large earth pressure values are anticipated below the excavation level, e.g. a) for very heavy foundations beside the excavation as shown in Figure R 10-2a or b) for a steep slope beside the excavation as shown in Figure R 10-2b and for retaining walls supported at the excavation level. The computed strut forces must be taken into consideration if they have an unfavourable impact on stability. If they act favourably, they may only be adopted at 80 % of the characteristic strut forces. 4. It may be necessary to investigate for heave at the excavation level for excavations deeper than 10 m and to demonstrate that the associated heave of provisional bridge supports or excavation coverings, or of intermediate supports for buckling protection devices, have no negative effects. See also [511and [52].

43

5

Analysis approaches for soldier pile walls

5.1

Load model determinationfor soldier pile walls with active earth pressure (R 12)

1. If the conditions mentioned in R 8 (Section 3.1) for reducing the earth pressure from the at-rest earth pressure to the active earth pressure are fulfilled, the earth pressure E, according to R 4 (Section 3.2) must be determined to the excavation level, if not differently stipulated in R 15 (Section 5.6), taking into consideration the soil self-weight, the unbounded distributed load and, if necessary, cohesion. 2. Classical earth pressure distribution must always be applied for unsupported soldier pile walls restrained in the ground. When superimposing the earth pressure from line loads or strip loads, R 71, Paragraph 2 applies for cohesionless soils and R 71, Paragraph 3 (Section 3.6) for cohesive soils. When investigating forced surfaces the starting point is generally assumed at the excavation level.

3. The probable earth pressure redistribution must be taken into consideration for supported soldier pile walls and the pressure diagram be defined accordingly conform to R 5 (Section 3.3). In the advancing and fully excavated states the selected pressure diagram may generally approach eh = 0 at the excavation level. If the entire earth pressure from ground level to the base of the soldier pile is incorporated into the redistribution according to R 15, Paragraph 5c (Section 5.6) for soldier pile walls with a free earth support, eh = 0 must be defined at the base of the soldier pile. R 15, Paragraph 6c also applies to soldier pile walls restrained in the ground. No differentiation need be made between a free earth support and restrained beams when defining the pressure diagram. R 72 (Section 3.7) applies with regard to superimposing earth pressure from line loads and strip loads. 4. The information given in Recommendation R 69 (Section 5.2) may be used as a guide for the choice of a realistic pressure diagram for supported soldier pile walls. If a uniformly distributed load is adopted in place of a more appropriate pressure diagram for braced soldier pile walls, Recommendation R 13 (Section 5.3) must be observed. 5 . If the soldier piles are embedded sufficiently deep in the ground, the toe support can be adopted as follows:

a) as free earth support according to R 14 (Section 5.4) or b) as an earth restraint according to R 25 (Section 5.5).

44

a1 Section through the excavation

bl Classical earth pressure and passive earth pressure dstribution

cl Load model for a uniformly dlstributed load

Figure R 12-1. Load model determination for soldier pile walls with active earth pressure and a free earth support

In the case of a free earth support, a pressure diagram as shown in Figure R 12-lc results when a uniformly distributed load according to R 13 (Section 5.3) is adopted. 6. Bored pile walls are treated as soldier pile walls. However, a small earth pressure redistribution should generally be anticipated, corresponding to the ratio of pile diameters to pile centres and depending on the stiffness of the piles.

Pressure diagrams for supported soldier pile walls (R 69)

5.2 1. If

a) the ground surface is horizontal, b) medium-dense or densely compacted, cohesionless soil or at least firm, cohesive soil is present, c) a non-yielding support according to R 67, Paragraph 3 (Section 1.2), is present, d) excavation does not proceed deeper than shown in Figure 69-1 before the next row of struts is installed, soldier pile walls according to R 5, Paragraphs 3 and 4 (Section 3.3) in the advancing and fully excavated states may employ the pressure diagrams described below when adopting the active earth pressure from soil self-weight, wide live loads and, if necessary, cohesion. However, this information should only be seen as a guide; it does not exclude other realistic pressure diagrams. See also [32, 52,69, 89, 901.

45

h

Figure R 69-1. Excavation level before installing support

5 L

--

a1 Support at hks0l.H

bl Support at O I.H< hk _C 02.H

cl Support at 02.H< hk103.H

Figure R 69-2. Pressure diagrams for single-propped soldier pile walls

i al Mgh supports

61 Central supports

q L

cl Low supports

Figure R 69-3. Pressure diagrams for double-propped soldier pile walls

---I a1 Trtple-propped wall

----I bl Quadruple-propped wall

cl Quintuple-propped wall

Figure R 69-4. Pressure diagrams for multiple propped soldier pile walls

46

2. The following pressure diagrams may be assumed as realistic for singlepropped soldier pile walls: a) a continuous rectangle as shown in Figure R 69-2a, if the row of struts or anchors is not lower than h, = 0.10.H; b) a stepped rectangle with eho: ehu= 1.50 as shown in Figure R 69-2b, if the struts or anchors are in the range h, > 0.10.H to h, = 0.20.H; c) a stepped rectangle with eho: ehu= 2.00 as shown in Figure R 69-2c, if the struts or anchors are in the range h, > 0.20.H to h, = 0.30-H;

3. The following pressure diagrams may be assumed as realistic for doublepropped soldier pile walls: a) a stepped rectangle with the load increment at the height of the lower row of struts and an ordinate ratio eho: ehu= 2.00 as shown in Figure R 69-3a, if the upper row of struts or anchors is approximately at ground level and the lower row is in the upper half of the excavation height H; b) a trapezoid as shown in Figure R 69-3b, if the upper row of struts or anchors is below ground level and the lower row is at approximately half of the height H of the excavation; c) a rectangle as shown in Figure R 69-3c, if both rows of struts or anchors are installed very low. 4. The trapezoid as shown in Figure R 69-4 may be assumed as a realistic pressure diagram for multiple propped soldier pile walls with approximately the same spans. The earth pressure resultant should be in the range z, = 0.50" to Z, = 0.55 .H.

5. The pressure diagrams recommended here do not take the previous construction stage into consideration. More precise definitions take the pressure diagram of the previous construction stage and the earth pressure increase from the additional excavation phase into consideration for the pressure diagram of the current construction stage. This earth pressure increase acts principally at the last installed support [89, 901. This is particularly important in stratified ground. Supports that are lower than 30 % of the wail height H have no appreciable impact on the shape of the pressure diagram.

Simplified earth pressure for braced soldier pile walls (R 13)

5.3 1. If

a) the ground surface is horizontal, b) medium-dense or densely compacted, cohesionless soil or at least firm,

cohesive soil is present or, c) a non-yielding support according to R 67, Paragraph 3 (Section 1.2), is present, 47

a realistic pressure diagram according to R 5 (Section 3.3) or R 69 (Section 5.2) need not be specified for braced soldier pile walls, and the earth pressure E,, approximately determined according to R 12, Paragraph 1 (Section 5.1) may be adopted as a uniformly distributed load independent of the strut configuration. The errors associated with this procedure when determining shear forces, support forces and bending moments must be corrected by applying the following surcharges.A corresponding conversion of the normal forces and correction of the embedment depth is not normally required. 2. If a uniformly distributed load is adopted for soldier pile walls with only one row of struts, instead of a realistic pressure diagram, the shear forces and the support force at the support computed on this basis must be increased in the ratio H : h,but to a maximum of the earth pressure value E,. The span moment may be reduced in the ratio h, : H (Figure R 13-la). The reduction of a cantilever moment at the wall top is not permissible.

bl Double-propped soldier pile wall

a1 Slngle-propped soldier pile wall

1 H

I

4 4 e;, IC

cl Trple-propped soldier pile wall

48

Figure R 13-1. Rectangular earth pressure for braced soldier pile walls

In other cases, adopting earth pressure as a uniformly distributed load is only expedient if the support is not deeper than h, = 0.70 H. If the struts are above ground level, conversion of shear forces, support forces and bending moment is not necessary. 3. If a uniformly distributed load is adopted for soldier pile walls with two rows of struts, instead of a realistic pressure diagram, the shear forces and the support forces in the upper row calculated on this basis must be increased at the ratio H : h, if the lower row is in the lower third of the excavation (Figure R 13-lb). If, on the other hand, the lower row is located in the central third of the excavation, the shear and support forces of the lower row determined with the uniformly distributed load must be increased by 30 %. A reduction in the bending moments is not permissible. 4. If a uniformly distributed load is adopted for soldier pile walls with three or more rows of struts, instead of a realistic pressure diagram, the shear forces and the support forces at the supports in the central third of the excavation must be increased by 30 % (Figure R 13-lc).A cantilever moment at the wall top may be reduced by 20 %. 5. If a uniformly distributed load as determined for the full excavation state is selected for the retreating states of soldier pile walls, instead of a realistic pressure diagram, the stipulations in Paragraph 2 are not applied to the retreating states.

5.4

Passive earth pressure for soldier pile walls with free earth supports (R 14) The passive earth pressure in front of soldier piles can generally be analysed on the basis of the analysis recommendation given in [20]. If the soldier piles are so closely spaced that the passive earth pressure influences overlap, the computed passive earth pressures must be reduced accordingly. Passive earth pressure must be determined with and without overlapping. The respectively smallest value is then decisive for analysis. See also [52]. If the analysis suggestion derived for cohesionless soils is adopted for cohesive soils, the proportion of the passive earth pressure resulting from cohesion must be reduced to half of the computed value. See also [21] and [93]. Generally, a factor of safety qp = 2.0 must be demonstrated for acceptance of the lower support force of a soldier pile wall with a free earth support and single- or multiple-props in cohesionless or at least firm, cohesive soils. The passive earth pressure may only be introduced into the analysis with 1 : qpof the value at the limit state for determination of internal forces and the embedment depth. For cohesive soils which display soft consistency on the one hand, but on the other hand are not considered to be soft soils in the sense of R 90, either 49

a) the safety factor qp is increased so that the anticipated toe deflection is compatible with the deflection of the rest of the wall, or b) the effects of the anticipated toe deflection on the loading of the soldier piles and struts or anchors must be taken into consideration. For example, instead of the support force deemed necessary by calculations, the passive earth pressure activated by consideration of the allowable deflection can be introduced into the analysis as a known external load. If the toe support can be lost due to boil in soft, cohesive soils, special measures are necessary, e.g. additional bracing of the wall at the excavation level or deeper penetration of the soldier piles. 4. The requirement of Paragraph 2 for a safety factor qp= 2.0 to demonstrate acceptance of the lower support force is based on the fact that otherwise toe deflection can occur which is not compatible with the deflections and deformations of the rest of the retaining wall. If it can be demonstrated that, a) the toe support deflections are not damaging, e.g. for single-proppedwalls, or b) these deflections are no greater than the deflections and deformations of the upper parts of the retaining wall, e.g. for dense, cohesionless soils or semi-solid cohesive soils at the embedment depth, a safety factor qp = 1.5 will suffice.

5. When determining the internal forces in medium-dense or dense soils, or of cohesive soils of least firm consistency, the factor of safety given in Paragraph 2 may be reduced to qP= 1.5 and analysis performed with a correspondingly smaller embedment depth. 6. The given safety factors may be further reduced a) from qp= 2.0 to qp = 1.5 or from qp= 1.5 to qp= 1.3 for Load Case HZ according to R 24, Paragraph 2 (Section 2. l), b) from qp= 2.0 to rlP = 1.3 or from qp= 1.5 to qp= 1.3 in the case of abnormal loads according to R 24, Paragraph 3 (Section 2), if the passive earth pressure is more heavily availed than in Load Case H according to R 24, Paragraph 1 (Section 2.1). 7. In the case of cohesionless or at least firm, cohesive soil, the point of acting of the effective passive earth pressure may be assumed to be at 0.60.t,,, below the excavation level (Figure R 13-1, Section 5.3). The effective passive earth pressure distribution may be assumed to be that for classical earth pressure theory for soft, cohesive soils. 8. If necessary, the anticipated toe deflection can be estimated with the help of the information given in [20] and [93].

50

5.5

Toe restraint for soldier pile walls (R 25)

1. If the beams of a soldier pile wall embed deeply enough in the ground below the excavation level, a degree of restraint in the ground can be adopted for determination of internal forces. The degree of restraint depends on the deformation behaviour of the soldier piles and the ground. The restraint of the soldier piles can be based on the load approach after BLUM[23], independent of the real stress conditions in the ground. This approach theoretically assumes that soldier pile walls are not subjected to deflection or torsion at the wall toe.

2 . The magnitude of the passive earth pressure in front of the soldier piles can be established according to R 14, Paragraph 1, (Section 5.4). It is generally expedient to distribute the effective passive earth pressure in front of the individual soldier piles uniformly across the whole length of the retaining wall being investigated in order to apply the analysis methods derived for sheet pile walls. The result is the same passive earth pressure as in front of the sheet pile wall if the passive earth pressure components in front of the individual soldier piles overlap and the wall friction angle is adopted as 6, = 0; in all other cases the passive earth pressure in front of a row of soldier piles is smaller than the passive earth pressure in front of a sheet pile wall [19,20].

3. If the passive earth pressure components in front of the individual soldier piles in cohesionless soils do not overlap, the result, based on the analysis suggestion in [20], is a parabolic passive earth pressure distribution increasing with depth [68].The ensuing passive earth pressure diagram can be transformed to an equal area triangle. The error resulting from the displacement of the resultant must be compensated for, as an approximation, by a reduction of the calculated passive earth pressure by 15 %, if no more precise analysis is performed. 4. A load model as shown in Figure R 25- l b results for unsupported soldier pile walls restrained in the ground. The passive earth pressure may only be introduced into the analysis with a maximum of half of the possible limit state value. If the head deflections anticipated using this approach give cause to doubt - e.g. with regard to damage to pipelines or road pavements, danger to road or rail traffic or with regard to restrictions in the projected workspace a factor of safety greater than 2.0 should be selected and, if necessary, a stronger section than determined by calculations also selected. This is particularly the case if loosely compacted, cohesionless soils or firm, cohesive soils are present in the restraint area. In soft soils, because of the large deformations, an unsupported wall that is only restrained in the ground is not normally expedient. 5. A load model as shown in Figure R 25-2b results for supported soldier pile walls, It may generally be accepted for densely compacted, cohesionless soil, or at least firm, cohesive soil, that the deformation conditions associated with full restraint after BLUMare approximately fulfilled if the passive earth pres51

a1 Section through the excavation

b) Load model with passive earth pressure and equivalent force t h

cl Bendlng moments

Figure R 25-1. System, load and moment distribution for an unsupported soldier pile wall restrained in the ground

ai Section through the excavation

61 Load model with passive earth pressure and equivalent force t h

cl Bendmg moments

Figure R 25-2. System, load and moment distribution for a double-braced soldier pile wall restrained in the ground (pressure diagram according to R 13)

sure is only introduced into the analysis with half of the possible limit state value when determining embedment depth. The corresponding safety factor of qp= 2.0 may be reduced to qp= 1.5 for determination of internal forces. The varying deformation behaviour of soldier piles and loosely compacted, cohesionless soil with stiff soldier pile sections can be adopted for analysis by an appropriate increase in the factor of safety of qp= 2.0 when applying the passive earth pressure. A restraint effect may not generally be applied for soft, cohesive soil or soil with a high organic content. 6. The embedment depth t,, which is theoretically required for restraint of an unsupported soldier pile wall as shown in Figure R 25- 1, must be increased

52

by at least Atl = 0.20.t, in order to accept the structurally required equivalent force c h , if no more precise analysis is performed. The same applies to supported soldier pile walls as shown in Figure R 25-2, if full restraint is assumed. Between the limit cases of full restraint and free earth support, intermediate cases with partial restraint are possible and can be adopted for supported soldier pile walls. As an approximation for partial restraint, the driving depth surcharge may be linearly interpolated between the decisive full restraint value Atl and the free earth support value Atl = 0, as a function of the embedment depth. If necessary, determination of the degree of restraint can also be based on elastic bedding. See also R 11, Paragraph 6 (Section 4.3). However, the restraint effect may not be applied at a greater degree than for the load approach after BLUM. Analysis of vertical force equilibrium must be performed according to R 9, Paragraph 4 (Section 4.1).

5.6

Equilibrium of horizontal forces for soldier pile walls (R 15) The earth pressure below the excavation level may be neglected for stability analysis of soldier pile walls if it has been demonstrated that this earth pressure, together with the support force from the soldier piles, is completely transmitted by the available passive earth pressure with a minimum factor of safety of 1.5. The passive earth pressure may be determined as for a closed wall with the wall friction angle 6, = - Q’, if based on curved slip surfaces. Analysis may only be dispensed with if the preconditions laid out in Paragraph 8 are fulfilled.

2. The magnitude of the neglected earth pressure for soldier piles with a free

earth support follows from the difference introduced into the analysis between the earth pressure at the soldier pile toe and the earth pressure to the excavation level. In the case of cohesive soil layers, earth pressure determination must be according to R 4, Paragraph 3a, and R 4, Paragraph 3b (Section 3.2). The larger value is decisive. The theoretical support point for soldier piles restrained in the ground replaces the actual toe point of the soldier pile. 3. The magnitude of the soldier pile support force follows directly from determination of permissible soldier pile stresses for walls with a free earth support (Figure R 11-1).The support force for walls restrained in the ground is equal to the numerically required passive earth pressure from the excavation level to the theoretical toe based on BLUM’S load approach (Figures R 25- 1 and R 25-2, Section 5.5). The support force E,, determined on the basis of BLUM’S load approach may be reduced by half of the computed equivalent

53

force C,, considering the magnitude of the actual anticipated passive earth pressure required for soldier pile restraint. See also Figure R 9- 1. If analysis with the selected embedment depth according to Paragraph 1 is not possible for unsupported soldier pile walls restrained in the ground then either a) the embedment depth must be increased or b) a further investigation must be performed in which the soldier pile wall is treated as a sheet pile wall. If analysis with the initially selected embedment depth and earth pressure adopted (Figure R 15-la) on the basis of Paragraph 1 is not possible for supported soldier pile walls with a free earth support, then either a) the embedment depth must be increased (Figure R 15-lb), or b) analytical embedment must be dispensed with (Figure R 15-lc), or c) the complete earth pressure from the surface to the soldier pile base must be taken up in the redistribution (Figure R 15-ld).

H

a1 Analysts of ZH = 0 not possible

bl Increase of embedment depth

d Analysts without embedment

dl Earth pressure redlstrtbutton to the wall toe

Figure R 15-1. Analysis of CH = 0 for soldier pile walls

54

6. If analysis with the selected embedment depth according to Paragraph 1 is not possible for supported soldier pile walls restrained in the ground then either a) the embedment depth must be appropriately increased, or b) full restraint must be dispensed with and analysis performed with partial restraint or with a free earth support, or c) the complete earth pressure from the surface to the theoretical support point must be taken up in the redistribution, or d) a further investigation must be performed in which the soldier pile wall is treated as a sheet pile wall. 7. The following analyses are additionally required for the solutions mentioned in Paragraphs 4 , 5 and 6:

a) If the embedment depth is increased on the basis of Paragraphs 4a, 5a or 6a, the internal forces for the altered span conditions must be redetermined. In addition, renewed analysis according to Paragraph 1 must be performed. b) For a soldier pile wall without sufficient embedment on the basis of Paragraph 5b, it must be demonstrated that the soldier piles and struts or anchors are capable of transmitting the horizontally acting earth pressure forces without soldier pile embedment. Analysis according to Paragraph 1 is dispensed with. However, see also R 10, Paragraph 1 (Section 4.2). c) The internal forces must be redetermined for the redistribution of earth pressure from ground level to the toe according to Paragraph 5c or to the theoretical support point according to Paragraph 6c. Here, only that portion of the earth pressure diagram above the excavation level need be applied. However, the component lying below the excavation level must be taken into consideration for analysis according to Paragraph 1, which must be renewed for the altered conditions. d) If a larger bending moment or larger embedment depth than that of the original analysis arises for a projected sheet pile wall according to Paragraph 4b or 6d, the respectively larger values must be adopted for soldier pile design. A renewed analysis according to Paragraph 1 is unnecessary. 8. Analysis on the basis of Paragraph 1 can be dispensed with if, simultaneously, a) cohesionless soil with a friction angle of $' = 32.5" is present below the excavation level and possesses approximately the same self-weight as the soil above the excavation level, b) no earth pressure from building loads needs to be taken into consideration below the excavation level, c) the embedment depth of the soldier piles is not less than one quarter of the excavation depth, 55

d) the width of the soldier piles is not more than one fifth of the soldier pile centres and e) the passive earth pressure in front of the soldier piles can be determined by applying a negative wall friction angle, given the prevalent conditions. 9. If a layer of soft, cohesive soil is present below the excavation level, additional investigations must be carried out to determine the deformation behaviour of the wall and the ground. It may then be expedient to decrease the strut centres or to analyse the soldier pile wall as a sheet pile wall.

56

6

Analysis of sheet pile walls and in-situ concrete walls

6.1

Load model determination for sheet pile walls and in-situ concrete walls with active earth pressure (R 16)

1. If the conditions given in R 8 (Section 3.1) for reducing the earth pressure from the at-rest earth pressure to the active earth pressure are fulfilled, the earth pressure ordinate e, according to R 4 (Section 3.2) must be determined down to the excavation level on the basis of classical earth pressure theory, taking into consideration the soil self-weight, the unbounded distributed load and, if necessary, cohesion. The thus defined active earth pressure is superimposed with the effective passive earth pressure e;. According to R 19, Paragraph 2 or Paragraph 3 (Section 6.4). This results from the division of the passive earth pressure limit state ordinate ep by the safety factor qp.Thus the circumstance is taken into consideration that - in contrast to applying the classical earth pressure distribution based on the Recommendations of the Working Committee for Waterfront Structures [2] for single-anchored sheet pile walls - when applying the earth pressure based on the information in Paragraph 3, the computed support force in the ground corresponds to the actual force which can be anticipated and the safety factor required for the earth support is therefore not already incorporated in the analysis.

2. The earth pressure distribution determined on the basis of Paragraph 1 must always be applied for unsupported sheet pile walls restrained in the ground and for in-situ concrete walls. When superimposing the earth pressure from line loads or strip loads, R 71, Paragraph 2 applies for cohesionless soils and R 71, Paragraph 3 (Section 3.6) for cohesive soils. When investigating slip surfaces, the starting point is generally assumed at the height of the theoretical wall toe. 3. For supported sheet pile walls and in-situ concrete walls the resultant load (Figure R 16-lc) determined on the basis of Paragraph 1 from ground level to the point of zero stress at a depth u below the excavation level must be converted to a simple pressure diagram according to R 5 (Section 3.3), which corresponds to the anticipated earth pressure redistribution. No differentiation is needed between a free earth support and full restraint of the wall. R 72 (Section 3.7) applies with regard to superimposing earth pressure from line and strip loads.

4. The information given in Recommendation R 70 (Section 6.2) can be used as a guide for the choice of a realistic pressure diagram for supported sheet pile walls and in-situ concrete walls. If a uniformly distributed load is adopted in place of a more appropriate pressure diagram for braced sheet pile walls and in-situ concrete walls, Recommendation R 17 (Section 6.3) must be observed. 57

t al Section through the excavation

bl Classical earth pressure and passive earth pressure distribution

ci Superimposed earth pressure and passive earth pressure

dl Load model for a uniformly distributed load

Figure R 16-1. Load model determination for sheet pile walls and in-situ concrete walls with active earth pressure and a free earth support

5. If the wall is embedded sufficiently deep in the ground, the toe support can be adopted as follows: a) As a free earth support according to R 19 (Section 6.4), b) as an earth restraint according to R 26 (Section 6.5). Using a uniformly distributed load according to R 17 (Section 6.3) a pressure diagram results as shown in Figure R 16-ld in the case of a free earth support.

58

6.2

Pressure diagrams for supported sheet pile walls and in-situ concrete walls (R 70)

1. If a) the ground surface is horizontal, b) medium-dense or densely compacted, cohesionless soil or at least firm, cohesive soil is present, c) a non-yielding support according to R 67, Paragraph 3 (Section 1.2) is present, d) excavation does not proceed deeper than shown in Figure 69-1 before the next row of struts is installed, sheet pile walls and in-situ concrete walls according to R 5, Paragraphs 3 and 4 (Section 3.3) in the advancing and fully excavated states may employ the pressure diagrams described below for the earth pressure approach from soil self-weight, wide live loads and, if necessary, cohesion. However, this information should only be seen as a guide; it does not exclude other realistic pressure diagrams. See also [52].

2. The following pressure diagrams may be assumed as realistic for singlepropped sheet pile walls and in-situ concrete walls: a) a continuous rectangle as shown in Figure R 70-la, if the struts or anchors are not lower than h, = 0.10.H; b) a stepped rectangle with eho: ehu= 1.20 as shown in Figure R 70-lb, if the struts or anchors are in the range h, > 0.10.” to h, = 0.20.”; c) a stepped rectangle with eho: ehu= 1.50 as shown in Figure R 70-lc, if the struts or anchors are in the range h, > 0.20.” to h, = 0.30.”;

1

ai Support at hk 01.W

bl Support at 01~H’shk=02~W

cl Support at 02.W _C hk = 03.W

Figure R 70-1. Pressure diagrams for single-propped sheet pile walls and in-situ concrete walls

3. The following pressure diagrams may be assumed as realistic for doublepropped sheet pile walls and in-situ concrete walls: 59

a) a stepped rectangle with the load increment at the height of the lower row of struts and the ordinate ratio eho: ehu= 1.50as shown in Figure R 70-2a, if the upper row of struts or anchors is approximately at ground level and the lower row is in the upper half of the excavation height H’; b) a quadrangular pressure diagram with eho: ehu= 2.00 as shown in Figure R 70-2b, if the upper row of struts or anchors is below ground level and the lower row approximately at half of the height H’ of the excavation; c) a tapered rectangle as shown in Figure R 70-2c, if both rows of struts or anchors are very low. as shown in Figure R 70-3 may be as4. The pressure diagram after LEHMANN sumed as realistic for multiple-propped sheet pile walls or in-situ concrete walls, but only with the bending points at the height of the support points and in the ratio eho: ehu= 2.00. The computed load resultant should be in the range z, = 0.45.” to z, = 0.50.”.

5. The pressure diagrams recommended here do not take the previous construction stage into consideration. More precise definitions take the pressure dia-

a1 Hgh supports

bl Central supports

cJ 1ow supports

Figure R 70-2. Pressure diagrams for double-propped sheet pile walls and in-situ concrete walls

-ai Triple-propped wall

bJ Quadruple-propped wall

cl Quintuple-propped wall

Figure R 70-3. Pressure diagrams for multiple-propped sheet pile walls and in-situ concrete walls

60

gram of the previous construction stage and the earth pressure increase from the additional excavation phase into consideration for the pressure diagram of the current construction stage. This earth pressure increase acts principally at the last installed support [89, 901. This is particularly important in stratified ground. Supports that are lower than 30 % of the wall height H have no appreciable impact on the shape of the pressure diagram.

Simplified earth pressure for braced sheet pile walls and in-situ concrete walls (R 17)

6.3 1. If

a) the ground surface is horizontal, b) medium-dense or densely compacted, cohesionless or at least firm, cohesive soil is present, c) a non-yielding support according to R 67, Paragraph 3 (Section 1.2), is present, arealistic pressure diagram according to R 5 (Section 3.3) or R 70 (Section 6.2) may be dispensed with for braced soldier pile walls and in-situ concrete walls, and the resultant loading approximately determined according to R 16, Paragraph 1 (Section 6.1), from ground level to the point of zero stress at the depth u below the excavation level may be adopted as a uniformly distributed load regardless of the strut configuration. The errors associated with this procedure when determining shear forces, support forces and bending moments must be corrected by applying the following surcharges. A corresponding conversion of the normal forces and correction of the embedment depth is not normally required.

2 . If a uniformly distributed load is used for sheet pile walls or in-situ concrete walls with only one row of struts, instead of a realistic pressure diagram, the shear forces and the support force at the support calculated on this basis must be increased in the ratio The span moment may be reduced in the (Figure R 17-la). The reduction of a cantilever moment at ratio-/, the wall top is not permissible. Selection of a rectangle as pressure diagram is only expedient if the support is no lower than ha = 0.70. H’. If the struts are above ground level, conversion of the shear forces, the support force and the bending moment is not necessary.

Jm.

3. If a uniformly distributed load is adopted for soldier pile walls or in-situ concrete walls with two rows of struts, instead of a realistic pressure diagram, the shear forces and the support forces in the upper row calculated on the basis of if the the uniformly distributed load must be increased in the ratio -/, lower row is arranged in the lower third of the height “between ground level and the point of zero stress (Figure R 17-lb).

61

ai Single-propped wall

bi Double-propped wall

Figure R 17-1. Rectangular earth pressure for braced sheet pile walls and in-situ concrete walls

cl Triple-propped wall

If, on the other hand, the lower row is in the central third of the height H’, the shear and support forces of the lower row determined with the uniformly distributed load must be increased by 15 %. A reduction of the bending moments is not permissible.

If a uniformly distributed load is adopted for sheet pile walls and in-situ concrete walls with three or more rows of struts, instead of a realistic pressure diagram, the shear forces and the support forces at the supports in the central third of the height H’ must be increased by 15 % (Figure R 17-lc). A cantilever moment at the wall top may be reduced by 20 %. If a uniformly distributed load as determined for the full excavation state is selected for the retreating states of sheet pile walls and in-situ concrete walls, instead of a realistic pressure diagram, the stipulations in Paragraph 2 are not applied to the retreating states.

62

6.4

Passive earth pressure for sheet pile walls and in-situ concrete walls with free earth supports (R 19)

1. If the CV = 0 condition and the relative movement between retaining wall and ground permit, the passive earth pressure can be determined as follows for sheet pile walls and pile walls with free earth supports: a) The wall friction angle may be adopted at 6, = - $ according to R 89 (Section 2.3), if curved slip surfaces after CAQUOT/&RISEL [70] or noncircular slip surfaces based on the modified approach after STRECK/ WEISSENBACH [71] are used as the basis for analysis. b) Planar slip surfaces may only be used as the basis for analysis if the ground surface is level, the friction angle is no greater than $' = 35" and the wall friction angle is reduced to 6, = -1/3 $'. In the case of diaphragm walls, a maximum wall friction angle 6, = -1/2 $' can be adopted based on curved or compound slip surfaces, or 6, = -1/3 9' based on planar slip surfaces, according to R 89 (Section 2.3), if detailed investigations do not allow a more favourable approach.

6, = 0 must always be applied for soft, cohesive soils. See Figure R 19-1 for sign definitions.

a) Negative wail friction angle

bi Positive wall friction angle

Figure R 19-1. Wall friction angle for passive earth pressure

2. A minimum factor of safety q, = 1.5 must be demonstrated for transmission of the support force to the ground if the passive earth pressure ordinate ep for superimposing the active earth pressure was not introduced into the analysis at 1 : q, of the value at the limit state, according to R 16, Paragraph 1 (Section 6.1). The safety factor must either be increased or the effects of the anticipated toe deflection taken into consideration for soft, cohesive soils. See also R 14, Paragraph 2 (Section 5.4). 3. When determining the internal forces and moments of the retaining wall elements in medium-dense or dense soils, or at least firm, cohesive soils, the safety factor given in Paragraph 2 may be reduced to q, = 1.2 and analysis performed with a correspondingly smaller embedment depth. 63

4. The given safety factors may be further reduced a) from qp= 1.5 to qp= 1.3 for Load Case HZ according to R 24, Paragraph 2 (Section 2.1), b) from qp = 1.5 to qp= 1.1 for abnormal loads according to R 24, Paragraph 3 (Section 2.1), if the passive earth pressure is more heavily availed than in Load Case H according to R 24, Paragraph 1 (Section 2.1).

5. The point of acting of the remaining effective passive earth pressure below the point of zero stress may be assumed as being below this point at 0.60.to for cohesionless or at least firm, cohesive soil, as shown in Figure R 16-Id, or at 0.50.t, for semi-solid, cohesive soil. The distribution of the remaining effective passive earth pressure for soft, cohesive soils must be applied according to classical theory. 6. If necessary, the anticipated toe deflection can be estimated with the help of the information given in [46,91, 94, 951. Generally, the same passive earth pressure as for closed walls can be applied for sheet pile walls and pile walls with staggered toes. However, without analysis, only every second double section or every second pile may be shortened by 20 % of the necessary computed embedment depth t, but by a maximum of 1.0 m. If such shortening is performed on the master (bearing) pile of combined sheet pile walls or on the reinforced piles of a pile wall manufactured with alternating reinforced and unreinforced piles, it must be demonstrated that the wall can accept the loads and that the passive earth pressure can accept the support force. 6.5

Toe restraint for sheet pile walls and in-situ concrete walls (R 26)

1. If sheet pile walls and in-situ concrete walls are embedded deeply enough below the excavation level, a degree of earth restraint can be applied for determination of internal forces. The degree of earth restraint depends on the deformation behaviour of the wall and the ground. The restraint of the retaining wall can be based on the load approach after BLUM[23], independent of the real stress conditions in the ground. This approach theoretically assumes that supported walls are not subjected to deflection or torsion at the wall toe.

2. A load model as shown in Figure R 26-lb results for unsupported sheet pile walls and in-situ concrete walls. The passive earth pressure may only be introduced into the analysis with a maximum of two-thirds of the value possible in the limit state. If the top deflections anticipated using this approach give cause to doubt - e.g. with regard to damage to pipelines or road pavements, danger to road or rail traffic or with regard to restrictions in the projected workspace - a factor of safety greater than 1.5 should be selected and,

64

if necessary, a stronger section also selected than is indicated as necessary by calculations. This is particularly the case if loosely compacted, cohesionless soils or firm,cohesive soils are present in the restraint area. Because of the large deformations, an unsupported wall restrained in soft soil only is normally not practical. 3. A load model as shown in Figure R 26-2b results for supported sheet pile walls. It may generally be accepted that the deformation conditions associated with full earth restraint for densely compacted, cohesionless soil, or at least firm, cohesive soil, are approximately fulfilled for a flexible sheet pile wall if the passive earth pressure is only introduced into the analysis with two-thirds of the value possible in the limit state when determining embedment depth. The corresponding safety factor of 1.5 may be reduced to qp= 1.2 for determination of internal forces. The varying deformation behaviour of

ai Section through the excavation

bl Load model with passive earth pressure and equivalent force t h

ci Bendlng moments

Figure R 26-1. System, load and moment distribution for an unsupported sheet pile wall or in-situ concrete wall restrained in the ground

ai Section through the excavation

bl Load model with passive earth pressure and equivalent force t h

cl Bending moments

Figure R 26-2. System, load and moment distribution for a double-propped, flexible sheet pile wall restrained in the ground (pressure diagram according to R 17)

65

loosely compacted, cohesionless soils can be adopted for analysis by an appropriate increase in the factor of safety of 1.5 when applying the passive earth pressure. A restraint effect may not generally be applied for soft, cohesive soil or soil with a high organic content. Generally speaking, this also applies to very stiff sheet pile walls and in-situ concrete walls regardless of the ground conditions, due to the low deformation properties.

4. Determination of the fixed-end moment for supported sheet pile walls with an earth restraint can generally be dispensed with because it is not decisive for design in average ground conditions. The magnitude of the fixed-end moment need only be determined and, if necessary, used as the basis for design for sheet pile walls in very dense cohesionless soils or semi-solid to stiff cohesive soils.

5. The embedment depth tl which is theoretically required for restraint of an unsupported sheet pile wall or in-situ concrete wall as shown in Figure R 26-1 must be increased by at least At, = 0.20.t, in order to accept the structurally required equivalent force C,, if no more precise analysis is performed. See also [2] and [24]. The same applies to supported sheet pile walls as shown in Figure R 26-2, if full earth restraint is assumed. See R 19, Paragraph 6 (Section 6) for staggering sheet pile walls and pile walls. 6. Between the limit cases of full earth restraint and free earth support intermediate cases with partial restraint are possible and can be adopted for supported sheet pile walls and in-situ concrete walls. As an approximation for partial restraint, the driving depth surcharge may be linearly interpolated between the decisive full restraint value At, and the free earth support value Atl = 0 as a function of the theoretically required embedment depth.

7. If necessary, the degree of restraint can also be determined with deformation resistance on the basis of elastic bedding. See also R 102, Paragraph 6 (Section 4.2). The deformation resistance may only be adopted below the point of zero stress. However, the restraint effect may not be applied at greater values than for the load approach after BLUM. 8. Analysis of vertical force equilibrium must be performed according to R 9, Paragraph 4 (Section 4.1).

66

7

Anchored retaining walls

7.1

Magnitude and distribution of earth pressure for anchored retaining walls (R 42)

1. The magnitude and distribution of the earth pressure on anchored retaining walls are principally dependent on whether or not anchors are prestressed and, if so, with what force. An earth pressure distribution based on determination of internal forces and deviating from the classical earth pressure, e.g. a pressure diagram as shown in Figure R 5-1 (Section 3.3), generally only results if the anchors are prestressed to at least 80 O/o of the active earth pressure design value or at 100 % for pressures higher than the active earth pressure for the forces computed for the respective construction condition. When prestressing for substantially lower forces, earth pressure distribution is predominantly dependent on the interaction of local factors such as live loads, building loads, ground type, wall stiffness, length of and strain on the anchors and flexibility of the toe support, and can no longer be unambiguously determined. 2. An earth pressure distribution of choice can be compelled, within certain limits, by the appropriate configuration and prestressing of the anchors, in particular depending on the stiffness of the retaining wall. If a large upward earth pressure redistribution needs to be achieved, e.g. an earth pressure diagram with the resultant in the upper half of the excavation, it is also necessary to design the upper anchors longer than the lower ones for retaining walls with more than one row of anchors. Otherwise, the length of the anchors depends on the stability analysis at low failure plane according to R 44 (Section 7.3), the general (global) stability analysis according to R 45 (Section 7.4) and, if necessary, on the results of the investigation of possible wall deflections according to R 46 (Section 7.5). 3. If an earth pressure rectangle is selected as an approximate pressure diagram for analysis of an anchored wall, the surcharge on the determined shear forces and support forces for braced excavations according to Recommendations R 13 (Section 5.3) and R 17 (Section 6.3) may be dispensed with. However, reduction of the bending moments determined with the earth pressure rectangle is no longer permissible.

4. In exceptional cases, determination of internal forces and embedment depths for relatively stiff walls can be based on classical earth pressure distribution if the anchor configuration, anchor lengths and prestressing are appropriately selected. With regard to adoption of cohesion and investigation of live load influence, the same considerations apply as for unsupported retaining walls restrained in the ground. See R 4, Paragraph 4 (Section 3.2), R 6, Paragraph 6 (Section 3.4), R 7, Paragraph 1 (Section 3 . 3 , R 12, Paragraph 2 (Section 5.1) and R 16, Paragraph 2 (Section 6.1). 67

5. If two opposing retaining walls are partially supported by anchors and partially by struts, earth pressure distribution may be selected similar to fully braced excavations. The anchors and struts must be prestressed appropriately. If necessary, the varying flexibility of the support points must be taken into consideration when determining internal forces. If a uniformly distributed load is used instead of a realistic pressure diagram, R 13 (Section 5.3) and R 17 (Section 6.3) must also be adopted to determine anchor forces. 6. It is generally permissible to prestress all anchors to 80 % of the forces computed for the fully excavated state for active earth pressure design and to 100 % for design with pressures greater than the active earth pressure. Only if this measure leads to overloading of the excavation structure or excessive deflection of the retaining wall top towards the ground, thereby representing a possible hazard to buildings or pipelines, it may be necessary to initially prestress the anchors for the forces prevalent in the construction stage following anchor installation and to later restress for subsequent construction stages.

7.2

Analysis of force transmission from anchors to the ground (R 43)

1. When determining the passive earth pressure in front of continuous anchor walls, the wall friction angle is assumed at 6 , = 0, if the only vertical force is the self-weight of the wall. Otherwise, the influence of anchor inclination must be taken into consideration, in particular for anchors that are inclined down into the wall. A minimum safety factor of 1.5 must be demonstrated for the passive earth pressure limit value in the ground failure state. If the anchor wall is covered with earth, the approximate passive earth pressure may be determined similar to walls beginning at ground level.

2. In principle, the passive earth pressure for anchor plates can be determined similar to anchor walls. See [35] and [20] for consideration of three-dimensional effects. Generally, a minimum safety factor of 2.0 must be demonstrated for the passive earth pressure limit value in the ground failure state, if the anchor plate edges are at least 20 cm thick. Less thick anchor plates, loosely compacted, cohesionless soil and soft to firm, cohesive soils require a correspondingly higher safety factor in order to consider yield phenomena.

3. The limit load for driven, non-grouted tension piles is generally determined by means of loading tests on at least two piles.If no precise investigations are carried out, the effective limit load in the force transmission zone outside of the earth pressure failure wedge must be proportionately computed in the ratio of the length of the force transmission zone to the overall length of the pile, if necessary considering the varying surcharge along the pile length. In simple cases the effective limit load can be numerically determined by assuming a skin friction value qr = 50 kN/mZon the developed skin, if in sufficiently load-bearing ground and not subjected to vibrations. Skin friction 68

may fall to qr = 10 kN/mZin loosely compacted sands. A minimum factor of safety of 1.75 against the effective limit load determined by loading tests or by analysis must be adhered to if the piles are raked at more than 45". If piles are raked at 1 : 1 or less, the safety factor may be reduced to 1.5 according to Recommendation R 26 of the Recommendations of the Committee for Waterfront Structures [2]. 4. Paragraph 3 applies accordingly for analysis of the bearing capacity of driven, grouted anchor piles based on loading tests. Otherwise, Recommendation R 66 of the Recommendations of the Committee for Waterfront Structures [ 2 ]must be observed. 5 . The bearing capacity of grouted anchors must be demonstrated according to Section 13.7 or EN 14199 for grouted piles.

7.3

Analysis of stability at low failure plane (R 44)

1. Generally, stability analysis at low failure plane is required for anchored retaining walls. This principally serves to determine the necessary anchor lengths. One must imagine that the anchors yield together with the surrounding ground and that the wall therefore moves towards the excavation (Figure R 44-1). When investigating, the anchor length is first selected and then the existing stability determined. 2. The procedure suggested in Recommendation R 10 of the Recommendations of the Committee for Waterfront Structures [2] is suitable for stability analysis of single-anchored retaining walls, that suggested in [ 171, based on the same principle, is suitable for stability analysis of multiple-anchored retaining walls. Both procedures assume either straight or compound slip surfaces. In special cases the applicable slip surface may be curved [36,48]. Otherwise, see [80]. 3. For continuous anchor walls, the rearward boundary of the earth mass under investigation is seen as a plane from the toe of the wall, reaching vertically to the ground surface. For individual plate anchors, an equivalent anchor wall must be assumed at a distance %.a, in front of the plate anchors, where a, represents the clear distance between the plate anchors. With driven piles, grouted piles and grouted anchors, the equivalent anchor wall is assumed to

Figure R 44-1. Stability at low failure plane

69

be in the centre of the effective or computed force transmission length, if this distance is not greater than half of the force transmission length. If it is greater, the weight of the earth mass involved must be limited according to Recommendation R 66 of the Recommendations of the Committee for Waterfront Structures. The slip surface for deep-seated stability of continuous anchor walls and plate anchors is taken from their lower edge, for driven piles and grouted piles from the centroid of the force transmission length. 4. The toe of the deep slip surface for free-earth support sheet pile retaining walls, in-situ concrete walls and soldier pile walls is at the base of the retaining wall or soldier pile. If the wall is embedded deeper than is necessary to accept the horizontal support force in order to accept vertical loads, the base is then taken as the depth which would be sufficient without considering vertical loads. If the wall is restrained in the ground, the toe for analysis is the shear force point of zero stress in the restraint area. If subsurface embedment is dispensed with, and thus the support below the excavation level, in reality or for calculation purposes only, e.g. for underpinning walls, for continuous or anchored element walls or for soldier pile walls according to R 15, Paragraph 5b (Section 5.6), the toe must be assumed at the depth at which the acting earth pressure force below the excavation level can be accepted by the unreduced passive earth pressure (cf. Figure R 15-lb). 5. According to Recommendation R 10 of the Recommendations of the Committee for Waterfront Structures, sufficient stability is given if it is demonstrated that the possible anchor forces are at least 1.5 times greater than the current anchor forces. If the safety factor related to the shear strength of the soil ([44, 48, 54, 5 5 ] ) , it must be demonstrated that for a reduction of the friction angle and the cohesion in accordance with the expressions tan@’ tan((, = 1.20

and c’ cq, = 1.60 the possible anchor forces are at least as large as the existing numerical anchor forces. All forces acting must then be recalculated with the reduced shear strength or accordingly converted. The procedure resulting in the smallest anchor length may be selected. 6. The failure state of the ground must also be utilised as the basis for deepseated stability analysis for walls designed for increased active earth pressure or for at-rest earth pressure, i.e. both the earth pressure forces and the anchor forces must be determined from the active earth pressure limit state. With regard to the equilibrium state, stability analysis using the safety factors given

70

in Paragraph 5 is sufficient. The associated wall deflections can be evaluated according to R 46 (Section 7.5).

7.4

Analysis of global stability (R 45)

1. Besides deep seated stability analysis according to R 44 (Section 7.3), anchored retaining walls must always be analysed for general stability. One must imagine that the anchors tie the retaining wall to the ground behind the wall to form a monolithic structure that slides on a curved slip surface (Figure R 45-1). Here, in contrast to deep seated stability analysis, the wall toe moves further forwards than the wall top. 2. General (global) stability analysis must be performed, generally assuming a circular slip surface, in special cases with non-circular or composite slip surfaces [45,54,55]. 3. The critical slip surface generally encompasses the anchored monolith. The end of the anchored monolith is: a) the toe of anchor walls, b) the toe of the equivalent anchor wall for anchor plates, driven piles and grouted anchors, and is defined by the intersection of the two planes as described in R 44, Paragraph 3 (Section 7.3). Otherwise, the critical slip surface generally contacts the toe of the retaining wall or the soldier pile. The critical slip surface may also be deeper for retaining walls with shallow embedment depths according to R 44, Paragraph 4 (Section 7.3). 4. For very long anchors and very deeply embedded retaining walls, it may also be necessary to investigate slip surfaces that intersect structural elements such as anchors or soldier piles. Besides the friction and cohesion forces in the slip joint, the shear forces acting against general failure in the intersected component can also be adopted in such cases. However, these shear forces may only be adopted, a) as permitted by the yield strength of the steel, taking the prevalent normal, bending and shear stresses into consideration, b) at a magnitude that allows the adopted shear force to be transmitted to the ground by the intersected component without large deflections.

Figure R 45-1. Global failure

71

Independent of this however, the additional friction forces generated in the slip surface by the prestressing force of the intersected anchors may be adopted in the analysis as supporting forces. The torque resulting from the axial force acting in the intersected anchor, with reference to the centre of rotation of the slip circle, may be taken into consideration if it acts as a support; if it reduces stability, it must be taken into consideration.

5. For analysis of global stability, retaining walls designed for active earth pressure only require a safety factor of q = 1.30 corresponding to Load Case 2, according to R 24, Paragraph 6 (Section 2.1). A safety factor r\ = 1.40 must be demonstrated for retaining walls designed for increased active earth pressure or at-rest earth pressure. 7.5

Displacements in anchored retaining walls (R 46)

1. As can be demonstrated from empirical values, displacements in anchored retaining walls cannot be completely ruled out even if the retaining wall and the anchor components are designed and prestressed for increased active earth pressure or for at-rest earth pressure. Critical in this respect are the displacements and deformations of the soil mass which is enclosed, similar to inside a cofferdam, by the retaining wall and a plane that connects the points assumed to transmit the anchor forces to the ground according to R 44,Paragraph 3 (Section 7.3) (Figure R 46-1).

2. The wall displacements principally consist of the following components: a) the elastic deflection of the wall, b) distortion of the cofferdam-like soil mass, c) shear deformation of the soil mass and the ground below it, d) horizontal deflection due to compaction of the ground below the excavation level, e) additional relaxing movements due to unloading of the ground when excavating. These deflections have been observed in excavations in at least medium-dense, cohesionless soil and at least firm,cohesive soil at depths of more than 10 to 12 m. They can be estimated on the basis of [74]. More precise investigations based on numerical analyses are possible. Otherwise, see [38] and [39].

Figure R 46-1. Development of a cofferdam-like soil mass

72

3. If an estimation or a more precise investigation of a wall anchored according to best practice results in excessive wall deflections, appropriate measures must be taken, e.g.: a) lengthening of the anchors, b) replacement of at least one row of anchors by bracing, c) replacement of the anchors by struts in some parts of the excavation to generate datum points [29], d) simultaneous manufacture of excavation and structure. Bracing must be designed for substantially higher loads than would correspond to the proportion of the computed earth pressure. 4. Independent of the measures according to Paragraph 3, it is recommendated to expand and stagger anchors in the region of the force transmission zone adjacent to other structures. Expanding can also reduce the mutual influence of the anchors. By staggering the anchor lengths, the danger of sudden jumps in settlement values behind the cofferdam-like monolith can generally be eliminated. Instead, a more shallow settlement depression can be anticipated.

5. If large deflections cannot be ruled out, it is always expedient to monitor at least the horizontal and vertical deflections of the wall top from the outset, so that counter-measures can be implemented at an early stage. Anchor force measurements and settlement measurements are also recommended for excavations in soft, cohesive soil and excavations adjacent to structures.

6. The wall deflections mentioned in Paragraph 2 cannot be substantially reduced by applying especially high prestressing. Such prestressing merely has the primary effect of generating internal stress conditions that prevent formation of an active earth pressure failure wedge and decompaction of the ground. Furthermore, high prestressing can lead to heavy lateral compaction of the ground mass and to especially large settlement at the rear of the anchored zone.

73

8

Excavations with special ground plans

8.1

Excavations with circular plan (R 73)

1. If the depth of a circular excavation is no greater than half of the diameter, the three-dimensional earth pressure distribution from soil self-weight and widearea live loads is only insignificantly different to the earth pressure on an infinitely long retaining wall. If, in flexible excavation structures, the depth is greater than the diameter, the three-dimensional earth pressure distribution is so far below the earth pressure based on classical earth pressure theory that the difference can generally no longer be ignored if economical methods are aimed for. 2. Similar to the earth pressure on an infinitely long retaining wall, the magnitude and distribution of the earth pressure from soil self-weight and widearea live loads depend on the construction methods, the stiffness of the wall and the flexibility of the supports. The following limitations are imposed by R 67 (Section 1.2), with respect to the flexibility of the system as a whole: a) Generally, diaphragm walls and bored pile walls can be seen as inflexible systems if they form an unbroken circle and simultaneously serve as a ring beam. A precondition for this is that the ground cannot relieve whilst manufacturing the retaining wall. b) Retaining walls can be seen as nearly inflexible if they possess a certain inherent flexibility, e.g. sheet pile walls and contiguous pile walls, but which are supported by stiff ring beams. c) Generally, all retaining walls in which the ground face is open before the infill walls are installed and which are supported by rings or other devices can be seen as slightly yielding, in particular soldier pile walls with timber infill walls. d) All retaining walls that rely solely on their restraint in the ground for stability can be seen as highly yielding, e.g. soldier pile walls or sheet pile walls without supports. Installation of segments or shotcrete linings can be seen as producing either slightly yielding systems or nearly inflexible systems, according to the excavation depth and ground stability. The same applies to soldier pile walls with concrete infilling, which ensures annular load transmission.

3. The following apply for determination of the earth pressure: a) The at-rest earth pressure E, may be applicable as the upper limit value for inflexible systems according to Paragraph 2a. An earth pressure of E = ?h. (E,, + EaR)can be assumed as the lower limit. E,, designates the three-dimensional active earth pressure according to the modified diskC 1, K 821. element theory after W A L Z ~ O [8 74

b) The upper limit value for nearly inflexible systems according to Paragraph 2b can be taken as an earth pressure of E = %.(E, + E,,) and the lower limit as the three-dimensional earth pressure E,, according to the modified disk-element theory. c) The upper limit value for slightly yielding systems according to Paragraph 2c can be taken as an earth pressure of E,, according to the modified disk-element theory; the lower limit value can be based on the simplified approach after BERESANZEW [83]. d) The earth pressure for highly yielding systems according to Paragraph 2d can be determined with the simplified approach after BERESANZEW. e) In cohesionless soils, the approach after STEINFELD [84] may be selected in place of the modified disk-element theory after WALZEIOCK, if based on the possible earth pressure distribution diagram. f) For the approach of the modified disk-element theory, the ring bracing factor must be adopted with k, = 0.5 for determination of the threedimensional earth pressure if the upper limit value is required, but with k, = 1.0 for determination of the lower limit value. The ring bracing factors h, = 0.7 and h, = 1.0 apply accordingly for the approach after STEINFELD. g) In order to assess the most unfavourable stresses at all points of the excavation structure, the internal forces must be determined in conjunction with the adopted live loads for both the upper and lower limit values for the case in question.

4. R 4, Paragraph 3 (Section 3.2) applies accordingly with regard to the minimum earth pressure. The disk-element theory after WALZ/HOCK [81, 821 is suitable for determination of the earth pressure from the three-dimensional active earth pressure acting on an excavation with a circular plan. Here, formally reduced vertical self-weight stresses are initially computed, which are then multiplied with the earth pressure coefficient associated with the selected equivalent friction angle in the region of the cohesive layers. 5 . It can be assumed that the earth pressure distribution only deviates slightly from a linear depth increase in inflexible systems according to Paragraph 3a. However, if the preconditions for active earth pressure are fulfilled, the total load generated by the three-dimensional active earth pressure must be distributed across the complete height of the wall based on the principles of Recommendation R 5, Section 3.3. If the total earth pressure lies between the at-rest earth pressure E,, and the three-dimensional active earth pressure E,,, the earth pressure distribution must be interpolated. Due to the lack of measurement data available for circular excavations and because theoretical considerations cannot exclude the possibility that upward redistribution of active earth pressure is less pronounced than for continuous, straight retaining walls, it is recommended to analyse using two limit distributions and to base the design of individual components on the greater internal forces. The presssure diagrams given in R 69 and R 70 can be selected as the upper limit.

75

max

e,

I

ai Load in plan

bi Earth pressure on developed surface

ci Earth pressure in plan

Figure R 73-1. Earth pressure from a bounded equivalent load p’ = 10 N / m 2

6. Unexpected deviations from the radial symmetry, e.g. inhomogeneity of the ground not recognised in exposures, or accidental geometrical imperfections, should be taken into consideration in the load approach. As an approximation, radially acting earth pressure from a bounded equivalent load p’ = 10 kN/m2, distributed in keeping with a cosinus function, may be adopted as shown in Figure R 73-1, e.g. in keeping with the function e,, = max eh.cos2a.In the atrest earth pressure limit case, the maximum value max eh results from the theory of elastic half-space; in the case of active earth pressure from the max eh = max eaph= p.K, approach, as for an infinitely long wall. If a value between the at-rest earth pressure and the active earth pressure is adopted for determination of the earth pressure, this also applies to earth pressure from the bounded live load. The recommended approach covers geometrical imperfections in an oval plan with a maximum deviation of the A and B principle axis dimensions of A : B = 1.05.This condition must be examined by onsite measurements, in particular for constructions without a ring beam. If the centres of bored pile walls or the longitudinal axes of individual diaphragm wall slices do not coincide with a possible pressure line, the imperfection must be corrected or compensated for by design or by structural measures.

7. If traffic or operating loads exceed the equivalent load p’ = 10 kN/m2 based on Section 6, only the actual load positions need be taken into consideration. Two cases may be considered: a) If the load is represented by a strip load p’, according to R 55, Paragraph 3 (Section 2.4), or R 57, Paragraph 4 (Section 2.6), as shown by Figure R 73-2a, the earth pressure must be determined similar to R 6 (Section 3.4) and R 7 (Section 3.5), as if a plane at a tangent to the circular excavation structure were the critical plane. As an approximation, the earth pressure determined can be adopted for one quarter of the circumference of the excavation as a radially acting load eh as shown in Figure R 73-2c. 76

a1 Load in plan

bl Load and earth pressure in section IExamplel

cl Earth pressure in plan

Figure R 73-2. Earth pressure from a strip load p’

b) If the load is represented by point loads according to R 55 (Section 2.4) or R 57 (Section 2.6), as shown in Figure R 73-3a, the earth pressure must be determined similar to R 6 (Section 3.4), as if a fictitious plane at a tangent to the circular excavation structure were the critical plane, taking the associated contact areas and the load distribution in the upper road layers and in the ground according to R 3 (Section 2.3) into consideration. The determined earth pressure can be adopted without precise analysis as a radially acting load eh based as an approximation on Figure R 73-3c with the same length 1 as the circle circumference which results from the load distribution as shown in Figure R 73-3a, but for a maximum of one quarter of the circumference.

1Reference plane

a1 1oad in plan

bl Load and earth pressure in section IExamplel

ci Earth pressure in plan

Figure R 73-3. Earth pressure for a point load

77

If the earth pressure from soil self-weight is adopted as the at-rest earth pressure, the earth pressure from live loads may also be determined according to the theory of elastic half-space; if a value between the at-rest earth pressure and the active earth pressure is adopted for the earth pressure from soil selfweight, this also applies for the earth pressure from live loads. 8. When determining the earth pressure from foundation loads for excavations adjacent to structures, the information in Paragraph 7 applies in principle: a) The load distribution and load length at the circumference of the excavation must be determined as shown in Figure R 73-3 for footing foundations. b) The earth pressure determined from strip loads must be applied to a quarter of the circumference as shown in Figure R 73-4c. Please observe Recommendation R 20 (Section 9.1). 9. The subgrade reactions resulting from bounded surcharges according to Paragraph 6 to Paragraph 8 must be adopted corresponding to the interaction between the load - deformation behaviour of the excavation structure and the load - deformation behaviour of the ground. As an approximation, earth pressure of the same magnitude and distribution as on the load side of the excavation may be adopted as a substitute for the corresponding subgrade reactions on the opposite side. More precise methods must be applied for higher demands on the precision of the determined internal forces and deformations, e.g. for excavations adjacent to structures. If the modulus of subgrade reaction method was utilised and no precise investigations were carried out, the modulus of subgrade reaction may be approximately determined using ks = E, : r from the constrained modulus of the ground and the outer radius of the excavation structure. However, values greater than eLh = 0.50.ePhare generally not permitted for the passive earth pressure.

ai Load in plan

bi Load in section lExample1

Figure R 73-4. Earth pressure from a strip foundation

78

d Earth pressure in plan

10. If the ground below the excavation level serves to support the wall, the passive earth pressure may be adopted as for an infinitely long wall, without the necessity for more precise investigation of the three-dimensional stress state. 11. Ring- or polygon-shaped, stiff bracing structures must be designed for bending considering the normal force. A stability investigation may generally be dispensed with if the contact with the retaining wall prevents ring deflection.

8.2

Excavations with oval plan (R 74) If the dimensions of the principle axes A and B of an excavation with a curved, but not circular, plan as shown in Figure R 74-1 deviate by more than 5 % from one another, the deviations in the subgrade reactions compared to those of a circular plan can generally no longer be neglected. These deviations increase rapidly with an increasing ratio A : B and reach a value for A : B = 1.5 for which assumptions and investigations are required which are beyond the scope of this Recommendation. Otherwise, the scope of this Recommendation is restricted to elliptically curved plans for which the radius of the larger curve is no more than 2.5 times the radius of the smaller curve. The following approaches apply to elliptically curved plans as shown in Figure R 74-1 with a ratio A : B < 1.5, if no more precise investigations are performed, e.g. with the help of finite element methods.

2. The magnitude and distribution of the earth pressure from soil self-weight and wide-area live loads depends on the type of construction project, the stiffness of the wall and the flexibility of the supports. The following limitations apply with respect to the flexibility of the wall in the region of the larger curve radius according to R 67 (Section 1.2) and R 73 (Section 8.1): a) Generally, diaphragm walls and bored pile walls can be seen as nearly inflexible systems if they form an unbroken arc and simultaneously serve as a ring beam. A precondition for this is that the ground on the outside of the retaining wall cannot unload whilst manufacturing the retaining wall.

I 2

Figure R 74-1. Excavations

A

with elliptically curved plan

79

b) Retaining walls can be seen as slightly yielding if they possess a certain inherent flexibility, e.g. sheet pile walls and contiguous pile walls, but are supported by stiff ring beams. c) Generally, all retaining walls in which the ground face is open before infill walls are installed and which are supported either by rings or other measures or not at all can be seen as highly yielding, in particular soldier pile walls with timber infill walls.

3. The decisive design earth pressure is principally dependent on the flexibility of the two elliptical curves with the smaller radius. For more precise analysis, an initial stress state of the undeformed system must be assumed, from which a final equilibrium state is developed in the context of the relationships between the earth pressure on the longer sides, the deformations of the excavation structure and the subgrade reactions on the shorter sides, if necessary iteratively. The initial stress state earth pressure must be assumed as for circular excavations as a function of the selected construction method. The radius of the section of the elliptical curve represents the respective circle radius. As an approximation, the stress reduction associated with the anticipated deformations in those areas with the larger curve radius according to Paragraph 4 also leads to an increase in those areas with the smaller curve radius according to Paragraph 10.

4. The following apply for determination of the earth pressure in the areas with the larger curve radius: a) The upper limit value for nearly inflexible systems according to Paragraph 2a can be taken as an earth pressure of E = %.(Eo+ EaR) and the lower limit as the three-dimensional earth pressure E,, according to the modified disk-element theory after WALZ/HOCK [8 1, 821. b) The upper limit value for slightly yielding systems according to Paragraph 2b can be taken as an earth pressure of E,, according to the modified disk-element theory; the lower limit value can be based on the simpli[83]. fied approach after BERESANZEW c) The earth pressure for highly yielding systems according to Paragraph 2c can be determined with the simplified approach after BERESANZEW. d) In cohesionless soils, the approach after STEINFELD [84] may be selected in place of the modified disk-element theory after W A L Z ~ O C if K based on the possible earth pressure distribution diagram. e) The ring bracing factor must be adopted at k, = 0.5 for determination of the three-dimensional earth pressure if the upper limit value is required, but with k, = 1.0 for determination of the lower limit value. The ring bracing factors h, = 0.7 and h, = 1.O apply accordingly for the approach after STEINFELD. f) In order to assess the most unfavourable stresses at all points of the excavation structure, determination of internal forces must be performed in conjunction with the adopted live loads for both the upper and lower limit

80

values for the case in question. If large stresses on the long side are unfavourable here, a smaller value than that resulting from Paragraphs a and c may be adopted if separate investigations demonstrate that the earth pressure as a function of the anticipated wall deflection justifies this. 5. R 4, Paragraph 3 (Section 3.2) applies accordingly with regard to minimum earth pressure. The disk-element theory after WALZ/HOCK [81,82] is suitable for determination of the earth pressure from three-dimensional active earth pressure acting on an excavation with a circular plan. Here, formally reduced vertical self-weight stresses are initially computed, which are then multiplied with the earth pressure coefficient associated with the equivalent friction angle according to R 4 in the region of the cohesive layers. 6. If the preconditions for active earth pressure are fulfilled, the total stress developed by the three-dimensional active earth pressure must be distributed over the wall height based on the principles of Recommendation R 5 (Section 3.3). If the total earth pressure lies between the at-rest earth pressure E,, and the three-dimensional active earth pressure E, the earth pressure distribution must be interpolated. Due to the lack of measurement data available for elliptically curved excavations and because theoretical considerations cannot exclude the possibility that upward redistribution of the active earth pressure is less pronounced than for continuous, straight retaining walls, it is recommended to analyse using two limit distributions and to base the design of individual components on the greater internal forces. The presssure diagrams given in R 69 and R 70 (Sections 5.2 and 6.2) can be selected as the upper limit.

7. If unfavourable loads are anticipated with regard to design of individual components of the excavation structure, a continuous distributed load of at least p = 10 kN/m2 similar to Recommendations R 55 to R 57 (Sections 2.4 to 2.6) must be adopted. The resulting earth pressure must be adopted for the whole zone of influence as a uniform, radially acting load ordinate as shown in Figure R 74-2, if it acts unfavourably. In the at-rest earth pressure limit state, this load ordinate follows from elastic half-space theory and in the case of active earth pressure, from the eh = eaph= p . K,, approach as for an infinitely long wall. If a value between the at-rest earth pressure and the active earth pressure is adopted for determination of the earth pressure, this also applies to the earth pressure from the live load. If site traffic or operating loads exceed the distributed load p = 10 kN/m* according to Paragraph 7, only feasible load configurations need be taken into consideration. Two cases may be considered: a) If the load is represented by a strip load p’ according to R 55, Paragraph 3 (Section 2.4) or R 57, Paragraph 4 (Section 2.6), as shown by Figure R 74-3a, the earth pressure must be determined similar to R 6 (Section 3.4) and R 7 (Section 3.5), as if a fictitious plane at a tangent to the excavation structure were the critical plane. 81

bl Earth pressure in plan

ai Load in plan

Figure R 74-2. Earth pressure from a bounded distributed load p = 10 kN/m*

al Load in plan

bl Load in section /Example1

cl Earth pressure in plan

Figure R 74-3. Earth pressure for a strip load p'

As an approximation, the determined earth pressure can be adopted as a radially acting load e,, as shown in Figure R 74-3c, for not more than 'Is of the circumference to each side of the tangent point and only inasmuch as the earth pressure acts unfavourably. b) If the load is represented by point loads according to R 55 (Section 2.4) or

R 57 (Section 2.6), as shown in Figure R 74-4a, the earth pressure must be determined similar to R 6 (Section 3.4), as if a fictitious plane at a tangent to the excavation structure were the critical plane, taking the associated contact areas and the load distribution in the upper road layers and in the ground according to R 3 (Section 2.3) into consideration. The determined earth pressure can be adopted without precise analysis as a

82

ai Load in plan

bi Load in section (Example1

ci Earth pressure in plan

Figure R 74-4. Earth pressure for a point load

radially acting load ehbased as an approximation on Figure R 74-4c, with the same length 1 as the total circumference which results from the load distribution as shown in Figure R 74-4a, but for a maximum of one quarter of the circumference, if it acts unfavourably. If the earth pressure from soil self-weight is adopted as the at-rest earth pressure, the earth pressure from live loads may also be determined according to the theory of elastic half-space; if a value between the at-rest earth pressure and the active earth pressure is adopted for the earth pressure from soil selfweight, this also applies for the earth pressure from live loads.

9. When determining the earth pressure from foundation loads, the information in Paragraph 8 applies in principle: a) The load distributions and load length at the circumference of the excavation must be determined as shown in Figure R 74-4 for footing foundations. b) The earth pressure determined from strip loads must be applied to a quarter of the circumference as shown in Figure R 74-5c, if it acts unfavourably. Half of the corresponding length must be adopted in each direction from the point which is closest to the foundation. Please observe Recommendation R 20 (Section 9.1).

10.The subgrade reactions in the region of the smaller curve radius may be adopted for determination of the internal forces from earth pressures according to Paragraph 3. The same applies if the earth pressure from bounded surcharges according to Paragraph 7 to Paragraph 9 acts on one side in the region of a large curve radius. As an approximation in such cases, an earth pressure of equal magnitude and distribution as on the load side of the excavation may be adopted as a substitute for the corresponding subgrade reactions on the opposite side. If an earth pressure in the region of the curve transition ensues from 83

ai Load in pian

bi Load in section (Examplei

ci Earth pressure in plan

Figure R 74-5. Earth pressure from a strip foundation

the loads according to Paragraph 7 to Paragraph 9, subgrade reactions will also ensue in the region of the large curve radius. The subsequent ground reactions must be adopted corresponding to the interaction between the load-deformation behaviour of the excavation structure and the load-deformation behaviour of the ground. If the subgrade reaction modulus method was utilised for this purpose and no precise investigations were carried out, the subgrade reaction modulus may be approximately determined using k, = E, : r from the constrained modulus of the ground and the decisive outer radius of the excavation structure. However, greater values than e’,h = 0.50 ephare generally not permitted for passive earth pressure. Tensional bedding must be excluded when determining internal forces for the decisive load combinations. 9

11. Upper and lower limit values of the modulus of subgrade reaction must be taken into consideration for estimating the deformations based on the modulus of subgrade reaction method. If necessary, more precise methods must be applied. The subgrade reactions on the opposing sides must be taken into consideration for bounded surcharges. 12. If the ground below the excavation level is utilised to support the wall, the passive earth pressure may be adopted as for an infinitely long wall according to R 14 and R 19, without the necessity for more precise investigation of the three-dimensional stress state. 13. Oval- or polygon-shaped, stiff bracing structures must be designed for bending considering the normal force. A stability investigation may generally be dispensed with if the contact with the retaining wall prevents ring deflection.

8.3

Excavations with rectangular plan (R 75)

1. In principal, the retaining walls and the bracing or anchors of excavations with square or rectangular plans can be designed and constructed similar to

84

those for elongated excavations. However, in the interests of economical design of structural members, it is also permissible to take the earth pressure reduction caused by the three-dimensional effect into consideration for cohesionless or at least firm, cohesive soil. The following procedures may be applied for determination of the reduced earth pressure: a) procedure according to Paragraph 2, which assumes shear forces in the flank faces of a slipping two-dimensional earth wedge, b) procedure according to Paragraph 3, which assumes a slipping threedimensional body. The procedures suggested here assume an excavation structure similar to R 67 (Section 1.2), which is either not supported, yieldingly supported or slightly yieldingly supported but sufficiently deformable, in order to facilitate reduction of the at-rest earth pressure to the active earth pressure. Where these displacements are prevented at the excavation corners for diaphragm walls and bored pile walls, the at-rest earth pressure may be locally retained; this may, however, generally remain unconsidered. 2. Where procedures are applied which assume shear forces in the flanks of slipping earth wedges, these are based on a conceptual model as shown in Figure R 75-la, whereby an earth wedge approaches the excavation from all sides and the corner regions are immovable. Friction forces and, where applicable, cohesion forces are thus mobilised in the boundary surfaces between the slipping earth wedges and the immovable comer masses and thus prevent slippage of the earth masses towards the excavation walls and reduce the total active earth pressure. Only procedures that do not overestimate the magnitudes of these forces may be selected. See [85, 861 and EN 1538. These may be suitable if the comers of the retaining wall are just as flexible as the middle sections of the excavation walls. The reduction of the total earth pressure can be implemented in the design of the individual components as shown in

ai 1ateral friction model

bi Arching model

Figure R 75-1. Models for determination of the three-dimensional earth pressure for rectangular excavations

85

-L-

-i t 1 tO'

E,

F

L

*

a'

*ai Earth pressure with chamfering

bi Earth pressure with steps

Figure R 75-2. Simplified earth pressure application for earth pressure reduction at excavation comers

Figure R 75-2a in the form of chamfering or as shown in Figure R 75-2b in the form of steps in the continuous earth pressure E, determined without the three-dimensional effect. 3. For those procedures which assume a three-dimensional failure body, the development of a arching effect as shown in Figure R 75-lb plays a decisive role for earth pressure reduction. Suitable procedures are those after KARSTEDT [53] and after PIASKOWSK~KOWALEWSKI [87]. The procedures based on threedimensional sliding bodies are suitable if the comers of the retaining walls are less flexible than the middle sections of the excavation walls. The difference between the total earth pressure for the continuous wall and the total earth pressure for the relevant area of the excavation side walls, corresponding to one of the procedures mentioned above, can be implemented in the design of the individual components as shown in Figure R 75-3a in the form

ai Earth pressure with chamfering

bl Earth pressure with steps

Figure R 75-3. Simplified earth pressure application for earth pressure reduction on excavation sides

86

of chamfering or as shown in Figure R 75-3b in the form of steps of the continuous earth pressure E, determined without the three-dimensional effect. 4. In Paragraphs 2 and 3, E, designates the earth pressure for a continuous wall from soil self-weight, cohesion and wide-area uniformly distributed load according to R 4 (Section 3.2) in combination with R 6 (Section 3.4), R 12 (Section 5.1) and R 16 (Section 6.1). 5. The earth pressure E, on each side of the excavation may be chamfered as shown in the Figures R 75-2a and R 75-3a or reduced as shown in the Figures R 75-2b and R 75-3b to %.Eh without further analysis. The wall lengths for which a reduction may be applied follow from WALZ[88], as a function of the depth H as follows aL = (0.35 - 0.06."/,).H on the sides with the length L, aB = (0.35 - 0.06.H/B).Hon the sides with the length B. Distribution of the earth pressure as shown in Figure R 75-2 is recommended if the preconditions according to Paragraph 2 are fulfilled: earth pressure distribution as shown in Figure R 75-3 is recommended if the preconditions according to Paragraph 3 are fulfilled. 6. If the excavation length for which the total earth pressure E, may be reduced results in 2aL > L or 2a, > B for the end walls of narrow excavations (from approx. H > 2.5 .B), the total load must be adopted with a minimum of

EiL = '/2.Eh.Lon the sides of length L, E ; ~= L/~.E,.Bon the sides of length B. The distribution on the sides of the excavation follows from Paragraphs 2 and 3 as one of the shapes represented in Figure R 75-4. When deciding on one of these shapes, the flexibility of the supports is decisive. The largest earth pressure should be anticipated where the displacement is smallest. The same applies accordingly for the longer sides of shaft-like excavations. 7. If, in exceptional cases, a retaining wall is designed for at-rest earth pressure according to R 23 (Section 9.6), an earth pressure reduction is not warranted. When applying increased active earth pressure to retaining walls adjacent to structures, interpolation may be performed between the at-rest earth pressure

a1 Reduction at the corners

bl Uniformly dlstributed load

cl Reduction to wards centre

Figure R 75-4. Earth pressure at narrow excavation end walls and deep shafts

87

\

ai Earth pressure with chamfering

bl Earth pressure with steps

Figure R 75-5. Earth pressure in rectangular excavations with increased active earth pressure and earth pressure reduction at the excavation comers

- _ _ - Le---

L

a1 Earth pressure with Chamfering

-1

bl Earth pressure with steps

Figure R 75-6. Earth pressure in rectangular excavations with increased active earth pressure and earth pressure reduction at the excavation sides

and the active earth pressure, just as in the area without reduction. See Figure R 75-5 and Figure R 75-6. Here, E, designates the component of the design earth pressure from soil self-weight according to R 22 (Section 9.5). Paragraphs 2 to 5 are decisive for applying simple active earth pressure in the area of structures according to R 28 (Section 9.3) or according to R 29 (Section 9.4). 8. The same pressure diagram may be selected for distribution of the earth pressure across the wall in the region of chamfers or steps as for the earth pressure E, in regions without reduction. 9. The earth pressure from point loads, line loads or strip loads from road and rail traffic according to R 55 (Section 2.4), from site traffic and operations according to R 56 (Section 2.5) and from excavators or lifting equipment according to R 57 (Section 2.6), as well as the earth pressure from building loads according to R 28 (Section 9.3), R 29 (Section 9.4), R 33 (Section 9.5) and R 23 (Section 9.6) may not be reduced.

10.If the ground below the excavation level is utilised as a wall support, the passive earth pressure may be adopted as for an infinitely long wall. A threedimensional effect at the corners may only be adopted on the basis of separate investigations. 88

9

Excavations adjacent to structures

9.1

Engineering measures for excavations adjacent to structures

(R20) 1. If a structure is within the zone of influence of an excavation, the impact with regard to stability and serviceability of the structure must be investigated. The necessary measures depend on the distance, the foundation depth, the structural condition of the building, the sensitivity to settlement and the use of the structure, and the ground conditions. Furthermore, for braced excavations, the elastic deformations, slippage and local deformations also play a role, in particular for long sets of struts consisting of a large amount of individual components. Struts or anchors are preferentially located in the area of the foundation load projection. Non-propped retaining walls that are only restrained in the ground are generally not permissible if the free wall height is within the projection zone of foundation loads. 2. Generally, retaining walls adjacent to structures can be manufactured as soldier pile walls. However, as a rule, it is generally necessary to eliminate as far as possible ground displacement caused by bending of the infilling or from the development of voids behind the infilling in the region below the foundation level by adopting appropriate measures. In suitable, temporarily stable ground, this can be achieved by excavating from soldier pile to soldier pile with the help of a template and by prestressing the individual components of the infilling to achieve a predefined curvature. These measures can be dispensed with if the infilling is manufactured using in-situ concrete for temporarily stable, cohesive soil. It may be expedient to install trench sheet piles as infilling in less stable ground and to wedge them against the waling to anticipate the computed deformations of the trench sheet piles and waling. 3. If a soldier pile wall is not possible or practical, e.g. - in uniformly grained, cohesionless soil, -

in soft, cohesive soil and soils that tend to flow,

- if dewatering is not desirable, - for small excavation - structure distances, - for particularly sensitive structures,

the installation of watertight and especially low-deformation retaining walls may be necessary, e.g. sheet pile walls, diaphragm walls or bored pile walls. In special cases it may be expedient to underpin the structure completely or in part or to apply soil stabilisation measures.

4. When selecting the excavation lining it must be observed that not every system is equally suitable due to the influence arising from the manufacturing process. The following may serve as examples: 89

a) When driving or vibrating sheet pile walls, loosely compacted, cohesionless soils are compacted and dragged by the piles. b) For pile walls in soft soils or soils subject to flow, settlement in the immediate area can ensue due to the soil being pushed into the void created by the drill bit projection. c) In suspension-supported diaphragm wall trenches, the intersection of voids, e.g. pipelines, can lead to loss of the suspension fluid; damage to adjacent structures may occur as a result of the lower trench support. Each case must be examined individually to determine the suitability of the construction method and manufacturing process.

5. In order to keep the anticipated wall displacement as low as possible it is practical to, a) select flexurally stiff sections or thick walls, b) use small spacings between the individual rows of struts and/or anchors, c) restrict the excavation advance to an unavoidable minimum before installing the struts and anchors, d) prestress the struts and/or anchors. The degree of prestressing should be carried out according to R 8, Paragraph 4 (Section 3.1) for active earth pressure design, according to R 22, Paragraph 6 (Section 9.5) for increased active earth pressure design and according to R 23, Paragraph 9 (Section 9.6) for at-rest earth pressure design.

6. For anchored retaining walls, it may be expedient to install all or at least some of the grouting lengthes behind the structure to be stabilised, in order to ensure that any ground displacements associated with a cofferdam effect cannot negatively impact the structure. See also R 46, Paragraph 4 (Section 7.5) and [29, 39, 721. 7. It may be expedient to carry out stabilising measures on the structure itself, independent of any measures for stabilising the excavation. These include, for example, measures to improve the connection between longitudinal and transverse walls, anchoring-back endangered sections of the structure to sections that are not within the zone of influence of the excavation, as well as placing brickwork in openings and installing binded double-walings in order to stiffen walls if the diaphragm action of these is in doubt.

8. The recommendations in R 20 must be applied accordingly to cases in which sensitive plants or installations may be endangered by the manufacture of the excavation. For example, such installations can be:

a) Railway installations, in particular badly positioned tracks or high train travelling speeds. b) Pipes without longitudinal tensional lock, in particular when associated with brittle material. c) Water or gas pipes, in particular with large diameters and breaked routes. 90

d) Masonry sewer pipes, in particular old or damaged pipes. e) Masts for illumination installations, signalling installations, electricity or overhead cables of rail vehicles, in particular if they are eccentrically loaded and restrained in the ground.

9.2

Analysis of retaining walls with active earth pressure for excavations adjacent to structures (R 21) If the struts or anchors of a retaining wall are not prestressed more than stipulated in R 8, Paragraph 4 (Section 3.1), it must be assumed that a horizontal wall deflection with a magnitude of 1 %O of the wall height will occur. Ground settlements directly behind the retaining wall that are twice as large as the horizontal wall deflections and only fade at large distances from the excavation may be associated with this wall deflection. If a structure is located within this region, it must be assumed that the resulting settlements will impact the foundations. As far as these settlements can be accepted, taking the condition and sensitivity of the structure into consideration, the excavation structure may be designed for active earth pressure.

2. Generally, the active earth pressure may also be determined on the basis of planar slip surfaces for excavations adjacent to structures. However, in individual cases, where very heavy building loads and unfavourable ground layering are prevalent, it may be necessary to determine the earth pressure on the basis of curved or non-circular slip surfaces. Horizontal building loads must always be taken into consideration. 3. The magnitude and distribution of the earth pressure on a retaining wall adjacent to a structure depend greatly on the distance and the foundation depth. Two cases can be differentiated here: a) Large distances to structures, see R 28 (Section 9.3). b) Small distances to structures, see R 29 (Section 9.4). The decisive factor for differentiation is whether or not a straight line running at an angle -9. and touching the front edge of the foundation is flatter (Figure R 21-la) or steeper (Figure R 21-lb) than the slip surface for soil self-weight and cohesion alone. The toes of these straight lines are viewed as a) the real toe of the wall for soldier pile walls with free earth supports, sheet pile walls and in-situ concrete walls, b) the theoretical toe of the wall for restrained soldier pile walls, sheet pile walls and in-situ concrete walls. 4. For soldier pile walls, only those portions of the earth pressure occurring

above the excavation level are incorporated into the redistribution pressure diagrams according to R 28 (Section 9.3) or R 29 (Section 9.4). When analysing the ZH = 0 equilibrium condition according to R 15 (Section 5.6), the 91

ai Large distance to structures

bi Small dlstance to structures

Figure R 21-1. Distance between retaining wall and structures

earth pressure from building loads occurring below the excavation level must be taken into consideration (Figures R 28-ld and R 28-le).

5. Application of passive earth pressure when analysing the embedment depth is a) according to R 14 (Section 5.4) or R 19 (Section 6.4) in the case of a free earth support, b) according to R 25 (Section 5.5) or R 26 (Section 6.5) in the case of an earth restraint. Furthermore, it is acceptable to determine the internal forces with the given reduced safety factors qp = 1.5 or qp = 1.2, if medium-dense or dense, cohesionless soil, or at least firm,cohesive soil is present beneath the excavation level. 6. See R 9 (Section 4.1) for vertical force equilibrium analysis.

7. See R 30 (Section 9.7) for design of retaining walls on opposite sides of braced excavations.

9.3

Active earth pressure for large distances to structures (R 28)

1. If the preconditions for a large distance between the retaining wall and other structures given in R 21, Paragraph 3 (Section 9.2) are fulfilled, the magni-

tude of the earth pressure from soil self-weight and wide-area live loads can be determined in the same manner as for retaining walls that are not subjected to earth pressure from building loads, taking cohesion into consideration where necessary. 2. The earth pressure from the building load may be determined from the difference between the total earth pressure on the one hand and the earth pressure 92

from soil self-weight and live loads on the other hand, taking cohesion into consideration where necessary, and be adopted as a uniformly distributed load. As an approximation, the zone of influence of the building load can be assumed as shown in Figure R 28-la. The upper boundary thus lies between the level of the foundation base and the point at which a straight line originating at the front edge of the foundation and projected at an angle = @’ to the horizontal intersects the rear face of the wall. The lower boundary is given by a straight line projected at an angle 6, to the horizontal from the rear edge of the foundation. The horizontal component must also be taken into consideration for inclined foundation loads. 3. Where large distances between the retaining wall and the structure have to be considered, the earth pressure redistribution from soil self-weight and unbounded distributed loads impacts across the whole wall height. For soldier pile walls, this earth pressure may be converted to a simple pressure diagram reaching from ground level to the excavation level according to R 12, Para-

ai Excavation, structure and load distribution

bl Nan-redistributed earth pressure from soil selfweight and live load

cl Earth pressure from building load as a rectangle

Figure R 28-1. Distribution of active earth pressure taking the influence of a buildine load with large distance between retaining wall and structure into consideration (example for a soldier pile wall with free-earth support) I

dl Total earth pressure in a pressure diagram with load increment

el Total earth pressure in a pressure diagram without load increment

93

graph 3 (Section 5.1) -if it is not a special case according to R 15, Paragraph 5c or Paragraph 6c (Section 5.6) - and reaching from ground level to the point of zero stress for sheet pile walls and in-situ concrete walls according to R 16, Paragraph 3 (Section 6.1). The earth pressure from the building load may be incorporated into this pressure diagram, taking the zone of influence according to Paragraph 2 into consideration, so that any sudden alteration in the earth pressure ordinate lies in the area of a support point (Figure R 28-ld), or so that no sudden alteration of the earth pressure ordinate occurs (Figure R 28-le). However, incorporation of the earth pressure from building loads in a continuous, rectangular pressure diagram according to R 13 (Section 5.3), from ground level to the excavation level, or according to R 17 (Section 6.3), from ground level to the point of zero stress, is not permitted.

9.4

Active earth pressure for small distances to structures (R 29)

1. If the preconditions for short distances between the retaining wall and other structures given in R 21, Paragraph 3 (Section 9.2) are fulfilled, it is convenient to determine the earth pressure from soil self-weight and wide area live loads separately for the following load components: a) for the self-weight of the soil above the foundation base between the retaining wall and the structure and for the effective live load between the retaining wall and the structure, b) for the self-weight of the soil below the foundation base, for the selfweight of the soil within the structure above the foundation base and the basement floor, and for a live load acting on the basement floor.

2. The earth pressure from the self-weight of the soil above the foundation base between the retaining wall and the structure and the earth pressure from the effective live load in this region are first determined down to the foundation base and then supplemented by the component resulting from an assumed slip surface as shown in Figure R 29-la at an angle 6,, projected from the front edge of the foundation (Figure R 29-lb). The earth pressure determined in this way is redistributed to the region between ground level and the intersection of the assumed slip surface with the retaining wall, according to R 12, Paragraph 3 (Section 5.1), orR 16, Paragraph 3 (Section 6.1) (Figure R 29-1d), if necessary taking cohesion into consideration. 3. The earth pressure determined from the self-weight of the ground below the foundation base is redistributed to the region between the foundation base and the excavation level for soldier pile walls and to the region between the foundation base and the point of zero stress for sheet pile walls and in-situ concrete walls, taking cohesion into consideration if necessary, unless it is a special case according to R 15, Paragraph 5c or Paragraph 6c (Section 5.6). 94

The soil self-weight above the foundation base in the region of the structure can be converted to a surcharge and adopted as a uniformly distributed load together with the self-weight of the basement floor and any live load in the basement. 4. The earth pressure from building loads is obtained with sufficient precision by first determining the earth pressure resulting from the sum of the loads given in Paragraph 3 and the building load and then subtracting the earth pressure determined using the loads given in Paragraph 3. The zone of influence of the building loads can be assumed approximately as shown in Figure R 29-la and the distribution of earth pressure from a building load as a uniformly distributed load as shown in Figure R 29-lc. If two or more foundations influence the earth pressure magnitude, the individual foundation earth pressure forces are first determined separately and then superimposed. The horizontal component must also be taken into consideration for inclined foundation loads. Earth pressure from the

ai Excavation, structure and load dstribution

bi Non-redlstributed earth pressure from soil selfweght and live load

- -4

--4

di Total earth pressure in a pressure dagram with load increment

el Total earth pressure in a pressure dagram without load increment

ci Earth pressure from building load as a rectangle

Figure R 29-1.Distribution of active earth pressure taking the influence of a building load with small distance between retaining wall and structure into consideration (example for a sheet pile or in-situ concrete wall with freeearth support)

95

5. The pressure diagrams determined according to Paragraphs 2 to 4 are superimposed. The ensuing total pressure diagram may be selected so that any sudden alteration in the earth pressure ordinate lies in the area of a support point (Figure R 29-ld), or so that no sudden alteration of the earth pressure ordinate occurs (Figure R 29-le). The earth pressure from the building load may be incorporated into the pressure diagram for the lower earth pressure proportion, taking the zone of influence according to Paragraph 3 into consideration. A continuous, rectangular pressure diagram from ground level to the excavation level or to the point of zero stress is not permissible for either the earth pressure from soil self-weight or the earth pressure from building loads. 9.5

Analysis of retaining walls with increased active earth pressure (R22)

1. If the horizontal deflection of a retaining wall, and thus the settlement behind the wall, needs to be more heavily restricted than stipulated in R 21, Paragraph 1 (Section 9.2), according to R 8, Paragraph 3 (Section 3.1), taking existing structures into consideration using the measures stipulated in R 20, Paragraph 4 (Section 9. l), the excavation structure must be designed for increased active earth pressure. Increased active earth pressure is defined as an earth pressure that is greater than active earth pressure but smaller than the at-rest earth pressure. The magnitude of this increased active earth pressure depends on conditions in the excavation and on the structure. It must be noted here that the calculated total at-rest earth pressure may be smaller than the total active earth pressure for structures close to the excavation. Because the validity of the elastic half-space theory is in question in this case, large and small distances to the structures are differentiated when defining the increased active earth pressure.

2 . For large distances to structures according to R 28, the mean value E, = 0.50.(E0, + E,) between the horizontal component of the active earth pressure E, and the horizontal component of the full at-rest earth pressure E, is generally sufficient. In simple cases the earth pressure E, = 0.25 .EOh+ 0.75 .E, is sufficient, in complex cases it may be necessary to adopt the earth pressure with E, = 0.75.Eo, + 0.25.E,,. The magnitude of the at-rest earth pressure E,, is determined according to R 23 (Section 9.6), the magnitude of the active earth pressure E, according to R 28 (Section 9.3). 3. The following approaches apply for small distances to structures according to R 29: a) E, = 0.25.Eo,, b) E, = 0.50.E,,, c) E, = 0.75.Eo,, 96

+ 0.75 .E, + Eaplhin simple cases, + 0.50.Eah+ in normal cases, + 0.25.E,, + Eapthin complex cases.

The magnitude of the at-rest earth pressure E,, must be determined according to R 23 (Section 9.6), the magnitude of the active earth pressure E, from soil self-weight and, if necessary, cohesion, as well as the active earth pressure E,,; from building loads according to R 29 (Section 9.4). 4. In the case of a design earth pressure lying between the active earth pressure and the at-rest earth pressure, it can be assumed that earth pressure redistribution occurs in a similar manner to the active earth pressure but with a tendency to decrease, the higher the proportion of at-rest earth pressure to design earth pressure. The design earth pressure may therefore also be converted to a simple pressure diagram with the bending points or sudden load alterations in the region of the support points (Figure R 22-ld). Here, structures with small and large distances to the retaining wall are differentiated, as for active earth pressure design. R 28 (Section 9.3) and R 29 (Section 9.4) also apply accord-

a1 Excavation, structure and load distribution

dl Earth pressure distribution when prestressing all props

bl Active earth pressure from soil self- weight, live load and building load

el Earth pressure dlstribution when prestressing the two lower props

rl At-rest earth pressure from soil self- weight. live load and building load

Figure R 22-1. Distribution of active earth pressure taking a building load with large distance between the retaining wall and the structure into consideration (example for a soldier pile wall with free-earth support)

97

ingly for increased active earth pressure. If only the struts or anchors in the zone of influence of the building load are prestressed, the earth pressure in this area is assumed to be more concentrated (Figure R 22-le). This can be recommended in particular if neighbouring basement walls, subsurface pipes or other structures are endangered by prestressing of the struts in the upper region of the retaining wall. 5. Even if the design earth pressure is based on the increased active earth pressure, it must be demonstrated that ZH = 0 for soldier pile walls, according to R 15 (Section 5.6). The earth pressure acting below the excavation level must be adopted in the same ratio as the active earth pressure and at-rest earth pressure acting above the excavation level. If the building load also acts below the excavation level, this must be taken into consideration. 6. When analysing the embedment depth the passive earth pressure is adopted a) according to R 14 (Section 5.4) or R 19 (Section 6.4) in the case of a free earth support, b) according to R 25 (Section 5.5) or R 26 (Section 6.5) in the case of an earth restraint, but with the proviso that a minimum factor of safety of q, = 3.0 against reaching the limit state is adopted for soldier pile walls or of qp= 2.0 for sheet pile walls and in situ concrete walls in order to reduce toe deflection in cohesionless soils or at least firm,cohesive soils. If the ground is a soft, cohesive soil, these safety factors must be increased further if the anticipated movements endanger stability or the serviceability of the structure. Regardless of which safety factor is adopted for analysis of the embedment depth, the reduced safety factors q = 1.5 or qp= 1.2 stipulated in Recommendations R 14 (Section 6.5), R f9 (Section 6.4), R 25 (Section 5.5) and R 26 (Section 6.5) may be adopted for determination of the permissible forces, if medium-dense or dense, granular soil or at least firm, cohesive soil is present beneath the excavation level.

7. In a similar manner to the horizontal component, the vertical component of the design earth pressure consists of the vertical components of the active earth pressure and the at-rest earth pressure. It must be demonstrated that the vertical component of the design earth pressure can be transmitted to the ground by the wall with a minimum factor of safety of 2.0, and that the subsequent settlements have no negative effects on the structure. It may be necessary to forgo the adoption of a wall friction angle when determining the active earth pressure and to demonstrate that the structure does not suffer from unacceptable settlement despite its restricted load distribution to the ground. See also Figure R 22-2. If this cannot be demonstrated for soils susceptible to settlement, appropriate measures must be provided for, either on the structure or in the ground. 98

Figure R 22-2. Stress redistribution for restricted load distribution

8. Generally, it is not necessary to prestress the struts and anchors for the computed load at each new construction stage. It is normally sufficient to prestress the struts and anchors for the support forces projected for the fully excavated stage from the outset, including in the advancing states. However, it is possible that the row above the last installed row unloads somewhat when the current row is prestressed. Post-stressing for possibly greater support forces occurring during the retreating states can generally also be dispensed with. However, monitoring the movements of the structure and the retaining wall by taking measurements is recommended where sensitive structures are involved, as well as monitoring the stresses on the struts or anchors, and providing for post-stressing measures where necessary. 9. See R 30 (Section 9.7) for design of retaining walls on opposite sides of braced excavations.

9.6

Analysis of the retaining wall with at-rest earth pressure (R 23)

1. It is generally advisable to apply the earth pressure with E, = 0.75.E,, + 0.2S.E,, for complex cases in accordance with R 22, Paragraphs 2 and 3. Then, the at-rest earth pressure is a calculation value, but not a design value. The following details on determination of the at-rest earth pressure therefore primarily serve for determination of this calculation value. Only in rare cases is it possible to acquire the at-rest earth pressure of the undisturbed soil in the field when installing the retaining wall. 2. If it can be guaranteed that the at-rest earth pressure is maintained when installing the retaining wall and, in addition, relieve of the ground is avoided by the use of an inflexible support according to R 67, Paragraph S (Section 1.2), it may be expedient in exceptional cases, e.g. for structures that are very high, poorly founded or in poor structural condition, to apply the at-rest earth pressure to the retaining wall. However, this does not exclude the possibility of settlements affecting these structures. 99

3. The magnitude of the at-rest earth pressure from soil self-weight can, at least for cohesionless soil in the case of a horizontal ground surface, be determined approximately with an at-rest earth pressure coefficient from the relationship I& = 1 - sin $’. For ground surface inclined at an angle p = $’, the relationship is KO= cos $’. For p < $’ linear interpolation may be performed [40]. The resultant at-rest earth pressure is directed parallel to the surface. The at-rest earth pressure can be assumed as increasing linearly with depth (Figure R 23-la). If, in exceptional cases according to Paragraph 2, the ground beneath the excavation level is utilised to a large degree for wall support, the full at-rest earth pressure can no longer act in this region, due to the unavoidable displacement of the wall toe.

In such cases, therefore, the earth pressure ordinate may be assumed as being constant from the lowest row of supports downwards for retaining walls with at least two rows of struts or anchors (Figure R 23-lb). For single-propped walls, the bending point of the pressure distribution can be placed approximately in the lower third of the free height h,, between the support and the excavation level. An equal area rectangle may not be substituted for the determined pressure distribution, neither for the full earth pressure at-rest as shown in Figure R 23-la nor for the reduced earth pressure at-rest as shown in Figure R 23-lb. In this case, R 17 (Section 6.3) may not be applied. 4. The at-rest earth pressure from an unbounded distributed load can be approximately determined with e,, = G . p and be assumed to act horizontally, independent of ground surface inclination. It is superimposed on the pressure distribution described in Paragraph 2, with a uniform ordinate for the complete wall height.

ai Earth pressure dtstribution for an inflexible wall toe support

bi Earth pressure dtstribution for a flexible wall toe support

Figure R 23-1. Earth pressure distribution determination for in-situ concrete walls with application of at-rest earth pressure

100

a1 Excavation and structure

di flultple bends in a pressure diagram

bi At-rest earth pressure from soil self-weght and service load

e) Single bend in a pressure diagram

ci At-rest earth pressure from the building load

Figure R 23-2. At-rest earth pressure distribution for a sheet pile wall or in-situ concrete wall with free earth support and consideration of building load influence (example of an in-situ concrete wall with free earth support)

5. The at-rest earth pressure from vertical or horizontal building loads can generally be determined and adopted according to elastic halfspace theory (Figure R 23-2c). Generally, v = 4 applies for the concentration factor after FROHLICH; for preloaded soils and for cohesive soils v = 3. In the v = 4 case, the horizontal at-rest earth pressure E,,, may be assumed at approximately 25 %; in the v = 3 case at 30 % of the total vertical load P. The vertical component EOPvof the at-rest earth pressure is to be introduced in both cases at 50 % of the total vertical load P, if no precise determination has been performed; e.g. based on [41] or [46].The pressure diagram selected for at-rest earth pressure from building loads should begin approximately at the height of the foundation level and the resultant should be approximately at the height of intersection with the rear of the retaining wall of a line at 45" from the horizontal, originating at the load axis of the base of the structure. See Figure R 23-3 for examples.

6 . To simplify analysis, the pressure distributions resulting from superimposing the individual at-rest earth pressure components may be simplified such that, 101

for an unchanged total load magnitude, a pressure diagram is created that demonstrates no sudden changes, or for which a sudden change lies at a support point (Figure R 23-2d) and (Figure R 23-2e). Application of passive earth pressure is according to a) R 19 (Section 6.4) in the case of free earth support, b) R 26 (Section 6.5) in the case of an earth restraint, but only on the condition that for medium-dense to dense, cohesionless soil or at least firm, cohesive soil, a limit state safety factor T~= 3.0 is adhered to. If loose, cohesionless soils are present below the excavation level, the safety factor must be increased in consideration of the anticipated toe displacement of the wall, or a construction method must be selected that does not require an earth support. The earth support must always be ignored for calculation purposes for soft cohesive soils beneath excavation level. Independent of the safety factor used for embedment depth analysis, it is permissible to use the reduced safety factor q = 1.2 given in Recommendation R 26 (Section 6.5) for determination of tke permissible stresses, if medium-dense or dense, cohesionless soil is present beneath the excavation base.

ai Excavation. structure and load dstribution

dl Staggered rectangular earth pressure dagram

102

bl Trapezoidal earth pressure dlagram

el Continuous rectangular earth pressure diagram

cl Triangular earth pressure dagram

Figure R 23-3. At-rest earth pressure approximations for building loads and inflexible retaining walls

8. Because no construction measures are capable of guaranteeing that deformations or deflections of the retaining wall or the ground will occur, the actual earth pressure distribution may deviate from the at-rest earth pressure distribution assumed as shown in Figure R 23-1. Therefore, it must be ensured that the struts or anchors located in the upper part of the wall are, if necessary, capable of accepting the active earth pressure determined according to R 21 (Section 9.2). In the place of a precise analysis it is permissible to design the struts and anchors in the upper third of the wall for support forces that are 30 % greater than the support forces determined using the at-rest earth pressure only. 9. It must be demonstrated that the vertical component of the at-rest earth pressure from soil self-weight (for inclined ground surface) and the at-rest earth pressure from the building load can be transmitted by wall friction with a minimum factor of safety of 1.5 at every point of the retaining wall, and from here transmitted to the ground with a minimum factor of safety of 2.0, without appreciable settlement. If this cannot be demonstrated, preservation of the original stress state is not guaranteed and application of the at-rest earth pressure is not justified. In this case it is necessary to either ensure the stability of the structure by further measures, e.g. by soil stabilisation, or to design the retaining wall for increased active earth pressure according to R 22 (Section 9.5).

10.In order to ensure that the retaining wall makes no undesired movements either towards or away from the structure, it is generally useful to continuously monitor its position during excavation work. Struts and anchors are prestressed to the full computed load during installation and then post-stressed, if monitoring according to R 34, Paragraph 6 (Section 13.4) shows an appropriate result. 11. See R 30 (Section 9.7) for design of retaining walls on opposite sides of braced excavations.

9.7

Mutual influence of opposing retaining walls for excavations adjacent to structures (R 30)

1. If a horizontally braced excavation is only subjected to earth pressures from

structures on one side of the excavation, but is lined equally on both sides by soldier pile walls, sheet pile walls or in situ concrete walls, both walls can generally be designed according to the analysis for the retaining wall adjacent to the structure, if no more precise analysis is performed. A precondition for this approach, however, is that the earth pressure acting on the loaded side produces strut forces in all rows of struts that are greater than those from the earth pressure acting on the unloaded side. For example, if, for structures close to the retaining wall (Figure R 30-1), lower strut forces arise from the loaded side of the excavation, the same deliberations should be made as if there are structures on both sides. See also Paragraph 3 and Paragraph 4. 103

ai Earth pressure on the left of the excavation

bl Excavation, structure and load dlstribution

cl Earth pressure on the rlght of the excavation

Figure R 30-1. Excavation with horizontal bracing and one-side loading from a structure

2. If the retaining walls in a horizontally braced excavation are subject to earth pressure from structures on one side only are differently designed, the retaining wall further away from the structure can, as an approximation, be designed for the same internal forces as the wall adjacent to the structure, if this wall does not substantially differ from the retaining wall adjacent to the structure with regard to stiffness and embedment depth. If they do differ substantially, it may be necessary to separately investigate the wall away from the structure. The support forces of the retaining wall subjected to building loads must be applied as loads to the retaining wall further away from the structure. The pressure diagram for this structure must then be selected as appropriate for the loads, stiffness conditions and earth pressure theory. 3. Each retaining wall must be investigated separately for horizontally braced retaining walls subject to building loads on both sides of the excavation (Figure R 30-2).

a1 Earth pressure on the left of the excavation

bl Excavation, structure and load distribution

cl Earth pressure on the rlght of the excavation

Figure R 30-2. Excavation with horizontal bracing and bilateral loading from a structure

104

If this procedure results in different pressure diagrams on each side of the excavation, the respectively larger load ordinates from each wall must be adopted for the opposing wall in the case of similar stiffness conditions and both walls, with the exception of the zone below the excavation level, designed for the same resultant pressure diagram (Figures R 30-3 and R 30-4). If the stiffness conditions are grossly dissimilar for the two retaining walls, the pressure diagrams must be respectively developed so that roughly similar support forces result. If one side of the excavation is fitted with a soldier pile wall and the other side with a sheet pile wall or in-situ concrete wall, the earth pressure adopted below the excavation level is according to R 15 (Section 5.6) for soldier pile walls and R 16 (Section 6.1) for in-situ concrete wall. See also Figures R 30-3c and R 30-4c. 4. Because of the effects of mutual influence, a larger earth pressure must be assumed for both sides according to Paragraph 3 than would be the case when determining the earth pressure for each side alone, if the earth pressure distribution is different for each side of the excavation, e.g. for one of the cases shown in Figures R 30- 1 and R 30-2. If this needs to be avoided, e.g. because it would represent a hazard to the stability of the basement wall for the case shown in Figure R 30-1, equal strut forces can be achieved with an appropriate Resultant envelope

tl-

dagrams

Simplihed a1 Orlginal pressure diagram for the left side

bl Superimposed rlght pressure dagram

cl Resultant pressure diagram

Figure R 30-3. Superimposing pressure diagrams on the left of the excavation

Resultant envelope

a1 Ortginal pressure dagram for the rlght side

bl Superimposed left pressure dagram

cl Resultant pressure dagram

Figure R 30-4. Superimposing pressure diagrams on the nght of the excavation

105

a1 Earth pressure on the left of the excavation

bl Excavation, structures and load dstribution

cl Earth pressure on the right of thhe excavation

Figure R 30-5. Excavation with inclined bracing and bilateral loading from a structure

configuration of the individual rows of struts, despite differing pressure diagrams on each side of the excavation, if the earth pressure magnitude is equal (Figure R 30-5). If this is not the case, and whenever the configuration of the rows of struts cannot be arranged according to these factors, the difference must be adopted as a surcharge on the wall with the smallest computed support force. The additional earth pressure mobilised by doing this must be selected according to the stiffness conditions of the wall. Otherwise, equilibrium of vertical forces according to R 9 (Section 4.1) must be demonstrated at all times for inclined struts.

106

10

Excavations in water

10.1 General remarks on excavations in water (R 58) 1. With regard to the varying modes of water associated with excavations, the following cases may be differentiated in principle: a) open water, e.g. lake, river b) free (phreatic) groundwater, c) perched groundwater.

2. With regard to the impact of the excavation structure and the water drawdown measures, the following cases may be differentiated in principle: a) If drawdown is performed as shown in Figure R 58-la, both horizontal and downward - directed seepage pressures occur in the soil mass critical to the excavation structure. In this context, R 59 (Section 10.2) must be observed when determining the seepage pressure and R 60 (Section 10.3) when analysing the stability of the excavation structure. b) Where percolation around the wall toe occurs as shown in Figure R 58-lb, upwardly directed seepage pressures also ensue. In this context, R 59 must be observed when determining the seepage pressure, R 61 (Section 10.4) when analysing the heave safety of the excavation level and R 63 (Section 10.6) when analysing the stability of the excavation structure. c) If a practically impervious layer of soil is present below the excavation level, e.g. where a sealing layer of grout is employed as shown in Figure R 58-lc, the flow of water is prevented and a hydrostatic pressure develops. In this context, R 62 (Section 10.5) must be observed when analysing the buoyancy safety of the excavation level and R 63 when analysing the stability of the excavation structure.

a1 lowered groundwater

bl Percolation around the wall toe

cl Now-hindered water

Figure R 58-1. Effects of water on the excavation structure

107

Furthermore, if the water is not pumped out of the excavation in situations b) and c), the water level inside and outside of the excavation may also be the same in some cases, It may even be expedient in special cases to keep the water level higher on the inside than on the outside, at least for a certain period of time. 3. Where retaining walls in cohesionless soils and soft, clayey soils are involved it may be assumed that the intimate contact between the retaining wall and the ground and thus the flow net are also retained if small displacements or deformations occur as a result of water pressure. However, if the ground behind the retaining wall does not possess sufficient lateral deformability, e.g. a rock-like ground or a hard or nearly hard, cohesive soil with low clay content, which is at least temporarily stable without support, the formation of a gap between the retaining wall and the ground is possible, in which the hydrostatic pressure can develop. 4. In loosely compacted sand and silt in particular there is a danger of erosion failure, which begins with increased local flow at the excavation level, progresses by flushing out soil particles in a tube-like formation (piping) and subsequently leads to a sudden inrush of water if a heavily water-bearing layer or open water is met. See Recommendation R 116 of the Committee for Waterfront Structures [ 2 ] for more details. Piping failure is difficult to assess numerically and can only be avoided by constructive measures. See also R 64 (Section 10.7).

5. A critical shortening of the flow path, presenting a hazard to the retaining wall, can occur if permeable zones arise between the individual elements when manufacturing the retaining wall and are not noticed in due time. A similar phenomenon can develop if water-bearing voids reaching deep into the ground in low-permeability, slightly cohesive soil occur, e.g. poorly backfilled boreholes or other voids caused by pulling out piles. In this case the water finds its way under high pressure, again in a tube-like formation similar to piping failure, to the excavation level. 6. The highest water level to which the excavation should remain stable must be defined. Appropriate safety measures against higher water levels must be provided for, e.g. controlled flooding according to R 64, Paragraph 7 (Section 10.7).

10.2 Seepage pressure (R 59) 1. Seepage pressure develops if a potential difference is present as shown in

Figure R 58-la or Figure R 58-lb, which induces the groundwater to flow. The seepage pressure is a mass force, which is transferred from the water to the soil skeleton due to the flow resistance in the water flow direction. In the special case of vertical flow this has the effect of altering the unit weight of 108

the percolated soil by the value of Ay’ = icy,. If the flow is directed from top to bottom, the unit weight increases, if it flows from bottom to top, the unit weight is reduced. 2. The seepage pressure at any point in the subsurface is determined from the flow net. The flow net is determined according to Recommendation R 113 of the Recommendations of the Committee for Waterfront Structures [2] a) by graphical methods based on the trial and error method, b) by model tests on an electric analogous model or c) by mathematical investigations with the help of potential theory. The excavation width must also be taken into consideration. If the seepage pressure at individual, specific points only is required, e.g. at the toe of a retaining wall, graphs and tables or simple numerical approaches may be utilised for uniformly permeable ground [26,56, 57, 581. 3. The seepage pressure in homogeneous soils is determined independent of the value of the coefficient of permeability. Not the amount of water flowing is critical, but the potential energy of the water as a result of height differentials between the inner and outer water level.

4. The following applies with regard to the permeability of the ground: a) Because a pressure drop is always concentrated in the less permeable layers, alternating vertical permeability of the ground must always be taken into consideration when determining the seepage pressure. See also R 61, Paragraph 5 (Section 10.4). b) The difference between the permeabilities in the horizontal and vertical directions need generally only be taken into consideration if the horizontal component of the total length of the critical stream tube is longer than the vertical component. 5 . For computation of the impact of groundwater flow on the positive water pres-

sure, active earth pressure and passive earth pressure see Recommendation R 114 of the Recommendations of the Committee for Waterfront Structures [2],

10.3 Dewatered excavations (R 60) 1. If the groundwater is lowered as shown in Figure R 58-la in order to dewater an excavation, investigations to determine the impact of seepage pressure on the stability of the excavation structure must be performed. If necessary, the seepage pressure must be taken into consideration for stability analysis. 2. In permeable soils the water surface profile is generally so flat that the groundwater exerts no impact on earth pressure. In fine-grained soils, however, the drawdown curve may be so steep that it intersects the decisive slip surface and influences the magnitude of the active earth pressure (Figure R 60-la).

109

ai Flow in the active earth wedge

bi Flow in the anchorage zone

Figure R 60-1. Flow resulting from groundwater drawdown

3. Complete groundwater drawdown is often impossible in stratified soil. The following effects ensue from the remaining water, as shown in Figure R 60-2a: a) Additional water pressures develop in the region of the retained water. b) It lowers the unit weight of the soil from y to y’ in the region of the retained water in the permeable layer. c) A gradient i = (h + d):d develops in the impermeable layer. The unit weight of the soil thus increases from yr = y’ + y, to ’I,= y’ + i.y, The effects on water pressure are shown in Figure R 60-2b, those on earth pressure corresponding to classical earth pressure theory in Figure R 60-2c.

4. When determining the passive earth pressure it must generally be assumed that the water level inside the excavation can be at the excavation level and the soil is therefore fully buoyant. The effects of groundwater drawdown and thus the adopted wet unit weight may only be taken into consideration if measures are taken against possible pump failure as specified in R 66, Paragraph 1 (Section 10.9), and then only if the anticipated drawdown curve justifies this. If a cohesive layer below the excavation level is subject to artesian pressure from below despite water management measures, the unit weight reduction

ai Section through the excavation

bl Water pressure surcharge

cl Earth pressure surcharge

Figure R 60-2. Impact of retained water in stratified ground

110

due to seepage pressure must be taken into consideration and the safety against base heave demonstrated according to R 61 (Section 10.4) or R 62 (Section 10.5).

5. If the drawdown curve intersects the soil region decisive for stability, as shown in Figure R 60-lb, the impact of seepage pressure must be taken into consideration for both the stability analysis at low failure plane according to R 44 and the global stability analysis of embankment failure according to R 45 (Section 7.4). 6. The effective unit weight of a saturated, cohesive soil is increased from y’ to yr by lowering the groundwater table or by groundwater relief. This has the same effect as applying a load at ground level and may cause considerable settlement of soft, cohesive soils, which may also be detrimental to more distant buildings. If necessary, dewatering measures must be dispensed with and different construction methods applied. See also R 62.

10.4 Analysis of hydraulic heave safety (R 61) 1. In permeable soils, the base of the excavation may fail by hydraulic heave if only sump pumping is utilised inside the excavation and no further measures are taken (see Paragraph 8). Hydraulic base failure occurs when cohesionless soils in front of the toe of a retaining wall become weightless as shown in Figure R 61-1 due to upward directed seepage pressure, or when the upward directed seepage pressure in cohesive soils is equal to the sum of the soil selfweight and additional restraining forces, e.g. ensuing from cohesion. 2. The seepage pressures acting in the area of the investigated failure mass must be determined according to R 59, Paragraph 2 (Section 10.2). An increase in the upward directed seepage pressure must be anticipated if the preconditions for three-dimensional effects are given, e.g. in round or rectangular excavations [56,57,59]. If the hydraulic heave safety needs to be equal at all points in a rectangular excavation, the retaining walls must be embedded deeper at the comers and possibly at the ends than at the centres of the longer sides.

Figure R 61-1. Restriction of flow cross-section in the region of an upward directed flow in narrow excavations

111

3. If necessary, the possibility of seepage path shortening, e.g. by fissure formation according to R 58, Paragraph 3 (Section 10.l ) must be taken into consideration. For staggered wall toes, the decisive depth for analysis of the hydraulic heave safety or is always the lesser embedment depth.

4. Hydraulic heave safety analysis for homogeneous soils is generally performed by means of the expression W’

q. = -

‘

s

(W’ = buoyant weight, S = flow force) after T E R Z A G H ~ E[60] C Kas shown in Figure R 61-2 for a rectangular soil mass, the width of which is half of the embedment depth. The simpler and more conservative stability analysis is performed by using an infinitely small strip after DAVIDENKOFF [61]. A more precise approach is the investigation of a failure body with a curved boundary line according to Recommendation R 115 of the Committee for Waterfront Structures [2]. Friction forces at the failure body and cohesion of cohesive soils or tensile strength may only be taken into consideration for special investigations.

5. If the ground is subject to variable permeability, the pressure drop is concentrated in the less permeable layers. With regard to the hydraulic heave safety, these layers have an unfavourable effect where they are present below the excavation level (Figure R 61-3a). In this case, only the seepage path through the less permeable layer may be adopted for the analysis. If the less permeable layer is located above the permeable layer as shown in Figure R 61-3b, the associated favourable effect may only be considered under certain conditions, because even slight disturbances in the subsurface structure can adversely effect the hydraulic heave safety at individual locations. The filter stability of the permeable layer must also be analysed [70]. 6. Excavations in groundwater exhibit less vulnerability to hydraulic heave than

excavations in open water if a drawdown curve develops and the positive

---_ Figure R 61-2. Analysis of hydraulic base failure safety after T E R Z A G H ~ ~ E C K

112

. . . . . . .

.

ai More permeable layer above

bi More permeable layer below

Figure R 61-3. Impact of ground stratification

water pressure therefore decreases in the region of the excavation. However, the short-term drawdown curve produced during the respective excavation phase is critical for hydraulic heave safety analysis. Generally, for lowpermeability soils, in particular for silt and fine sand, the non-lowered groundwater table is taken as the basis for analysis.

7. The required hydraulic heave safety factor is dependent on the properties of the soils present. The following applies: a) qi = 1.5 for gravel, gravely sand and at least medium-dense sand with grain sizes larger than 0.2 mm (medium sand and coarse sand), b) T~= 2.0 for loosely compacted sand and fine sand, silt and soft, cohesive soil, c) q, = 1.5 for at least firm, clayey, cohesive soil. The safety factor increase for loosely compacted sand and for fine sand and silt is justified by the fact that these soils demonstrate a tendency for hydraulic heave failure. However, even in these cases a safety factor of qi = 1.5 is sufficient if the surcharge W’ principally ensues from a loaded filter, the filter stability of which has been demonstrated. For analysis of abnormal loading conditions according to R 24, Paragraph 3 (Section 2.1), a safety factor of vi = 1.3 is sufficient for cases a) and c) or vi = 1.5 in case b). 8. For excavations and trenches up to 5 m deep in homogeneous, groundwaterbearing soil, analysis of the hydraulic heave safety can be dispensed with if the following conditions are adhered to with the designations in Figure R 614:

a) where B 2 2.h: b) where B = h: c) where B = 0.5.h:

t 2 0.4.h t 2 0.5.h t 10.7.h

Intermediate values may be linearly interpolated. 113

Figure R 61-4. Simplified analysis of hydraulic base failure safety

9. If investigations do not demonstrate sufficient hydraulic base failure safety, the following measures may be taken in addition to enlarging the embedment depth: a) installation of spill wells within the excavation, b) partial or complete dewatering or soilwater relief, c) installation of a surcharge filter. Furthermore, an impermeable layer in the subsurface may be manufactured or an impermeable excavation base of underwater concrete as specified in R 62 (Section 10.5), or an excavation cover utilising compressed air.

10.5 Analysis of buoyancy safety (R 62) 1. If the retaining walls form a closed body with an impervious layer at the excavation level or lower, sufficient buoyancy safety must be demonstrated. This is principally the case in the following circumstances: a) The excavation possesses a concrete base (Figure R 62-la). b) There is a sufficiently thick, impervious sealing layer at the toe of the retaining wall, manufactured by grouting (Figure R 62- lb). c) The retaining walls are so deep that they embed in a practically impervious layer (Figure R 62-lc). d) There is a sufficiently thick, practically impervious soil layer below the excavation level (Figure R 62- Id). A soil layer is viewed as practically impervious if it has a permeability at least two orders of magnitude less than the permeability of the surrounding ground.

114

ai Concrete base

bi Sealing layer

ci Embedment in a practically imper vious layer

di Practically impervious layer below the excavation level

Figure R 62-1. Forces adopted for analysis of buoyancy safety

2. Buoyancy safety analysis is to be performed with the

approach and the

approach

115

Where: A W

Zult WE R qw

qz q, qR

buoyant force self-weight of the excavation structure, the practically impervious layer and any soil above this sum of limit loads of tension piles or grouted anchors self-weight of the anchored soil mass sum of effective friction forces self-weight safety factor tension force safety factor anchored soil mass safety factor friction force safety factor

See also Figure R 62- 1. 3. The full hydrostatic pressure y'., h, acting on the practically impervious layer or the concrete base, derived from the design water level according to R 58, Paragraph 6 (Section 10.l), must be applied for determination of the buoyant force A. For natural soil and sealing layers, in the analysis the decisive lower edge must be assumed at a height that allows conservative consideration of any possible unevenness.

4. The following applies for determination of the self-weight W: a) If the actual unit weight is not verified by soil investigation or by soil sampling, only the reduced values specified in Appendix A4 may be adopted for natural soil and sealing layers. b) If the actual unit weight is not verified by soil sampling, the unit weight of the concrete may be assumed at a maximum of 23 kN/m3,that of reinforced concrete at a maximum of 24 kN/m3. c) The water level within the excavation must be estimated conservatively, taking into consideration any drawdown due to water management measures. d) The self-weight of the retaining walls may only be taken into consideration if appropriate force transmission to a sealing layer or to the concrete base is ensured.

5. The limit load Zultof tension piles and grouted anchors must be taken from the relevant regulations. If, in exceptional cases, the loading capacity of prestressed and non-prestressed tension piles or grouted anchors must be utilised, their load-displacement behaviour must be investigated. 6. The following applies to determination of the self-weight WE: a) The self-weight WE of the anchored soil mass may be assumed equal to the limit load for sufficiently large spaced tension piles or grouted anchors. b) The overlap of the individual soil masses must be taken into consideration for smaller spacing of tension piles or grouted anchors. In such a case, the 116

external boundary of the complete soil mass must be assumed to lie inside the retaining wall. If the actual unit weight of the soil is not verified by soil investigation or by soil sampling, only the reduced values given in Appendix A 4 may be adopted. The conservative vertical component of the effective earth pressure force or a corresponding skin friction may be adopted as the friction force R, a) if sufficient force transmission in the concrete base, or in the practically impervious layer, is ensured and b) if the concrete base, or the soil mass between the retaining walls, is capable of accepting the ensuing loads.

8. The safety factors qG,qCEand q,must be applied as follows:

Load case

1.05

HZ Abnormal loads ~

_

_

_

_

1.05 _

_

~

~

1.35 1.20

1.30

~

See R 24, Paragraphs 1 to 3 (Section 2.1) for load case definitions.

9. In addition to the buoyancy safety analysis, analysis of the hydraulic heave failure safety according to R 61 (Section 10.4) must be performed, a) if the retaining walls are only shallowly embedded in the practically impervious layer, b) if the retaining walls embed in a layer with a permeability less than two orders of magnitude smaller than the surrounding soil.

10. If the practically impermeable layer is fine-grained and the layer above coarsegrained, the filter stability must be analysed [70].

10.6 Stability analysis of retaining walls in water (R 63) 1. If the groundwater is not lowered, but percolation around the wall toe is prevented, the full hydrostatic water pressure from the open water surface or groundwater level to the wall toe on the outside, or the hydrostatic water pressure from the lowered groundwater level to the wall toe on the inside, must be adopted (Figure R 63-lb). This approach may generally also be selected as an approximation if actually seepage is taking place around the wall toe. 117

ai Destgnations

bi Water pressure

d Active earth pressure

dl Total ioad

and passive earth pressure Figure R 63-1. Load model determination for a non-percolated retaining wall in water (simplified representation)

If water does percolate around the wall toe, the flow impact must be considered as follows for more precise investigations: a) The water pressure on the outside of the retaining wall decreases, on the inside it increases (Figure R 63-2b). b) The earth pressure on the outside of the retaining wall increases as a result of the increase in unit weight due to seepage pressure (Figure R 63-2c); however, this impact may generally be disregarded. c) The passive earth pressure on the inside decreases considerably due to the decrease in unit weight; this impact must always be taken into consideration. See also R 59 (Section 10.2). Earth pressure redistribution as specified in R 5 must also be anticipated if the soil is completely or partially buoyant. The increase of earth pressure enforced by seepage is not encluded yet. If the water pressure on the retaining wall is greater than the earth pressure on the wall, the anticipated earth pressure redistribution may be disregarded for determination of internal forces and, where necessary, replaced by appropriate surcharges to the determined support forces (Figures R 63-lc and R 63-2c). When adopting the passive earth pressure, R 19 (Section 6.4) applies without restrictions for selection of the safety factor qpand the location of the resultant.

118

.....

.: . . . . . . . .

a) Deslgnations

****

-c----

(y'-A y '1.

bi Water pressure

-$t ,

(y'+A y 7 .Koh .Id+t )

rl Active earth pressure

di Total load

and passive earth pressure Figure R 63-2.Load model determination for a percolated retaining wall in water (simplified representation)

5. In order to avoid wall displacements if the external water level subsequently increases, anchors may generally be prestressed at a minimum of 80 % of the calculated load of the respective excavation phase and at a maximum of 100 % of the design load. 6. For excavations in open water, the following must also be applied as surcharge loads according to R 24, Paragraph 2 (Section 2.1), or as abnormal loads according to R 24, Paragraph 3, besides water pressure and earth pressure where they can occur and are not excluded by measures as specified in R 64, Paragraph 8 (Section 10.7): a) b) c) d)

wave action, berthing forces of ships, ice floe impact forces (see [62]), sheet ice pressure (see [631).

Informations on adopting wave action are contained in Recommendation R 135 of the Recommendations of the Committee for Waterfront Structures [2], information on adopting berthing forces is contained in Recommendations R 12 and R 38, information on the adopting ice loads can be found in Recommendation R 177. 7. For stability analysis of cellular and box cofferdams, see Recommendations R 100 and R 101 of the Recommendations of the Committee for Waterfront Structures [2]; for elastic dolphin piles see recommendation R 69. 119

10.7 Design and construction of excavations in water (R 64) 1. Only retaining walls providing sufficient imperviousness may be used for excavations in open water or in groundwater, e.g. sheet pile walls, diaphragm walls and bored pile walls. If the normal sealing properties of sheet pile interlocks are not sufficient, they can be improved in open water by tipping fly ash, sawdust or similar materials, and by driving in wooden wedges below the bed of the water body. If it is expected that sheet piles will run out of the interlocks to a large extent because of obstructions in the soil, it is useful to carry out soil replacement in the driving region before driving the piles. If such areas or open joints in diaphragm walls or pile walls are noticed during excavation, securing measures must be immediately initiated, e.g. installation of a second wall or manufacture of a grout curtain. 2. The retaining walls must reach the design depth at all points. If individual sheet piles, diaphragm wall slices or piles cannot be installed to the projected design depth, additional measures must be provided for or additional analyses performed to ensure safety against hydraulic heave failure.

3. For the analysis of buoyancy safety according to R 62 (Section 10.5) of an excavation with walls embedded in a practically impermeable layer, the walls must form a watertight unit with this layer. It can generally be assumed that a watertight connection is given if a retaining wall is embedded at least 0.5 m in firm to semi-solid, cohesive soil or in rock, if this is not heavily jointed. If necessary, the watertight seal must be subsequently manufactured, e.g. by grouting the region around the wall toe. Grouting should preferably be carried out before lowering groundwater in the excavation, but at the latest before flow disturbances occur. 4. A practically impermeable layer according to R 62, Paragraph l b is generally at least 1.0 m thick. The grouting tubes must be spaced so tightly that grout overlapping is guaranteed. The erosion safety of the grouting medium must be demonstrated. If initial piping phenomena are observed during excavation, countermeasures must be implemented immediately, e.g. a) soil placement, b) partial flooding of the excavation, c) water pressure relief measures. The weak area can then be regrouted. The measures given here may be dispensed with if rapid-curing chemical substances are used for grouting.

5. In excavations that are to be provided with a concrete floor, the water level within the retaining walls may be initially lowered as far as hydraulic heave safety considerations and the strength of the excavation allow. During installation of the concrete and the time of hardening the water level within the excavation may not be lower than outside. The relevant regulations apply for installation of impermeable and underwater concrete. If the friction at the

120

Figure R 64-1.Securing an excavation with cofferdams

retaining wall needs to be taken into consideration for analysis of the safety against buoyancy, complete force transmission between the base and the wall must be guaranteed, e.g. by grooves in in-situ concrete walls or welded steel pieces on sheet pile walls. Any projected tension piles must be sufficiently embedded in the concrete base.

6. As security against sudden ingress of water through defects in the retaining wall and against the possibility of fissure development behind the retaining wall mentioned in R 58, Paragraph 3 (Section l O . l ) , the configuration of a cofferdam as shown in Figure R 64- 1 has proven useful. As a minimum measure, securing of the bed with sandbags along the retaining wall should be planned.

7. In cases where a hazardous situation cannot be sorted out by other means, measures for purposefully flooding of the excavation must be taken, in particular for excavations in open water. Occasionally it may be expedient for economical reasons to flood an excavation instead of designing for exceptionally high, rarely occurring water levels. In both cases, it must be ensured that the inflowing water cannot cause damage. Furthermore, elongated excavations must be provided with intermediate walls in order to restrict sudden water ingress to limited sections of the excavation. It must be demonstrated, that the embedment depth of the intermediate walls and the immediately adjacent side walls is sufficient. 8. If surcharges and abnormal load conditions according to R 3, Paragraph 6 (Section 10.6)need to be avoided, the following measures may prove useful:

a) Configuration of dolphin piles for accepting berthing shocks of ships or placing of a sandbank to keep ships at a distance, b) continuous icebreaking along the retaining wall, c) configuration of dolphin piles and floating beams to deflect ice floes and similar objects, d) protection of the bed against scour formation according to Recommendation R 83 of the Recommendations of the Committee for Waterfront Structures [ 2 ] .

121

10.8 Water management (R 65) 1. The following principle methods of water management can be considered: a) b) c) d)

avoiding ingress, sump pumping, dewatering (gravity or vacuum drainage), groundwater relief.

The filter criteria must be observed here.

2. Ingress of groundwater into the excavation can be prevented by a) watertight retaining walls embedded in an impermeable formation, see R 62 (Section 10.5), b) installing a sealing curtain outside of the excavation, which is embedded in an impermeable formation, see Recommendation R 56 of the Recommendations of the Committee for waterfront Structures [2]. These measures can be expedient if groundwater drawdown is not permitted or would lead to settlement damage in the surroundings or there is no possibility of disposing of the pumped water economically.

3. Sump pumping involves the water entering from the sides and the bottom of the excavation being collected in drains, sent to pump sumps and pumped away. Sump pumping is suitable for small drawdown depths and limited water ingress. In soils with a tendency to boil, special measures are necessary, e.g. soil replacement methods, whereby only small areas are laid free for short periods and immediately covered by filter material. 4. For dewatering the water is collected in wells, which may be arranged inside or outside of the excavation, and pumped away. In principle, two types are differentiated: a) Gravity wells are used if the water flows into the wells as a result of gravity, e.g. in sand and gravel. b) Use of vacuum wells is necessary if the gravity is not sufficient to allow the water to flow into the filter well, e.g. in fine-sand and silt. See also [I] and [64]. The lowered groundwater table should generally be around 0.50 m below the excavation level.

5. Groundwater relief may be necessary, a) if a cohesive layer below the excavation level is not capable of bearing positive water pressure acting from below, see R 62 (Section 10.5); b) if the safety against hydraulic heave according to R 61 (Section 10.4) cannot be demonstrated in any other way.

122

In these cases it may be sufficient to arrange overflow wells with adequately small spacing within the excavation, where the groundwater can rise as far as the excavation level and then be pumped away.

6. See [65]and [66]for groundwater reclamation by means of injection wells.

10.9 Monitoring excavations in water (R 66) 1. The following facilities must be provided if the stability of the excavation is endangered or heavy economical losses can be anticipated if the water management facilities fail at short notice: a) two independent power sources, e.g. from the public utility network and emergency generators, b) switching facility for the well power supply, c) automatic switching if one pump fails, d) optical or acoustic signals, e) display equipment for evaluation of pump performance. Facilities b) to e) are generally integrated into one switching and control centre. This control centre must be constantly monitored, be equipped with a reliable warning system and have a sufficient supply spare parts available. If short-term faults or interruptions do not pose a hazard, less complex facilities for power supply, switching and monitoring will suffice.

2. All influences important for the assessment of water management facilities should be regularly observed and monitored, e.g.: a) the water level of open water bodies, b) the drawdown achieved within the retaining wall and in the surroundings, c) the amount of water pumped. Where there is a danger of violating water rights, the range of the drawdown must also be controlled. The same applies if there is a danger of settlement occurring. In this case, settlement measurements on buildings and the surrounding areas should also be provided for.

3. During excavation, it may prove useful to continuously measure the water level in the ground below the excavation level, or the porewater pressure in low-permeability grounds, in order to facilitate timely recognition of irregularities. If piping becomes apparent at any stage the soil must be immediately refilled to prevent subsurface backward erosion.

4. If local conditions cannot exclude the possibility of fissure formation behind the retaining wall according to R 58, Paragraph 3 (Section l O . l ) , it is useful to tap the retaining wall in the endangered area and to install transparent hose to display the local water pressure. Areas of the wall displaying large deformations are particularly threatened. 123

Excavations in unstable rock

11

11.1 General Recommendations for excavations in unstable rock (R38) 1. Rock is a consolidated mass of mineral material formed in-situ and consisting of similar or dissimilar individual components. Stability is demonstrated by means of rock-mechanical investigations based on rigid body mechanisms. If these indicate that a cutting is instable, supports are needed either a) by means of securing individual rock masses in danger of slipping by targeted or distributed installation of rock nails or rock anchors, or b) by means of a distributed, supported lining, in particular if heavily fractured or decomposed rock indicates that further fracture mechanisms may act in addition to the kinematics predetermined by the principal joint and bedding structures. The force with which a rock cutting must be supported will be subsequently known as the rock support force.

2. Although a wall displacement is necessary to allow the at-rest earth pressure to fall to the active earth pressure level when determining the active earth pressure according to R 8, Paragraph 4 (Section 3.1), when determining the rock support pressure it must be assumed that deformations must be prevented as far as possible in order to retain the initial strength or the strength of the untouched rock. If displacements are allowed, the initial strength can be exceeded and the residual shear strength becomes the decisive value with the consequence of a possible increase in the rock pressure. The excavation lining and its supports must therefore be designed to prevent displacement. All support components must be installed immediately after cutting the rock and connected tightly to the exposed face. All struts and anchors must be prestressed to their complete computed loads immediately after installation. The rock support force to be transmitted to the rock due to prestressing must generally be large enough so that the safety against slipping of the critical rock mass on the corresponding slip surface is at least q = 1.3. Here, the restraining and exciting forces acting parallel to the slip surt%ceare decisive. A smaller safety factor is permissible in this configuration if sufficient stability can be demonstrated on the soils of other boundary condition considerations. See also [77]. 3. In order to realistically estimate the rock mass properties for planning and construction of the excavation the following must be investigated in exposures:

a) extraction particulars, for example extraction by excavating, scraping, ripping, boring, blasting, 124

b) the mineralogical composition and geological development of the rock, for example magmatic, metamorphic, sedimentary rocks, c) the degree of weathering, for example sound, partially weathered, disintegrated, decomposed, d) nature, extent and spatial arrangement of jointing, e) the nature of the joint walls or joint infill, and f) any faults and be continuously examined during excavation, if possible in advance, e.g. by trenching. See also [76].

4. Regardless of the supports and the retaining wall lining, the magnitude and distribution of rock support force is primarily dependent on: a) the spatial distribution of the joints, b) the extent of jointing in the rock, c) the size, unevenness, waviness or roughness of the joints, d) the shear strength of the joints and/or bedding and/or the joint infill, e) the degree of weathering. 5. For determination of the shear resistance in possible slip surfaces, large-scale testing according to the procedures laid out in Recommendation No. 4 of Working Group 19,“Testing Procedures in Rock” (“Versuchstechnikim Fels”), of the DGEG [78] (German Society for Geotechnical and Foundation Engineering), in order to correctly assess jointing irregularities. If this is not possible, the rock shear strength in the unripped state and the residual shear strength of the ripped rock, as well as the shear strength of the bedding or joint infill should be determined as a minimum. If the extracted amount of soil is not sufficient for this purpose, at least the grain-size composition of the bedding or joint infill should be determined. 6. The properties of the undisturbed rock can be altered by external influences. For example, a) vibrations from blasting, b) disintegration or swelling phenomena caused by access of air or water or by releasing movements of the rock, c) alterations in porewater pressure in the joint infill and associated plastic flow caused by pressure redistribution can all influence the magnitude and distribution of the rock support pressure. The notes in R 4, Paragraph 5 (Section 3.2) must also be observed. 7. Disposal of the water from layers and joints in completely lined excavations must be provided for. Otherwise, both the rock support pressure and the water pressure must be taken into consideration. Generally, the whole water pressure must be applied to the complete wall surface. 8. The length of rock nails or rock anchors depends on the rock in which the grouted sections are embedded: 125

a) If the complete wall is installed on a rock face, it is sufficient if the grouted sections are located behind the decisive slip surface. b) If the grouted sections are in soil or in completely weathered, decomposed rock, the anchor length follows from the analysis of low failure plane stability according to R 44 (Paragraph 7.3).

11.2 Magnitude of rock support pressure (R 39) 1. Generally, determination of the rock support pressure is based on the existing joints. Three slip surface types must be differentiated:

a) slip surfaces in bedding planes (Figure R 39-la), b) slip surfaces in and parallel to existing joints (Figure R 39-lb), c) stepped slip surfaces in bedding planes and joints (Figures R 39-2a and 2b). For small distance of the bedding planes and high degree of fissures, and consequently small rock blocks compared to the size of the sliding body, it can be necessary to determine earth pressure as for soil. 2. For continuous slip surfaces, which run in a bedding plane as shown in Figure R 39-la, the residual shear strength of the jointed rock in the slip surface is decisive and the residual shear strength of the weaker layer if varying rock types are present. These may be only a few millimetres thick and be decomposed to soil, and may act as a slip surface between the stronger rock strata. This also applies to stepped slip surfaces if the sliding occurs in the intersecting planes as shown in Figure R 39-2a. 3. The following possibilities must be differentiated for slip surfaces parallel to jointing as shown in Figure R 39-lb: Bedding plane \

ai Slip surface in a bedding plane

Jomt plane \

bi Slip surface in and parallel to joint planes

Figure R 39-1. Continuous slip surfaces in excavations in unstable rock

126

a) If an appropriate retaining wall lining and the supports guarantee that no movements occur in joint and bedding planes in all construction stages, and that therefore the material interfaces are not severed, the rock shear strength in the material interfaces may be adopted as the decisive measure for determination of the rock support pressure. b) If these conditions are not met, it must be assumed that the material interfaces are severed as a result of unavoidable movements. In their respective proportions, the shear strength of the joint infill in existing joints and the residual shear strength of the material interfaces after severing are then decisive for determination of the rock support pressure. The shear strength of the joint infill alone is decisive for high fracture degree. c) In both cases, it must be proved that the rock support force from a stepped slip surface as shown in Figure R 39-2b can be accepted by the bracing. The shear strength of the joint infill is decisive for determination. If there is no joint infill in cases b and c, but a high degree of fractures, analysis may be based on the residual shear strength of the severed rock. 4. The shear strength of the rock and the bedding or joint infill is generally determined according to R 38, Paragraph 5 (Section 11.1).If the appropriate investigations have not been carried out, the friction angle of the infill may be estimated as follows as a function of the grain size distribution: a) 4' = 30" for sandy material, b) 4' = 20" for silty material, c) @' = 10" for clayey material, Generally, cohesion values are not adopted. It may be necessary to adopt the shear strength at 4, = 0 for slip surfaces subject to porewater pressure. The undrained shear strength c, may only be adopted for these slip surfaces on the basis of special investigations. Beddlng plane

ai S l i d q movement in beddlng planes

Joint, plane

bi Sliding movement in andparallel tojoint planes

Figure R 39-2. Stepped slip surfaces in excavations in unstable rock

127

5. If the dip is not perpendicular or the strike is not parallel to the retaining wall as viewed in plan, the same analysis assumptions used for determination of the earth pressure from soil self-weight and adoption of a forced surface as shown in Figure R 6-lc can be applied (Section 2.3). A wall friction angle may only be adopted if complete transmission of the vertical forces into the ground is guaranteed. See also R 4 (Section 3.2). 6. If the dip is perpendicular or the strike parallel to the retaining wall as viewed in plan, the required rock support pressure is reduced. In case the right angle is deviated from, a force component parallel to the retaining wall arises, which must be safely transmitted to the ground. In this case additional investigations must be performed to determine whether intersections occur, due to existing bedding or joint planes, which dip perpendicular or at an angle to the lining. The thus developed partial sliding bodies can exert locally higher pressures on the retaining wall than was calculated for the complete sliding body. See [33] and [34] amongst others.

7. Independent of the numerical determination of the rock support pressure according to Paragraph 2 or Paragraph 3, a computed minimum design pressure for the excavation lining should be adhered to in analogy, based on the definitions in R 4, Paragraph 4 (Section 3.2), which give QEquiv = 40" or QEquiv = 4.5" according to the stipulations for earth pressure. This also applies to anchored retaining walls and in cases where the strike is at an angle to the retaining wall. 8. If a greater rock support pressure ensues on one side of a braced excavation because of the different development of the slip surfaces, the higher load must be adopted for the design of the whole excavation structure, if this is not lower than the calculated minimum dimensioning pressure.

11.3 Distribution of rock support pressure (R40) 1. Because the magnitude and distribution of the rock support pressure is a func-

tion of the fault density of the rock, concrete rules such as for determination of earth pressure in soil cannot be given. The loads for the rock support pressure must be conservatively adopted on the basis of the determined local conditions. 2. If the complete wall is installed on a rock face, the rock support pressure determined according to R 39 (Section 11.2) is generally adopted as a rectangle due to the rigid body mechanisms mainly assumed for this case. In soil zones above the rock itself and for disintegrated, completely weathered or decomposed rock, an earth pressure distribution according to the rules for soil may generally be adopted. Because of the possible pressure redistribution it is recommended to at least determine the support forces in the upper half of the wall and/or in the rock transition zone for a load from a rectangular pressure diagram. 128

3. The permissible stresses determined according to Paragraph 2 must usually be increased by about 30 % to compensate for the relatively rough assumptions on the distribution of the rock support pressure, independent of the type of excavation lining. These surcharges may only be dispensed with if measurement results for the distribution of the rock support pressure and the pressure diagram that is based on it were obtained under comparable conditions and are confirmed by further measurements.

4. It is recommended a) to prestress all struts and anchors to the computed maximum load according to Paragraph 3, b) to carry out measurements in representative sections to facilitate timely recognition of deviations from the analysis assumptions.

11.4 Bearing capacity of rock for support forces at the wall toe (R 41) 1. The resistance of the rock in front of the toe of a continuous retaining wall can be determined analogous to the rock pressure. Either the slip surface in a bedding plane or a slip surface parallel to the joint planes is critical here. The investigation according to R 39, Paragraph 2 is decisive in the one case and according to R 39, Paragraph 3 (Section 11.2) in the other. 2. To avoid deformation, boreholes must always be backfilled with hydraulically curing material, e.g. concrete or lime mortar. The diameter of the borehole is then decisive for determination of the rock resistance in front of the soldier piles. A three-dimensional effect may only be adopted if degree of fracture, joint density, joint infill and joint orientation justify this. Not more than half of the embedment depth, or a maximum of double the borehole diameter of the concreted boreholes, may not be adopted as equivalent width for the threedimensional effect without special analysis. 3. Soldier pile walls and retaining walls with comparable support below the excavation level must be examined for seams caused by the existing bedding and joint planes, which run upwards from the soldier piles and/or from the concreted borehole to the excavation level. The thus developed partial sliding bodies can be decisive for determination of the rock resistance, particularly for a small embedment depth. 4. A negative wall friction angle may only be adopted when determining the rock resistance where this is allowed by the CV = 0 condition according to R 9 (Section 3.1). 5. The location of the support force for a retaining wall supported below the excavation level may be adopted according to R 14, Paragraph 3 (Section 4.3), and/or R 19, Paragraph 3 (Section 5.4) as for cohesionless soil. 6. When analysing the support force that can be accepted by the rock resistance, a safety factor of 1.5 is generally sufficient. 129

Excavations in soft soils

12

12.1 Scope of the Recommendations for excavations in soft soils (R 90, Draft) 1. The Recommendations R 90 to R 101 apply to excavations in which soft, fine-grained soils, occasionally including organic constituents are prevalent - in favourable cases above the excavation level, - in less favourable cases only below the excavation level, - in unfavourable cases both above and below the excavation level.

The designation “soft soil” should be seen as a generic term, which is not connected to the consistency index I,. 2. The soft soils referred to here are primarily layered, uniformly grained soils, e.g. lacustrine clays and basin silts. Furthermore, softened boulder clays and flood plain loams, as well organic soils such as lacustrine chalk, digested sludge, tidal mud deposits and decomposed peat may be considered. These soils are generally normally consolidated but on occasion are still not completely consolidated under their own weight. 3. Each of the following soil properties taken on its own generally indicates the presence of a soft soil according to Paragraph 1:

very soft or liquid consistency corresponding to a consistency index I, < 0.50, undrained shear strength cUI 3 0 kN/m2, high vibration sensitivity, determined by the ratio of ultimate shear strength to residual shear strength in a vane test, or water content w 2 35 % for soft soils without organic constituents or w 2 75 % for soft soils with organic constituents.

4. The following soil properties indicate that a soft soil according to Paragraph 1 may be present: -

soft consistency corresponding to a consistency index of 0.75 > I, 2 0.50,

- undrained shear strength of 40 kN/m2 2 cu > 30 IcN/m2, - complete or almost complete saturation, - proneness to flow, - slightly plastic properties, -

thixotropic properties, or organic constituent content.

In individual cases, a decision for soft soil on the basis of these Recommendations should not be dependent on a single criterion given here. However, if 130

two of the criteria are fulfilled it can generally be assumed that a soft soil according to Paragraph 1 is present.

5. In all cases, the situation is aggravated if more permeable soil layers or bands are intercalated in the soft soil, e.g. fine-sands, which are subject to excess pore water pressure, regardless of whether this was already present before commencing with construction measures or occurs as a result of excavation work or drawdown measures.

12.2 Slopes in soft soils (R 91, Draft) 1. Slopes in soft soils as defined in R 90 may be manufactured without stability analysis for excavation depths of up to 3.00 m and at an angle of up to p = 45" if the following conditions apply:

a) The undrained shear strength of the soil cu must be 2 20 kN/m2. b) If water-bearing layers or layers or bands subject to excess porewater pressure are present in the soft soils, they must be drained by means of vacuum water drowndown. c) No heavy vibrations may occur, e.g. from traffic, driving work, compaction work or blasting. d) No buildings, pipelines, other structures or traffic areas may be endangered. e) The soil beside the slope crest may not climb more than 1.20 m for a width up to five times the excavation depth, but for a maximum of twice the depth of the soft layer below the excavation level. A live load of p = 10 kN/mZ at a distance of at least 1.00 m from the slope crest is permissible. f) On ground level, no earth fill inclined at more than 1: 1 and no higher than 1S O m may be used beside a protective strip of at least 1.O m width. g) Road vehicles and construction equipment up to and including 12 t gross weight must adhere to a distance of at least 1.00 m between the outer edge of the contact area and the slope crest if load-bearing layers, e.g. a road pavement or natural soil with a total thickness of at least 0.50 m, are present above the soft soil or are built up to this level. Otherwise, the distance must be increased to 2.00 m. h) Road vehicles and construction equipment of more than 12 t up to and including 40 t gross weight must adhere to a distance of at least 2.00 m between the outer edge of the contact area and the slope crest if loadbearing layers with a total thickness of at least 0.50 m are present above the soft soil or are built up to this level. Otherwise, the distance must be increased to 3.00 m. i) A berm immediately adjacent to the slope may not be stressed by horizontal support loads from a retaining wall. 131

k) Any soil movements associated with manufacturing the slope must remain within acceptable limits. The additional engineering measures required to ensure stability must be in accordance with Paragraphs 2 to 4.

2. If the ground a) is above groundwater at least as far as the excavation level, b) is classified as soft due to a consistency index of 0.75 > I, 2 0.50, c) is not classified as particularly difficult on the basis of any further criteria according to R 90 and d) does not demonstrate properties below the excavation level that are less favourable than those above, no special measures are generally necessary for short-term construction stages. However, if the slope is exposed to weathering for an extended period, the slope surface should be protected against erosion. 3. If the ground down to the excavation level is above groundwater and demonstrates no less favourable properties below the excavation level than above, but

0.75 > I, 2 0.50 and at least one other criterion according to R 90, Paragraphs 3 or 4 indicates particularly difficult soil conditions, or - is classified as very soft due to a consistency index of I, < 0.50,

- must be classified as soft because of the consistency index of

excavation may only be carried out in short stages with slope stabilisation following immediately, i.e. a minimum slope toe stabilisation by means of a loaded filter or support element, e.g. of single-sized aggregate concrete on a geotextile base. 4. A slope that intersects the groundwater table is generally only sufficiently

stable if the soil can be stabilised by implementing vacuum drawdown measures.

5 . If the boundary conditions stipulated in Paragraphs 1 to 4 are not adhered to, slope stability must be verified on the basis of the shear strength according to R 94. The safety factors for load case 2 are only valid, if the expected deformations do not endanger buildings, conducts, other constructions or traffic areas. If a risk cannot be excluded because of the local situation, the slope stability must be investigated demonstrating a safety factor q 2 13. It is also recommended to select larger safety factors for heavily organic material. After [ 1lo], a safety factor of = 1.70 has proven reliable for North German tidal mud deposits with a loss of ignition of vLoI > 15 % and a water content of w > 75 %.

132

12.3 Lining systems in soft soils (R 92, Draft) 1. If a slope is not possible in soft soils as defined in R 90, due to space, consideration of buildings, pipelines or other structures, or for other reasons, the excavation must be lined and braced as far as this is possible or be secured by anchoring. Only walls that will not cause appreciable settlement of the ground or other structures during manufacture may be utilised as excavation linings. There is a danger of settlement if the soil liquefies or is displaced during installation or manufacture of the wall. Primarily, for excavations in soft soils a) sheet pile walls, b) bored pile walls and c) diaphragm walls are suitable. See also Paragraphs 2 to 4. Soldier pile walls and bored pile walls with infilling installed between the piles during excavation are generally unsuitable as an excavation lining in soft soils.

2. Care should be taken to keep the effects of vibrations on neighbouring buildings to a minimum when installing sheet pile walls. The usual guide values for allowable vibration velocities are generally too high for the boundary conditions stipulated in R 90, because neighbouring buildings on shallow foundations in soft soil have often previously been subjected to deformations associated with an increased internal stress state and therefore only have low deformation reserves. Moreover, vibration-sensitive soils can suffer strength losses up to and including liquefaction. The hazard presented to neighbouring buildings from liquefaction and thus from settlement is greater for vibratory techniques than for impact driving. The following demands must be placed on the projected installation methods: a) When installing the sheet piles with a pile hammer, the driving energy per impact and the impact frequency should be defined on the basis of previous piling tests according to Paragraph 5. Cautious installation in soft soils can generally be achieved if vibrations are allowed to fade between two separate impacts. b) Vibration techniques are unsuitable if the soil is very vibration-sensitive, displays a proneness to thixotropic behaviour or has interbedded, saturated bands of fine sand. Sheet pile walls can only be vibrated-in in soft, highly plastic soils with low vibration sensitivity. Even when favourable conditions for the use of vibration techniques apply in this regard, the vibration velocities must be kept to a minimum. Driving tests according to Paragraph 5 are required for this purpose. Empiricism demonstrates that vibrations in neighbouring buildings are lowest at operating speeds of more than 2000/min. Moreover, particularly heavy vibration effects caused by switching on and off must be avoided by using vibration hammers with variable balance weights.

133

c) The jacking method is particularly suitable in homogeneous, soft soils without obstructions. Covering soil layers with a large jacking resistance, e.g. made soil with construction waste, must be prepared for jacking by pre-drilling or soil replacement.

3. EN 1536 applies for the manufacture of bored pile walls. The following points must also be observed:

A low-vibration drilling method should be selected for the manufacture of the individual piles of a bored pile wall. Heavy-impact or vibratory drilling is generally impractical. Soil displacement caused by pile drilling must be avoided by, e.g. - selecting larger pre-penetration of the casing tube as compared to that given in EN 1536, - dispensing with a bit that protrudes outside of the diameter of the casing tube, - using bits which do not possess teeth but a cutting edge, - using drilling tools that exert as low a suction effect as possible at the bottom of the borehole. It may also prove expedient to maintain a constant excess water pressure in the borehole. b) In principle, the following types of implementation may be considered: - bored pile walls with secant piles, - bored pile walls with sealing piles, i.e. piles with small diameters in the rear interstices of the neighbouring bored piles, - tangent bored pile walls with subsequent closing of the spaces during excavation. The unreinforced piles or sealing piles must be installed to the depth below the excavation level which is indicated by the analysis of safety against base heave or against hydraulic heave. c) The following points should be observed when selecting the execution method: - With bored piles, drilling of the primary piles without a protruding bit is only possible as long as the concrete is not completely cured. Furthermore, pre-penetration of the casing tube below the bottom of the borehole is not possible. - If the sealing piles are manufactured by drilling techniques, there is a danger of lateral displacement, in particular at local projections on the wall piles. If they are manufactured using jetting techniques, the surrounding soil may be locally softened, thus presenting a settlement hazard for neighbouring structures. The cement suspension curing process is not guaranteed in organic soils. - By driving in wooden wedges, for example, squeezing of the soft soil through the unavoidable gaps can often be avoided for tangent bored 134

piles, but not a limited groundwater drawdown outside of the retaining wall. Moreover, there is a danger of strong impacts and heavy vibrations if the drill bit catches on protrusions on a neighbouring pile. Furthermore, this type of pile installation requires that soft soil be present above the excavation level only. d) Uncased boreholes supported by a suspension must be manufactured in accordance with the stipulations for diaphragm walls. CFA piles are less suitable due to the hazard of uncontrolled soil displacement. e) If the shear strength in an undrained shear test is cu = 15 kN/mz or the consistency index IC= 0.25, direct concreting with the soil is not permissible. This stipulation can be ignored above the excavation level if the pile wall is carefully examined for defects areas during excavation.

4. EN 1538 applies for the manufacture of diaphragm walls. The following points must also be observed: a) If possible, the distance to neighbouring buildings, in particular to heavily loaded gable foundations, should be more than half of the trench depth, but at least 5.0 m, or the trench be outside of the failure body. b) When analysing trench stability, the fact that no arching effect is assumed must be taken into consideration. Therefore, the suspension pressure 0 s = yF'z

in regions with soft layers at a depth z must be at least 10 % greater than the total horizontal pressure

oh= e,

+w

from earth pressure e, and water pressure w. Here e, = 0;-2.c" as the initial condition of the unconsolidated soil and e, = 0;. K, - 2 c' . as the final condition of the consolidated soil 9

&,

must be investigated. The effective overburden pressure 0;follows from the unit weight of the wet or buoyant soil. The water pressure is given by the unit weight of water and the depth below the groundwater level from the equation w = yw.z If necessary, the suspension pressure must be increased, e.g. by deepening the guide walls or by use of a high-density support fluid. c) The safety against slipping of single grains or grain groups, the safety against slipping of a failure wedge into the trench and the most appropriate composition of the suspension fluid should be tested on a test trench. 5. Although not previously mentioned in Paragraphs 2, 3 or 4, the selected installation or manufacturing method should always be tested on the site in 135

question before starting work, but at a reasonable distance from existing neighbouring structures, in order to optimise the process on the basis of parallel investigations on such things as concrete requirements, integrity tests, and vibration and settlement measurements. 6. In principle, bracing is less flexible than anchors. If anchors are nevertheless adopted, the grouting sections must be located in soil of sufficient bearing capacity. The same applies to the grouted sections of anchors with which a concrete base slab is anchored down. The anchor installation method must ensure that soil displacement, softening and decompaction is avoided.

7. Regardless of the type of lining construction selected, the working level must be manufactured so that the soft soil does not lose its bearing capacity when machinery and equipment are operating on it and does not begin to boil. If an existing layer of fill cannot be employed for this purpose, the soft soil must be protected as deemed necessary or be replaced by a load-bearing layer. Furthermore, only equipment exerting small loads, e.g. from bearing pressure or vibrations, should be utilised to install or manufacture walls or to manufacture wall or base anchorages. If necessary, load distributing excavator mattresses must be used.

12.4 Construction procedure in soft soils (R 93, Draft) 1. Because the anticipated displacements in excavations in soft soils only allow an earth restraint of the retaining wall in the initial excavation stage with very small excavation depth, - only a limited free earth support of the retaining wall below the excavation level, -

the following procedures are useful [ 1001, depending on the excavation depth and dimensions and the soil and groundwater conditions [loo]. They assume the most unfavourable case of soft soil from ground level to far below the excavation level. If more favourable soil conditions prevail, the measures described may be correspondingly adjusted.

2. Regardless of the excavation depth, a continuous head beam in the shape of waling or a wale runner, which is capable of redistributing the earth pressure from the area of the excavated strip to neighbouring areas, must be installed for sheet pile walls. This also serves to limit the anticipated head deflections. In this regard, it is particularly useful to arrange this head beam as waling for a row of struts at ground level. The same applies to in-situ concrete walls, if constructive measures are not taken to ensure that the individual diaphragm slices or individual piles cannot move separately. 3. The following procedure can be adopted for excavations with small depths, generally to 3 m, and small plan dimensions: 136

a) Within a daily shift - an approximately 2 to 3 m wide, laterally sloped trench, parallel to the narrow end of the excavation at the height of the projected excavation level is excavated and - a stiffening strip of blinding concrete manufactured below the projected foundation level of the new building. b) By continuing the blinding concrete strip as shown in Figure R 93-1, a concrete slab is produced at the height of the excavation level, stiffening the retaining walls. In the comer regions of the excavation it can be useful to arrange the blinding concrete slab diagonally. c) If the retaining wall head displays unacceptable deflections when using this method, - the trench must be manufactured with vertical walls and lined, - bracing must be installed at soil level according to Paragraph 4, or - the core method as described in Paragraph 6 must be selected. It may be practical and sufficient to gradually manufacture blinding concrete strips at wider intervals in lined trenches and to thus achieve an effective bracing of opposing retaining walls, before supplementing the intermediate blinding concrete strips in trenches sloped on one side only. 4. For excavations of medium depth, generally 3 to 5 m, a low-deformation bracing construction must be installed in two or more excavation stages. If the excavation plan allows, bracing can be installed directly against the opposite wall. In principle, the construction procedure is as follows:

a) If the uppermost row of struts is not already installed approximately at ground level as part of the head beam installation according to Paragraph 2, but lower instead, in an initial stage based on Paragraph 3a, the soil can be excavated in strips as far as the excavation level of the first advancing stage and a strut installed at each stage. b) The soil can also be excavated in stages to the final excavation level on the basis of Paragraph 3a and a stiffening blinding concrete strip manufactured at each stage. c) If a second or more rows of struts are projected, the procedure repeats according to Paragraph a, before the final stage according to Paragraph b concludes the excavation phase. In Figure R 93-2a single-propped retaining wall is shown, after manufacture of the first blinding concrete strip. 5. The following must be observed for manufacture of the blinding concrete strip according to Paragraph 3 or Paragraph 4:

a) The stiffening blinding concrete should be manufactured with quick-curing concrete to facilitate a rapid work schedule. 137

b) The thickness of the blinding concrete depends on the structural analysis, but should not be less than 0.20 m. c) The blinding concrete should be reinforced where necessary. 6. If the construction procedure according to Paragraphs 3 or 4 is not possible due to the large plan dimensions of the excavation, the following procedure applies :

a) In an initial stage the central section of the foundation slab is manufactured in a sloped or lined excavation. In a sloped excavation, sufficiently wide berms must be provided to enable reliable and low-deformation support of the retaining wall. b) In a second stage an intermediate bracing is installed against the completed central foundation, the berms removed in strips and the bracing slab extended to the retaining walls. In this manner each wall gradually receives a continuous support at the excavation level.

7. In deep excavations, generally more than 5 m in deep, soft soils, it may be necessary to create a toe support for the retaining wall by means of a bracing base, e.g. using jet grouting techniques [102, 1041. Support of the retaining wall above the excavation level in the course of excavation is by means of struts, where necessary by anchors or by means of the core method based on Paragraph 6. 8. Soft soils are particularly sensitive to dynamic loading and to changes in the initial stress condition due to excavation. In order to minimize the risk of boiling the soft soil at the excavation level may not be traversed by vehicles and in extreme cases may not even be traversed unprotected on foot. Excavation must always be carried out from a higher level. If this is not possible in all areas a sufficiently thick working subgrade must be installed to protect the soft soil. 9. Soft soil and especially banded soils below the groundwater level are particularly susceptible to boil. The construction procedures described in Paragraphs 3, 4 and 6 with temporary slopes and berms can often only be realised after stabilisation of the soft soil by means of vacuum wells or vacuum lances [103]. See also R 100, Paragraph 3 and Paragraph 4.

10. Because soil arching cannot reliably develop in soft soils and stress redistribution in the soil is directly associated with wall displacements, all strut removal and strut redistribution measures must be implemented with little or no deformation, generally with the help of hydraulic presses and in small stages.

11. In excavations in soft soil, there is a permanent risk of heave or hydraulic heave. This can lead to large rises in the ground level within the excavation and to large settlements outside of the excavation. Depending on the boundary conditions, one or more of the following measures must be provided for 138

in order to minimize the associated danger of settlement damage to neighbouring structures: a) Construction in small stages based on Paragraph 4. b) Downward extension of the wall beyond that required for a toe support. c) Implementation of blinding concrete installed in stages as a vault or a reinforced flexural beam from retaining wall to retaining wall. d) Anchoring of blinding concrete installed in stages, a base slab installed in stages or a previously manufactured, jet grouted base utilising tension piles. 12. Because the behaviour of excavation structures in soft, cohesive soil and the deformations of the ground outside of the excavation cannot always be predicted with the required reliability, it is absolutely necessary to monitor and measure the individual components of the excavation structure, the ground and the neighbouring structures from the outset. See R 31 to R 37 and DIN 4123 (2000). If the measurements demonstrate that unacceptably large movements are anticipated, with regard to neighbouring buildings, other structures or traffic areas, a different construction procedure must be selected or additional measures implemented.

12.5 Shear strength of soft soils (R 94, Draft) 1. A geotechnical expert must be consulted for the soil investigation of excavations in soft soils in the sense of R 90 if the soft soil a) is to be excavated for a height of more than 3.00 m, b) the excavation depth is more than 5.00 m, or c) impacts on neighbouring buildings, pipelines, other structures or traffic areas are anticipated. 2. Knowledge of the prevalent and the anticipated porewater pressures is of particular importance for the design of excavation structures in soft soil. The geotechnical expert must therefore investigate, a) whether the soft soil is already consolidated under its own weight or whether excess porewater pressure has resulted from previous excavation measures, b) whether excess porewater pressure is to be expected during excavation. Based on these findings, it must be decided for each individual case whether the analysis must be based on the shear strength of the drained or the undrained soil, or on a shear strength lying between these two limits. 3. The criteria for anticipated excess porewater pressure in a soil normally consolidated under self-weight and therefore subject to undrained conditions are given in [ 105, 111, 1191. Approximately drained boundary conditions can often be anticipated:

139

a) The investigations in [ 11I] have demonstrated that drained conditions can very often be anticipated for the boundary conditions usually prevalent in practice. b) Investigation of the effective stress paths in [ 1191 demonstrated that in the most ground areas an unloading situation occurs and the existing shear stress is not substantially increased. c) Analyses of previously executed excavation structures in soft soils with the numerical methods in [ 1181 shows that conformity with the measured deformations can largely only be demonstrated with the adoption of effective shear parameters and therefore under the assumption of drained conditions. However, depending on the field of application and the local conditions, cases are also possible in which excess porewater pressure can be anticipated and therefore undrained conditions are decisive. In this regard, the local experience of the geotechnical expert should also be incorporated into the assessment.

4. According to the boundary conditions, the following should be differentiated: a) the shear strength of the drained soil with the parameters $’ and c’ (however, see R 95, Paragraph 3 ) , b) the angle of total shear strength $: of the drained soil according to DIN 18137- 1 with friction and cohesion components, c) the shear strength of the undrained soil with the parameters @u and c,, whereby 9, = 0 is generally assumed. The shear parameters $’ and c’ and the angle of total shear strength $: of the drained soil generally follow from triaxial tests according to DIN 18137-2 or from direct shear tests according to DIN 18137-3. These tests are only of limited suitability for determination of the shear strength of very soft soils, see Paragraph 5.

5. Determination of the shear strength of drained and undrained soils in laboratory test can be heavily influenced by random and systematic errors: a) Sampling errors can lead to strength reductions as can errors installing the sample in the shear box or triaxial cell. b) Apparent strength increases can be simulated in direct shear tests due to frictional resistance in the shear box. c) The resistance of the rubber membrane in triaxial tests can lead to an apparent strength increase. For these reasons, the values determined in laboratory tests, in particular for the cohesion c‘ of the drained soil and for the shear strength c, of the undrained soil, should be carefully assessed when specifying the calculation values. A cohesion of c’ = 0 is normally anticipated in any case for normally consolidated soils without organic constituents, giving 0’ = $:. 140

0

20

40

60

Plastirity index /p

80 in

%

100

r

120

Figure 94-1. Reduction factor p when using the shear vane for determination of shear strength c,

6. Failing the relevant experience for excavations in soft soils, the local shear strength c, of the undrained soil should be determined by vane shear tests in addition to the usual soil investigation measures and laboratory tests. The vane shear tests should be to a depth at which the soil strength noticeably improves, but at least three times the depth of the excavation for thick, soft soil layers. The shear vane provides the value T~ In order to consider the differing loading rates during the shear vane test and the shear stresses during excavation, the associated value c, must be determined with the help of a correction factor p using c, = zf p. Long-term and short-term construction stages may be differentiated: a) The lower curve in Figure R 94-1 represents the relationship between the

factor p and the plasticity index I, for long-term construction stages according to [ 1131. b) The corrected upper curve in Figure R 94-1 represents the relationship between the factor p and the plasticity index I, for short-term construction stages based on [ 1161. Constructions stages in which a locally limited critical state is redressed on the same day by installation of effective stabilisation are considered shortterm.

7. If the implementation of vane shear tests does not appear to promise success, e.g. in fibrous organic soils, the shear strength c, of the undrained soil may be estimated as follows, in consultation with the geotechnical expert: 141

a) If the relevant regional experience or confident correlations are available [ 1121, the shear strength c, can be derived with the help of a coefficient A,, with c, = ?L,,.o: from the effective overburden pressure. This gives 0;= y’.z

if the groundwater level is at ground level. If it is lower, the unit weighty of the non-buoyant, wet soil must be adopted, or the unit weight yr of the saturated soil. b) In addition, indirect determination of the shear strength c, from cone penetration tests according to [ 1201, using c, = (0.05 to O.lO).q,, can be considered. For further information on the relationship between the shear strength c, and the cone resistance qc see [ 1141 and [ 1151. Deriving the shear strength c, by means of correlation with the consistency index I, is not recommended [ 1061.

8. Taking the variance of the measurement results into consideration, the decisive calculation value for the shear strength c, should be adopted so that analyses provide conservative results. In this way it represents a cautious estimate of the mean value in the associated soil region. 9. Because of the anisotropy of the soil due to sedimentation and the alteration of the principal stress directions due to soil excavation, the shear strength c, of the undrained soil must normally be increased when determining the active earth pressure and reduced when determining the passive earth pressure [ 105, 1131. As this influence can only be estimated with difficulty, but both effects partly cancel each other out and an analysis with varying shear strengths would lead to difficulties, it is recommended that consideration in the analysis be dispensed with and the impacts thus not assessed be compensated for by increasing the safety factor when adopting the passive earth pressure. 10. If excess porewater pressure is anticipated in certain concrete situations, the shear strength c, for each layer should be converted to an equivalent friction angle @Ln as follows: a) If the shear strength c, increases approximately linearly with depth as shown in Figure R 94-2a: sin @&

CU,l

= - applies above the groundwater level and 06,1

sin@& = 142

below the groundwater level. 0 : , 2 - 0:,1

I_

cusl

-I

I-

& kTk

Shear strength c,

I_Acu2

ai C, increasing with depth

strength c,

-

bi c, constant in each layer

Figure 94-2. Determination of equivalent friction angle $&

b) If the shear strength c, is approximately constant above and below the groundwater level as shown in Figure R 94-2b: C",l

applies above the groundwater level and sin@&1= 7 O"Ill,l

sin @du,2 =

% below the groundwater level. (3",,2

An additional layer boundary must be introduced at the height of the excavation level, if this substantially improves correlation for the best-fit line. In all cases, further analysis may be performed using the equivalent friction on the basis of effective stresses, although the shear strength of the angle undrained soil is assumed.

@:,

11.If excess porewater pressure can be ruled out in certain concrete situations and the effective shear strength with the shear parameters $' and c' was not determined in the laboratory, the angle of total shear strength @; for the drained condition may be determined from the shear strength c, of the undrained soil defined in Paragraph 8, based on [ 1191. 12. Larger angles of total shear strength than $' = 27.5" or larger equivalent friction angles than $,: = 27.5" may only be adopted if they are confirmed by a geotechnical expert. 143

12.6 Earth pressure on retaining walls in soft soils (R 95, Draft) 1. The same principles apply for determination of the earth pressure magnitude and for selection of the earth pressure distribution on retaining walls in soft soils as for excavations in firm to semi-solid soils, if no other stipulations are made in the following passages. The same applies accordingly for excavations adjacent to structures.

2. According to [lo51 and [112], determination of the earth pressure acting on retaining walls in soft, unconsolidated soils may be based on the shear parameters of the undrained soil $, and c,. In addition to this, the total stresses in the soil may be assumed and thus earth and water pressure be applied in a single computation. The following points speak against this procedure: a) Merely determining the active earth pressure and passive earth pressure alone, taking the water pressure into consideration separately, often produces unreliable results because - arithmetically, no earth pressure acts to a depth depending on the parameter c,, - active earth pressure and passive earth pressure increase equally with depth, therefore leading to decreasing numerical safety against reaching the passive earth pressure limit state with increasing embedment depth. b) In stratified soils, an analysis using total stresses may only be adopted for the soft layers, but not for the stiffer layers. Different procedures are therefore used for a single computation. Accordingly, this concept will not be further pursued here. 3. The following regulations assume that the shear strength is always adopted as a friction angle according to R 94, either - as the total shear strength angle $: determined in a shear test according to R 94, Paragraph 4b, - as an equivalent friction angle $,; according to R 94, Paragraph 10, based on the shear strength c,, - as a total shear strength angle according to R 94, Paragraph 11, based on the shear strength c,.

+:

Analysis should be performed with effective stresses in all cases. See R 97 for excess water pressure and, if required, excess porewater pressure approaches. 4. The starting point for an investigation of a retaining wall in soft soil is the atrest earth pressure eog = Y.KO. z a as for firm or semi-solid soil. In saturated soils g is replaced by - "J, above the groundwater table and - y' below the groundwater table. 144

The following empirical approximations are available for the at-rest earth pressure coefficient of a soil consolidated under self-weight: a) The usual approach as a function of the friction angle & = 1-sin@ b) As a function of the plasticity index, the approach & = 0.24 + 0.310 log I, after LEE/JIN (1979), see [ 1121. The plasticity index I, is given in %. c) As a function of the water content wL at the liquid limit, the approach & = 100,00275.(~,- 20) - 0,2676 after SHERIF/KOCH (1970) applies, see [ 1121. The water content wLis given in %. An evaluation of the given approaches results in the following at-rest earth pressure coefficients for normally consolidated, cohesive soils: a) KO= 1 - sin 4

b) LEE/JIN

c) SHERIFKOCH

Q

Kfl

1,

Kfl

WL

Kl

30" 25" 20" 15" 10"

0.500 0.577 0.658 0.741 0.826

5% 15 % 25 % 35 % 45 %

0.456 0.605 0.673 0.719 0.752

10 % 20 % 30 % 40 % 50 %

0.507 0.540 0.575 0.613 0.653

A geotechnical expert, taking all relevant points into consideration, should decide which value the analysis is based on. Any prevalent excess porewater pressure must be adopted according to R 97,

Paragraph 6 .

5. The following apply when adopting the at-rest earth pressure: a) At-rest earth pressure may only be adopted above the excavation level as shown in Figure R 96-3b if wall deflections at the top or at the excavation level are almost completely avoided due to the construction procedure selected. This can be the case if e.g. - a low-deformation wall is installed, - a stiffening concrete base is manufactured from ground level by jet grouting or soil stabilisation and - the first supports are installed and prestressed without appreciable excavation. 145

b) At-rest earth pressure may only be adopted below the excavation level as shown in Figure R 96-3 if wall deflections towards the excavation at the toe or at the excavation level are almost completely avoided due to the construction procedure selected, or if a wall deflection against the ground is anticipated. This can be the case, for example, if a stiff wall is installed and a stiffening concrete base is manufactured from the original ground level using jet-grouting techniques. c) Because of their deformability, it may be practical to only adopt an increased active earth pressure for sheet piles walls above and, if necessary, below the excavation level as shown in Figure R 96-4a, in the sense of R 22, for cases in which the at-rest earth pressure should be adopted for low-deformation walls. If the supports are also heavily prestressed, a large wall top deflection may develop, which in turn leads to a rotation of the wall toe towards the excavation below the stiffening base slab, in particular in conjunction with earth pressure from building loads and water pressure, so that only the active earth pressure acts in this zone as shown in Figure R 96-4b. Always check that the selected earth pressure approach approximately conforms to the computed deformations and deflections of the wall. At the least, it should not obviously contradict the determined deformations and deflections. 6. The following apply for determination of the active earth pressure: a) As in firm or semi-solid soil, the magnitude of the active earth pressure in soft soil follows from eag= y'.Ka.za In saturated soils y is replaced by

- ?I, above the groundwater table and - y' below the groundwater table.

b) In soft soils it may be assumed that adhesion acts between the retaining wall and the ground. As a simplification, it is permissible to adopt a wall friction angle 6, = 'I3@ instead of the adhesion. 7. The following apply for adoption of the active earth pressure:

a) The active earth pressure must be adopted if the measures mentioned in Paragraph 5a are not implemented. This is particularly the case, - if the first support is installed relatively deeply, - if passive earth pressure is present at a wall support, - if a stiffening base according to R 93, Paragraph 3 is installed in strips. b) If undrained conditions are assumed when determining the earth pressure and the equivalent friction angle $hu is determined according to R 94, Para146

graph 10, the active earth pressure may be greater than the at-rest earth pressure for very low shear strength values. In this case, determination of the wall loading may be based on the at-rest earth pressure. 8. For excavations in soft soils, classical earth pressure must generally be assumed, in particular if the wall top can suffer greater displacements than at the excavation level due to the projected course of construction. However, if an upper strut is prestressed on the one hand, but a strut at the wall toe loaded by passive earth pressure on the other hand, earth pressure redistribution must be assumed. The following must then be observed. a) The earth pressure from ground level to the excavation level must be converted to a trapezoid or, at the most, a rectangle. b) Surcharges on the strut forces determined with the redistributed earth pressure according to R 17 are not necessary. 9. These stipulations are valid for a homogeneous soil and a groundwater table at or below ground level. The following must be observed: a) These stipulations only apply for determination of earth pressure below the groundwater table in conjunction with R 97. b) For consideration of changes in soil layering see R 99, Paragraph 6.

12.7 Soil reactions in soft soils (R 96, Draft) 1. The soil reactions below the excavation level can take on any value between the active earth pressure and the limit state passive earth pressure, depending on the displacements occurring. The following cases are differentiated when adopting these soil reactions: a) Construction phases without a stiffening base as shown in Figure R 96-1. b) Construction phases with a stiffening base installed in strips in the course of excavation as shown in Figure R 96-2. c ) Construction phases with a stiffening base previously installed from ground level as shown in Figure R 96-3 and Figure R 96-4. The load models assume that not only the active earth pressure and the at-rest earth pressure, but also the passive earth pressure according to R 95, Paragraph 3, were determined using the angle of total shear strength (( or the equivalent friction angle .,;pC 2. In construction phases without a stiffening base slab as shown in Figure R 96- 1, equilibrium of horizontal forces can only be achieved if the soil reaction anticipated in front of the wall toe was defined on the basis of the passive earth pressure according to R 19. Assuming a buoyant soil, this follows in the limit state from epgh

= Y'Kpgh'

'p.

147

GW

GW

I

H

eagh

ai Without earth pressure redistribution

eagh

bi With earth pressure redistribution

Figure 96-1. Possible load models for single-propped walls without support at the excavation level

The following must be observed: a) On the basis of R 95, Paragraph 6b, the wall friction angle 6, = - 1/3 4 may be adopted as a simplification instead of the adhesion if the special case shown in Figure R 99-2 does not apply. b) To take the required limitation of wall deflections into consideration, the passive earth pressure according to Paragraph 5b may only be adopted at a fraction of the value possible in the limit state. c) The passive earth pressure distribution must be assumed as linear with depth. d) Due to the anticipated top deflections, an earth restraint in soft soil is only possible with shallow excavation depths for unsupported walls. e) Because of the stiffness of retaining walls and soil, no earth restraint may be adopted in soft soils for supported walls.

3. In construction phases with a stiffening base slab installed in strips as shown in Figure R 96-2, force equilibrium is primarily ensured by the bearing capacity of the base slab. Otherwise, procedure may be as follows: a) Because the base slab should already be installed before appreciable wall deflections occur at this depth, it may be assumed, as an approximation, that the original at-rest earth pressure is largely retained below the base slab even after excavation is complete. Assuming an originally buoyant soil, it follows that eOg= y’.K,. (H + zp).

148

GW

GW

w

w a) Without strut prestressing

I

bi With strut prestressing

Figure 96-2. Possible load models for single-propped walls with a base slab installed in stages

However, because of the reversal of the principal stresses, only the limit value of the passive earth pressure e determined using S, = 0 may be effective in the zone directly below tghbase slab. b) If only the active earth pressure from soil self-weight is effective on the exterior of the wall as shown in Figures R 96-2a and R 96-2b, the effective at-rest earth pressure determined according to Paragraph a must be reduced to a value equal to that which impacts on the other side of the wall. If, on the other hand, the sum of the actions below the base slab is greater than the at-rest earth pressure determined according to Paragraph a, e.g. as a result of the impact of building loads or of excess water pressure, the soil reaction in excess of the at-rest earth pressure may be determined as follows: - A soil resistance is adopted below the intersection of eogand ePgh. - Depending on the constrained modulus Es,hfor horizontal loading, the modulus of subgrade reaction is based on the constant value k, = Es,h : tB, whereby tB is the distance from the bending point of the remaining E, area to the lower edge of the wall. - The soil reaction mobilised by the modulus of subgrade reaction may not be greater than Rph

= (Epgh - EOg)/qp

with qp= 1.50 according to Paragraph 5b. 149

ai Without strut prestressing

bl With strut prestressing

Figure 96-3. Possible load models for single-propped, low-deformation walls with a base slab installed by jet grouting

If the intersection of eogand ephlies beneath the base of the wall, analysis using the modulus of subgrade reaction procedure is not possible because the largest possible soil reaction ePghis already available to take up support forces without substantial displacement.

4. In construction phases with a stiffening base slab installed by jet grouting as shown in Figures R 96-3 and R 96-4, the force equilibrium is primarily ensured by the bearing capacity of the base slab as stated in Paragraph 3. However, the following points must also be observed: a) Because the retaining wall moves towards the soil when installing the base slab, the ground below the base slab can relax so that only the active earth pressure is effective as shown in Figures R 96-3 and R 96-4a. Assuming a buoyant soil and considering the surcharge p of the base slab, it follows that

b) The adoption of a soil reaction in excess of the at-rest earth pressure determined according to Paragraph 3a, as shown in Figure R 96-4b, or an at-rest earth pressure according to Paragraph 3b can only be justified if a flexible sheet pile wall is installed, the struts or anchors are heavily prestressed and the sum of the impacts below the base slab is so large that the wall bends back towards the excavation, e.g. due to the effect of building loads or excess water pressure. 150

GW

eagh

eh

%p'h

bi With strut prestressing

ai Without strut prestressing

Figure 96-4. Possible load models for single-propped, flexible sheet pile walls with a base slab installed by jet grouting

Two conditions are critical for determination of the utilisable passive earth pressure: a) Taking the influence of anisotropy into consideration, see R 94, Paragraph 9, with qp2 2.00, the 'Lgh

'

'pgh'qp

condition must be adhered to. b) Taking the serviceability state into consideration, with qP2 1.50, the Ekgh

+ ('pgh

- 'Og)/qp

condition must be adhered to. In the second case, the safety factor must be defined on the basis of the local experience of a geotechnical expert or based on large-scale in-situ tests so that the anticipated deflections of the wall in the embedment zone remain at an acceptable level. However, a reduction of the passive earth pressure ELgh to a value corresponding to a passive earth pressure coefficient q h 5 1.00 is not practible. These stipulations are valid for a homogeneous soil and a groundwater table at or below ground level. The following must be observed: a) These stipulations only apply for determination of passive earth pressure below the groundwater table in conjunction with R 97. b) For consideration of changes in soil layering see R 99, Paragraph 6. 151

7. The approaches mentioned are suitable for determination of internal forces, but not for determination of the necessary embedment depth below the stiffening base slab. The following apply for demonstration of sufficient embedment depth: a) In construction phases with a stiffening base slab installed in strips in the course of excavation it may generally be assumed that the total length of the wall, resulting from the state prevalent before installing the stiffening concrete slab according to Paragraph 3, in conjunction with R 98, Paragraph 2, is sufficient. b) In construction phases with soil stabilisation below the excavation level or a stiffening base slab installed by jet grouting, the necessary minimum embedment depth follows from analysis of the safety against base heave according to R 99, Paragraph 2, analysis of safety against hydraulic heave according to R 99, Paragraph 3 or, where required, analysis of safety against general failure according to R 99, Paragraph 4. In individual cases it may be useful to increase the embedment depth if this leads to a more favourable distribution of internal forces.

12.8 Water pressure in soft soils (R 97, Draft) 1. If it is not possible or no measures are taken to dewater a deep, permeable layer, it must be assumed that saturated soft soil is buoyant and a hydrostatic water pressure acts. If the wall is embedded in a load-bearing, impermeable layer, the procedure is as follows: a) The water pressure on the outside of the wall follows from w, =;y z, if the groundwater table is at ground level, or from w, ,=:.;y if the groundwater table is below ground level. b) The water pressure on the inside of the wall follows from w, = yw.zp, if the groundwater table is not lowered below the excavation level. c) The water pressure on both sides of the wall is determined separately and subsequently superimposed so that only the positive water pressure need be considered for further analysis. See Figure R 97-lb. 2. If the wall is not embedded in an impermeable layer, different approaches for treatment of water pressure may be used according to R 63:

152

GW

WG

bi Hydrostatic positive water pressures

ai Hydrostatic water pressures

Figure 97-1. Water pressure for embedment of the wall in an impermeable layer

a) Using the simplified approach, the procedure assumes, as an approximation, that the wall is embedded in an impermeable, load-bearing layer. The actual percolation around the wall is not considered. Therefore, the decisive approach is that according to Paragraph 1 (Figure R 97-1). b) In more precise procedures the percolation around the wall is considered. See also R 63, Paragraph 2. For a water level at ground level and at the excavation level the water pressure is w, = (yw- i,. yw). z, on the outside and wP = (yw - ip.yw)'zp on the inside of the wall. The two components are superimposed so that only the positive water pressure wp need be considered for further analysis. See Figure R 97-217.

c) The procedures given in R 59 for determination of the seepage pressure provide differing results according to the simplification they are based on. In Figure R 97-2 the simplified case of i = i, = i, = i,= AhA = H/(H

+ 2 t)

is shown. Here, the ordinates of the effective water pressure at the toe of both sides of the wall are equal. 3. By considering the water pressure the following stipulations apply for determination of the earth pressure below the water table:

153

GW

ai Effective water pressures

bl Effective positive water pressures

Figure 97-2. Water pressure for percolation around the wall toe (simplified representation)

a) In the approximate solution according to Paragraph 2a the active earth pressure and the at-rest earth pressure are determined using the unit weight y’according to the stipulations in R 95. b) In the more precise analysis according to Paragraph 2b the effective unit weights deviate from R 95 and

yl= y’ + ia.ywon the outside and y’P = y‘+ i,. yw on the inside of the wall are decisive. In the following two Paragraphs these approaches are transferred to the passive earth pressure.

4. If the simplified water pressure approach without consideration of percolation around the wall according to Paragraph 2a is used, the following analysis approaches apply for the soil reactions on the inside of the wall: a) Construction stages without a stiffening base slab: e’pgh= y’.qgh.zp/qeflcorresponding to Figure R 97-3a. The effective safety factor qeflhere represents a substitute for the safety analysis according to R 96, Paragraph 5a and Paragraph 5b.

154

GW

GW

4 i

GW

i

1 ai Without stiffening base slab

Bagh

eap’h

bi With stiffening base slab

Figure 97-3. Load models for single-propped walls and positive hydrostatic water pressure

b) Construction stages with a base slab installed in strips or a stabilised soil layer below the excavation level: eog= y’.&.(H epgh= y‘.$,,.z,

+ z,), but at most corresponding to Figure R 97-3b

and not greater than the total load from earth and water pressure adopted on the outside of the wall, below the excavation level. If the total load is greater, a soil resistance may additionally be adopted according to R 96, Paragraph 3b. c) Construction stages with a base slab manufactured by jet grouting: eah = y”Kagh‘Zp 4- P’Kagh for a low-deformation wall, otherwise R 96, Paragraph 4b applies.

5. If the more precise water pressure approach with consideration of percolation around the wall according to Paragraph 2b is used, the following analysis approaches apply for the passive earth pressure or the at-rest earth pressure on the inside of the wall: a) Construction stages without a stiffening base slab as shown in Figure R 96-1: erpgh= y;l.Kpgh.zp/qeffcorresponding to Figure R 97-4a.

155

e’pgh

eah

ai Without stiffening base slab

bl With stiffening base slab

Figure 97-4. Load models for single-propped walls and seepage pressure (simplified representation)

The effective safety factor qeffhere represents a substitute for the safety analysis according to R 96, Paragraph 5a and Paragraph 5b. b) Construction stages with a base slab installed in strips or a stabilised soil layer below the excavation level as shown in Figure R 96-2: eOg= yk.&.(H

+ z,),

but at most

epgh= y;.Kpgh.zpcorresponding to Figure R 97-4b. and not greater than the total load from earth and water pressure adopted on the outside of the wall, below the excavation level. If the total load is greater, e.g. as the result of surcharge earth pressure from a building load, a soil resistance may additionally be adopted according to R 96, Paragraph 3b. c) Construction stages with a base slab installed by jet grouting as shown in Figure R 96-3 1 = YL’Kagh’zp + P ’ Kagh for a low-deformation wall, otherwise R 96, Paragraph 4b applies. eah

The necessary drainage layer for percolation around the wall is not shown in the Figures R 96-2 to R 96-4. 6. If the settlements caused by a fill or a foundation adjacent to the planned excavation are not complete and a porewater pressure therefore acts, the 156

hydrostatic water pressure over the complete effective height must be increased by the porewater pressure. 7. In Paragraphs 2 to 5 and in the corresponding Figures R 97-2 to R 97-4 a simplified case of a groundwater table at ground level is assumed. This is generally not the case. Additionally, the effect of ring drainage recommended in R 100, Paragraph 2 must be considered. The ordinates for the earth pressure, at-rest earth pressure and water pressure in Figures R 97-3 and R 97-4 change accordingly.

12.9 Determination of embedment depths and internal forces for excavations in soft soils (R 98, Draft) 1. According to R 11, Paragraph 1, all construction stages during excavation and backfilling of the excavation must be investigated. The following must be observed: a) R 95 for determination of earth pressure, b) R 97 for determination of water pressure, c) R 96 for adopting soil reactions. In contrast to R 11, Paragraph 2, the computed deformations of the retaining wall and the impacts, in the shape of support point displacements at the height of the subsequent support in the following construction stage, must generally be taken into consideration due to the great influence on the internal forces.

2 . For analysis of the construction stages that develop, and are locally and temporally limited according to R 93, Paragraphs 3 and 4, the following apply: a) Two conditions must be investigated: - the condition in which the first trench is sloped on both sides, - the condition in which the excavated strip is bounded by blinding concrete on the one side and sloped on the other. b) During analysis -

a temporary arching effect in the soil,

- the load transmission by the head beam according to R 93, Paragraph 2, - and the load-bearing effect of parts of the retaining wall, which are

supported by a blinding concrete strip, may be taken into consideration. c) In addition to this analysis, the displacements of the wall during excavation must be monitored. If the results are unsatisfactory, the originally selected trench width must be reduced or one of the construction procedures given in R 93, Paragraph 3c must be selected. 3. The analysis according to Paragraph 2 may be dispensed with if the following procedure is adhered to:

157

a) The retaining wall must have a minimum embedment depth that follows from - the stability analysis assuming an equivalent level according to Paragraph 4, - the base heave analysis according to R 99, Paragraph 2, without the subsequent blinding concrete surcharge, - the hydraulic heave safety analysis according to R 99, Paragraph 3, and - where required, the general failure safety analysis according to R 99, Paragraph 4. b) Work should start at a non-critical location. A small trench depth should be initially selected and can then be optimised in the course of work on the basis of monitoring and measurements. c) Whilst manufacturing the blinding concrete strip or installing additional bracing, the displacements, settlements and heave of the wall and its surroundings must be carefully monitored. d) If, due to unforeseen circumstances, it is not possible to install the blinding concrete strip or other additional bracing within the course of one daily shift according to R 93, Paragraphs 3 and 4,a condition that is numerically demonstrated as being stable must be reached by other means, e.g. by reinstating the condition prevalent before commencement of the planned measures. If the results are unsatisfactory, one of the construction procedures given in R 93, Paragraph 3c must be selected.

4. Regardless of whether stability is demonstrated according to Paragraphs 2 or 3 for the short-term construction condition before installation of a blinding concrete strip, the construction condition after excavation of the first trench as shown in Figures R 93-1 and R 93-2 must be analysed for an arithmetic equivalent level, which lies at two thirds of the height of the intended trench depth as shown in Figure R 98-1. In this manner, determination of the necessary embedment depth considers that - on the one hand, the first trench or its strip-wise extension possesses the

full excavation depth, - but on the other hand that lateral regions occur that are either still sup-

ported by soil or already supported by the load-bearing blinding concrete. For this analysis, the groundwater level within the excavation must be adopted at the actual projected excavation level.

5. For a single-propped wall with a free earth support, both the classical earth pressure distribution and an earth pressure redistribution can be decisive according to R 95, Paragraph 7. If in doubt whether analysis should be performed using earth pressure redistribution both cases should be investigated. 158

However, additional determination of internal forces and embedment depths with redistributed earth pressure as shown in Figure R 96-1 may generally be dispensed with if the support force determined for the row of struts or anchors is increased by 30 %.

___________________---------__-----------------. ai L ongitudmal section

bl Cross-section

Figure R 93-1. Unsupported retaining wall in soft soil after installing the first strip of blinding concrete

X

X

X

/ Is' advancing state

,' --__---. #'

'I

______________________________________ ai 1ongitudlnal section

bi Cross-section

Figure R 93-2. Single-propped retaining wall in soft soil after installing the first strip of blinding concrete

1.t

3

2.t 3

Figure R 98-1. Arithmetical equivalent plane for the intermediate stage with trench

159

6. See R 99 for further safety analyses, in particular for base heave, hydraulic heave and general stability, as well as additional investigations for stratified soil.

12.10 Further stability analyses for excavations in soft soils (R 99, Draft) 1. In Recommendations R 95 to R 98 the most unfavourable case is dealt with, assuming soft soil from ground level to the base of the wall or deeper as shown in Figure R 99-la. More favourable conditions are prevalent if, as shown in Figure R 99-lb, load-bearing soil is present in the upper layer and soft soil only deeper. Even more favourable conditions are prevalent if, as shown in Figure R 99-lc, soft soil is only present in the upper layer and load-bearing soil deeper. The change in layers may be at the excavation level or at a point above or below this. In some of the cases mentioned, further stability analyses to those described in R 98 must be performed. See also Paragraphs 2 to 6. 2. Excavations in homogeneous soft soil as shown in Figure R 99-la are greatly endangered by base heave. The same applies to a lesser extent for excavations in stratified soils as shown in Figure R 99-lb. Base heave analysis is generally performed using the shear strength c, of the undrained soil. For excavations with a depth H and width B > 0.30 H in homogeneous, saturated soil:

q = - 5.0. c 2 1.50 Y;H The following must be observed: a) If the soil beneath the excavation is buoyant, the safety factor is reduced in the ratio y’ly or yf/yr. b) For analysis of base heave safety for excavations with a width B = 0.30 H or in stratified soils see [52] and [117].

Soft soil

mq -

U

ai Soft soil above, load-bearing soil below

r -E

Soft soil

dbearing soil

ft soil

U

bl Load-bearing soil above, soft soil below

cl Homogeneous soft soil

Figure R 99-1. Excavations in stratified soils (without representation of supports)

160

Because of the anisotropy of the soil due to sedimentation and the change in principal stress directions due to soil excavation, the shear strength c, of the undrained soil must normally be increased when determining the earth pressure and reduced when determining the bearing capacity [ 105, 1131. As this can only be estimated with difficulty, but both effects partly cancel each other out, it is recommended to disregard it according to common practice. 3. For high groundwater levels in particular, excavations in stratified soils as shown in Figure R 99- l b are greatly endangered by the possibility of hydraulic heave failure. The same applies to a lesser extent for excavations in homogeneous soil as shown in Figure R 99-la. See also R 61.

4. For excavations in homogeneous soft soil as shown in Figure R 99-la and excavations in stratified soils as shown in Figure R 99-lb, general stability must be demonstrated. The following must be observed: a) For braced excavation walls, slip surfaces that terminate within the excavation as shown in Figures R 10-2a and 10-2b must be particularly carefully investigated. b) Higher safety factors should be adopted for soft soils than for load-bearing soil types. See also R 91, Paragraph 5. However, these higher safety factors are not required for the load-bearing layers involved in a slip mechanism. c) The normal force and the shear resistance of a stiffening base slab may be taken into consideration in the analysis as acting favourably.

5. The low failure plane stability of anchored retaining walls must be analysed according to R 44 using the safety factors given there. The following must be observed: a) The starting point for the deep failure plane is generally the retaining wall toe. b) In excavations with a change of soil layers at the excavation level as shown in Figure R 99-lb, the anchored block must generally be supported by a low set of struts, blinding concrete installed in stages or a base slab installed by jet grouting. c) In excavations with alternating layer of soft soils and load-bearing soils, a deep failure plane may develop, the course of which is not a straight line from the centre of gravity of the grouted section to the wall toe, but instead is interrupted by a lengthy horizontal section in one of the soft layers. 6. In excavations in which - the soft layer as shown in Figure R 99-2 is below the wall toe and there-

fore - the development of a toe support in the covering layer is possible, the

following must be observed: 161

a) When determining the active earth pressure, the wall friction angle must be adopted with 6, = 0 because transmission of vertical forces to the subsoil is not guaranteed. b) In braced excavations as shown in Figure R 99-2a, sufficient embedment depth in the load-bearing cover layer must be demonstrated. Here, the wall friction angle must be adopted with 6, = 0 so that no slip surface through the soft soil reaches critical status. c) For anchored retaining walls as shown in Figure R 99-2b, sufficient sliding safety

must be verified. Greater safety may be necessary in order to limit the anticipated displacements. The passive earth pressure in the cover layer must be determined using the wall friction angle 6, = 0. For the sliding resistance either

R = W.tan@ where @ =

@: or where @ = @iu according to R 95, Paragraph 3, or

K = c,*L The smaller value has to be taken.

Figure R 99-2. Excavation with soft soil below the wall toe

162

12.11 Water management for excavations in soft soils (R 100, Draft) 1. Substantial settlements must be anticipated in soft soils if the groundwater is lowered so that the buoyant effect is lost and the weight of the saturated soil can act. Drawdown or relief of the groundwater table is therefore only permissible within strict limits.

2. The groundwater table is generally subject to seasonal fluctuations. Because of the extremely unfavourable influence of positive water pressure on the determination of embedment depth and the internal forces of the retaining wall, it is recommended to lower the groundwater table to the lowest known previous level by arranging a ring drainage system around the outside of the excavation. It may generally be assumed that the soil is consolidated at this level.

3. Within an excavation lined according to R 92, Paragraph 1, it is generally acceptable to dewater intercalated bands of fine-sand or coarse silt or to lower an existing confined groundwater table. Here, the wells should terminate above the retaining wall toe in order to restrict the effects of dewatering outside of the excavation. Vacuum filter wells should be implemented if gravity dewatering is insufficient or if additional compaction is aimed for. 4. The localised use of vacuum lances for stabilisation of slopes, e.g. when manufacturing trenches for installation of blinding concrete strips according to R 93, Paragraphs 3 or 4, generally presents no problems with regard to neighbouring structures.

5. Residual perched water and surface water should always be collected in filter stable surface drains according to DIN 4095 and directed to pump sumps. The pump sumps should be operated long enough to rule out heave of a readymade part of the structure. 6. The effects of water management measures must be constantly monitored inside and outside of the excavation.

12.12 Serviceability of excavation structures in soft soils (R 101, Draft) 1. The serviceability of excavation structures depends on - the accurate investigation and assessment of the soil situation,

- the selection of a suitable wall and floor construction, - the selection of a suitable construction method, - realistic approaches for analysis and design, - technically correct implementation of monitoring and construction work.

If deficits in only one of these points occur it must be assumed, in contrast to excavations in soil of good load-bearing capacity, that grave impacts on the 163

surroundings will result and may extend far deeper than the depth of the excavation. In addition to the stipulations in the previous Recommendations the following points must be observed.

2. The demands on the serviceability of excavations in soft soils must be defined with the manufacture of the structure in the excavation and the effects on the surroundings in mind: a) If it is certain that no structures in the vicinity of the excavation are impacted, it is sufficient to draft the design according to R 95 to R 99 and to guarantee, by implementing a working space or by selecting a sufficiently large degree of tolerance when installing the retaining wall in excavations without a working space, that large soil movements do not compromise the serviceability of the retaining wall. b) In excavations in the vicinity of settlement- and deformation-sensitive structures, it is of prime importance to limit soil movements, as well as wall deformations. Here, the only possibility in soft soil is to maintain, as far as possible, the primary stress state in the soil. It is of decisive importance to restrict relaxation and decompaction of the soil below the excavation floor.

3. The most obvious measures for maintaining the primary stress state are the selection of a stiff retaining wall and the implementation of stiff supports at ground level. In addition, stiff support of the wall toe and measures to prevent base heave may be considered. In principle, the following impacts can be anticipated: a) In large excavations with a load-bearing cover layer next to the wall toe as shown in Figure R 101- l a there is a hazard of the toe support in the cover layer to slip and the excavation floor being subjected to heave, which leads to extensive relieve of the ground behind the retaining wall and thus to settlements and deformations. This is not substantially influenced by the use of struts instead of anchors. b) If additional floor bracing is installed as shown in Figure R 101-lb the toe support is largely free from deformation, but even with a numerically adequate safety against base heave it is possible for the excavation floor to lift, leading to subsequent settlement behind the retaining wall. c) If additional stabilisation of the excavation floor against heave is implemented by means of ballast, floor doming or tension piles/anchors to a sufficient depth as shown in Figure R l o l - l c , base heave can be avoided to the extent that settlement behind the retaining wall is greatly reduced or unloading heave occurs. The favourable influence of floor bracing or ground anchors is increased if these are installed before excavation begins instead of in stages after reaching the excavation level. 164

Settlement

Figure R 101-1. Soil movements as a function of base stabilisation

165

In Figure R 99-1 three cases with varying depths of the layer boundary between load-bearing soil and soft soil are shown, representing increasing demands on stabilisation measures.

4. Conservation of evidence measures should be carried out at an adequate radius around the projected excavation and on existing structures before starting construction work, and the groundwater level recorded. All subsequent work sequence phases which impact on the soft soil should be accompanied by settlement measurements in the vicinity at sufficiently short intervals during the course of work. It is necessary to repeatedly inspect the structures in the immediate vicinity of respective work areas whilst installing retaining walls and during excavation work. As soon as critical settlements in the vicinity or wall deformations or cracks in neighbouring structures which are perceptible to the naked eye appear, excavation work must be stopped immediately and, for advanced excavations, supportingberms tipped or the excavation partly backfilled until settlement processes cease. 5 . The manufacture of excavations in soft soils without settlement processes on

neighbouring buildings is only possible with very favourable boundary conditions. Deformations as a result of the excavation process are generally unavoidable, in particular with increasing excavation depth. These deformations cannot be determined with sufficient precision using classical analysis methods. Analysis using numerical methods, in contrast, e.g. based on finite element methods (FEM) according to R 103, can provide approximately correct deformation figures when used with realistic material parameters. If possible, the projected FE model should be calibrated on the basis of measurement results from an excavation executed in similar soil conditions. The implementation of FE analyses is particularly valuable if the deformation reducing effect of any additional support measures needs to made visible. The plausibility of adopted earth pressure distributions or earth pressure redistributions can also be represented in an FE analysis.

166

13

Dimensioning of structural elements

13.1 Dimensioning of soldier pile infilling (R 47) Irrespective of whether permissible soldier pile stresses were determined using an earth pressure rectangle according to R 13 (Section 5.3) or by means of a more realistic pressure diagram, either may serve for dimensioning the infilling, provided the various allowable stresses are taken into consideration. In case of a triangular or classical earth pressure distribution being selected, either the tip, as shown in Figure R 47-la, or the maximum value, as shown in Figure R 47-1b, may be truncated. However, the remaining earth pressure ordinate must equal at least two thirds of the original. Infill wall thickness can be adjusted to the respective pressure diagram. For the infill wall dimensioning, the earth pressure from live loads or building loads may be superimposed with the pressure diagram from soil selfweight earth pressure, unbounded distributed load and cohesion in such a way that a uniformly distributed load (as shown in Figure R 47-2) is created in the region of the load distribution boundaries according to R 7, Paragraph 2 (Section 3.5) or R 28, Paragraph 2 (Section 9.3), respectively.

+hy ai Triangular pressure diagram

ai Load distribution

bi Classical pressure diagram

Figure R 47-1. Reduction of earth pressure from soil self-weight, unbounded distributed load and cohesion when dimensioning infilling

bi Possible pressure diagrams (Examples)

Figure R 47-2. Earth pressure from live loads for infill wall dimensioning

167

3. The dimensioning earth pressure must generally be adopted as a uniform load from soldier pile to soldier pile. The impact of the soil shear stress between soldier piles and the resulting decrease in stresses on the infill wall may be taken into consideration i f a) either a medium-dense or densely compacted, cohesionless soil or at least firm, cohesive soil is present, b) soldier piles are driven or, in the case of soldier piles set into boreholes, the backfill material is compacted in such a way that a friction bond is developed between soldier piles and the native soil, c) the infilling is installed, without pre-bending, behind the flanges on the excavation side and d) sufficient bending deformation is ensured under the calculated load. See also DIN 4085 and [53].When using triangular pressure diagrams and the classical pressure distribution, the impact of shear stress can be related to the remaining earth pressure ordinate as shown in Figure R 47- 1.

4. In general, the vertical earth pressure component may be neglected when dimensioning infill walls, except in the following cases: a) if soil shear stress is taken into consideration according to Paragraph 3, or b) individual infill elements are arranged vertically, supported by nogging pieces. It may be necessary to consider the vertical earth pressure component arising from the horizontal component multiplied with the tangent of the of wall friction angle, determined on the basis of Paragraph 1 or Paragraph 3.

5. Paragraphs 1 to 4 apply to all infill types accordingly. 6. Maximum allowable stresses for timber infill must comply with Appendix A 6 and values for steel infill must comply with Appendix A 7.

13.2 Dimensioning of soldier piles (R 48) 1. When dimensioning soldier piles, the self-weight of the excavation structure may be neglected. Besides normal stresses, however, shear stresses and effective stresses must be analysed in every case (see Appendix A 7). 2. If no vertical forces other than the self-weight of the excavation structure and the vertical earth pressure component need to be transmitted, a general stress analysis will suffice to determine edge stresses. In the case of other vertical forces to those previously mentioned, e.g. from excavation covers, provisional bridges or inclined anchors, a stability analysis must be undertaken, in particular for single-propped retaining walls and for the individual retreating states of multiple-propped retaining walls.

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In the case of a soldier pile wall infill wedged behind the front flange faces, it can be assumed that the front soldier pile flanges are secured against deflection by the infill whilst flanges on the soil side are protected against deflection by the surrounding soil. Otherwise, additional continuous waling must be linked to soldier piles in such a way that it provides sufficient strength to counteract lateral torsional buckling of the flanges on the excavation side. Double [-sections must be connected by battens both on the excavation and the soil side, ensuring the battens are positioned close enough together. A torsional stress analysis may be dispensed with if the batten spacing does not exceed 1.5 m. Stability analysis is not required if sections, except their facing sides, are fully embedded in concrete. Maximum allowable stresses must be determined according to Appendix A 7. With regard to limit load design, see R 27, Paragraphs 1 and 4 (Section 4.4). R 50 (Section 12.4) applies accordingly to the dimensioning of reinforced concrete piles with infilling installed in the spaces.

13.3 Dimensioning of sheet piles (R 49) 1. The self-weight of the excavation structure may be neglected when dimensioning sheet piles. Analysis of normal stresses is sufficient for sheet piles with interlocks inside the flanges, unless the major part of loads acting on retaining walls results from water pressure. 2 . If no vertical forces other than the self-weight of the excavation structure and the vertical earth pressure component need to be transmitted, a general stress analysis will suffice to determine edge stresses. In case of other vertical forces to those previously mentioned, e.g. resulting from excavation covers, provisional bridges or inclined anchors, a stability analysis must be undertaken, in particular for single-propped retaining walls and for the individual retreating states of multiple-propped retaining walls. 3. A zero-line shear force transmission analysis must be performed for sheet pile walls consisting of U sections i f a) the sheet pile wall is located in open water or if a significant portion of it is driven through peat, tidal mud deposits, mud or soils with high clay content, b) grease or a bitumen-based sealant e.g. is applied to reduce interlock friction prior to the driving process, or if interlocks are appropriately protected against penetration of soil particles or c) connecting elements between individual sections exceed tolerances as stated in Recommendation R 97 of the Recommendations of the Committee for Waterfront Structures [2]. 169

Continuous wall section properties may be taken fully into consideration even if shear force transmission is only projected for every second interlock. 4. Maximum allowable stresses must be determined according to Appendix A 7, taking increased stresses into consideration (see R 24, Paragraph 4, Section 2.1). Sheet pile steel grade StSp 37 must be classified as structural steel grade S 235 and sheet pile steel grade StSp S as structural steel grade S 355. Sheet pile steel grade StSp 45 is classified according to Recommendation R 20 of the Committee for Waterfront Structures [2]. With regard to limit load design, see R 27, Paragraphs 1 and 4 (Section 4.4).

13.4 Dimensioning of in-situ concrete walls (R 50) 1. Besides reducing the computed maximum support moment according to R 11, Paragraph 5 (Section 4.3), the moment diagram may be smoothed out at each point of support if concealed beams or reinforced concrete waling are installed. For rolled section walings, the full flange width may be considered as support only if web stiffeners are designed to sufficiently prevent flange deflection and if the space between waling and retaining wall is filled with concrete. 2. When determining shear reinforcement, diaphragm wall slices with thicknesses greater than one fifth of their width must be treated as beams unless individual slices are friction bonded with dowels. 3. The relevant regulations for reinforced concrete apply for the dimensioning of in-situ concrete walls, with the following additions: a) The computed internal forces may be reduced by 15 % for cross-section dimensioning and for analysis of partial area loads. b) The computed internal forces may be reduced by 25 % when testing, overstressing or unloading struts or anchors.

4. With regard to reinforcement positioning and concrete cover, the requirements of EN 1538 must be met for diaphragm walls and EN 1536 for pile walls. 5 . When analysing anchoring lengths, the bonding properties of horizontal rebars

should be classified as “moderate” and those of vertical rebars as good.

13.5 Dimensioning of walings (R 51) 1. If waling subject to bending stresses is utilised for transmission of axial forces, it must be analysed for both bending and buckling. Only deflections on the excavation side need be taken into consideration when determining the buckling length. 170

2. Shear stresses and effective stresses must always be analysed for steel section waling subject to bending. 3. If a cantilever effect is considered when determining bending moments, the impact of unintentional displacement of load transmission or support points must be assessed.

4. If web stiffeners are welded on at load transmission or support points of steel section waling, or if waling is concreted, it can be assumed that flanges are sufficiently protected against deflection. This also applies to friction fit steel plate or timber web stiffeners, provided they are installed with reasonable care and accuracy.

5. Maximum allowable stresses for waling and steel sections are given in Appendix A 7. With regard to limit load dimensioning, see R 27, Paragraphs 1 and 4 (Section 4.4).

6. R 50 (Section 13.4) applies accordingly to the dimensioning of reinforced concrete waling.

7. Excavations adjacent to structures may require limitation of waling bending and appropriate choice of sections depending on the degree of deformation that the structure may tolerate.

8. Walings designed to prevent collapse of the excavation structure only for a limited period following complete failure of an anchor or strut may be designed, if analysis is required in exceptional cases, taking full consideration of the reserves inherent in the supporting structure and soil, e.g. arching effects and the steel yield strength.

13.6 Dimensioning of struts (R 52) 1. With regard to exposure to stresses and risk of failure, struts constitute the most sensitive elements of an excavation structure. Therefore, dimensioning always be based on conservative assumptions. In case of any doubt as to whether the pressure diagram selected for a specific row of struts provides safe support reactions, appropriate surcharges must be incorporated. 2. Generally, strut dimensioning must take eccentric force transmission into consideration in addition to the normal force and bending moment. For steel struts, deflection due to dead and live loads must also be considered. Furthermore, tilting, buckling and, where necessary, lateral torsional buckling must be investigated for rolled section struts.

3. If no specific force transmission eccentricity is defined and ensured by appropriate procedures, the stability analysis carried out for steel struts must include the following additional vertical eccentricities: 171

a) an eccentricity of one sixth of H-pile height for rolled sections or one sixth of the tube diameter for tubes without end centring, b) one tenth of the tube diameter for tubes with end centring. Eccentricity must be added to deflection.

4. If the buckling length of struts must be reduced, the waling and bracing required must be fitted to the top and bottom of the struts. In place of the bracing on the bottom other, similarly acting constructions may be installed. If buckling safeguards are not desirable or must be avoided as far as possible for operational reasons, the use of tubular sections is recommended.

5. The buckling length is defined as the length of the strut excluding wedges, packing pieces and waling. If the strut ends are not restrained according to design, it must be assumed that they can rotate freely. Where applicable, this is also valid for points where the buckling length is shortened by an antibuckling element. 6. The influence of temperature increases must generally be considered according to [92]: a) at long-term construction sites with large seasonal temperature fluctuations, b) when using slender I-beam struts without anti-buckling elements, c) when using short steel struts with anti-buckling elements and relatively stiff abutments, such as provided by rocky ground or in-situ concrete walls, except for the cases mentioned below. Dimensioning aids for IPB sections can be found in [92]. Analysis may be dispensed with: a) for IPB steel struts for soldier pile walls, b) for tubular sections, c) for trench sheeting with shoring struts, d) for timber struts.

7. For narrow excavations, frost action must be taken into consideration if frostsusceptible soils lead to the assumption that the strut forces may increase considerably when the soil freezes. 8. Constructions that serve to reduce the buckling length of struts, such as central supports, waling and bracing, must be designed for loads perpendicular to these struts, which may be assumed at of the sum of the normal forces occurring in the struts. If two or more of these constructions are arranged side by side, each one must be dimensioned for the given load. The same applies to common bracing. Rigid connections, e.g. welding and high strength screw connections, must be designed for twice the calculated loads, taking possible constraining forces into consideration.

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9. The allowable stresses for round timber struts are given in Appendix A 6, and in Appendix A 7 for steel struts. Analysis of the load-bearing capacity of shoring struts is according to the relevant regulations.

13.7 Dimensioning of grouted anchors (R 87) 1. The tensile strength limit value F of the anchor is given by a) the tensile strength F, of the anchor head construction and the steel tension member according to Paragraph 3, b) the tensile strength FK of the grouted element when transmitting the tensile force into the ground according to Paragraph 4. 2. The tensile strength limit value F, of the anchor according to Paragraph l a is given as follows: a) The tensile strength F, of the anchor head construction must be confirmed by a technical approval certificate. b) The characteristic tensile strength Fs of the steel tension member is given by F, = A,*fy with A, = cross-sectional area of the tension member fy = nominal yield strength 3. The tensile strength limit value F, of the grouted element according to Paragraph 2b is the smallest value from three individual qualification tests according to Paragraph 4. For anchors with a dimensioning life of up to two years a renewed qualification test may be dispensed with if a comparable test has previously been performed in comparable soil conditions. 4. The stipulations given in EN 1537 apply for the extent, implementation and evaluation of qualification tests. The following definitions must be observed:

a) The test force F, is given by the working force F, according to Paragraph 6 with Fp

= qK*FW

qK= 1.50 for anchors which are designed for active earth pressure or for buoyancy forces, - using the factor r\, = 1.33 for anchors which are designed for at-rest earth pressure. - using the factor

However, the test force may not exceed 90 % of the anchor tensile strength value F, according to Paragraph 2. 173

b) The tensile strength FK of the grouted element in an individual test is the force that generates a creep of k, = 2 mm.

5. All anchors must be subjected to an acceptance test before being stressed to the prestressing force F,. The acceptance test should demonstrate that each anchor installed can take the projected design load. Here, each anchor should be prestressed to the test force: Fp= = 1.25.FW The acceptance test should be carried out according to EN 1537.

6. The working force F,, which is the basis of the acceptance test, is the force that results from the determination of internal forces, taking the pre-stressing into consideration.

7. The allowable anchor force allow. F must be determined: a) For the grouted element with: FK allow F I 7K

b) For the steel tension member with:

FS allow F I ‘IS

The respectively smaller values have to be taken. The safety factors 7, and qs may be taken from Appendix A 8. For increased active earth pressure, interpolation may be carried out according to the respective proportions of active earth pressure and at-rest earth pressure.

13.8 Dimensioning of trench sheeting and bracing (R 53) 1. R 47, Paragraphs 1 and 2 (Section 13.1), apply for determination of the pressure diagram used for the design of sheets of horizontal trench sheeting, unless already based on rectangular pressure diagrams. 2 . For vertical trench sheeting, R 49 (Section 13.3) applies for the design of timber planks, trench sheet piles, driven sheet plates, curtain sections or lightweight sheet pile walls accordingly. 3. R 5 1 (Section 13.5) applies for the design of waling for vertical trench sheeting. R 51 applies accordingly for the design of soldier beams for horizontal sheeting. 174

4. Irrespective of the specific material used, the cantilever effect of projecting ends and the continuous beam effect may be taken into consideration in case of multiple-propped elements of horizontal or vertical trench sheeting. 5. R 52 (Section 13.6) applies for strut dimensioning.

13.9 Dimensioning of provisional bridges and excavation covers (R 54) 1. The determination of internal forces of structural elements of provisional bridges and excavation covers must take into consideration the following loads in addition to self-weights: a) for provisional bridges and excavation covers designed to accommodate road and rail traffic; loads according to R 55, Paragraph 7 (Section 2.4), b) for provisional bridges and excavation covers for site traffic, as well as for excavation covers provided to create storage or work spaces; loads according to R 56, Paragraph 5 (Section 2.5), c) for operating areas of excavators and lifting equipment; loads according to R 57, Paragraph 7 (Section 2.6), d) for pipe bridges; self-weights of lines, pipes, protective elements and, if applicable, materials or substances inside pipes, including resultant deflection and surge forces, e) for protective covers; wind loads and snow loads according to the relevant regulations and, if applicable, loads resulting from build-up of water pockets on sheet coverings.

2. Maximum allowable stresses as set out in Appendix A 6 and Appendix A 7 apply to the dimensioning of provisional bridges and excavation covers unless, in the case of rail traffic, regulations and guidelines of the respective transport company prevail. If main walings also act as stiffening elements, R 52 (Section 12.6) must also be considered. 3. In addition to standard analyses prescribed in generally accepted regulations and guidelines, e.g. stability and allowable stress analyses,the following analyses must usually be undertaken for provisional bridges and excavation covers: a) transfer of vertical and horizontal loads from road pavements into the ground via the supporting structure and retaining wall and, if necessary, by means of intermediate supports and load distributing bearing structures, b) safety of road pavements and supporting structures against uplift, also with a view to anticipating noise impact that may be caused by pavement detachment. 175

4. In certain cases it may be necessary to restrict deflection of provisional bridges and excavation covers and to select their dimensions as a function of the maximum tolerable deflection. The following criteria may serve to determine such dimensions:

a) for provisional bridges and excavation covers designed for road and rail traffic, maximum permissible speed, potential hazards to the pavement, driving comfort, or impact on vehicles, b) for pipe bridges involving rigid pipes, strength and deformation behaviour of pipes and sleeves if deflection cannot be compensated for by appropriate structural design, c) for protective covers, the amount of water drainage or prevention of ponding. For provisional bridges and excavation covers designed for road and rail traffic, it is widely accepted practice to restrict live load related deflection to 1/500 of the structure’s span. In addition, dead-load related structural deflection is often compensated for by appropriate superstructure design, which is of particular relevance if the structure must accommodate rail traffic. In the region of points, it may be necessary to reduce deflection even further and to restrict the potential rotation angle at the ends of main walings to a tolerable level.

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14

Measurements on excavations

14.1 Purpose of measurements (R 31) 1. Although current understanding of the mutual influence of ground and retaining walls allows predictions on their behaviour in general and suggestions for approximate analysis concepts, there are still a number of questions that are not fully understood. Therefore, measurements should, if at all feasible, be performed at the excavation structure in order to expand the knowledge base and to gather more reliable data to serve as a basis for economical and safe design, even if measurements have already been carried out under similar conditions.

2. Usually, current knowledge still leaves room for interpretation with regard to assumptions on the magnitude and distribution of earth pressure. If, in individual cases, this tolerance is utilised almost to the full and no more hidden load-bearing reserves consequently exist, measurements are always recommended as an appropriate method to investigate available safety. 3. Under uncertain conditions, such as the presence of soft or highly cohesive, or stratified or highly faulted soils, measurements may be required in order to ensure an economically viable solution. Instead of proceeding according to R 5, Paragraph 7 (Section 3.3), i.e. to calculate several possible load approaches and apply the least favourable internal forces as the basis for design, it suffices in many cases to apply only the most probable approach and to verify the reliability of assumptions by measurements. Should measurements verify deviations from assumed parameters, these can be specifically addressed by implementing suitable structural measures.

4. Measurements are also recommended if site-specific subsoil properties can only be determined in the course of excavation activity. It is then possible to plan and design certain construction procedures, such as the installation of struts or anchors, depending on the established behaviour of the excavation structure wall. This also applies to analysis of the magnitude and distribution of earth pressure during the individual retreating states according to R 68, Paragraph 3 (Section 3.8). 5. Long term excavations are subject to the risk of earth pressure magnitude and distribution changing with time. This particularly applies if exposed to intense vibrations, and to cohesive soils in general. In such cases, measurement of support reactions and wall deflections is recommended. 6. Measurements may also be helpful to ensure that a retaining wall neither moves away from nor toward an adjacent structure. In this respect, other variables of interest are settlement and, where applicable, heave of the retaining wall, adjacent structures and the surrounding site.

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7. For excavations in partly cohesive soils, dewatering measures may be assessed for suitability. This particularly applies to confined groundwater below the excavation level. See R 66, Paragraph 3 (Section 10.9).

14.2 Preparation, implementation and evaluation measurements (R 32) 1. Measurements must be carefully prepared in order to preclude failure due to unpredictable circumstances. These include, amongst others: a) defining the measurement area, taking operational and measurement-specific factors into consideration. See R 35 (Section 14.5), b) gathering information on soil types and groundwater tables found in situ, c) determination and co-ordination of all technical details deemed relevant from a structural and procedural point of view, d) coordinating relevant dates with all parties involved, with particular emphasis on construction progress and execution, e) timely provision of the required measuring instruments, including prior checking of proper working order, if necessary, f) preparation of a work schedule, including detailed instructions to all parties involved.

2. When installing girders, sheets, or struts to be used for measurements, care must be taken that design dimensions are precisely adhered to. If this is not possible, deviations must be determined. When installing measuring instruments, utmost care must be taken that they are not damaged. Required protective devices must be installed as early as possible in the process. Competent and experienced companies or institutes should install sensitive measuring systems. 3. On-site measurements should be assigned only to competent persons sufficiently trained and experienced with regard to the measuring equipment used. If required, suitable specialist companies or institutes should be consulted. If possible, personnel to whom measurements are assigned should not change throughout the process.

4. All stresses, forces, bending moments, water levels, deformations, displacements etc. derived from measurement data should be continuously recorded, using an easily legible format, in order to facilitate detection of irregularities at an early stage. It is recommended to record data as a function of time, complemented by notes on the construction procedure and measured temperatures. In addition, detailed information on soil stratification and properties, excavation structure, adjacent structures etc. will usually be required for evaluation.

5. Preparation, execution, monitoring and evaluation of measurements should all be provided by the same contractor. 178

14.3 Measured variables (R 33) 1. As a minimum requirement, measurements in excavations should generally yield data on the total acting earth pressure. The easiest and most economical way to achieve this is to measure strut or anchor forces. Methods appropriate for this purpose are either direct load measurements according to R 34, Paragraph 1 (Section 14.4) or indirect load determination by means of measurement of elastic length changes in struts or anchors according to R 34, Paragraph 2 (Section 14.4). Only in exceptional cases should struts or anchors be loosened to determine support reactions. Furthermore, because of the probable ambiguity, direct earth pressure measurement for determination of earth pressure according to R 34, Paragraph 3 (Section 14.4) is not recommended. However, in order to accurately delineate earth pressure and ensure thorough support reaction measurements, it may be expedient to expose the wall toe or to cut off soldier piles or sheet pile walls and subsequently install load cells between the blinding concrete and the retaining wall.

2. In the case of retaining walls with four or more rows of supports, the magnitude and distribution of the earth pressure can generally be determined with sufficient precision on the basis of strut and anchor force measurements. Measurement evaluation should take the continuous beam effect into consideration. If only determination of an appropriate pressure diagram is required, strut and anchor force measurements will also suffice in the case of retaining walls with two or three rows of supports. Otherwise, earth pressure distribution in retaining walls with three or less rows of supports can generally only be determined with sufficient precision if, in addition to strut and anchor forces, bending stress or deformation of soldier piles, sheet piles, piles or diaphragm wall slices are measured, or additional direct earth pressure measurements are carried out. If requirements for determination of earth pressure distribution are more demanding, in particular if an earth pressure reduction between supports must be determined, this must also be applied to retaining walls with four or more rows of supports. In this case, the influence of the continuous beam effect on support reactions must always be taken into consideration. 3. In case of excavations in water, measurement of the magnitude and distribution of the water pressure may be required. If only water pressure from the water level to the excavation level is required and conditions are not too complex, measurement of strut and anchor forces according to Paragraph 1 should meet requirements for open-water retaining walls with three or more rows of supports. In such cases, earth pressure is generally low enough to be calculated with sufficient precision and to be deducted from the total measured load acting on the wall, in order to arrive at water pressure loads. For retaining walls with less than three rows of supports, several groundwater storeys, excavations in groundwater and in any other case where greater measurement precision is required, water pressure loads can be determined with sufficient 179

accuracy only by means of direct measurement according to R 34, Paragraph 4 (Section 14.4). Effective wall friction can most easily be determined on the basis of acting vertical forces. For this purpose, the following methods are principally used: a) Edge strain measurements according to R 34, Paragraph 2 (Section 14.4) can serve to not only determine bending moments but also normal forces acting in soldier piles and sheet piles. b) After reaching the final excavation depth, soldier or sheet piles located within the measurement area may be fixed to neighbouring wall sections by means of hinged diagonal bracings, and subsequently cut through at excavation level. Elongation measurements may now serve to determine the stress acting on the suspension structure and to deduce the magnitude of the acting vertical force. c) Soldier piles, sheet piles or in-situ concrete walls within the measurement area are only installed as deep as the excavation level and are hung onto the neighbouring sections by means of highly stiff cross beams. The vertical force acting in the measurement area can then be measured directly by means of load cells positioned at the cross beam support points. Direct wall friction measurement by means of shear force load cells is problematic. Besides loads acting on the retaining wall, stresses on soldier and sheet piles, in-situ concrete walls and soldier pile infill walls can be measured. Generally, the following methods can be applied: a) edge strain measurements according to R 34, Paragraph 2, b) bending line measurement according to R 34, Paragraph 5, c) curvature measurement according to R 34, Paragraph 5 (Section 14.4). Under less complex circumstances, it may be sufficient to measure the deflection at specific points, e.g. at unsupported or single-propped retaining walls. In all cases, the accuracy of stress determination not only depends on measurement precision but also on the precision of determination of the modulus of elasticity of the specific material used. Without specific reference to related loads acting on individual sheeting elements, in certain cases it may be necessary to monitor horizontal and vertical movement and deformation of the retaining wall and the surrounding area, e.g. if adjacent structures are exposed to the risk of damage. In such a situation, it often suffices to measure the distance between two opposite walls or between a wall and a defined axis. In particular cases, measurement of wall toe movements may be necessary, using a tube installed within the retaining wall. Parallel to determining loads, stresses and deformations, air temperature in the shade must always be recorded. It may also be useful to determine the temperature of building elements subjected to measurement. Here, tempera180

ture changes that occur as a function of the distance from ground level must be considered. Otherwise, it is recommended to gather key weather data, such as precipitation and cloud.

14.4 Measurement methods (R 34) 1. The following systems are mainly used to directly measure tensile and compressive forces: a) b) c) d)

mechanical load cells, hydraulic load cells, electric vibrating wire load cells, electric strain gauge load cells.

Mechanical and hydraulic load cells offer the advantage that readings can be taken without requiring any further measuring equipment and by non-skilled staff. This is generally subject to free access to the point of measurement. Without the use of specific converter systems, separate points of measurement and reading are only possible with hydraulic load cells, and only over limited distances. With electric load cells, separate reading and measurement points are the rule, with no distance restrictions. Readings may also be automated.

2. The following systems are available to measure length variations: a) settlement strain gauges, b) vibrating wires, c) strain gauges. The mechanical settlement strain gauge is the most economical method in terms of the amount of preparatory work necessary at the point of measurement. However, this point must be freely accessible for the actual measurements, in contrast to the two electrical systems, which allow remote measurement to be carried out and also require less time. Greater accuracy is another point in favour of electrical systems. On the other hand, strain gauges are sensitive to moisture and vibrating wires are sensitive to intense vibration, at least if not welded but only clipped into position. This potential disadvantage is outweighed by the fact that these systems are well suited for longer-term measurements. The installation of strain gauges requires a high degree of experience and care; such work should therefore be contracted to experienced companies only. Otherwise, points of measurement fitted with strain gauges are generally irrevocable if the feed line is damaged. In contrast to this, strain gauges also offer the advantage that battery-powered display units are available, thus dispensing with the requirement for mains connections. 3. Direct earth pressure measurements are very difficult to perform, and so far have not always been successful. Nevertheless, it may be useful in individual

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cases to carry out direct earth pressure measurements in order to support other methods. Such measurements are primarily executed by means of the following pressure cell types: a) b) c) d)

hydraulic pressure cells, operating in closed-circuit, hydraulically operated valve sensors, electric vibrating wire pressure cells, electric strain gauge pressure cells.

Results may be impaired when using any of these pressure cells if cell compressibility significantly differs from the elastic behaviour of the soil. Only valve sensors are less sensitive in this respect.

4. The following systems are available for measuring water pressures acting on retaining walls: a) standpipes, b) electric pore water pressure sensors. Electric measurement of pore water pressure is recommended in less permeable soils in particular. In addition, this method is appropriate if points of measurement are difficult to access, or not accessible or if automated reading is intended. In general, pore water pressure gauges based on the vibrating wire principle are preferred; their display accuracy is independent of feed line length and is not impaired by long-term operation. In this respect, pore water pressure measurement systems fitted with strain gauges are considered less suitable. Such systems are suitable options if they can be calibrated after installation, based on a known groundwater table, and if they are not intended to remain in place for significantly more than a year. They offer the advantage of being autonomous if battery-powered data readers are used.

5. The following methods may be used for determination of the bending line: a) measurement of distance between two opposite retaining walls, b) measurement of distance between one of the retaining walls and a defined axis, c) measurement of change in wall inclination as a function of depth. The method specified under a) is only appropriate for measuring bending moments in soldier piles and sheet pile walls if these can be installed without pre-bending, i.e. without stress, which is not always the case for driven soldier and sheet piles. The same applies to the method given under b), if the measurement axis is located within the excavation and measurements follow excavation activity. If, on the other hand, measurements are performed by means of a tube cast in concrete, the progression of the bending line can always be estimated. However, the method described under c) is better suited; here, a mobile inclinometer is used to measure variations in wall inclination as a function of depth. It is often sufficient to use simple methods to only measure curvature at mid-span for elements propped or supported at their 182

ends only, such as retaining walls with a single support at ground level, and steel or timber infill of soldier pile walls.

6. Surveyor’s tape, metre rule, calliper scale and surveyor’s level or theodolite generally suffice for measuring movements, e.g. if the following parameters are measured: a) horizontal deflection of the retaining wall top at various construction stages, b) horizontal retaining wall deflection at the level of a row of struts or anchors during prestressing, c) retaining wall bending upon removal of a row of struts or anchors, d) settlement or heave of the retaining wall, of the adjacent ground surface or of a structure, e) heave of level benchmarks. In this case it is important to appropriately secure individual points of measurement and to monitor benchmarks. For the definition of height gauges, exposed beams, such as intermediate supports of anti-buckling devices or provisional bridge supports, can be used if heave must be measured only at final excavation level. Monitoring of vertical movements at several levels may require mounting of extensometers, slide micrometers, or comparable systems.

7. No measurement method is reliable enough to be applied without any control. Such control may include: a) comparison of the zero value of an unstressed element prior to installation and subsequent to removal, b) comparison of measurement results in several adjacent sections, c) installation of redundant systems employing the same instruments systems, d) installation of redundant systems employing different instrument systems, e) measurement of various parameters to facilitate comparison of results. In some cases, random control measurements may suffice.

14.5 Requirements on the measurement area (R 35) 1. It is recommended to always obtain a detailed picture of ground conditions in the intended measurement area at an early stage in order to correctly delineate the anticipated loads, stresses and deformation parameters of the excavation support structure, and to identify suitable measuring instruments and provide for their most appropriate use. Besides soil samples extracted from boreholes prior to commencing construction activity, further samples should be taken throughout the excavation process. In order to arrive at a correct interpretation of measurement results, it is generally necessary to determine at least the particle grain size distribution, unit weight, water content, and shear strength, as well as the relative density of cohesionless soil and the 183

consistency of cohesive soil. In addition, it may be necessary to determine the modulus of elasticity and the permeability.

2. The measurement area should meet the following requirements if plane stress conditions are applied to the evaluation of load and deformation measurement results: a) identical ground conditions and groundwater table within the measurement area and adjacent zones, b) for braced excavations, identical structural conditions at the left and right of the measurement area, extending over a minimum length of half of the excavation depth, c) for anchored retaining walls, identical structural conditions at the left and right of the measurement area, extending over a minimum length equal to the excavation depth. When interpreting measurement results obtained in excavations of limited length, three-dimensional influences must be taken into consideration.

3. The design of lining and bracing or anchoring elements located within the measurement area should be identical to neighbouring sections. For example, soldier pile wall infill in the measurement area may not be located behind the external flanges if they are located behind the internal flanges in adjacent areas. If at all possible, weakening of soldier pile or sheet pile wall sections or reinforcements must be avoided. If additional structures for protecting measuring instruments and cables are unavoidable, they should be linked to loadbearing sections such that they either carry the full load or no load at all. Measuring instruments and protective installations must not interfere with the measuring process.

4. Construction procedures in the measurement area must be identical to those in adjacent zones. For example, measurement results are questionable if measurement piles are set in boreholes whilst adjacent soldier piles are driven. Struts in the measurement area must not be treated differently to neighbouring struts. They must be wedged and prestressed identical to, and roughly concurrently with, the remaining struts. Otherwise, measurement results may be affected by horizontal force redistribution in the soil.

14.6 Number of measurement areas and measurement sections (R36) 1. A measurement area is only representative for a given excavation length if the soil and groundwater conditions, surrounding structures and construction methods remain roughly identical. If these criteria contain major variances, and measurements that go beyond random sampling are planned, measurements should be carried out in more than one area.

184

2. In excavations whith low prestressed struts or anchors, measurements within a defined area must normally include simultaneous measurements in three adjacent sections in order to ensure that data is representative and to avoid the possibility of random results. This facilitates mutual control of measuring instruments in case of specific points of measurement becoming dysfunctional due to damage or failure and allows mean values to be computed in order to compensate for result variance. As a minimum, however, measurements must be carried out in two adjacent sections. Only in exceptional cases, e.g. if only one control measurement is carried out to verify data gathered by a fully valid measurement, may it be sufficient to measure only in one axis. 3. In excavations where struts are prestressed to a level leading to an anticipated earth pressure higher than the active earth pressure according to R 8, Paragraph 3 (Section 3.1), measurements at two adjacent sections will normally suffice. In exceptional cases, measurement at only one section may be sufficient. The same applies to anchored retaining walls if the anchors are prestressed with a force not significantly less than 80 % of the force calculated for the fully excavated condition. 4. For measuring systems susceptible to faults or inaccuracy, such as direct earth pressure measurements or settlement strain gauge measurements, hydraulic presses or surveyor’s tape etc., it is generally useful to add a further section to the number of sections specified in Paragraphs 2 and 3 in order to develop a better basis for the calculation of mean value to compensate for result variances. 5. When performing simple measurements, such as measuring the distance

between two opposing soldier piles at ground level, it may be useful to do this over large excavation lengths. This offers the possibility of establishing retaining wall behaviour as a function of such factors as soil strata, the construction procedure, and site operating loads acting on the wall or other parameters.

14.7 Reading measurement values (R 37) 1. If no automated reading systems can be installed, the number of readings must generally be limited due to the personnel requirements associated with the task. In order to assess the impact of individual construction procedures, time and temperature, the following minimum readings should be taken:

a) before and after to installation or before and after to loading a structural element to be measured, b) before and after to any change of the construction state in the measurement area or neighbouring zones, c) in case of any artificial change of the equilibrium state, e.g. cutting off soldier piles or sheet pile walls, or post-stressing or relaxation of struts and anchors, 185

d) before and after to removal or before and after to relieving a structural element at which measurements have been performed. If the excavation does not display any changes, minimum readings must be taken once a week in the morning and at midday.

2. If it is not possible to measure continuously, it is recommended to take readings either every hour or every morning at low temperature and at midday at peak temperature in order to better assess the impact of construction procedures or temperature. This should be adhered to for a limited period at least. 3. Soil types encountered during excavation activity must be continuously monitored and entered into the soil profile. If the person to whom readings are assigned is not constantly on site, site management or building officials should be requested to observe and monitor any irregularities at the points of measurement, e.g. exceptionally high loads caused by vehicles or construction equipment, vibration due to blasting, or other impacts. Such incidents must be recorded in writing.

186

Appendix A 1: Unit conversions a) Forces New units MP

kP

MN

100 10 1 0.1 0.01 0.001 0.000

100000 10000 1000 100 10 1 0.1

1 0.1 0.01 0.001 0.0001 0.00001 0.00000

kN

-j 1000000 100000 10000

1000 100 10 1 0.1

0.01 0.00

b) Stresses Old units

Mp/m2 10000 1000 100 10 1 0.1

0.01

New units

kp/cm2

1000 100 10

MN/mZ N/mm2

100 10 1

bar

1000 100 10 1

1 0.1

0.1 0.01

0.1

0.01 0.001

0.001 0.0001

0.01 0.001

N/cm2

W/m2

:I

100000 10000 1000 1000000 10 100 100000 1 10 10000 1 0.1 0.01 0.1

10000 1000 100

187

A 2: Relative density of cohesionless soils a) Relative density definition D=

1 1

maxn - n - Pd - minPd Yd - min Yd max n - min n max pd - min pd max Yd Yd

I Iu

soil group

I I

SE,GE

Uniformity coefficient Very loose compaction Loose compaction Medium dense compaction Dense compaction Very dense compaction

2

3

D > 0.00 to D < 0.15 D 2 0.15 to D < 0.30 D 2 0.30 to D < 0.50 D 2 0.50 to D < 0.75 D 2 0.75 to D = 1.00

SE, SW, SI, GE, GW u>3 D > 0.00 to D < 0.20 D 2 0.20 to D < 0.45 D 2 0.45 to D < 0.65 D 2 0.65 to D < 0.85 D 2 0.85 to D = 1.00

b) Criteria for medium dense compaction Soil class

Relative density

Proctor density

CPT cone resistance

SE, SU GE, GU, GT

u23

D 2 0.30 D,2 95 % q, 2 7.5 MN/m2

SE, SW, SI, SU GE, GW, GT, GU

u>3

D 2 0.45 Dpr298 % q, 2 7.5 MN/m2

Soil class

1

Coefficient of uniformity

I of uniformity I

Coefficient

Relative density

u>3

D20.65

I

Proctor density

I

CPT cone resistance

SE, SU GE, GU, GT SE, SW, SI, SU GE, GW, GT, GU 188

DPr

q, 2 15 MN/m'

I

d) Explanations Grain size: G: Gravel S: Sand U: Silt Grain size distribution: W: well-graded grain size distribution E: uniform grain size distribution I: intermittent grain size distribution d60 Uniformity coefficient: U = -

dl0

189

A 3: Consistency of cohesive soils a) Definition of terms The consistency depends on the water content w. With decreasing water content, cohesive soil changes its state from liquid to plastic to semi-solid to stiff (hard). Transitions from one state to another were defined by ATEREERGand are known as consistency limits [ 1231: a) The liquid limit wL is the water content at the transition from the liquid to the plastic state. b) The plastic limit w, is the water content at the transition from the plastic to the semi-solid state. c) The shrinkage limit ws is the water content at the transition from the semisolid to the stiff state. d) The plasticity index I, is the difference between the liquid and the plastic limit: I, = wL - w,. e) The range between the liquid and the plastic limit is sub-categorised into very soft, soft and firm.

b) Determination of consistency in laboratory tests Based on the water contents at the liquid limit wL and the plastic limit w,, the consistency index I, is computed using the soil water content w:

I, =

WL

-w

WL - w -

WL - w , 1, The following I, values correspond to the plastic state sub-categories [ 1231: a) I, = 0.00 to 0.50: very soft, b) I, = 0.50 to 0.75: soft,

c) I, = 0.75 to 1.00: firm.

c) Determination of consistency in field tests The following criteria must be applied to field tests in order to determine the cohesive soil state:

a) A soil that is squeezed through the fingers when making a fist is very soft. b) A soil that is easy to knead is soft. c) A soil that is difficult to knead but can be formed to 3 millimetre thick rolls in the hand without cracking or crumbling is firm. d) A soil that cracks and crumbles when attempting to form 3 millimetre thick rolls but is still moist enough to be re-formed to a clod is semi-solid or firm to stiff respectively. e) A soil that has dried out and generally appears light-coloured is stiff (hard). This soil can no longer be kneaded but only broken apart. Subsequent balling of individual pieces is not possible. 190

A 4: Soil properties a) Properties of cohesionless soils (calculationvalues) ~~

Soil type

4bbreviation

State of compaction

I

- - of

moist

t

saturated

3E and SU with U I 6

Gravel, pebbles, 3E stones with low sand content, uniformly graded Sand, gravel sand, gravel, widely or intermittently graded

sw, SI, su

Sand, gravel sand, gravel, slightly silty gravel, widely

sw, SI, su

3W, GI with

5 < U S I5

3W, GI with U > 15 and 3U

buoy- friction ant

Y’

m/m3

Sand, slightly silty sand, gravel sand, uniformly graded

Angle

Unit weight

kN/m’

Deg.

loose medium-dense dense

17.0 18.0 19.0

19.0 20.0 21.0

9.0 10.0 11.0

30 32.5 35

loose medium-dense dense

17.0 18.0 19.0

19.0 20.0 21.0

9.0 10.0 11.0

32.5 35 37.5

loose medium-dense dense

18.0 19.0 20.0

20.0 21.0 22.0

10.0 11.0

12.0

30 32.5 35

loose medium-dense dense

18.0 20.0 22.0

20.0 22.0 24.0

10.0 12.0 14.0

30 32.5 35

or intermittently

graded

Given table values apply to both natural and tipped soil. In both cases, the decisive state of compaction may be achieved by artificial compaction. Values may not be applied to soils with porous particles, such as pumice gravel and tuff sand. If no relevant experience or studies on compactness are available, loose compaction must be assumed when determining earth pressure and buoyancy factor of safety, and medium-dense compactness for determination of surcharges. Otherwise, medium-dense compactness may only be assumed if justified by knowledge of site conditions. When determining shear strength, higher than medium-dense

191

compaction can only be assumed as a result of specific tests, such as static or dynamic penetration testing. Values given in lines 1 to 9 apply to rounded particles only. If angular particles dominate, friction angle values may be increased by 2.5 degrees. When analysing safety against buoyancy or uplift, unit weights must be reduced

by 2.0 kN/m3 for earth-moist soil and by 1.0 kN/m3 for saturated or buoyant soil. b) Soil properties of cohesive and organic soils (calculation values) Soil type

Abbreviation

State

Unit weight

-

I

Shear strength

1 1

4bove Under Fricwater water tion

I

I

Cohesion

1

kN/m3 Deg. kN/m2 !+Urn2

Inorganic cohesive soils with high degree of plasticity (w, > 50 %)

TA

TM Inorganic cohesive soils with and medium plasticity UM (50 70> w L >

soft firm semi-solid

18.0 19.0 20.0

soft firm semi-solid

19.0 19.5 20.5

35 5%)

Inorganic cohesive soils with low plasticity (WL< 35 96)

TL and UL

soft firm semi-solid

20.0 20.5

Organic clay, organic silt

oz

soft firm

14.0 17.0

and

ou

Peat without pre- HN load, peat under and moderate preload HZ

192

21.0

11.0 13.0

Given values apply to natural, consolidated, cohesive soils. The given unit weights and shear strength parameters are permissible for tipped cohesive soils if they are compacted to a relative compaction not less than 95 % of the normal Proctor density. Otherwise, cohesion may only be adopted on the basis of separate investigations. Cohesion may only be considered if the soil does not become liquid when kneaded and if it is certain that the soil state will not change compared to its original condition, e.g. when thawing following a period of frost. For cohesive soils with particularly flat grading curves, such as boulder clay, with particle sizes ranging from clay to sand or gravel (mixed-grained soils of groups S o , ST, ST , G O , GT and GT pursuant to DIN 18 196), the unit weights given in lines 1 to 9 must be increased by 1.0 kN/m3. When calculating safety against buoyancy or uplift, unit weights given in the table must be reduced by 2.0 kN/m3 for soils above the groundwater table or by 1.O khVm3 for soils under water. For unconsolidated shear strength, only table values for cohesion c, are given. The associated angle of internal friction must be assumed at = 0.

193

A 5: Guide values for the modulus of subgrade reaction ks,h for soil above the groundwater table a) Subgrade reaction modulus of cohesionless soils Degree of mobilisation

Ebgh:Ep,= 25 % ELgh:Epgh = 37.5 % ELgh:Epp,, = 50 % Ebgh:Epg,, = 75 %

Degree of mobilisation Ekh:Ep,= 25 % Ebh:Eph = 37.5 % Ekh:Ep,= 50 % ELh:Eph = 75 %

Relative density Loose

Medium-dense

Dense

15.0 MN/m3 3.0 MN/m3 1.2 MN/m3 0.5 MN/m3

30.0 MN/m3 6.0 MN/m3 2.5 MN/m3 1.O MN/m3

60.0 MN/m3 12.0 MN/m3 5.0 MN/m3 2.0 MN/m3

Subgrade reaction modulus 9.0 MN/m3 5.0 MN/m3 3.0 MN/m3 2.0 MN/m3

Note: The values given must be halved for soils below the groundwater table.

194

A 6: Allowable stresses for timber bracing Type of stress

European softwood Grade I11

Grade I1

Grade I

Averagequality oak or beech wood

General bending of sawn timber: a) for rectangular earth pressure diagram over entire height of retaining wall b) for more realistic earth pressure diagram c) for excavation covers and provisional bridge cover planks

8.4 N/mmz 12.0 N/mm2 15.6 N/mmz 13.2 N/mm2 10.5 N/mmz 15.0 N/mm2 19.5 N/mmz 16.5 N/mmz -

12.0 N/mm2 15.6 Nlmm’

13.2 N/mm2

Sawn timber bending over internal supports: a) for rectangular earth pressure diagram over entire height of retaining wall b) for more realistic earth pressure diagram c) for excavation covers and auxiliary bridge cover Dlanks

13.2 N/mm2 17.2 N/mm2 14.5 N/mm2 16.5 N/mm2 21.5 N/mm’

18.2 N/mm2

13.2 N/mm2 17.2 N/mm2 14.5 N/mm2

Round timber bending without weakening of edge zone: a) for rectangular earth pressure diagram over entire height of retaining wall b) for more realistic earth presssure diagram c) for struts

10.1 N/mm2 14.4 N/mm2 18.7 N/mm2 15.9 N/mm2 12.6 N/mm2 18.0 N/mm2 23.4 N/mm2 19.8 N/mm2 14.4 N/mm2 18.7 N/mm2 15.9 N/mm2 ~~

Compression parallel to grain: a) for sawn timber b) for round timber without weakening of edge zone ~

8.5 N/mm2 11.O N/mm2 10.0 N/mm2 10.2 N/mm2 13.2 N/mm2 12.0 N/mm2

~

Compression perpendicular to grain:

’)

a) for less than 10 cm overhang b) for 10 cm overhang or more

4.0 N/mm2 5.0 N/mm’

4.0 Nlmm’ 5.0 N/mm2

6.4 N/mm2 8.0 N/mm2

Shear parallel to grain:

1.1 N/mm2

1.1 N/mm2

1.2 N/mm2

Only for soldier pile infilling.

195

The given allowable stresses are based on the use of new or practically new timber. The given allowable stresses apply to Load Case H (principle loads). For Load Case HZ (principle and secondary loads), the given values may be increased by 25 %. For definitions of Load Cases H and HZ, see R 24 (Section 2.1). The stresses acting during testing, overstressing, or loosening of struts or anchors need not be analysed.

196

A 7: Maximum allowable stresses for steel bracing a) Allowable stresses in Load Case H (Principle loads) Type of stress

Steel grade S 235 S 240

Compression if buckling and tilting analysis is required

S 270

s 355

140 N/mm2

160 N/mm2 2 IO N/mm2

160 N/mm2

180 N/mm2 240 N/mm2

160 N/mm2

180 N / m 2 240 N/mm2

a) for sheet pile walls b) for soldier piles with sheeting behind front flanges c) for waling and provisional bridge girders with sufficient web stiffening

180 N/mm2

204 N/mm2 270 N/mm2

180 N/mm2

204 N/mm2 270 N/mm2

180 N/mm2

204 N/mm2 270 N/mm2

Shear

104 N/mm2

117 N/mm2 156 N/mm2

Effective stress

192 N/mm2

2 16 N/mm2 288 N/mm2

General tension and bending compression: a) for soldier piles with sheeting behind rear flanges and insufficient protection against flange torsion b) for waling and provisional bridge girders without sufficient web stiffening General flexural tensile stress and bending compression:

197

b) Allowable stresses in Load Case HZ (Principle and secondary loads) Type of loading

Steel grade S 235 S 240

Compression, if buckling and tilting analysis is required

160 N/mmz

S 270

s 355

180 N/mmz 240 N/mmz

General tension and bending compression: a) for soldier piles with sheeting behind rear flanges and insufficient protection against flange rotation b) for waling and provisional bridge girders without sufficient web plate stiffening

180 N/mmz 204 N/mmz 270 N/mmz 180 N/mm2 204 N/mmz 270 N/mmz

General flexural tensile stress and bending compression: a) for sheet pile walls b) for soldier piles with sheeting behind front flanges c) for waling and provisional bridge girders with sufficient web plate stiffening

192 N/mmz

Shear

11 1 N/mm2

Effective stress

204 N/mmz 230 N/mmz 306 N/mmz

2 16 N/mmz 288 N/mmz

192 N/mmz 2 16 N/mmz 288 N/mmz 192 N/mm2 2 16 N/mmz 288 N/mm2 124 N/mmz

166 N/mmz

The allowable stresses given require that any weakening of steel sections due to drilling, transverse welding or significant corrosion b e taken into consideration in zones with large bending moments. When testing, overstressing, or loosening struts or anchors, bending compression, flexural tensile and effective stresses may achieve up to 90 % of yield strength. With regard to allowable stresses for connections, see the relevant regulations for permanent structures. For definitions of Load Cases H and HZ, see R 24 (Section 2.1).

198

A 8: Grouted anchor safety factors 1 Load case

1 2 3

2

3

Grouted element: q K

4

I

5

Steel tension member: qs

Typical

At-rest earth pressure

Typical

At-rest earth pressure

1S O 1.33 1.25

1.33 1.25 1.20

1.75 1.50 1.33

1.33 1.25 1.20

The safety factors qKand qs must be applied as a function of the applied load according to the table. For permanent anchors, typical values must always be used while, for temporary anchors, safety factors can be chosen according to the applied load. Otherwise, the following must be applied to temporary anchors: a) If increased active earth pressure is assumed, the safety factors must be interpolated between typical table values and at-rest earth pressure. b) If typical loads occur in addition to at-rest earth pressure or increased active earth pressure, safety factors may be interpolated corresponding to their respective proportion of the total load. The load cases for grouted anchors are defined as follows according to R 24, Paragraph 6 (Section 2.1): a) Load case 1: Denotes the completed structure stage; the final excavation stage for excavations. b) Load case 2: For excavations, all advancing states prior to the final excavation stage and all retreating states up to backfilling. c) Load case 3: For rock anchors, construction stages may also be treated if anchor forces are regularly monitored on a representative sample.

199

A 9: Allowable compressive loads for soldier piles a) Allowable compressive loads for driven steel section piles Allowable load

Embedment depth in load-bearing soil

t=5m t=6m t=7m t=8m

300 mm

350 mm

allow. P = 450 kN allow. P = 550 kN allow. P = 600 kN allow. P = 700 kN

allow. P = 550 kN allow. P = 650 kN allow. P = 750 kN allow. P = 850 kN

Note: The given allowable loads apply to wide I-beams with a heighuwidth ratio of 1 : 1, such as IPB or PSp sections.

b) Allowable compressive loads for bored piles

I

I

Bored piles without toe

I

Bored piles with toe

Pile diameter

Allowable load

Toe diameter

Allowable load

d = 300 mm

allow. P = 200 kN

d = 700 mm

allow. P = 380 kN

d = 350 mm

allow. P = 250 kN

d = 800 mm

allow. P = 470 kN

d = 400 mm

allow. P = 300 kN

d = 900 mm

allow. P = 550 kN

d = 500 mm

1 allow. P =4OOkN I d = 1OOOmm 1

Minimum embedment depth t=3m

allow. P = 650kN

I

I

Minimum embedment depth t = 2.5 m

Note: The given allowable loads are empirical values for a 20 mm settlement at the beginning of the sinking process.

200

c) Soil requirements

Cohesionless soils must have a relative density of D 2 0.4 with U < 3 or a relative density of D 2 0.55 with U 2 3, respectively. The required compactness is achieved if a CPT cone resistance of not less than 10 MN/mZis measured in the foundation stratum. For cohesive soils, a texture close to semi-solid is required. The values in the table can be exceeded by up to 25 % if load-bearing strata consists of either cohesionless soils of particularly high load-bearing capacity or stiff, cohesive soils. Bearing capacity of cohesionless soils is particularly high if they possess a relative density of D 2 0.5 with U < 3 or a relative density of D 2 0.65 with U 2 3. or if a cone resistance of not less than 15 MN/mZis achieved.

20 1

Further work programme The Working Group for Excavations is currently preparing Recommendations covering the following subjects: a) Adaptation of recommendations to the new safety concept. b) Excavations with exceptional longitudinal or cross sections. d) Construction and execution.

Expert opinions and suggestions are welcome.

202

Further work programme The Working Group for Excavations is currently preparing Recommendations covering the following subjects: a) Adaptation of recommendations to the new safety concept. b) Excavations with exceptional longitudinal or cross sections. d) Construction and execution.

Expert opinions and suggestions are welcome.

202

List of Recommendations in numerical order R 1: R 2: R 3: R 4: R 5: R 6: R 7: R 8: R 9: R 10: R 11: R 12: R R R R

13: 14: 15: 16:

R 17: R 18: R 19: R 20: R 21: R 22: R 23: R 24: R 25: R 26: R 27: R 28: R 29: R 30: R 31: R 32: R 33: R 34: R 35: 212

Engineering requirements for applying the Recommendations Determination of soil properties General requirements for adopting live loads Magnitude of active earth pressure without surcharge loads Distribution of active earth pressure load without surcharges Magnitude of active earth pressure from live loads Distribution of active earth pressure from live loads Magnitude of earth pressure as a function of the selected construction method Equilibrium of vertical forces Stability analysis in special cases Determination of internal forces Determination of load models for soldier pile walls with active earth pressure Simplified earth pressure for braced soldier pile walls Passive earth pressure for soldier pile walls with free earth supports Equilibrium of horizontal forces for soldier pile walls Determination of load models for sheet pile walls and in-situ concrete walls with active earth pressure Simplified earth pressure for braced sheet pile walls and in-situ concrete walls Not yet allocated Passive earth pressure for sheet pile walls and in-situ concrete walls with free earth supports Engineering measures for excavations adjacent to structures Analysis of retaining walls with active earth pressure for excavations adjacent to structures Analysis of retaining walls with increased active earth pressure Analysis of retaining walls with at-rest earth pressure Analysis load cases and allowable stresses Toe restraint for soldier pile walls Toe restraint for sheet pile walls and in-situ concrete walls Limit load design method Active earth pressure for large distances to structures Active earth pressure for small distances to structures Mutual influence of opposing retaining walls for excavations adjacent to structures Purpose of measurements Preparation, implementation and evaluation of measurements Measured variables Measurement methods Requirements on the excavation measurement area

Number of measurement areas and measurement sections Reading measurement values General Recommendations for excavations in unstable rock Magnitude of rock support pressure Distribution of rock support pressure Bearing capacity of rock for support forces at the wall toe. . Magnitude and distribution of earth pressure for anchored retaining walls R 43: Analysis of force transmission from anchors to the ground R 44: Analysis of stability at low failure plane R 45: Analysis of global stability R 46: Displacements in anchored retaining walls R 47: Dimensioning of soldier pile infill walls R 48: Dimensioning of soldier piles R 49: Dimensioning of sheet piles R 50: Dimensioning of in-situ concrete walls R 51: Dimensioning of walings R 52: Dimensioning of struts R 53: Dimensioning of trench sheeting and bracing R 54: Dimensioning of provisional bridges and excavation covers R 55: Live loads from road and rail traffic R 56: Live loads from site traffic and site operations R 57: Live loads from excavators and lifting equipment R 58: General Remarks for excavations in water R 59: Seepage pressure R 60: Dewatered excavations R 61: Analysis of hydraulic heave safety R 62: Analysis of buoyancy safety R 63: Stability analysis of retaining walls in water R 64: Dimensioning and construction of excavations in water R 65: Water management R 66: Monitoring excavations in water R 67: Support of retaining walls R 68: Earth pressure in retreating states R 69: Pressure diagrams for supported soldier pile walls R 70: Pressure diagrams for supported sheet pile walls and in-situ concrete walls R 71: Superimposing earth pressure components for unsupported retaining walls R 72: Superimposing earth pressure components for supported retaining walls R 73: Excavations with circular plan R 74: Excavations with oval plan R 75: Excavations with rectangular plan R 76 to R 86: Not yet allocated

R 36: R 37: R 38: R 39: R 40: R 41: R 42:

213

R 87: R 88: R 89: R 90: R 91: R 92: R 93: R 94: R 95: R 96: R 97: R 98:

Dimensioning of grouted anchors Not yet allocated Wall friction angle Scope of the Recommendations for excavations in soft soils (Draft) Slopes in soft soils (Draft) Lining systems in soft soils (Draft) Construction procedure in soft soils (Draft) Shear strength of soft soils (Draft) Earth pressure on retaining walls in soft soils (Draft) Soil reactions in soft soils (Draft) Water pressure in soft soils (Draft) Determination of embedment depths and internal forces for excavations in soft soils (Draft) R 99: Further stability analyses for excavations in soft soils (Draft) R 100: Water management for excavations in soft soils (Draft) R 101: Serviceability of excavation structures in soft soils (Draft) R 102: Modulus of subgrade reaction method (Draft) R 103: Finite element method (Draft)

214