Slope Stability Engineering: Proceedings of the International Symposium on Slope Stability Engineering : Is--Shikoku'99 : Matsuyama, Shikoku, Japan, 8-11 November 1999, Vol. 2

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Author: Norio Yagi | Takuo Yamagami | Jing-Cai Jiang

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Figure 2. Tlie rocltfall sinmlatioii apparatus at tlie Hoiig Kong Polytechnic University million of HK dollars (OmniSpeed HSl00 manufactured by Speed Vision Technologies. USA) of capability of capturing 222 frame per second and exposure time up to 32000-tli of a second for each frame were purchased for the rocltfall tests in our laboratory. Two pentiiiuin PCs were used as the data recorders. The boulders used iii our experiments are of various shapes (spherical, hexagonal, cubic, aiid cylindrical) and various sizes (50, 60 aiid 70niin) and were casted using plaster. The artificial slope surfaces were made of either plaster or soil, depending on the slope types that we want to simulate. The calculation of the coefficients of restitution can be found by recording tlie time and position of the boulders before, at, and after tlie impact. Without going into the details, we compile only our results here. Results of iiiany of our ongoing experiments will be published in our forthcoming papers. 2.3. Boulders filling onto soil slopes

As far as we know, coefficient of restitution for boulders falling onto soil slopes has been investigation comprehensively in laboratory. Therefore, the soil slope is made by compacting a colluvium obtained from Tsing Shan into a wooden

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box of about 35cm x50cm x l lcm. The soil slope of thicltness of about l l c m is formed by three compacted soil layers, and for each layer different amount of impact energy has been used to compact the soil. A standard rammer used in the Proctor test (i.e. 2.5 kg) is employed for the compaction. Each location for each layer is impacted 5, 8, 10, 12, 15 times respectively in five independent experiments, In addition, for each compaction test four different water contents were used in the soil mixing (therefore a total of 20 combinations of different water contents and coinpaction levels). The results show that the coefficient of restitution is sensitive to both the dry density and the water content of the soil slopes. Before we discuss the results, it is essential for us to show the results of the Proctor test for the soil. In particular, Fig. 3 plots the dry density after Proctor test versus the water content of the soil. It is observed that the maximum dry density is about 2.34 Mg/mi and the optimum water content w,,/,, is about 10.5%.

this observation is that when the optimum water content is exceeded, the soil is about fully saturated and, thus, is overall incompressible (as water is incurnpressible).

0.25

&

0.21 -

0.17

4

0.08

0.40

1

0.30

-

0.20

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-I

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0.12

0.14

1

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1.90

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moisture constent

1

2s0

1

0.10

0.12

0.14

moisture constent 0.08

0.12

0.16

moisture content

o.28

Figure 3 . Dry densities versus the water content for the soil used in making the soil slope in rockfall experiments.

1

0.26 -

t X 0.24 0.22 -

Figure 4 shows the coefficients of restitution R,, , R, and R, versus the water content of the (defined in ( 1 ) and (3)) for various degrees of freedom. It is interesting to note that the variation of the coefficient of restitution depends on whether the optimum water content w ~ , / ~is, exceeded or not when the soil is compacted. More specifically, RI, is found relatively insensitive to the water content of the soil when 11’ is less than ~r,),,,and R,, increases linearly with

11)

when

M!,,~,,is

exceeded. The main reason for

0.20

!

0.07

I

0.09

0.1 1

0.13

moistuer content

Figure 4. The coefficients of restitution R,, . R, and R,, defined in (1 -3) versus the water content of the soil slopes.

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3. CONCLUSION In this paper, we have summarized briefly 'our recent experimental effort for the determination of the coefficient of restitution for boulders falling onto soil slopes. We find that normal and tanegntial components of the coefficient of restitution (R,, and R, j increases with the dry density of the soil when the moisture content is less than the optimum water content, which leads to optimum soil compaction. When the optimum water content is exceeded, both R,, and R, remain roughly constant regardless of the values of the dry density of the soil. ACKNOWLEDGEMENT This paper was supported by RGC's CERG and HKPolyU research grants. REFERENCES Azzoni, A., & de Freitas, M.H. 1995. Experiinentally gained parameters, decisive for rock fall analysis. Rock Mech. Rock Engng. 28(2): 111-124. Bozzolo, D., & Painini, R. 1986. Simulation of rock falls down a valley side. Acta Mechanicci 63: 113130. Budetta, P., & Santo, A. 1994. Morpliostructural evolution and related kinematics of rocltfalls in Cainpania (southern Italy): A case study. Engng. Geol 36: 197-210. Chau K.T. 1997. Rocltfall, landslides and slope failures. Dej'ormation and ProgressivP Failure of Geonzechanics, (ed. by A. Asaolta, T. Adachi and F. Olta), IS-NAGOYA'97: 907-92 1, Pergamon: Oxford. Chau, K.T., Wong R.H.C., & Lee C.F. 1998. Rocltfall problems in Hong Kong and some new experimental results for coefficient of restitution. Int. J. Rock Mech. Min. Sci., 35(4-5): 662-663, Paper No. 007. Chau K.T., Wong R.H.C., Liu J., Wu J.J. & Lee C.F. 1999. Shape effects on the coefficient of restitution during rocltfall impacts. 9th Internationcrl Congress O H Rock Mechanics, ISRM Congress, Paris (in press). Descoeudres, F., & Ziminermaiin, TH. 1987. Threedimensional dynamic calculation of rockfalls. In: Pi-oc., 6 t h Int. Congress on Rock Mech.: 337-342. Evans, S.G., & Hungr, 0. 1993. The assessment of rocltfall hazard at the base of talus slopes. Cun. Geotech. J. 30: 620-636.

Flageollet, J.C. & Weber, D. 1996. Fall. In Landslide Recognition: Identification, Movement and Causes (ed. R. Dikau, D. Brunsden, L. Schrott and M.L. Ibsen), John Wiley, Chichester: 13-28. Fomaro, M., Peila, D. & Nebbia, M. 1990. Block falls on rock slopes-Application of a nuinerical simulation program to some real cases. In: Proc., 6-th Int. Congress IAEG, (D.G. Price ed.j Amsterdam, Balltema, Rotterdam: 2 173-2 180. Gerrard, A.J. 1990. Mountain Environments: An Exainination o j the Physical Ceogruphy of' Mountains. Belhaven Press, London. Giani, G.P. 1992. Rockfalls, topples and buckles. In: Rock Slope Stability Analysis. Chapter 7: 191207,, Rotterdam: A.A. Balltema. Habib P. 1976. Notes sur le rebondissenient des blocs rocheux. Meeting on Rockfall Dynamics and Protective Works Efectiveness: 20-2 1. Hoek, E. 1990. Rocltfall-a program in BASIC for the analysis of rocltfalls from slopes, Unpublished notes, Golder Associates/University of Toronto. Hungr, 0. & Evans, S.G. 1988. Engineering Aspects qj' Rockfall Hcrzurds in Cunadci, Geological Survey of Canada, Ottawa, Open File 206 1. Japan Road Association 1983. Rocltfall Handbook. Maruzen Publisher, Tokyo: 1-359 (in Japanese). Kobayashi, Y., Harp, E.L., & Kagawa, T. 1990. Simulation of rockfalls triggered by earthqualtes. Rock Mech. Rock Engng. 23: 1-20. Paronuzzi, P. 1989. Probabilistic approach for design optimization of rocltfall protective barriers. Quurt. J. Engng. Geol. 22: 175-183. Pfeiffer, T.J., & Bowen, T.D. 1989. Computer simulation of rockfalls. Bull. Assoc. Eng. Geol. 26(1): 135-146. Richards, L.R. 1988. Rocltfall protection: A review of current analytical and design methods. In: Meeting on Rockfall Dyncimics and Protective Works Eflectiveness, Bergaino: 1 1- 1- 11- 13. Spang, R.M., & Rautenstrauch, R.W. 1988. Empirical and mathematical approaches to rocltfall protection and their practical applications. In: Landslides: Proc., 5th Int. Symp. on Lnndslides (ed. C. Bonnard), Vol. 2, 1237-1243, Rotterdam: Balkema. Spang, R.M. & Sonser, H. 1995. Optiniized rocltfall protection by "ROCKFALL". In; Proc. 8th Znf. Congi.. RockMech., Tokyo. Vol. 3: 1233-1242. Wu, S.-S. 1985. Rockfall evaluation by coniputer simulation, Transportution Reseurch Record, 1031 : 1-5.

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Slope Stability Engineering, Yagi, Yamagami & Jiang 0 1999Balkema, Rotterdam, ISBN 90 5809 079 5

The May 5th 1998 landsliding event in Carnpania, Southern Italy: Inventory of slope movements in the Quindici area D.Calcaterra - Section of Applied Geology, Department oj'Geotechnica1 Engineering, Federico II University of Naples, Italy M.Parke-C,E.R.I.S.T, C.N.R., Bari, Italy

B. Palma - Vico Equense, Italy L. Pelella - Somma Vesuviana, Italy

ABSTRACT: On May 5'", 1998, more than 300 soil slide - debris flows originated in the Quindici territory, and moved downslope along pre-existing drainage ways, killing 11 people and destroying or severely damaging some tens of houses. The mass movements resulted fiom failure of Late Quaternary-Holocene shallow pyroclastic cover, resting on Mesozoic carbonatic bedrock. The preliminary results deriving from landslide inventory of the 1998 slope failures are here presented: following identification, surveying and mapping of the soil slide - debris flows, local geologic, geomorphologic and morphometric characters are described. Moreover, some considerations are presented concerning the role of man-made cuts and roads in the development of the slope movements.

1 INTRODUCTION

Local bedrock consists of Cretaceous to Tertiary limestones, with subordinate pelites and conglomerates. The carbonatic sequence, mostly dipping toward the northern sectors, was involved in Tertiary to Quaternary tectonics, which produced pervasive faulting and jointing in the rocks; the main faults in the area border the structural depression of Lauro, where Quindici and other towns are located (Fig. 1).

On May 5th, 1998 the territory of Quindici (Campania, Italy) was severely affected by some hundreds of slope failures. They began in the late morning, after some 20 hours of continue rainfall, during which 103 min of rainfall were recorded, with a peak intensity of 15 m d h and an average intensity of 5.1 m d h . In a time-span of about 12 hours, more than 300 soil slides originated from the hillslopes above the town, rapidly turning to debris flows, which moved downslope following the major drainage ways. Some of these flows catastrophically reached the inhabited area, and caused 11 victims. At the same time, three other towns located nearby were struck by similar events; 148 more casualties were counted. This catastrophic event forced the national scientific and political community to a greater awareness about debris flows involving loose volcaniclastic deposits in Campania: about 3000 kin' of the regional territory, as a matter of fact, are susceptible to these phenomena which, therefore, have to be considered one of the main landslide hazard in the region. The present paper deals with the stability conditions in the Quindici area, focusing in particular on the prelimiiiary results from analysis of the 1998 slope failures. 2 GEOLOGY AND GEOMORPHOLOGY QuiIldici is placed at the northern foot Slope of Mt. PizzO d'Alvano, a SE-NW bending carbonatic ridge (Fig. 1).

Figure 1. Location map and geology of the Quindici area. Legend: 1 ) Pyroclastics (Holocene - Late Quaternary); 2) Alluvial and fan deposits (Holocene - Quaternary); 3) Carbonate bedro& (Te1-tial-y - Mesozoic), mantled by pyroclastics and epiclastics; 4) Main fault (presumed where dashed).

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Figure 2. Distribution of slope failures during the May 5'h, 1998 event. Legend: I ) Soil slide - debris flows; 2) Inhabited area; 3 ) Unmappable landslide; 4) Location of open cracks on the ground; 5) Identification of drainage basins (see Table I ) . Contour interval is 100 m.

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Tlie above described morphologic and topographic features point out, as a whole, to the existence in the Quindici territory of geoniorphie conditions which, in case of slope failure, axe likely to exert low dissipation of energy in the moving material, and axe, therefore, strongly favourable to higlily-mobile landslides (Nicoletti & Sorriso Valvo 199 l), which uiifortunately is what actually occurred during the 1998 event.

Recent alluvial and fan deposits, mixed with epiclastics and interbedded with volcaniclastics, fill the valley with thickness of several tens of metres. The carbonate rocks are mantled throughout the area by volcaniclastic deposits, from some centimetres to a few metres thick. These products also crop out with greater thickness on the suinmit plateau of ltarstic origin. The volcaniclastic deposits come from the major volcanic centres in Campania: tlie Mt. Somma-Vesuvius and the Phlegraean Fields (Orsi et al. 1998). The Vesuvian products, locally present in greater amount, have been referred to at least three major explosive events, altogether occurred in the last 9000 years. As regards the Phlegraean activity, Campanian Ignimbrite (37,000 yrs. BP) and Agnano-Mt. Spina products (41 00 yrs. BP) have been recognized. The present geomorphologic setting is the effect of the tectonic uplift which started at the end of the Tertiary age and lasted up to the late Quaternary; some stages of intell-uption during the uplifting process have been inferred from the recognition of remnants of at least three planation surfaces on tlie slopes. Tlie morphologies shaped in the carbonate bedrock were successively modified by ltarstic processes, and then covered by tlie late Quaternary to I-Iolocene products of the volcanic activity. The suriicial Iiydrograpliic network consists of deep and narrow incisions that have mostly developed following the main fault and joint sets in the area; longitudinal profiles of these incisions show high to very high gradients.

3 LANDSLIDE INVENTORY Analysis of the 1998 slope failures at Quindici was performed through the approach generally followed in the implementation of a landslide inventory, which is the basis for landslide hazard zonation (Wieczorek 1984; Soeters & Van Westen 1996). This approach consists of: 1) collection and validation of historical information concerning the occurrence of slope failures in the Quindici territory; 3) photo interpretation of the 1998 air photos, and of other available photo data sets as well; 3) ground survey; 4) interpretation of collected data; 5 ) production of thematic maps. This session deals essentially with tlie preliminary results coming from the analysis of the 1998 landslide inventory; to provide the reader with inforniation about historic landslides which have occurred in the study area, and to present the preliminary correlation among these phenomena and the 1998 slope failures, the January 1997 event is also mentioned. The inventoiy implemented consists of more than

between brackets are reference for drainage basins in Figure 2. n.c. = not calculated. 1997 Drainage basin Vallone della Cantarella (A) Vallone Mercolino (B) Vallone Colafasulo (C) Vallone Cisierno (D) Lagno Cisierno (E) Pietre della Valle (left channel) (Fl) Pietre della Valle (right channel) (F2) between Pietre d. Valle and S. Francesco (G) Vallone S. Francesco (H) Vallone della Connola (left channel) (11) Vallone della Connola (right channel) (I2) Inserto di Prato (J) Bocca dell’Acqua (K) Liporeta (L) Vallone del Tocco (M) Lagno di Quindici (N) other drainage basins Tntnl

j Detachment Freqiiency area vOlUJlle : (m3)

3 4 4 6 4 3 5 1 1 4 2 1 2

8 48

i

2950 3325 >8000 j 9200 j 3000 2075 j 3680 [ 2000 i 2000 8450 i 1900 i 250 6675 j

/

i

i

~

1363

18,050 71,555

1998 j

Detachment j Frequency area volume ! Total volume : (m3) i (m3) 38 i 44,530 j n.c 3 j 435 j t1.c

/

33 26 5 18 20 10 12 22 13 2 12 16 11 37 30 308

i j

i j j

j

i 1 i

i

27,020 29,225 4725 15,270 23,800 11,775 30,925 93,380 36,250 1550 12,200 16,500 2150 17,700 15,820 383,255

i j j

i j j j j j

i j ~

n.c 11.c n.c 61,681 137,834 73,800 122,124 264,058 191,780 n.c n.c n.c n.c n.c n.c 851,277

300 slope failures activated during the 5'h May, 1998 event. Each slope failure was analyzed by means of photo interpretation, and mapped on 1 5000 scale maps. Most of the slope failures identified on tlie photos were successively visited during the ground survey; some were not, due to difficulties in accessibility. A data form was compiled for each landslide, indicating its location and catchment basin, and describing inorphoinctric parameters of the landslide (delined according to the IAEG Coiiiiiiissioii on Landslides 1990) such as shape, length, width, vertical relief, area; thickness of the pyroclastic cover. presence and type of vegetation, presence of any break on the slope, either natural or man-made, were also objects of investigation. All the above features were examined and measured exclusively in tlie source area of tlie slope failures: this choice was dictated by the main aim of tlie work, that is tlie need in understanding which factors played a prominent role in the development of slope failures at Quindici. The May 1998 soil slide - debris flows affected with particular severity the northern slopes of Mt. Pizzo d' Alvano, but involved materials froiii all the main drainage basins in the Quindici territory (Fig. 2). 'The highest number of slope failures was registered in two of the western basins (38 and 33 cases, respectively, in Vallone della Caiitarella and Valloiie Colafasulo) and along the Lagno di Quindici (37 cases). Table 1 lists frequency and volume of tlie 1998 events per drainage basins; the last coluinn in the table presents tlic available data about total volume of landslide materials at tlie mouth of the basin in tlic main valley, estimated by comparison of preand post-landslide topography. Table 1 also coinpares the 1998 slope failures with those which occurred in the same drainage basins in January 1997. The Mt. Pizzo d'Alvano ridge is elongated in a SENW direction, therefore it faces the north with its slopes uphill from the town of Quindici. This orientation exerted a clear control upon tlie a k n u t h distribution of slope failure source areas, as shown in the rose diagram of Figure 3: the inaiii peaks in frequency of azimuth distribution are concentrated in the northern sectors, with the highest values toward NE and NW: a secondary peak is also present in tlie E quadrangle. However, no unique geolob' 'lC structure in the local bedrock seemed to control slope failure distribution throughout the area: as a matter of fact. soil slide - debris flows formed on dip slopes as well as on cross-dip slopes. As above stated, morpliometric parameters refer to laiidslide source areas. Length varied froin the minimum value of 5 metres to the highest of 135

1364

Figure 3. Rose diagram of the azimuth of slope failure source areas, plotted in 22,Y arc segments.

metres, with mean of about 30 i n (Table 2). Maximum width, measured perpendicular to the length, shows values between 2 and 100 metres, with mean slightly longer than 20 i n . Vertical relief, that is the difference in elevation between tlie two points deTable 2. Main morpliometric parameters in the sotirce areas of

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Length

(111)

I~IUI

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clelev.

I 1 I e nn

Slope (")

iiiin 171CIY

51 dell

33.5 35.9 26 45 45

21.4 36 12 56 6.I

Figure 4.Frequency of 1998 slope failures with relation to the highest elevation of the source areas.

limiting tlie length, is comprised between 2 and 120 metres, mean value being about 23 ni (Table,2). Frequency of the slope failures with respect to elevation of tlie top of source areas shows that the highest frequencies are concentrated from elevation of 600 ni up to 800 m a.s.1. Frequency decreases moving both downslope and upslope from this range (Fig. 4). The sector from 600 to 800 ni a.s.1. approximately corresponds to tlie sector of highest slope gradients in the study area. In a typical transverse profile of the northern slopes at Mt. Pizzo d'Alvano, three sectors can be identified: the upper one is characterized by smooth morphologies in the more than 5 metres-thick pyroclastic deposits, with slope gradients of few degrees, only locally increasing up to about 30".The lower sector, where talus and fan deposits are present, also shows similar slope gradient values. These two sectors are connected by the central portion of the slope, with the highest value in gradients, above 30"; moreover, this portion is characterized by a thin cover of pyroclastic materials over the carbonate rocks, usually lower than 3 metres and in most areas limited to a few decimetres. Therefore, high slope gradient values in the 600800 in elevation range, combined with the widespread presence of pathways, probably affected location and distribution of the slope failures. that at several sites foiined at the same elevation along the ridge. Slope gradients in the landslide source areas cover a wide range, from a minimum value of 12" up

Figure 5 . Frequency of 1998 slope failures with relation to slope gradient in the source areas.

to a maximum of 55" (Fig. 5 ) . However, the overall distribution of slope gradients shows a Gaussian pattern, and the highest concentration of values in the range 31"- 43", with the peak corresponding to 39". This pattern is consistent with previous observations on debris flows in Campania and elsewhere by other scholars (Jibson 1989; Guadagno 1991; Calcaterra et al. 1997), which pointed out to the occui-rence of the majority of slope failures in colluvium or pyroclastic deposits mantling carbonate bedrock on slope angles between 34" and 37". The pyroclastic deposits mobilized as planar slabs whose thicltness usually was limited to less than 2-3 metres. Shape of the source areas ranged from circular or disk-shaped to triangular, rectangular, and linear. The most typical shape, already observed in similar slope failures in pyroclastic deposits of Campania (Lazzari 1954), was a triangular one: it is characterized by width of the source area progressively increasing downslope by adding further material from both the sides, so that the overall shape is a triangle, with the apex usually in proximity of pathways. Regardless of the shape, source areas showed surfaces planar and parallel to the ground, wliich represents an uncommon but previously noted feature in debris-flow detachment areas (Jibson 1989). Source areas range broadly in size, from volumes of only a few cubic metres up to volumes exceeding 16,000 m3. The smallest features originated debris flows which moved only a few metres, and then stopped on the slopes; however, the great majority of slope failures (81%) fed debris flows which moved down pre-existing gullies and valleys for several hundreds of metres. In their downslope movement, the debris flows scoured away most of tlie vegetation and loose debris down to solid bedrock. Availability of several sets of air photos since the fifties helped in ascertaining the presence of instability signs in the area affected by the 1998 slope failures; the data obtained through air photo interpretation were also integrated with historic information concerning past landslide occurrencc. This analysis pointed out to a perccntage of about 20% of the 1998 slope failures that showed previous landslides and/or erosion signs. Twelve percent of the 1998 slope failures occurred in correspondence of a break in the slope, either natural or man-made. The former is usually represented by steeply inclined to subvertical carbonate wall; the latter. on the other hand, are essentially mountain pathways. Since the very first days following the event, it appeared that there was a clear coniiection between mountain pathways and source areas of slope failure (Del Prete et al. 1998). This connection was the con-

1365

sequence of the high increase in both number and length of pathways on the Mt. Pizzo d’Alvano slopes during the last 50 years: in particular, the lack of any surficial drainage work, and the accumulation of the removed material on the downslope side of pathways as well, are considered to have been among the more likely causes for development of slope failures.

ACKNOWLEDGMENTS Research partly supported by the Italian University Ministry (MURST) - Research Projects of National Interest (funds granted to prof. de Riso, University of Naples). M. Di Vito (Osservatorio Vesuviano) is gratefully acknowledged for his valuable observations on the pyroclastic deposits.

4 CONCLUSIONS

REFERENCES

The May 5‘”, 1998, event at Quindici and nearby towns has to be considered among the most catastrophic landsliding events ever occurred in the Campania region, as regards frequency and size of the slope movements, and the damage they produced as well. A clear sign of the hazard posed by the 1998 slope failures is given in Figure 6, where the travel distances reached by the 1998 Quindici soil slide - debris flows are compared to those of similar historical phenomena occurred in Campania in the last decades. In Campania, location of many elements at risk (inhabited areas, communication routes, lifelines) in areas characterized by geologic and geoinorphic conditions similar to those where slope failures occurred at Quindici, highlight the need for the scientific community to perform a strong effort aimed at identifying the most susceptible areas to soil slide debris flows, and to transfer such knowledge in a simple and usable form to the local administrators and planners. With this main aim, our group is at present working in the Quiiidici area at a three-fold research project: 1) understanding of local geologic and geoniorphologic conditions that predisposed the slopes to failures; 2) reconstruction of the stratigraphic and sedimentologic history, recorded in the fan deposits in the main valley; 3) assessment of the anthropogenic influence on failure development.

Calcaterra, D., A. Santo, R. de Riso, P. Budetta, G. Di Crescenzo, I. Franco, G. Galietta, R. Iovinelli, P. Napolitano & B. Palma 1997. January, 1997 intense rainfall related landslides in Sorrentine Peninsula - Lattari Mts. : first contribution. Proc. 9‘” Nut. Congr.. of Geologists, Rome, 17-20 April 1Y97, 223-23 1. (In Italian). Del Prete, M., F.M. Guadagno & A.B. Hawkins 1998. Preliminary report on the landslides of 5 May 1998, Campania, southern Italy. Bull. Eng. Geol. Env. 57: 113-129. Guadagno, F.M. 1991. Debris flows in the Campanian volcaniclastic soils (Southern Italy). Proc. Int Conf.’ on Slope stability engineering developments and applications, Isle of Wight: 1091 14, London, Thomas Telford. IAEG Commission on Landslides 1990. Suggested nomenclature for landslides. Bull. Int. Ass. Eng. Geol. 41: 13-16. Jibson, R.W. 1989. Debris flows in southern Puerto Rico. In: Schultz, A.P. & R.W. Jibson (eds.), Landslide processes of the eastern United States and Puerto Rico. Geol. Soc. Am., Spec. Paper 336: 29-55. Lazzari, A. 1954. Geological aspects of events occurred in the Salerno area as a consequence of the October 25-26, 1954 storm. Boll. Soc. Natur. in Napoli 63 : 131- 142. (In Italian) Nicoletti, P.G. & M. Soniso Valvo 1991. Geomorphic controls of the shape and mobility of rock avalanches. Geol. Soc. Am. Bull. 103: 1365-1373. Orsi G., M. Di Vito & R. Isaia (eds.) 1998. Volcanic hazards and risk in the Parthenopean megacity. Field excursion guidebook Int, Meet. on “Cities on Volcanoes”, Rome-Naples, 28 June-4 July 1998: 206 pp. Soeters, R. & C.J. van Westen 1996. Slope instability recognition, analysis, and zonation. In: Turner, A.K. & R.L. Schuster (eds.), Landslrdes. Investigation and mitigation. Transp. Res. Board, Spec. Rep. 247, Nat. Res. Council, Washington, D.C.: 129-177. Wieczorek, G.F. 1984. Preparing a detailed landslideinventory map for hazard evaluation and reduction. Bull. Ass. Eng. Geologists 21 (3): 337-342.

Figure 6. Relationships between vertical height and travel distance for soil slides - debris flows at Quindici (1998 event) and elsewhere in the Campania region (in a period 1960-1997).

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13 Simulation and analysis of debris flow

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Slope Stability Engineering, Yagi, Yamagami & Jiang 0 1999 Balkema, Rotterdam, ISBN go 5809 079 5

A proposed methodology for rock avalanche analysis R.Couture & S.G. Evans Geokogicul Survey of Ccinadcz, Ottuwa, Ont., Cunnda

J. Locat & J. Hadjigeorgiou L m n l Universiy, Suinte-Foy, Que., Canucla

l? Antoine IRIMG-LGM, Uniwrsitk Joseph-Fourier, Grenohle, Frunce

ABSTRACT: This paper presents the methodology developed while analyzing selected rock avalanches. The methodology can be divided into four major steps which includes 1) gathering documentation, 2) field work including field testing, 3) laboratory testing and specific interpretation, and 4) analysis related to stability, mobility to the post-failure behavior, and energy balance. The proposed methodology allows an evaluation of block size distribution in the detachment zone and the deposition zone as well as the boundary conditions along the travel path. This methodology is applied to a case study, the La Madeleine Rock Avalanche (Savoie, France).

INTRODUCTION

METHODOLOGY

Rock avalanches are characterized by high mobility with potential for devastating effects on population and economic infrastructures. One of the tasks of geological engineers is to analyze this kind of hazard and reduce the associated risks. Within a framework analysis on large gravity movements (Leroueil et al. 1996), we propose a methodology used for rock avalanche investigations. This methodology concerns a global characterization of the rock mass involved in the rock avalanche, and the material itself, as well as the debris generated. Physical and mechanical aspects of rock avalanches are covered by this methodology. Grain size distribution of rock avalanche debris have not been studied so much in the past (Cruden & Hungr 1986). Fragmentation has become a subject increasingly investigated by researchers. It has been rarely studied with respect to a comparison with block size distribution in the detachment zone (Couture et al. 1996). Static, kinematic and dynamic analyses are also included in the methodology. We illustrate this proposed methodology by applying this to a case study of rock avalanche located in the French Alps.

The methodology presented here for rock avalanche analysis consists of four steps which include 1) documentation, 2) field work, 3) laboratory testing, and 4) analysis related to stability, energy balance with emphasis on the fragmentation process, mobility and post-failure behavior (Figure la). Geometrically, we divide a rock avalanche path into three major sections, which are the detachment zone, the deposition zone and the transition zone (Figure lb). The detachment zone corresponds to the volume of rock mass that would fail or has failed from the slope. The transition zone starts at the base of the detachment zone and mainly corresponds to the travel path of the mass in motion. The deposition zone corresponds to the area covered by the debris of the rock avalanche. Documentation A complete description of a rock avalanche sta-ts with gathering information concerning recorded historical events, geological and structural aspects, geomorphology, former investigations and reports, and any iconographic documents. Topographic maps, geological maps and air photos are usually used when avalaible.

1369

Figure 1. Methodology used for rock avalanche analysis.

A Digital Terrain Model (DTM) also helps the morphology representation. Climatic data are helpful too in historical cases for which failure could be related to climatic factors. Documents and data as a whole represent basic elements of the first step in the analysis, which will be followed by more detailed field surveys and laboratory works.

direction, trace length and position of all joints and discontinuities encountered along a line into an observation window are measured (Figure 2). The observation window should represent a sampling area showing similar characteristics to the entire bedrock face. The scanlines could be placed anywhere within the window. However, their location has to be where the bedrock reflects the fracturing characteristics of the entire rockmass. Scanlines are horizontal along the longer axis of the observation window. Data from the scanlines survey are plotted in a stereonet, to give a more visual repesentation of the different joints and average values of dip and direction. Detailed surveys in the transition zone give indications of the geomorphology (channeled flow path or presence of local uneven topography), the nature of the substratum (hard if bedrock, or soft if surficial deposits), and on the erosion of surficial deposits by the flowing mass. Surveys may also provide indications of the thickness of the flowing mass given by marks of impact on trees. Velocity can be estimated by run-up on the opposite

Field survey Mapping and scanlines survey With the help of air photos and topographic and geological maps, large-scale mapping is performed in the detachment zone. Geological formations and their limits are defined. The objective is to establish a detailed map of the area affected by the detached mass and estimate the volume (V) as accurately as possible. From a structural point of view, regional lineaments can be easily seen on air photos. At the scale of an outcrop, the scanlines technique should still be better than air photo-interpretation as a useful tool for measuring joint sets (Hudson & Priest 1979, 1983; Priest & Hudson 1981, Pahl 1981). Dip, 1370

Figure 2. Technique used for the estimation of block size distribution in the detachment zone.

slope or in bends of a winding flow path (Chow 1959, Evans et al. 1989). Debris in the deposition zone of a rock avalanche is often the easiest identifiable feature in landscape. Mapping of the deposition zone is achieved using air photo interpretation coupled with topographic maps. Geometrical parameters such as travel distance (L), length of deposit (Ld), excessive travel distance (Le) and width (W) of debris area are measured. Detailed site investigations may give information on features such as inverse grading of debris seen in cross section or the presence of molards or alignments of ridges on top of the debris.

Figure 3. Technique used for evaluation of the grain size distribution of rock avalanche debris.

Physical and meclzanical properties Intact material and joint properties can be evaluated either in-situ or in the laboratory. Compressive strength (0,) of rock is estimated in the field in using the Rebound Index (R) in Schmidt hammer test (Deere & Miller 1966) and by the point load test performed in the field. Field work determined the properties of joints, discontinuities and failure plane. Joint Roughness Coefficient (JRC) is estimated using profile measurements from joint roughness profilers and correlated to a JRC chart (Barton & Choubey 1977). Moreover, the Joint Compressive Strength (JCS) is given by Schmidt hammer rebound index values (R).

Photographic sampling A photograph-sampling technique was carried out to provide a special type of sampling for rock avalanche debris (Figure 3). Photographsamples might be taken at the ground surface, for instance in debris cross section, by air at low altitude and low speed flight, or using large-scale air-photos where blocks can be identified and measured. A graduated frame, or a known size element such as a ball, acts as a reference element or as a scale when photographs are taken. These special samples of rock avalanche debris are used for determining the grain size distribution of particles in the deposit.

Laboratory Rock properties The point load test performed in the laboratory determines the tensile strength (00. Based on wave velocity properties we are able to evaluate dynamic properties such as the modulus of elasticity (E), the shear modulus (G) and the coefficient of deformation (v). This supposes that the rock is elastic, isotropic and homogeneous, 1371

varies between 0.0001 and 1000. Q value and RMR can be correlated by the following equation (Bieniawski 1984):

which is, however, rarely the case for a rock mass. Measurements are made in laboratory by piezoelectric crystals coupled to an oscilloscope. Such measurements may also show the influence of schistosity, foliation and fracturing on dynamic properties.

RMR = 9 InQ + 44

A difference can be seen between both systems: the RMR-system does not consider the stress condition of the rock mass, while the Q-system does not consider joint orientation and intact rock strength as independent parameters (Goel et al. 1996)

Frictiori parameters Shear strength of discontinuities or friction angle (@)can be estimated by the in-situ tilt test. However, for more accurate values of the friction angle, we performed laboratoiy shear tests with samples showing the same type of plane as those associated with the failure plane, such as bedding, foliation or schistosity planes. Shear tests are achieved in dry (@d) and wet conditions (@w). Friction angles are measured in residual conditions ($.), that is to say after large displacements, or on During tests, normal load, saw cut planes.),I@( shear load, vertical and horizontal displacements are measured.

Block size distribution Joint sets cut up the rock mass in a multitude of blocks. The size and the shape of blocks depend on the number, the spacing and the trace length of discontinuities. Block size and shear strength along discontinuities control the mechanical behaviour of the rock mass. Block size can be estimated by the Block Size Index (Ib) or by the Volumetric Joint Count (Jv). However, these simple methods do not take account of the tridimensional aspect of a rock mass. To achieve this we used a joint set model, STEREOBLOCK, to evaluate block size distribution based on Beacher stereological principles and on the statistical analysis of joint sets (Hadjigeorgiou et al. 1995). Measurements taken from scanlines provide 1) identification of joint sets using stereonets; 2) statistical analysis that is carried out giving discontinuity normal spacing and trace length distributions. STEREOBLOCK provides 4 types of information: an output stereonet to compare with initial data, a statistical analysis of joint set spacing and length, a 2-D representation of joint sets at any location in the rock mass, and, essential data to trace block size distributions (Figure 2). The major advantages of STEREOBLOCK are the unlimited number of joint sets and the fact it takes into account discontinuity trace length. However, scanlines coupled with STEREOBLOCK do not allow the simulating of fractures or weakness planes that are not visible on the rock face or those intrinsically related to mineralogy.

Geo-nzechanical classifications Field measurements, such as scanline surveys, and results from laboratory tests are integrated in a global evaluation of the rock mass in the detachment zone. Spacing between two joints obtained by scanline surveys lead to Rock Quality Design (RQD) values for the rock mass (Sen & Kazi 1984). RQD values range between 0% and 100%. Quality of rock mass can also be evaluated by geomechanical classifications. Bienawski (1974, 1984) proposed the CSIR classification based on 6 criteria: compressive strength (oc),RQD, joint spacing, joint conditions, water, and orientation of slope, which they have their own rating. Summation of these criteria (RRI to RR6) gives a quality parameter, RMR (Rock Mass Rating) that ranges between 0 and 100, which can be used to evaluate @. The Slope Mass Rating (SMR, Romana 1988) has also been computed. An other parameter of rock quality is given by Q from the NGI system where Q is defined as follow: Q=(RQD/Jn) x (Jr/Ja) x (Jw/SRF)

(2)

(1)

Grain size distribution of debris material The photograph-sampling of debris is analyzed to provide an estimate of the grain size distribution. Each block in a sampling window on the photographs is traced on translucid paper (Figure 3), then measured manually (Kellerhals & Bray 1971) or using an image analysis technique (Doucet & Lizotte 1992). Blocks have to be traced

where Jn is the number of discontinuities, Jr is joint roughness, Ja is the level of weathering on joint , which can give an estimation of residual friction angle, and where Jw and SRF are respectively a pore pressures paremeter and a strength factor (Barton et al. 1974). Value of Q

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since the system can not recognized the block contours, especially those that overlap. The translucid paper is hung on a white wall and photographed by a cam-recorder. The image analysis system converts the analog signal to numerical values, which can be measured once the scale of reference element is calibrated with the scale in pixels. Then each block can be measured and its diameter estimated. Statistical analysis are carried out to provide grain size distribution of rock avalanche debris (Figure 3). The shape coefficient, or any dimensional parameter, may also be measured by this system.

1990).The general equation describing the shear strength (z)is as follows~ T = zc + o(l-ru)tg$’ + q(6v/6y)‘

(3)

where the first term zc is the yield strength, which is equal to zero for rock avalanche (Locat 1993). The second term is the Coulomb plasticity, where o(1-ru) is the effective stress, and $’ is equivalent to the static friction angle (Hungr Morgenstern 1984)* The pore pressure ratio (rU) is estimated from the results of the stability analysis or interpretation of slide reconnaissance. The last term of (3) corresponds to the viscous component where q is a viscosity parameter and r is the exponent affecting the dispersion pressure term. Equation (3) is solved with a finite difference numerical model, SKRED (Irgens 1988). Calculations give the location of the front and the tail of the rock avalanche, the flow front height and the flow tail height, the average flow front velocity, the normal stresses and the shear stress.

’

Analysis Stability Using a lower hemisphere stereographic projection we carried out a kinetic analysis of the slope. This approach leads to the determination of slope failure modes and the degree of freedom of the rock mass (Hoek & Bray 1981). Results from field work and laboratory testing are integrated into a stability analysis, an analysis of energy balance with an emphasis on debris fragmentation, mobility and rock avalanche dynamics. For instance, the stability analysis of the detached mass includes geometrical parameters and friction angle both measured in-situ and in laboratory. The objective in the stability analysis is to evaluate the slope conditions at failure in terms of the role of water, seismicity and apparent cohesion due to the overlapping and interlocking of blocks and the failure plane roughness. This phenomenon of imbrication can be represented by the angle i in the Patton criterion (Patton 1966). We performed stability analysis of initial with the failure limit equilibrium method.

Mobility Factors affecting mobility can be related to the boundary conditions in the detachment zone, in the transition zone and in the deposition zone, and is also related to geomorphic control (Nicoletti & Sorriso-Valvo 199 1). Mobility can be evaluated in terms of empirical relationships linked to geometric parameters (e.g. Scheidegger 1973; Hsu 1975; Davies 1982; Corominas 1996). Mobility of rock avalanches may be affected by topography, but also by the presence of water in the flow path. Fraginentcctioiz Comparison between block size distribution in the detachment zone and in the deposition zone leads to an evaluation of the fragmentation process during a rock avalanche. Our analysis puts emphasis on the communition process in a rock avalanche where the energy of fragmentation (EF) is evaluated using a classical relationship for rock breakage:

Post-fai1U re beh avior Although detailed in situ surveys give relevant information on the post-failure behaviour of rock avalanches, the use of numerical modelling may improve our knowledge of velocities reached by the mass in motion, the topographic influence on velocity, and the thickness of the flowing mass. Modelling can also give additional information on dynamic parameters that are difficult to evaluate otherwise. The proposed approach is based on a visco-plastic model sustained by Newton’s Second Law. This 2-D model has been tested and calibrated on other granular flows, such as snow avlanches and submarines flowslides (Norem et al.

(4) where Kb is a constant (Bond 1952; DeMatos 1988). This approach takes into account the comparison between block size distribution (D) in the detachment zone and the block size distribution of the debris (d).

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Table 1. Characteristics of La Madeleine rock avalanche. Parameter

Svmbol

Volume Horizontal Travel Distance Vertical Travel Distance Fahrboschung Equivalent Coefficient of Friction Excessive Travel Distance Run-up Density Compressive Strength Tensile Strength Coefficient of Elasticity Coefficient of Deformation Shear Modulus Residual Friction Angle, dry Residual Friction Angle, wet Joint Roughness Coefficient Rock Quality Design CSlR Classification: Rock strength

The energy of fragmentation is one of the elements included in the energy balance equation. EP (mgh), the potential energy available in the detachment zone, is transformed in the kinetic energy (EK),in the energy lost by friction (Ef), and in energy due to fragmentation. The latter is could be evoked for contributing to high mobility of rock avalanches (Couture 1998). The methodology presented above attempts to describe the boundary conditions in the three zones of a rock avalanche and to analyse, at different scales, the rock mass and the debris involved in the rock avalanches. A novel aspect of this methodology is both the application of geomechanical methods and the evaluation of the blocometry in the detachment zone to compare the size distribution of debris in order to evaluate fragmentation energy. This methodology was applied to 7 rock avalanches located in the French Alps and the Canadian Rockies (Couture 1998). One of the rock avalanches studied in the French Alps is described hereafter.

LA MADELEINE (FRANCE)

ROCK

L H F f

Le

90x10~m3 4500 m 1250 m 15.5 O 0.28 2000 m

Ld h

Length Deposit

Figure 4.View of the La Madeleine rock avalanche (LMRA), Savoie (France).

v

Value

RQD Spacing Joint Condition Water Condition Orientation to Slope

3500m

130m 26 kN/m3 oc 70-100 MPa 0 1 2 MPa E 23 GPa v 0.36 G 8.5 GPa b 35 $w 29 O JRC 9 RQD 97%

Y

O

RRi: 9 RRz: RR3: RR4: RR5: RR6:

20 10 25 0 -20

Class

RMR 44 -(25"-35") Ill Fair SRM 26

RQD Joint Sets Alteration Roughness Water Condition Stress Reduction Factor

RQD:97 Jn: 15 Ja: 1 - (25"-35") Jr: 1.5 Jw: 0.1 SRF: 1

Slope Mass Rating NGI Classification:

Q

0.97

Mean Block Size Distribution: Detachment zone Deposition zone

DS0 dS0

2.48 m 0.138 m

flank of Pignes Mountain (3061 m). The rock mass went down the slope, ran up at least 130 m on the opposite slope (Le Collet on Figure 4), and then flowed down along the valley for a total horizontal travel distance (L) of about 4500 m and a vertical distance (H) of 1250 m (Table 1). A lake was formed upstream the debris, and then the Arc river found a passage through the debris forming a deep gorge (Figure 4). A I4C dating of organic matter found in the lacustrine clay deposit dated the LMRA older than 7625 f 65 BP (Couture 1998). Former investigations concerned geomorphologic aspects (Onde 1938; Blanchard 1918), Quaternary deposit (Letourneur et al. 1983; Hugonin 1988) and the landslide was mentioned in discussions on

AVALANCHE

Introduction The La Madeleine Rock Avalanche (LMRA) is located near the Italian border (Figure 4) in the Maurienne valley midway between the hamlets of Lanslevillard and Bessans. The Arc river was dammed by about 90 x 106 m3 of schistose material originating from the north-east 1374

natural disasters (Goguel 1980; Monjuvent & Marnezy 1986).

100

80

Detachment zone The detachment zone corresponds to a large rentrant in the S-E slope of the valley. The Mont Cenis Glacier extends as a glacier tongue down to Pignes Mountain and supplies continued water flow toward the starting zone. The LMRA area is covered by a thrust sheet of lustreous schists overlaying gneissic Paleozoic bedrock. More massive beds can be seen in the schist formation which constitute the detachment zone. The failure plane corresponds to the base of one of these massive beds. Regional orthogonal structural lineaments can be recognized on air photos. These lineaments are oriented NW-SE and about E-W. The limits of the detachment zone are defined by these lineaments. The ubiqitous schistosity, striking parallel to the valley and dipping 20" toward the valley bottom. Scanline surveys carried out in the detachment zone shows 4 discontinuity sets includinp the schitositv. Schmidt hammer test performed in the filed on the schists gave a compressive strength (oc)of 100 MPa 5-40 MPa (Table 1j. The csCvalue decreases to 70 MPa if the point load test is performed parallel to the schistosity. The tensile strength (oJ is low, 2.3 MPa. Results from laboratory tests showed low values of dynamic properties. The Young's modulus equals 23 GPa, the value of shear modulus is 8.5 GPa and Poisson's ratio is 0.36 (Table 1). Shear tests performed on planes of schistosity gave residual friction angles in dry (Cprd) and wet (Cpnv) conditions respectively of 35" and 29". JRC values estimated by Barton's 10 typical profiles show an average value of about 9. Scanline surveys, for a total length of 35 m, were performed in massive beds of schist in the detachment zone. They gave an average joint spacing of 0.37 m and a RQD value about 97 which gives an excellent quality rating for the rock mass. However, this high value does not reflect the quality of the entire mass. Geomechanical classifications gave a more realistic quality rating. The summation of parameters RR, leads to a RMR value equal to 44 giving a fair quality rating, whereas Q is 0.97 giving a very poor quality rating for the rock mass (Table 1). These values fit very well with equation (2). The friction angle, Cp,

.p

l!

60

40

2 20

n

0.01

0.1

1

10

100

Blodc size, diameter (rn)

Figure 5. Block size distribution in the detachment zone (dashed line) and grain size distribution o f debris material (continuous line) for La Madeleine rock avalanche.

evaluated by RMR and by the parameter Ja gives a value ranging between 25" and 35O, which is equivalent to values found by shear tests. Kinematic analysis showed that plane slide failure occurred locally along schistosity planes when the dip angle was higher than the friction angle for the dry condition. Toppling and wedge failures may have occurred but did not generate a huge volume o f rock as illustrated by small wedge scars seen in the detachment zone. However, massive planar sliding occured most probably on one joint set striking sub-parallel to the valley and dipping 51" toward the valley bottom. Water have played an important role in the failure. Melting of a glacier, upstream from the detachment zone, still supplies an important amount of water in the fractured rock mass. Data from scanline surveys were integrated into STEREOBLOCK to perform bIocometry analysis. Simulations used an external volume with dimensions 4 m x 4 m x 15 m. The average block size best-fit distribution is shown on Figure 5 (dashed line). Diameter of blocks at 50% passing (Dso) equals 2,48 m (Table 1). A important limitation of the estimation of block distribution comes from the length of the scanline and the size of the observation window, which appears too small for an accurate representation of the entire detachment zone. Moreover, scanline surveys performed normal to the rock face would bring a better 3-D representation of the fractured mass.

1375

equal to 19" reflecting the roughness of the failure plan and the imbrication of blocks in the detachment zone. However, this value appears much too high considering the intermediate value of JRC measured in the field. Post failure behaviour of the rock mass was analyzed by SKRED for the first part of the flow path, i.e. from the detachment zone to Le Collet (Figure 4). This back-analysis integrated pore pressures ratio (r,,) values, derived from the stability analysis (ranging between 0.18 and 0.36), and used a friction angle of 29" (wet conditions). Simulations provided average front velocity and the thickness of the mass in motion. Flow front velocity reached a maximum value of 80 d s . The irregular velocity profile reflects the uneven flow path. Average velocities range between 47 d s and 63 d s . Front thickness shows a steady profile until the very last part of the flow path, whereas the tail thickness increases considerably when it reachs the valley bottom. Simulated travel distances also show a good relationship with insitu observations. However, simulations still include non-measured parameter values such as viscosity, which is based on the value given in the literature (Locat 1993). Results from modeling are also sensitive to the shape of the failed mass as illustrated by Locat (1993). The model assumes a constant volume and does not take into account the erosion/deposit effect. La Madeleine rock avalanche can be described as a low-mobility rock avalanche determined by high-energy-dissipative control (Nicoletti & Sorriso-Valvo 1991). Despite its high excessive travel distance (Le), LMRA does not show a great mobility. Much kinetic energy was lost by running up the opposite slope of the narrow valley. The potential avalaible energy calculated is 2.76 x 10ls J. Based on Francis & Baker (1977), 69% of the available energy was lost by friction. Comparison between block size distribution (D) of the rock mass and those in the debris (d) gave a fragmentation ratio (DjO/ds(J of about 9. The schistose state of the materials and the long travel distance gave rise to such a degree of fragmentation.

Transition zone After the failure, the fragmented mass ranup 130 m on the opposite site of the valley, and then ran down the confined Arc River valley. Based on the run-up (h), velocity is estimated to be 50 d s . In the upper part of the flow path, debris flowed on a hard substratum (bedrock), then on soft subtratum (pre-rock avalanche surficial deposits) in the valley. Surficial deposits beneath the rock avalanche debris corresponds mainly to colluvium generated by the failure of the toe of the slope. Deposition zone The deposition zone is easily recognizable by its chaotic surface and by the numerous large blocks scattered on the bottom of the valley. Near the distal part of debris, some large blocks are more than 20 m in diameter. Calculations based on digital terrain models of pre- and post-failure in the deposition zone gave a volume about 90 x 106 m3. Pre-failure topography and the thickness of the debris deposit are based on borehole logs (Hugonin 1988) and interpolation of bedrock topography. Table 1 gives value of geometric parameters such as L, Le and Ld, which are respectively, 4500 m, 2000 m and 3500 m. The calculated equivalent coefficient of friction (f = H/L) and the fahrboschung ( F = tg-'f) are 0.28 and 15.5". A steep deep gorge in cemented debris gives excellent cross sections that show inverse grading in debris. Thirteen photograph-samples taken in the deposition zone allow evaluation of the block size distribution of the debris. The diameter of debris at 50% passing (dSO) shows values ranging from 0.07 m to 0.30 m, with a mean value equal to 0.138 m. The best fit curve showing the distribution is illustrated by the continuous line (open circles) in Figure 5. Analysis and discussion Stability analysis was carried out with limit equilibi.ium using the SARMA method (Sarma 1979) to take into account the irregular failure plane and shape of the destabilized rock mass. The effect of water pressures in the detachment zone is examined according to different scenarios of ground water table positions. Results indicated that the slope failure was mainly related to the loss of a block at the toe of the slope rather than by high pore pressures. Nevertheless, the pre-failure profile of the detachment zone remains hypothetical and may lead to a miscalculation of the safety factor. Back-calculations showed an average value of i

CONCLUSION The methodology proposed above was used to analyze Canadian and French rock avalanches. One case studied, the La Madeleine rock avalanche, is presented herein. This methodology

1376

was concerned by the complete characterization of the detachment zone, the transition zone and the deposition zone of the rock avalanche. Data collected from rock mass and debris were integrated in a framework study including stability, post-failure behaviour, mobility and fragmentation analysis.

d’kcroulements rocheux. Proc. 7’;’ Int. Symposium on Landslides, Trondheim, Senneset (Ed), Balkema, Rotterdam : 11771182. Cruden D.M., Hungr 0. (1986). The debris of Frank Slide and theories of rockslide avalanche mobility. Can. J. of Earth Sciences, 23, 3: 425432. Davies T.R. (1982). Spreading of rock avalanche debris by mechanical fluidization. Rock Mechanics, Vol. 15: 9-24. Deere D.U, Miller R.P. (1966). Engineering classification and index properties for intact rock. Technical Report No. AFNL-TR-65-116 Air Force Weapons Laboratory, New Mexico (USA). DeMatos M.M. (1988). Mobility of soil and rock avalanches. Ph.D. Thesis, University of Alberta, Edmonton: 360 pages. Doucet C., Lizotte Y. (1992). Rock fragmentation essment by digital photography analysis. Rapport MRL 92-116 (TR) CANMETLaboratoire de recherche minikre, EMR Ottawa (Canada), 42 pages. Evans S.G, Clague J.J., Woodsworth G.J. and Hungr 0. (1989). The Pandemonium Creek rock avalanche, British Columbia. Can. Geotechnical J., 26: 427-446. Francis P.W., Baker M.C. (1977). Mobility of pyroclastic flows. Nature, 270: 164-165. Goel R.K., Jethwa J.L., Paithankar A.G. (1996). Correlation between Barton’s Q and Bieniawski’s RMR - A new approach. Int. J. Rock Mech. Sci. & Geomech. Abstr., VoI. 33, No.2: 179-181. Goguel J. (1980). Les risques de grands tboulements. La Recherche, 1 1: 620-628. Hadjigeorgiou J., Lessard J.-F. And Flament F. (1995). Characterizing in-situ block size distribution using a stereological model. Canadian Tunnelling, Annual Publication of Tunnelling Ass. of Canada: 111- 121. Hoek E., Bray J.W. (1981). Rock Slope Engineering. 3rd Edition, Institution of Mining Metallurgy, London, 358 pages. Hsu K.J. (1 975). Catastrophic debris streams generated by rockfalls. Geological Society of American Bulletin, Vol. 86: 129-140. Hudson J.A., Priest S.D. (1979). Discontinuities and rock inass geometry. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., Vol. 16: 339362. Hudson J.A., Priest S.D. (1983). Discontinuity frequency in rock masses. Int. J. Rock Mech.

AKNOWLEGEMENTS This paper is part of the senior author’s Ph.D. thesis carried out within a research agreement between Laval University and CEMAGREF (Grenoble). This work was supported by FCAR, NSERC, Geological Survey of Canada and the P81e Grenoblois des Risques Naturels (PGRN). Collection of data at LMRA would not have been possible without the support of CEMAGREF and field assistance by C. Gagnon. REFERENCES CITED Barton N., Choubey V. (1977). The shear strength of rock joints in theory and practice. Rock Mechanics, 10: 1-54. Barton N., Lien R., and Lunde J. (1974). Engineering classification of rock masses for the design of tunnel support. Rock mechanics, Vol. 6 (4): 183-236. Bieniawski Z.T. (1974). Geomechanics classification of rock masses and its application in tunnelling. Proc. 3rd Int. Cong. Rock Mech., Denver (USA), Vol. 11A: 27-32. Bieniawski Z.T. (1984). Rock mechanics design in mining and tunnelling. Balkema, Rotterdam: 272pa.e~. Blanchard R. (1918). Comparison des profils en long des vallkes de Tarentaise et Maurienne. Revue de GCographie Alpine, 6: 26 1-331. Bond F.C. (1952). The third theory of comminution. Mining Engineering, New York. Chow, V.T. (1959). Open-channel hydraulics. McCiraw-Hill, New York (USA). Corominas J. (1996). The angle of reach as a mobility index for small and large landslides. Can. Geotechnical J., Vol. 33: 260-271. Couture R. (1998). Contributions aux aspects physiques et mkcaniques des Ccroulements rocheux. Ph. D. Thesis, Dept. of Engineering Geology, Laval University, Quebec (Canada), 573 pages. Couture R., Locat J., Hadjigeorgiou J., Evans S.G., Antoine P. (1996). Dkveloppement d’une technique de caracterisation des dCbris 1377

Min. Sci. & Geomech. Abstr., Vol. 20, No.2: 73-89. Hugonin F. (1988). Le Quaternaire de la HauteVallte de 1’Arc (Stratigraphie, stdimentologie et chronologie). Thkse de doctorat, Universitt Joseph-Fourier, Grenoble (France). Hungr O., Morgenstern N.R. (1984). Experiment on the flow behaviour of granular materials at high velocity in an open channel. Geotechnique, 34, (3): 405-413. Irgens 1;. (1988). A continuum model of granular media and simulation of snow avalanches flow in run-out zones. 17thCongress of Theoretical and Applied Mechanics, Grenoble (France). Kellerhals R., Bray D.I. (1971). Sampling procedures for coarse fluvial sediments. J. of the Hydraulics Division, ASCE, 97, No. HY8: 1165-1180. Leroueil S.,Vaunat J., Picarelli L., Locat J., Lee H., Faure R. (1996). Geotechnical characterization of slope movements (Invited Lecture). 7th International Symposium on Landslides, Trondheim, 1996: 53-74. Letourneur J., Monjuvent G. and Giraud A. (1983). Ecroulement de la Madeleine et le Lac de Bessans - Contributions B l’histoire quaternaire ricente de la Haute-Maurienne (Savoie). Travaux scientifiques, Parc National de la Vanoise, 13: 31-54. Locat J. (1993). Fra fjell til fjord: Considerations on viscous flows. Proc. Pierre-Beghin Workshop on rapid gravitational mass movements, Grenoble (France), Buisson & Brugnot (Ed.): 197-207. Monjuvent G., Marnezy A. (1986). Processus d’ivolution des versants dans les Alpes franpises. Gtologie Alpine, 62: 87- 104. Nicoletti P.G., Sorriso-Valvo M. (1991). Geomorphic controls of shape and mobility of rock avalanches. Geological Society of America Bulletin, Vol. 103: 1365-1373. Norem H., Locat J. and Schieldrop B. (1990). An approach to the physics and the modelling of submarine flowslides. Marine Geotechnology, Vol. 9: 93- 1 11. Onde H. (1938). La Maurienne et la Tarentaise (Etude de Gkographie physique). Ed. A. Arthaud, Grenoble, France Pahl P.J. (1981). Estimating the mean length of discontinuity traces. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. Vol. 18: 221-228. Patton F.D. (1966). Multiple modes of shear failure in rock. Proc. lst Int. Cong. of Rock Mechanics, Lisbon (Portugal), Vol. 1: 509-5 13.

Priest S.D., Hudson J.A. (1981). Estimation of discontinuity spacing and trace length using scanline surveys. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. Vol. 18: 183-197. Romana M. (1988). Practice of SMR classification for slope appraisal. Proc. 5‘h Int. Symp. on Landslides, Vol. 2, Balkema, Rotterdam: 12271232. Sarma S.K. (1979). Stability of analysis of embankment and slopes. J. Geotechnical Engineering Div., ASCE, 105 (GT12): 151 11524. Scheidegger A.E. (1973). On the prediction of the reach and velocity of catastrophic landslides. Rock Mechanics, 5: 23 1-237. Sen Z., Kazi, A. (1984). Discontinuity spacing and RQD estimates from finite length scanlines. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., Vol. 21, No.4: 203-212.

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Slope Stability Engineering, Yagi, Yamagami & Jiang 0 1999 Balkema, Rotterdam, ISBN 90 5809 079 5

The Otari debris flow disaster occurred in December 1996 H.Kawakami Nagano Study Center, University of the Air, Japan

H.Suwa Disaster Prevention Research Center, Kyoto University, Uji,Japan

H. Mami, 0.Sat0 & K. Izumi Research Institute for Hazards in Snowy Areas, Niigata University, Japan

ABSTRACT: The debris flow that occurred in the upper stream of the Gamahara-zawa killed 14 construction workers and injured 9 others. The debris flow was triggered by a small slope failure. It flowed down 2.7 km downstream with a grade of 20 degrees. The volume of the slope failure was about 33,000m3. According to the chemical analyses of the water, calcium and sulfate iron components are predominant. It showed a lot of groundwater discharged through the volcanic products. The precipitaion of the day before disaster was comparatively small even if the melting snow was considered. Conclusively, it is supposed that the slope failure was easy to occur due to the existence of big slope failures caused in the preceding year and due to the geologic conditions. 1 INTRODUCTION The debris flow that occurred in the Gamahara-zawa Stream which is one of the branches of the Hime River, occurred at about 10:40 a.m. on December 6, 1996. The debris flow swept away 14 construction workers and injured 9 others working at the junction of the Gamahara-zawa Stream and the Hime River. The Otari debris flow was triggered by a small slope failure and by slight precipitation. The reasons for cause of the debris flow and characteristic features of it are investigated.

above 700m in elevation. The formation alternates among gravelstones, sandstones and mudsones. The strike of the formation is E-W or NE-SW and the dip is to the South. The right bank of the stream is steep because of the dip opposing to slope. The left bank is gentle due to the direction of dip nearly coinciding with the slope. The upper stream area above 1300m in elevation is covered by the volcanic products originating from MtKazafuki and shows a gentle landscape.

2 GEOLOGIC CONDITION The Gamahara-zawa Stream is situated in the northern part of Nagano Prefecture and forms part of the border between Nagano Prefecture and Niigata Prefecture as shown in Figure 1.The Gamahara-zawa Stream is very steep and has a mean inclination of about 20 degrees. The area of the drainage basin is 4.0 km2 and the length of the stream watercourse is 4.6 km. The left bank of the basin is long and gentle. On the other hand, the right bank is short and steep reflecting its geologic condition. The geologic map around the Gamahara-zawa Stream is shown in Figure 2 and Figure 3. The Palaezoic consists of sandstones, clayshales and cherts distributed along the Hime River. Serpentines exposes above the elevation of 450m. Moreover, the Kuruma formation of the Jurassic age distributes

Figure 1. Location of the debriss f lo w

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Figure 3. Geologic section along the Gamahara-zawa stream

3 SLOPE FAILURE

Figure 2. Geologic map around the Gamahara-zawa stream (Shiraishi.1992)

The trigger of the debris flow occurred at the right bank slope of the Gamahara-zawa Stream. The slope failure is shown in Figure 4. A topographic map of the collapsed slope shown in Figure 5 is drawn by using the aerial photograph taken on July 23, 1995 and on December 7,1996. The latest slope failure occurred at the previous location of the slope failure that had been caused by the heavy rainfall on July 11, 1995. The slope failure went back 40m to the hillside and a small ridge was disappeared by the slope failure. The longitudinal section of the failured slope is shown in Figure 6. The inclination of

Figure 4. The slope failure at upstream of the Gamahara-zawastream taken by Kyodo-SokuryoCo.on December 7, 1996. A black part is new slope failure. That is 120m inlength and 60 m in width. White parts around the black are the preceding slope failure caused in the preceding year.

1380

the slope reaches 50 degrees at the upper part of the slope. The sliding mass is 120m in length, 60m in width and 20m in depth. Its volume is about 33,000m3. The volume of the colluvial deposit is 13,000m3 at the lower part of the slope. Therefore, 20,000m3 of sliding mass flowed down along the stream. The old slope failure that occurred on July 11, 1995 was 47,000m3 in volume. The previous slope failure was bigger than the latest. The previous debris flow was 100,000 m3 in volume. A lot of floods, debris flows and landslides were caused on July 11, 1995 by a heavy rainfall of 400mm or more in two days in the Otari and Hakuba districts. Moreover, the old slope failure occurred on the left bank of the Gamahara-zawa stream on July 11,1995 as shown in Figure 5. A few slope failures were concentrated at 1300 m. They depend on the geologic condition that there is the boundary between the Kuruma Formation and the volcanic products at this elevation. The volcanic products are exposed 10m or more at the scarp of failed slope. The volcanic products consist of many large andesite rocks and soft clayey matrix. Thick volcanic products of 80 meters or more were confirmed by boring. They were carried out at the site 50 meters apart from the south of scarp. Several holes from which groundwater was flowing were found in the scarp. The shalestones and sandstones of the Kuruma Formation were confirmed at the foot of the failured slope. It is theorized that the slope failure that occurred at the boundary between the Kuruma Formation and the volcanic products was caused by groundwater that flowed out through the volcanic products.

Figure 5. Plan of slope failure

4 CHARACTERISTICS OF THE DEBRIS FLOW A part of the debris flow flooded at the junction of the Gamahara-zawa stream with the Hime River. Other debris flowed down along the Hime River. Dams, bridges and channel works have been constructed at the junction as shown in Figure 7. The largest rock in the debris flow was as large as 4m or more in diameter. Most of large rocks were serpentine and others are shales or sandstones. However, the larger part of the debris flow was gravely soil less than 100 mm in diameter. The typical grainsize distribution of samples obtained in the debris flow deposit is shown in Figure 8. The grain size distribution of several samples obtained from debris flow deposit is similar to Figure 8. The deposits of the older debris flows are found at the left cliff of the stream as shown in Figure 7. But its depositional age is unknown. The grain size distribution is also same to Figure 8. The coarse grains are also classified depending on their material as shown in Figure 8. It was expected

Figure 6. Section of slope failure

that the sample would contain much volcanic materials, as the hilltops of the stream are overlayed by volcanic products. However, debris flow deposits contain little volcanic rocks because volcanic rock might b e crushed while the debris flows down along the stream. It is shown in Figure.8 that two thirds of the gravel were shales and sandstones originating in the Kuruma Formation and one third of the gravels were serpentine.

5 SCALE OF DEBRIS FLOW Based on the surveying of the deposition of the debris flow on the Gamahara-zawa stream, the following became clear. 1)The maximum cross sectional area of the debris 1381

flow passing through the Sabo-dam is about 170m2. 2)The first debris flow flooded over the channel works at the alluvial fan as shown in Figure 7 but later debris flows went down the channel work and flowed along the Hime River. 3)Deposit volumes of the debris flow were: 6000m3on the alluvial fan and 18,000m3 between the junction with the Hime River and 200m downward site along the Hime River. The volume of debris flowing downstream is unknown. According to construction workers and a television broadcast reporter who arrived at the site immediately after the debris flow, three or more surges occurred and later eight or more relatively small within one hour. Based on the surveying on the alluvial fan and the video records of the debris flows, the dimensions of the debris flow were investigated at the channel with a gradient of 5 degrees. a)The running depth h is less than 0.5m. b)Front velocity Vr is less than 3.8mh.

100 No.

80-

0

4

5 10 grain size in m m

1

50

3

100

Figure 8. Grainsize distribution and classification

a)

before debriss flow

b) after debriss flow

-4

-2

0 aeq/l

2

4

Figure 9. Chemical analyses of the stream water

c)Peak discharge is less than 20m3/s. d)Velocity coefficient Vf/u*’nearly equal to 6. Traces of the debris flows were found on the channel walls at the alluvial fan. The mean velocity of the debris flow can be obtained by using the super elevation dh in the debris flow. From the traces of debris flows on the sidewall of the channel, dh=0.8m at T2 site and dh=OSm at T3 site. These sites are shown in Figure 7. After some calculations, it was analyzed that the discharge velocity of the debris flow was 5-11 m/s and the peak discharge was 200-500 m3/s. The midvalue of velocity was 8 m/s and that of discharge was 350 m3/s. 6 CHEMICALANALYSES OF STREAM WATER

Figure 7. Construction work at the junction of two rivers

Chemical analyses were carried out on the water samples obtained from the stream water before and after the debris flow. One sample a) was taken on October 22, 1996 before the debris flow in the Gamahara-zawa stream water and the other b) was extracted by using a centrifugal separator from debris flow deposits taken on December 7, 1996. Analytical results are shown in Figure 9 as pattern diagrams. The electric conductivity of b)sample is higher than a)sample. b)sample contains more soluble iron components. Particularly, calcium iron and sulfate iron are predominant. 1382

It is theorized that soluble iron components were supplied by the groundwater, which permeated through the volcanic products and flowed out along the boundary layer between the volcanic layer and the Kuruma Formation. Similar phenomena had been observed in the stream water of the Ura River that is famous for having many debris flows. That is situated 10 km south from the Gamahara-zawa. Andesites containing pyrites were found in the upper stream basin of the Ura River. Depending on the oxidation of pyrite, a highly concentrated solution of sulfate iron flowed into the Ura River, (Aoki.1982-86).

7 WEATHER CONDITION Figure 11. Relation between rainfall and elevation

Weather conditions of the day before the debris flow was investigated in detail. An atmospheric depression was developing and proceeded northeast on 5 December. Then, it rained in Nagano, Niigata and Toyama Prefecture. The temperature rose, and snow coverage melted rapidly in this area. 7.1 Temperature Condition The relation between the elevation of the observationsite and the maximum and minimum temperature on 5th December were investigated. Both maximum and minimum temperatures decreased with the increase of elevation as shown in Figure 10. Depending on the inflow of warm weather, maximum temperature rose to 7 degreeC even at the elevation of 1200 m. Accordingly, decreased snow coverage also proceeded at the site of slope failure. A ski field in Nagano Prefecture being about 8 km south from the Gamahara-zawa has measured weather conditions in detail including solar radiation and wind velocity. According to the results of calculation, melting snow was 30 mm on December 5 at the ski field where the elevation is 1300 m. Melting snow mainly depends on the sensible and latent heat.

Figure 12. Rainfall record by a construction company

7.2 Rainfall on December 5 The relation between the daily rainfall on December 5 and elevation at observation sites is shown in Figure 11. The daily rainfall in Nagano Prefecture had shown as the A-line increases linearly with elevation. However, the daily rainfall increases largely in the left bank side of the Hime River as the B-line shows. According to the B-line, it is theorized the rainfall might be 70 mm in the area of slope failure. It thought the precipitation reached 100 mm including melting snow 30 mm.

8 MANAGEMENT OF CONSTRUCTION WORK

I

0

I

500 1000 elevation in m

1500

Figure 10. Relation between temperature and elevation

The labor safety associations of the construction companies working in this field have determined the rainfall standards for disaster prevention and evacuation. At first, a standard rainfall for evacuation was 20 mm in an hour or 60 mm in six hours. After a heavy rainfall on June, the standards level was raised to 15 mm in an hour or 50 mm in six hours. 1383

of Research Institute for Hazards in Snowy Areas, Niigata University. No.4-8. (in Japanese). Japan Sabo Association. 1993-1995: Sabo Handbook (Sabo-Binran). (in Japanese). Shiraishi,S. 1992. The Hida Marginal Tectonic Belt in the middle reaches of the River Hime-kawa with special reference to the lower Jurassic Kuruma Group. Earth Science (Chikyu Kagaku). 46(1): 120. (in Japanese).

Also, the Nagano prefectural office showed the rainfall standards for evacuation to construction companies on May 28, 1996. The standards were 75 mm for a continuous period, 60 mm in a day or 15 mm in an hour. But melting snow had not been factored into the standard. An example of rainfall records by a construction company is shown in Figure 12. Accumulated rainfall reached 71.5 mm before the debris flowed on December 6. And rainfall did not reach the refuge standard decided as in advance. It has been considered in general that rainfall in December was the lowest in a year and debris flows happened to occur in the rainy season, i.e. July, August and September. Within the past five years 251 debris flows occurred in the rainy season and was never found in December (Sabo-binran.1995). According to the results of headline surve,y of the Asahi newspaper, the words debris flow’ were found 202 times in the past 50 years. However, debris flow occurring in December has only happened once on the hillside of Mt.Fuji. Rainfall on December 5 did not reach the refuse standard. And the debris flow disaster has never happened on December in the past. 9 CONCLUSIONS The precipitation on the day before the debris flow was 100 mm including the snow melting. The debris flow was triggered by a small slope failure, which was 33,000 m3 in volume and occurred in the old slope failure of 1300 m in elevation. The failure was affected by groundwater flow due to the geologic condition of the slope. The debris flow contained a lot of calcium and sulfate iron components. It showed that the effect of groundwater flow was predominant. Depend on the unseasonable debris flow and a little precipitation, which does not reach to the refuge standard, disaster prevention for debris flow could not be carried out. ACKNOWLEDGEMENTS Financial support was provided by the Grant-in-Aid for Scientific Research by the Japanese Ministry of Education, Science and Culture. The Authors are grateful to the cooperative researchers of the Grantin-Aid for Scientific Research. REFERENCES Aoki,S. et al 1982-1986.Geologicaland geomechanical studies on the slope failures and debris flows in the Ura river basin, Nagano Prefecture. Ann, Rep.

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Slope Stability Engineering, Yagi, Yamagami & Jiang 0 1999 Balkerna, Rotterdam, ISBN 90 5809 079 5

Dimensional analysis of a flume design for laboratory 'debris flow simulation L.C. K h a n & K.T.Chau Department of Civil and Structural Engineering, Hong Kong Polytechnic Universi@ Kowloon, People's Republic of China

ABSTRACT: Dimensional analysis for a flume design for laboratory debris flow simulation was proposed. Four geometric scaling factors and three constitutive scaling parameters were used to scale down the Tsing Shan debris flow event occurred on Sept. 1 1, 1990 in Hong Kong. In this paper, we focus on the effect of granular contents on the shape of debris fan and the maximum runout distance of debris flow. The configuration of debris fan and the velocity of debris flow surge were experimentally studied. Experimental results show that debris material with a richer sand content (60% sand and 40% gravel by weight) gives a longer runout compared to the debris material with (50% sand and 47% gravel by weight).

1 INTRODUCTION

Hong Kong is not immune from debris flow. The largest debris flow reported in Hong Kong occurred on Sep. 11, 1990 at Tsing Shan involved the movement of 19,000 m' of boulders and soils, and debris was deposited into the "Area 19 Tuen Mun", a designated site for further development (King, 1996a). The total path measures 1035m long along a V-shaped gully of slope angle 16-27 degree. The path of the debris flow narrowly misses a nearby squattering area (San Shek Wan San Tsuen) by less than 200m. On June 13, 1992, another debris flow of smaller scale (250m3) flowed down a nearby gully at Tsing Shan and covered a concrete footpath (King, 1996b). On Nov. 4-5 1993, over 800 debris flows occurred on Laiitau Island (Wong et al., 1996). An abandoned school located at a mountain side of about 1 km west of Lo Wu in Hong Kong was also destroyed by debris flow in 1996. As population of urban areas grows, infrastructures and building development tend to spread into areas adjacent to natuaral hillsides. The risk of debris flow to the community inevitably increases. Therefore, much attention has been drawn to scientists and engineers to study debris flow by experimental, empirical and numerical modelling, especially its characteristics of deposition process and travelling distance (Liu, 1996; Major, 1997; Tverson and LaHusen 1993; Shieh and Tsai 1997;

Debris flow, a flowing mixture of water, mud, soil, boulders, and woods has always been a threat to mankind even before the historical time. Its composition is always nonuniform and ranges from clay to boulders. Debris flow is also known as lahar ( in case of volcano ), debris torrents (mainly used in Canada), or mud flow. The geographical appearance of debris flow is extremely widespread, it occurs in most of the mountainous areas of the world. Debris flow is one of the most threatening natural hazards in some regions in the world, such as Japan (about 90 lives a year on average are claimed by debris flow, Takahashi, 1981) and China (occurred in almost two-third of mountaineous regions in China, Zhang, 1993). The worst debris flow of this century occurred on Nov. 13, 1985 at Nevado del Ruiz volcano in Colombia, which claimed the nearby Armero city, killed 22,000 people, and spread debris of volume 48,000,000m' over an area of 30 km' (Takahashi, As a 1991; Garcia and Savage, 1993). conservative estimate, over 60 countries of the world have been attacked by debris flows, including China, Japan, Canada, USA, Switzerland, New Zealand, U.K., Philippines, Peru, Colombia, Brazil, Sweden, Tanzania, Indonesia, and many other countries.

1385

Mizuyama and Uehara 1983; Benda and Cundy 1990; Hungr 1995; Johnson and Rodine 1984; Takahashi 1991; Wong and Ho 1996). Since most debris flows occur under adverse condition of severe rainstorm and/or earthquake, except in some designated areas, such as Mt. Yakedake in Japan (e.g. Okuda et al, 1981; Takahashi, 1991), Jiangjia Ravine in Yunnan China (e.g. Wu et al., 1990), and Mount St. Helens in USA (e.g. Pierson, 1995), very few field observations have been made on debris flow. A common recourse to study debris flow dynamics is the use of experiments under well-designed conditions in laboratories, and these observations can be used in assessing both theoretical and numerical debris flow models. However, to date all previous experiments on debris flow has been done on an ad-hoc manner (except those by Iverson and LaHusen, 1993), to the best of our knowledge no complete dimensionless analysis has been proposed and used in flume design. Thus, scaling problem may lead to the observed phenomena in laboratory differing from the real debris flow in field. In view of such deficiency in the experimental approach, this paper is going to discuss our recent effort in designing a flume and a debris material for modeling the real scale debris flow, based upon the consideration of seven scaling numbers (the Bagnold number B, Savage number S, friction number F, velocity ratio nv, flow ratio nQ, stress ratio nT,and viscosity ratio nJ. The first three of these are proposed by Iverson and LaHusen (1993) for modeling the debris material and the last four of these are proposed by Hua (1989) for modeling the size of flow. The use of the Bagnold number, Savage number, friction number has been adopted in designing the debris material for the 95m long and 2m wide flume located in Eugene, Oregon USA (Iverson and LaHusen, 1993). However, due to financial and space limitations, this kind of full scale flume is not feasible and practical in our laboratory. This is the reason why the four scaling parameters for flow size proposed by Hua ( I 989) is adopted in this study. Such design of flume is believed to yield debris flow phenomenon which is similar to the real debris flow.

2 A NEW EXPERIMENTAL DESIGN FOR DEBRIS FLOW FLUME A newly-designed flume of 3in long, 20cm wide and 30cm depth is manufactured. The side wall of channel is made by transparent plastic board so that 1386

the movement of debris flow can be visualized and measured. The cross section of our flume can be changed in a flexible manner (U-shaped, V-shaped, circular and rectangular-shaped). Topography of channel can be adjustable by allowing three Im long sub-channels, namely upper channel, middle channel and lower channel. The inclination of these channels (i.e. upper channel, middle channel and lower channel) can be adjusted from 10°-45', 10°-350 and 10°-350 respectively. Horizontal length markers are placed along the lower channel while vertical length markers are also placed at three specified locations at the lower end, middle and upper end of the lower channel. The bed of channel is made rough by gluing particles of 2.68n1m mean diameter on the base of the channel. The deposition board at the toe of flume is also made adjustable form 0"-10", and grid lines were drawn so that the deposition area and the fan development can be studied. A supply tank of debris (maximum volume 3 5,000cm') is designed such that it can be moved along the profile of the channel. A water-tight opening gate is attached with hinges to the front of the supply tank. The schematic configuration of debris flow flume is shown in Figure 1. In our study, we coinbine the scaling factors proposed by Hua (1989) for velocity, flow rate, yield strength and viscosity and the scaling parameters proposed by Iverson and LaHusen (1993) in terms of Bagnold, Savage and friction numbers. Hua( 1989) proposed four dimensionless scaling factors for velocity v, flow rate Q, yield strength z,and viscosity p :

(4)

where L and p are the length scale and density of the debris. Therefore, the ratio of velocity, flow rate, shear strength, and viscosity can be expressed in terms of the ratio of the length scale and the density scale only. Iverson and LaHusen (1993)stated that the solid friction, liquid viscosity and particle collisions can play an important role to the

Figure 1. Schematic configuration of debris flow flume

mechanism of debris flow, and proposed three dimensionless number can be used as scaling parameters by equating the friction number, Savage number and Bagnold number between prototype and model :

Hua (1991) r[,

lverson and Laliosen (1993) 10.5

v/v,,,

%

9 1

S

0.3963 5 . 2 7 10.' ~

-

= (y.?*&?)/(u*g*I-I)

I .78x I 0-* 96.8

1:

7.048

-

(p*u*g*H)/(y*p)

= (P/P,,J*(L/L"J

(7)

0.2642

*&'*XI 12yp 126905.9

= QIQ,,, = (L/L,$

XT = TIT,,,

B = (Y.*P

= = (L/L,,,)"

15.858

1014.8

= PiPm

-

(P~P,,J*(L~L",)'

where H is the typical flow depth, y is the typical shear strain rate, 6 i s a typical grain diameter, 2 is linear grain concentration and g is the gravitational acceleration, p is debris viscosity, p is density of reconstituted debris, U is granular volume fraction of debris. By using of these scaling parameters, we try to model the debris flow event occurred on September 19, 1990 in Tsing Shan of Hong Kong. In addition, following the discussion by Johnson and Rodine (1984), the shear strength, viscosity, flow rate, velocity of Tsing Shan debris flow can also be estimated. With reference to field parameters of Tsing Shan debris flow, the required scaling paranieters for the Tsing Shan debris flow and the design parameters of our flume can be computed and tabulated as shown in Table 1 and Table 2 respectively.

As the experimental debris flow properties are measured and their scaling parameters are calculated and be comparable to the real Tsing Shan debris flow, a complete dimensionless analysis can be performed to yield real debris flow phenomenon.

3 EXPERIMENTAL CONDITIONS Most of debris used as our experimental samples, collected by the Geotechnical Engineering Office of Hong Kong, were obtained from Tsing Shan debris flow. Four experimental runs were conducted using varying particle size distributions in laboratory under well controlled conditions. 1387

4 EXPERIMENTAL RESULTS

The three channels were set to be 32’ and tlie deposition board was set to be 0’. The cross section of flume was set to be rectangular. Tlie supply tank of debris is inclined to 40’all the time. The volume of debris supply is fixed to be 10,000c1n3 in each experiment. The weight of debris is measured in each experiment before and after pouring into debris supply tank so that the density of reconstituted debris can be obtained. The velocity of debris flow surge at the lower channel was measured for each experiment. In order to capture the transient iiiotions of the flow at the lower channel aiid the evolution process of debris fan at the deposition board, two digital video cameras (with maximum shutter speed of 1/10000 sec) were used in our study.

Four experimental runs were conducted using varying particle size distributions (materials S 1, S2, S3 and S4) in laboratory under well controlled conditions. We focus on tlie effect of granular contents on the shape of debris faii and the maximum runout of debris flow. Figure 2 shows the configuration and extent of debris flow fan. When the gate of supply tank was opened and tlie flow issued, the path of debris flow is forced to be a straight line. As the flow reached downstream at the mouth of lower channel, it began to spread out aiid deposit. The iiiaxirnum widths of deposition fails were within 3 times of tlie width of channel. Tlie faii length quicltly reached its maximum value in the earliest stage. Then the width of fan began to develop aiid spread out to the sides of debris fan as the flow can no longer proceed downstream aiid increase its fan length. At the same time, it deposited the sediiiieiits aiid the thickness of debris faii developed. Coarser particles were found to be more coilcentrated at the boundary of tlie debris flow fan. This corresponds to the tendency of bigger particles coming together toward the surge when it flows along the channel. The runout of material S2, S3, S4 were 42~111,18cm and 65 ciii respectively. The runout of material S1 cannot iiot be obtained as the debris flow can iiot travel to the outlet aiid stopped at tlie lower end of tlie middle channel. It showed that debris material (S3) with a richer sand content (60% sand and 40% gravel by weight) gives a longer runout ( 3.6 times longer) compared to the debris material (S4) with (50% sand and 47% gravel by weight) and that the material (S2) with 80% sand aiid 20% gravel by weight produces a shorter longitudinal runout than material (S3). The particle size distribution of each materials are plotted aiid shown in Figure 3. The maximum lateral spread widths of debris fan of materials S2 and S4 are longer than their corresponding longitudiiiai runout aiid the faii are in shape of elliptic in general. For material S3, the longitudinal runout, on the other hand, is longer than its maximum lateral spread width of debris fan aiid its fan shape is in a shape of ‘pear‘. The velocity distribution of debris flow was also studied as shown on Figure 4. It is clear that the material S3 with (15% fine sand) has a higher velocity than material S2 and S4 with (8% and 5% fine sand) respectively.

Table 2. The design parameters of our flume compared to the size of the Tsing Shan debris flow. Values in

1

L

= channcl

length

‘Ising Shan Debris FlOW (prototype) 330111

W

= cliannel

width

20111

20cm

II

= llow

depth

4 - 6111

3.6 - 5.5cm ( 1 .8c111)

Q

= llow

rate

699.8111’/s

55 14 cm’/s (6696cm’/s)

V

=

12.51ii/s

1 . I 9 m/s ( I .86 m/s)

I8723 N/mZ

179 Nlm’

I872 Nsim’

2 Ns/ln’ (0.9 Ns/m’)

2.08 - 3.13

21.85 - 32.78 ( I 03s.’)

0.56

0.6

10

1 1.44

T =

velocity

sliear strength

11 = viscosity

y =

shcar strain rate

llleall volume fraction of granular phase A = linear grain concentration

U =

6

typical graiii diameter [i.e. dsol p = density of reconstituted debris B = Bagnold Nuniber =

200

111ll1

1877 kg/m?

Experimental Si in ula tion (model) 3111

1 - 3llllll ( I .6mm)

1972 kg/m’ (1 796 kg/m’)

0.2642 - 0.3963

0.1069 -0.9617 (1 h 7 )

I S = Savage Number

5 . 2 7 ~ 1 0 - ’ -1.78s10-’

2 . 7 9 10” ~ - 2.5 1x 1 0.’ ( I .36x I 0-2)

I: = Friction Number

7 048 - 15.858

9.449 - 14.176 (34.21)

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5 CONCLUSION A dimensional analysis of a flume for laboratory simulation of debris flow was designed in this paper. Four geometric scaling factors and three constitutive scaling parameters were used to scale down the Tsing Shan debris flow occurred on Sept. 11, 99 in Hong Kong. Experimental results showed that debris material (S3) with a richer sand content (60% sand and 40% gravel by weight) gives a longer runout ( 3.6 times longer) compared to the debris material (S4) with (50% sand and 47% gravel by weight) and that material (S2) with 80% sand and 20% gravel by weight produces a shorter longitudinal runout than material (S3). In summary, the particle size distribution plays an extremely important role in affecting the longitudinal runout of debris fan. Thus, empirical data fitting of runout distance to slope angles (or angle of reach) irrespective to the local soil and boulder conditions should be avoided.

Figure 2. Stable shapes and extents of deposition fans

REFERENCES Benda, L.E. and Cundy, T.W. (1990). Predicting deposition of debris flow in mountain channels. Can. Geotech. J., v27, pp.409-417. Garcia, J.A. and Savage, S.B. (1993). Kinetic-theory approach to the Nevado del Ruiz I985 debris flow. Hydmzslic Engineer.ing'93. V2, pp.1408 -1413, ASCE, New York. Hua G. (1 989). Classifications of Bingham debris flow and similarity rules. In Collected Papers of the 2nd National Confirence on Debris Flow, pp. 1-9, Science Press, Beijing (in Chinese). Hungr, 0. (1995). A Model for the Runout analysis of Rapid Flow Slide, Debris Flows, and Avalanches. Can. Geotech. J., v32, pp6 10-623. Iverson R.M. and LaHusen R.G. (1 993). Friction in debris flows: inferences from large-scale flume experiments. Hydratr1ic Engineer ing'93. Vo1. 2, pp. 1604-1609, ASCE, New York. Johnson A.M. and Rodine J.R. (1984). Debris ,flow. In Slope Instability (ed. D. Brunsden and D.B. Prior), Wiley, New York. King, J.P. (1996a). The Tsing Shim debris .flow. Vol. 1-3, GEO Special Project Report, SPR 6/96, GEO. Hong Kong King J.P. (1996b). Tsing Shan debrisflood ofJune 1992. GEO Special Project Report, SPR 7/96, GEO. Hong Kong

Figure 3. Particle size distribution of debris flow materials used in our study

Figure 4. Velocity distribution of debris flow at the lower chan ne 1

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Liu, X. (1996). Size of a debris flow deposition: model experiment approach. Environmental Geol. 28,70-77. Major, J.J. (1997). Depositional Processes in largescale debris flow experiments. The Journal of Geology, v105, pp.345-366. Mizuyama T. and Uehara S. (1983) Experimental study of the depositional process of debris flows. Trans. Japan. Geomorph. Union, v4, n l , pp.49-64. Okuda S., Suwa H., Okunishi K., Yokoyama K. and Ogawa K. (198 1). Synthetic observation on debris flow,part 7, Annual Disast. Preventive Res. Inst. Kyoto U. No. 24B-1, 411-488 (in Japanese). Pierson T.C. (1995). Flow characteristics of large eruption-triggered debris flows at snow-clad volcanoes: constraints for debris flow models. J. Volcanology Geothermal Res. 66,283-294. Shieh C.L. and Tsai Y.F. (1997). Experimental Studies on the Configuration of Debris-flow Fan. Proc. Ist Int. Conf Debris-Flow Hazards Mitigation: Mechanics, Prediction, and Assessment. ASCE, pp.133-142. Takahashi T. (1981). Debris Flow. Ann. Rev. FluidMech, v13, pp57-77. Takahashi, T. (199 1) Debris Flow. IAHR-AIRH Monograph Series, A.A. Balkema, Rotterdam. Wong H.N., Chen Y.M. and Lam K.C. (1996). Factual Report on the November 1993 Natural Terrain Landslides in Three Study Areas on Lantau Island, (3 Vols.) Special Project Report No. SPR 10196, GEO. Hong Kong Zhang S. (1993). A Comprehensive Approach to the Observation and Prevention of Debris Flows in China. Natural Hazards, v7, pp. 1-23. Wong H.N. and Ho K.K.S. (1996). Travel Distance of Landslide Debris. Proc. of the Seventh International Symposium on Landslides, v 1, pp4 17-422. Wu J., Kang Z., Tian L. and Zhuang S. (1990). Observation and Study of Debris Flow in Jiangjia Gully, Yunnan. Science Press, Beijing.

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Slope Stability Engineering, Yagi, Yamagami & Jiang (c; 1999 Balkema, Rotterdam, ISBN 90 5809 0795

Shear characteristics at the occurrence and motion of debris flow Yuichi Yamashita Aratani Civil Engineering Consultant Company Limited, Hiroshima, Japan

Norio Yagi, Ryuichi Yatabe & Kinutada Yokota Department of Civil and Environmental Engineering, Ehime University,Matsuyama, Japan

ABSTRACT : The purpose of this paper is to make clear mechanisms of a debris flow from a soil mechanical view point. Then shear characteristics of saturated soils were investigated in use of a newly designed ring shear apparatus which can measure the pore water pressure. The accuracy of this ring shear apparatus has been made sure for the Toyoura standard sand and so on. The shear tests were done for the decomposed granite soils, Shirasu and the debris flow sediments. The results of the shear tests indicate that these soils are extremely decreased the angle of shearing resistance in terms of total stress during high-speed shearing and therefore the debris flow easily occur due to the generation of excess pore water pressure. The excess pore water pressures increase with the progression of particle crushing. 1 INTRODUCI'ION

In Japan, the slope failure often occurs in heavy rain at the close of rainy season and typhoon every year and furthermore it causes the debris flow which gives heavy damage. The ordinary direct shear test and triaxial compression test are not enough to investigate the shear characteristics of the debris flow because the maximum displacements of these tests are no more than a few centimeters. A high-speed ring shear apparatus which can give unlimitedly the shear displacement is recommended. The study using a high-speed shear apparatus has mainly been performed by Sassa and so on ), 2, . Some shear characteristics to find the mechanics of the occurrence and motion of debris flow have been made clear. The experiment of saturated soils is performed under the condition of low and high speed in use of the newly designed ring shear apparatus to investigate the shear characteristics of soils such as the decomposed granite soil and the samples from the debris flow sediments. From a soil mechanical consideration the relation between the excess pore water pressure and the particle crushmg during motion of the debris flow is estimated.

2

THE NEWLY DESIGNED RING SHEAR APP&TUS AND EXPERIMENT METHOD.

The structure diagram of a newly designed ring shear apparatus is shown in Fig. 1. A detail shear box is shown in Fig.2. The shear box is composed of the inside ring and the outside ring, and of the upper and

Fig1 Structureof a newly designed ring shear apparatus 1391

Fig.2 The section of the shear box lower parts respectively. An opening between the upper and lower parts is about OSmm and is shut by the installation of 0-ring. The outside and the inside diameter of shear box are 21.5cm and lO.Ocm, the area of shear plane is 284.5~111~ and the sample height is average l.Ocm. Shear test is performed by revolving the lower part of shear box. A saturated sample is set into the shear box. In this study normal stress of 0.5 or l.0k@cm2 was loaded. The shear test was performed under the undrained condition after the consolidation. The shear speed was 66.7mm/sec. The shear plane is formed horizontallybetween the upper and lower parts. The pore water pressure is measured by the pore pressure meter connected to the porous stone of diameter 35mm buried in the base of shear box. The accuracy of the newly designed ring shear apparatus has been confirmed by obtaining reasonable angle of shear resistance for the Toyoura standard sand and other soils3b4!

3

Fig.3 Grain size distribution curves of samples Both the high and the low speed ring shear tests were d e d out to investigate the shear characteristics during debris flow. The high-speed tests were done using the apparatus mentioned above in undrained condition. The low-speed ring shear tests were carried out in drained condition to investigate dilatancy characteristics of soils. The low-speed ring shear apparatus is smaller type, whose diameters of outside and inside of specimen are 16.2cm, 10.2cm respectively. The shear speed is 0.008mrdsec. Here the rate of strength decrease is defined by the following equation to estimate the difference of the peak shear strength and the residual shear strength.

S H E A R CHARACTERISTICS DURING DEBRIS FLOW.

The rate of strength decrease = peak shear strength-residual shear strength peak shear strength

3-1 Prepared samples and kinds of test.

Test samples for shear tests are the decomposed granite soil, the Shirasu and the three debris flow sediments, that are the Gamahara debris flow at Niigata prefecture, the Harihara debris flow at Kagoshima prefecture and the Unzen pyroclastic flow at Nagasaki prefecture. The grain size distribution curves of prepared samples are shown in Fig.3. These samples are mainly composed of the sand, but sediments of the Gamahara and Harihara debris flow are composed of the wellgraded grain size that ranges from the gravel particle to the fine grained soil. All samples for the shear test are adjusted smaller than 2mm.

The following explains the shear characteristics of the composed granite soil and the Harihara debris flow sediment as example. 3-2 The composed granite soil The composed granite soil belonging to the Ryoke granite was collected in Matuyama City. The result of low-speed shear test for the composed granite soil is shown in Fig.4. The shear strength reaches the peak strength immediately after the start of shear test and decreasesgradually to the residual strength.

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Fig.4 Results of low-speed ring shear test (composed of granite soil)

The rate of strength decrease at normal stress (7" = 0.5 kgf/cm2is 0.429, which is nearly same to 0.490 at = l.0kgf/cm2. From the results of shear test the angle of shearing resistance for the peak strength dd was 30.9" . The angle of shearing resistance for the residual strength cbr was 19.3" .The volume changes tend to compress at both normal stresses and compressive volume changes were between 1.5% and 2.0%. The result of the high-speed ring shear test under the consolidated undrained condition is shown in Fig.5 . The shear resistance reaches the peak strength immediately after the start of the shear test, and then indicates the tendency to converge a fixed value. The rate of strength decrease is 0.433 at consolidating normal stress (7, = 0.5 k@cm2 and 0.526 at 0, =1.0 kgf/cm2. The pore water pressures trend to increase after the start of shear test. The pore water pressure of 0.1kgf/cm2generated during 20 second after the start of shear test at (7, = 1.0 kgf/cm2. The pore water pressures increased gradually after the end of test and reached finally 0.16 and 0.29 kgf/cm2 at normal stress of 0.5 and l.0kgf/cm2 respectively. This is due to the delay of pore water pressure measurement. A diagram of effective stress path obtained from the high-speed shear test is shown in Fig.6. The angle of shearing resistance in terms of effective stress d'm correspond to the peak stress was 30.6" . The angle of shearing resistance in terms of effective stress d',w correspond to the residual stress was 24.0" . The maximum pore water pressure was used to obtain the effective stress when the residual strength state. The large difference between d ',,, and d 'mr was

$I ',=30.6"

ulV

Fig5 Results of high-speed ring shear test (composed of granite soil)

Fig.6

1393

d ',,,- d

>

(kgf/cm2)

diagram of composed granite soil by high-speed ring shear test

recognized even in the undrained shear test. It is said that this fact occurred when very large deformation is given. The angle of shearing resistance in terms of total stress d w correspond to the residual stress was 17.3" . It is so low because of the generation of pore water pressure due to large deformation. It is considered to be due to particle crushing that the pore water pressure continue to increase until large deformation. The grain size distribution curves before and after the shear tests were compared to make sure particle crushing. This is described in detail in the next chapter. 3-3

Harihara debris flow sediment

The shear characteristics of Harihara debris flow sediment obtained from low-speed ring shear tests under the consolidated drained condition are shown in Fig.7. The shear resistance reaches the peak strength immediately after the start of the shear test and then the residual strength decrease a little from the peak strength. The rate of strength decrease is 0.11 at LTv = 0.5kgE/cm2and 0.10 at ITv -1.0 kgE/cm2, is so small comparing with ones of the decomposed granite soil. From the result of shear tests, the angle of shearing resistance for peak strength dd was 32.6" and the angle of shearing resistance for residual strength d r was 29.99 The volume changes trend to compress at both normal stresses. Compressive volume changes were bigger than one of the composed granite soil. This is considered that the shear test has done under the loose condition. The results of the high-speed ring shear test under the consolidated undrained condition are shown in Fig.8. The shear resistance reaches the peak strength immediately after the start of shear test and then indicates the tendency to converge a fued value. The rates of strength decrease were 0.425 at consolidating normal stress O c = 0.5kgE/cm2 and 0.500 at LTc l.0k@cm2. The pore water pressures trend to increase with time at both LTc = 0.5 and 1.0 kgf/cm2 . The pore water pressures increased after the end of shear test reached finally 0.23 and 0.39 kgf/cm2at normal stress of Oc = 0.5 and 1.0 kgf/cm2 respectively. This is due to the delay of pore water pressure measurement. A diagram of effective stress paths obtained fiom the high-speed shear test is shown in Fig.9. The angles of shearing resistance in terms of effective stress d ',n correspond to the peak strength and d ',,,, correspond to the residual strength are 29.3 " and 27.7" respectively.

Fig.7 Results of low-speed ring shear test (Harihara debris flow sediment)

Fig.8 Results of high-speed ring shear test (Harihara debris flow sediment)

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Table 1 AFc ( % ) (increase rate of fine particle fraction by shear test)

Fig.9

As the samples the particle crushing characteristics of the composed granite soil and Harihara debris flow sediment are explained. The rate of particle content less than 75 ,urnof the composed granite soil was 1.9% before the shear test became 8.5% after the low-speed shear test and 11.0% after the high-speed shear test. Accordingly AFc of the composed granite soil is 6.6 % under the low-speed shear condition and 9.1% under the highspeed shear condition. It is considered that composed granite soils possess the crushabilityoriginally. On the other hand the rate of particle content less than 75 ,um of Harihara debris flow sediment was 12.5% before the shear test became 13.1% and 2 6 8 % after the low and high speed shear test respectively. AFc is 0.8% under the low-speed shear condition, but is 14.5% under the high-speed shear condition. The cause of low crushability under low-speed shear and of very high crushability under high-speed shear is not clear. But this fact is considered to be one of causes of Harihara debris flow. Therefore Harihara debris flow deposit are considered the soil that occur the particle crushing with the increase of the shear speed in spite of the soil that possess little the crushability when low-speed shear.

@ ' m - d 'm diagram of Harihara debris flow sediment by high-speed ring shear test

The maximum pore water pressure was used to obtain the effective stress when the residual strength. As the result the angle of shearing resistance didn't almost decrease for the peak strength to the residual strength. The angles of shearing resistance in terms of total stress d mcorrespond to the peak strength and c3,,r correspond to the residual strength were 31.7" and 16.7" respectively. The angle of shearing in terms of total stress decreased very much from the peak to the residual strength and this means that much pore water pressure generated after the peak strength. This may be one of the causes of Harihara debris flow.

4 CHARACTERISTICS OF PARTICLE CRUSHING DURING SHEAR TEST. In order to express the effect of particle crushing on the shear characteristics, it is necessary to give a quantitative index of particle crushing. In this paper, A F c is defined as the amount of increase of fme particle( less than 75pm ) fraction after shear test 5! 4-1

Characteristicof AFc

The results of grain size analysis before and after the shear tests are indicated in Table 1. Samples after shear tests were collected near the shear plane. The difference of content of fme particle fraction for Toyoura standard sand before and after the shear test was little recognized. The samples after the shear deformation of 140cm was given when the low-speed shear test and of 270cm when the high-speed shear test.

4-2

The relation between AFc and A Umax

The soils that possess the crushability are considered to generate excess pore water pressure when shear. The relation between AFc and the maximum generation quantity of the pore water pressure A Umax at the normal stress oC= 1.0 kgf/cm2 during the high-speed ring shear test is shown in Fig. 10. According to Fig. 10 it is recognized the tendency that AUmax increase linearly with the increase of AFc. This makes clear the mechanism of debris flow which causes the generation of excess pore water pressure with the increase of the particle crushing and become further easily the fluidizationitself. 1395

speed ring shear apparatus , journal of Japan landslide society , Vol. 29 ,No.4, pp. 1-8 , 1993 Y.Yamashita, N.Yagi, 0.Futagami : Shear characteristics of soils during rapid motion, Journal of Shikoku Branch in 1997, The Japanese Geotechnical Society,pp49-50, 1997. YYamashita , H.Inoue ,N.Yagi ,0.Futagami : Shear characteristics of soils depending on a newly designed shear apparatus, Journal of Shikoh Branch in 1998, Japan Society of civil Engineers, ~~234~235,1998. S.Miura , N.Yagi , S.kawamura : Static and cyclic shear behavior and particle crushing of volcanic coarse grained soil in Hokkaido : Journal of Geotechnical Engineering, No.547/a-36 , pp159 -170,1996.

0 Debris Flow Sample

6

0.4

c m '

2

3 d

A F C (%)

Fig.10 Relation between AFc and

A Umax

5 CONCLUSION 1, A newly designed ring shear apparatus with the pore water pressure measurement was produced to investigate the mechanism of debris flow. 2. The composed granite soil having the crushability caused the particle crushing in the low and the high speed shear test. And it was made clear that the excess pore water pressures occurred during shear and the angle of shearing resistance in terms of total pressure decreased fairly. 3. Harihara debris flow sediment didn't cause the particle crushing in the low-speed shear test but caused the particle crushing and more pore water pressure generated in the high-speed shear test. As a result the angle of shearing resistance in terms of total stress decreased more than 10 degrees from the peak strength. 4. AFc (the increase content of fiie particle fraction due to shear) was used as the index of the particle crushing. It was indicated that A Umax was increased linearly with the increase of AFc. 5. These results made clear the mechanism of debris flow that caused the generation of excess pore water pressure with the increase of the particle crushing and became further easily the fluidization itself.

REFERENCES H.Fuhoka, KSassa, M.Shima : Shear characteristics of sandy soil clayey soils subjected to the high - speed and high - stress ring shear tests , Annuals, Disas. Prev. Res. Inst., Kyoto Univ. , N0.33 B-1 ,pp. 179--190,1990 K. Sassa, H. Fukuoka: Measurement of the internal fiction angle of soils during motion by the high -

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H.Chen & C. E Lee Department of Civil Engineering, University of Hong Kong, People’s Republic ojChina

ABSTRACT: The Lagrangian Galerkin finite element method (FEM) is formulated in this paper for simulating general unsteady muddy debris flows, in which the viscoplastic Bingham model is coded. The introduction of lumped mass matrix turns out to be a volume-weighted procedure. The second pass mass-lumping not only facilities an explicit solution, but also automatically releases the continuity constraint. The current numerical model is validated and a substantial agreement is produced in comparison to experimental results. Application to the Lai Ping Road landslide of Hong Kong, July 2, 1997, reasonably simulates the landslide process.

1 INTRODUCTION Debris flows are the flows of sediment-fluid mixtures. It is generally recognized that rainfall-induced debris flows are caused by a temperately and spatially increased pore water pressure and seepage forces in their source area during periods of intense rainfall, which leads to a largely decrease in effective stress. When there is a triggering event such a heavy rainstorm to initiate instability of a slope, debris flows may be caused. The unpredictable and destructive occurrence of debris flows leads this kind of disasters to be one of the most threatening natural hazards in some region in the world. Solid particles in debris t o w can coiiide, rub, rotate, and vibrate during their physical movement. Therefore, fluid viscosity, particle sliding/rolling friction, particle collision and turbulence are the major factors contributed to energy dissipation. Considerable attentions have been drawn to understand the mechanism and physical process of debris flows (e.g. Chen, 1987, Takahashi, 1991 & Laigle & Coussot, 1997). From the view point of rheological constitutive relationships, three categories are generally classified as: granular fluid models with negligible fluid effect; yield-stress viscoplastic fluid models with negligible particle collision; and a combination of different dissipation models with consideration of both fluid effect and particle interactions.

Laboratory experiments generally investigate the sliding profiles, deposition fan and the relationship of different dynamic parameters of debris flows. However, in the prediction of potential runout distance or the extent of the hazard area caused by debris flows, it is unavoidably hedged by geometrical scale effects. Researches on numerical modeling have been focused on two-dimensional (2D) finite differential method for the descriptions of the profiles of moving mass along the sliding direction (e.g. Savage & Hutter, 1989 & Hungr, 1995), which yet cannot mimic the correlative characteristics in multidirections. By practical consideration, Lagrangian FEM is more suitable for this kind of problems because the computational grid is adverted with the highly unsteady moving mass. Bingham model has been popular to interpret the motion features of viscous mudflows or muddy debris flows (e.g. O’Brien & Julien, 1988 & Major & Pierson, 1992). The primary purpose of the present study is an extension of the Lagrangian FEM (Chen 8L Lee, 1999) originally for the granular flows. Under the Bingham model, this three-dimensional (3D) dynamic procedure is developed for general unsteady muddy debris flows in a multi-direction sliding process. The method, moreover, is validated by experimental results and applied to the simulation of the Lai Ping Road Landslide in Hong Kong.

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2 MATKEMATICAL MODEL 2.1 Fundamental equation In spite of the existence of particles, the mixture of debris is usually treated as the movement of a continuum for simplicity. Subsequently in the current mathematical model, a finite moving mass is represented by a number of columns contacting each other, which are free to deform and retain fixed volumes of debris mixtures during their sliding down a slope with the assumption of constant bulk density. The solution here is referred to a fixed Cartesian = (x,y,z) in space, with z- pointing coordinates upward opposite to the direction of gravity. For significant landslide debris flows, the spread is more dominant than the depth in scale. If no overturning occurs, it is reasonable to assume that the momentum equations are integratable along z-direction within a column. Denoting the unit net force acting on a typical column as p and the velocity 0= (u,v,w), the equations can then be reduced to 2D depthaveraged ones in the x-y plane D ( 0 h) f---"-=Eh, Dt in which h is the depth of the typical column. The zero sign denotes the averaged value within the column for a given variable CD that 1 I' CD, = -J@dz (2-2) ho and is omitted hereinafler for brevity. Zero depth boundary condition is specified along the margin of the moving debris.

remarkable finding of a series laboratory experiments (Hungr & Morgenstern, 1984a,b). The formulation of shear resistance force can be deduced from various rheological models. Shear stress at various rates of angular deformation forms relevant stress-strain relationships, i.e. different rheological models. Bingham model has been popular for non-Newtonian fluids, in which mudflows or muddy debris flows remain laminar. The Bingham fluids exhibit a linear stress-strain relationship at shear stresses, meanwhile a finite threshold value of shear stress ryreldmust be exceeded prior to the fluid motion. The stress-strain relationship in a Bingham fluid is dU = 'yreld (2-5)

'

'77x,

in which zis the shear stress; 17 the dynamic viscosity; and 0= U .S . When IzI 2 zyleld,the flow exhibits a parabolic velocity profile, otherwise it moves in a uniform one. Integrating (2-5) along the normal direction A ( E . S = 0 ) and neglecting the higher order term in the expression of the cross-sectional mean velocity, the resistance shear force is expressed as

Therefore, for a given column, the components of the unit net force l@ in equation (2-1) in x- and yaxes are (2-7a) (2-7b)

2.2 Force analysis

where q = (1 + B:

Referred to a moving column, the unit net forcep acting on the column consists of the weight force @, inter-column force P and basal resistance force ?;. The component of @ along the sliding direction 3 , W,= p g s i n 8 , (2-3 1 in which 8 is the inclination of the intersection between S and x-y plane. The inter-column force P on vertical sides of a column is the difference of the lateral earth pressure acting on the both sides of column like in the Janbu's slope stability analysis, which is expressed by using the lateral pressure ratio k, (Sassa, 1988) dh P =-kgW-, (2-4)

the basal elevation hnction B ( x y ) and the horizontal plane, and B,, By are the first derivations of B ( x y ) with respect to x and y respectively; the total velocity absolute value U = (u2+ v 2 + w2)'", and 0 . E = 0 which results in IY = uBx +vBy; k, and ky are the

+ B:)'"

is the inclination between

anisotropic lateral pressure ratios in x-,y- directions. At macroscopic scale, regarding the flowing mixtures as an homogeneous viscous non-Newtonian fluid and treating the fluid as a continuum, the fimdamental equations for the description of flowing characteristics consist of mass conservation and momentum equations (2-1) with the net force components of (2-7a) and (2-7b).

ds

The constitutive relationship is in terms of the widely accepted Mohr-Coulomb yield criterion based on the 1398

3 FINITE ELEMENT ANALYSIS The moving mass is discretized into a finite number of quadrilateral elements which are hrther bilinearly transformed into canonical squares in the computational plan (67) by shape fbnction N({,v). With the Galerkin residual weighting procedure, we reach at the discretized formula

I(%

- Fh)N,dQ = 0 .

(3-1)

i.4

Bearing in mind of the mass conservation that the volume of each element should keep constant, we hVl3

Vole (heJ,)"+' = (heJ,)" = -= Const., (3 -2) 4 where J is the Jacobian determinant, and the subscript "e" denotes the averaged value at an element center. With explicit Euler time integration, Lagrangian scheme of discretization in space and the advantage of the midpoint quadrature principle, equation (3- 1) can be written as the matrix form (3-3a) MO"+' = MO" + BAt , in which = vole ~ , d < d >v (3-3b)

/

n

Bj = VolejnFN,dCdq,

Therefore, the momentum and mass conservation is closed within columns (elements) in terms of nodal velocity and depth. The basal elevation fbnction B(xy) is represented in an uneven spacing coordinate grid. To achieve a higher-order basal inclination, the gradients B, and By are computed a priori by the finite difference of B ( x j ) before bilinearly interpolated for the elemental or nodal ones.

(3-3c)

where M is the consistent mass matrix, B is the nodal force vector, and the continuity constraint is released. In practice, the following two-step predictorcorrector procedure is adopted in order to avoid an implicit solution of the algebraic system: (3-4a) U* = (MO" + A t @ M - d , on+] = 20*- ~ o * ~ - d , (3-4b) in which 0' is the intermediate velocity, and M-d the inverse of lumped mass matrix. Worth special mention is that the correction step is quite crucial for unsteady problems, otherwise spurious dissipation will be introduced. Moreover, the volume of an element is unchanged during motion and hence accumulative error can be avoided. When element vertices proceed to new positions = 2"+ AL\t(B"" + U " )/ 2 , (3-5) the mean height at an element center is readily updated by (3-2). The nodal height is then redistributed from the least squares approximation QhnC'= R , (3-6a) in which Q, = J,"+'JN,N,d
4 MODEL VALIDATION A series of flume experiments have been conducted by Jeyapalan (1980) in a 1 foot wide, 6 foot long glass walled flume filled with oil. Impoundment of viscous oil were retained by a vertical gate. Ponding with various heights of oil, the experiments were performed by pulling the gate upward quickly until it was raised above the surface level of the oil. Meanwhile a high speed photo camera is equipped so as to take a sequence of snapshots of the oil movement. In the present computation, the inclination of the downstream bed slope is horizontal, the initial height of the impoundment is 0.25 feet. The tested oil has a unit weight of 8.79 kN/m3 and a viscosity of 0.0039 Was. The lateral pressure ratios are regarded as 1.0. Unstructured grid and Lagrangian FEM are used to simulate the oil to be mobilized, in which 414 elements are discredited by 459 nodes. Coinparison is made for time history of the tip displacement shown in Figure 1, in which the trend of the calculated curve matches fairly well with the tested values. The sequence of simulated profiles after loss of impoundment is in Figure 2, where different profiles in typical time levels e h b i t the changing features of the surface, After the loss of the impoundment, the oil progressively elongates along the downstream bed, first rapidly and afterwards more slowly, to reach finally a steady state. The agreement between the calculated and experimental results is excellent, which gives the numerical model an applicable potential in practical simulations.

z"+'

n

1 R, = -Vole Nid{d7 4 Q

J

i

TIp displacement (ft)

1399

~

/

'

'

I

~

~

__

Present calculation

Experiment(Jeyapalan,

'N 0

(3-6~)

'

4

10

20

30

40

Fig.1: Comparison of calculation with experimental results ' ) (Initial height of impoundment= 0.25 ft, bed slope = 0

~

~

tufT The main scarps, which is highly irregular in plane with 0.5 - 7m high and variably subvertical to 30°, are the jointing features within the original cut slope in a total volume of 2250m3. Like a polylobate sheet, the fine-grained debris flows in a ‘‘slurry form” spread out on the Lai Ping road about 1.5m thick, deposited in a smooth lobe up to 1.5m thick against the wall of the services reservoir. 5.2 Numerical simulation

Fig.2: Sequence of simulated profiles after loss of impoundment (Initial height of impoundment = 0.25 ft, bed slope =Oo)

5 CASE STUDY In the past 30 years, rapid developments in Hong Kong lead to abundant construction on steep slopes. The tropical to subtropical climate provide Hong Kong rich rainfall in raining season every year from May to September so that extreme rainstorms trigger hundreds of slope failures. Even though most of the failures are minor and shallow in depth with a volume of usually less than 50m3, many buildings are constructed so close to slopes and the occurrences of failures are rather unpredictable that these failures can be very destructive. Since the introduction of the Slope Safety System by the government of Hong Kong, the trend of landslip fatalities has dropped significantly. However, landslide failures are still rather difficult to completely avoid. 5.1 Site description A recent landslide occurred on the roadside cut slope at Lai Ping Road in Hong Kong on 2 July 1997, which comprised of several discrete failures along a 135m-long section of the cut slope. The Lai Ping Road was completely blocked. Rainfall on the preceding days was not particularly high, however, on 2 July the maximum rainfall was close to the site. Post-failure investigation indicates that the principal trigger of the landslide is likely caused by the buildup of pore water pressure due to elevation of the main ground water table in a complex hydrogeological regime, following heavy rainfall. No fatalities or injured were reported in this incident (GEO Report, 1998). The landslide site lies in an area of undifferentiated tuff and tiffite, where some alluvial, debris flow deposits and fine-grained granite are shown. The debris is mainly comprised of soft clayey sand silt and loose brown variably clayey to very silty to very sandy gravel with cobbles and occasional boulders of moderately to completely decomposed

The numerical model described in the previous sections is applied to analyze the extent of hazard area of slurry flows initiated by the main jointing scarps. Actual topography of the landslide ground surface before and after failures are digitized according to the topographic surveys, so is the mobilized debris. The simulated initial occupied surface area is 1694m2 and the volume of the debris mixtures is 2395m3. Totally, 414 elements are discredited by 459 nodes within the mobilized debris with the yield stress ryreld = 0.8kPa and the dynamic viscosity 77 = 0.5OkPa.s. In description of the simulated flowing process, Figure 3 is the calculated sequence of debris flow, which perspectively visualizes the entire landslide process. The front of the failure mass rapidly stretches downslope, followed by a flow-like displacement. As time increasing, the front part progre5sively elongates towards downstream, first rapidly and afterwards more slowly. Meanwhile the rear part in a small quantity remains on the incipient rupture surface. When the flowing debris comes across Lai Ping Road and encounters the reservoir, part of the debris deposits aside, the front of debris turns its flowing direction to the east side of the road in lower elevation. The phenomena hlly reflects the actual movement of natural debris in multi-directions. Moreover, it can be seen that the elements deform differently in different time levels, i.e. compression or elongation. In other words, different parts of the mixture continuously reshape themselves: the rear part elongates meanwhile the front accumulates during the whole mixture stretches downslope. Figure 4 shows the depth contours of the sliding debris. It is noticeable that the flowing track of the muddy debris is in a substantial agreement with the field recorded landslide scar. The depth and the occupied area of the failed patch simultaneously change during the sliding process. The contour distributions indicate that the debris deposits on Lai Ping Road and against the wall of reservoir can build up to 1.5m thick, which is again consistent with the

1400

Fig. 3: Sequence of simulated debris flow

Fig. 4: Depth contours of sliding debris

1401

field observations. In more quantitative terms, the maximum mean velocity, about 12.41m/s, occurs 3.5 seconds after the incipient failure. The main flowing process lasts about 33.0 seconds with a polylobate sheet of deposition fan of 3140m2. After that, the deposition fan still redistributes slowly and locally. Natural debris flow features not only a mixture of water and random sorted particles, but also a distinctive movement process: transient, with free surface and heavily dependent on the topography of sliding surface. Two main parameters in Bingham model, the yield stress and dynamic viscosity, play a very influential role in momentum exchange. Comparative study with the friction modal tested by Chen & Lee (1999) shows that Bingham model is more suitable for simulating the clayey rich and more saturated debris flows. 6 DISCUSSION AND CONCLUSION With Lagrangian FEM, the 3D dynamic model coded with Bingham model has been formulated to simulate the general unsteady muddy debris flow. The second pass mass-lumping not only facilities an explicit solution, but also automatically releases the continuity constraint. To achieve a higher-order basal inclination, the basal gradients are obtained a priori. Bilinear interpolation is directly carried out without using any basal filter, which authentically and effectively reproduces the multi-directional slope characteristics. The flowing process of gravity-driven debris is a temporal accumulative result. Gravitational force is the most important one among the others. Therefore, the input data of the site topography should be imprudent. Furthermore, the accuracy, robustness and generality of the numerical method have been validated by experimental results, in which the substantial agreement suggests a good potential in simulating practical muddy debris flows. Application to the Lai Ping Road landslide gives reasonable results in comparison to the field observations. Sliding features simulated by Bingham model are as well demonstrated.

Hungr, 0. & Morgenstern, N.R., 1984a. Experiments on flow behavior of granular materials at high velocity in an open channel flow. Geotechnique 34:405-413. Hungr, 0. & Morgenstern, N.R. 1984b. High velocity ring shear tests on sands. Geotechnique 34: 415-421. Hungr, 0. 1995. A model for the runout analysis of rapid flow slides, debris flows and avalanches. Can. Geotech.J 32:6 10-623. Jeyapalan, K. 1980. Analysis of flow failures of mine tailings impoundments. Ph.D. thesis, University of California, Berkeley. Laigle, D. & Coussot, P. 1997. Numerical modeling of mudflows. J Hydra. Eng 123(7):617-623. Major, J.J. & Pierson, T.C. 1992. Debris flow rheology: experimental analysis of fine-grained slurries. Water Resozrr. Res. 28(3):841-857 O’Brien, J.S. & Julien, P.Y. 1988. Laboratory analysis of mudflow properites. J Hydra. Eng. 114(8): 877-887. Sassa, K. 1988. Geotechnical model for the motion of landslides (Special lecture). Proc. 51hInt. Symp. on Landslides 1;37-56. Savage, S.B. & Hutter, K. 1989. The motion of a finite mass of granular material down a rough incline. J FluidMech. 199:177-215 Takahashi, T. 1991. Debris flow. A.A. Balkema Publishers, Rotterdam.

REFERENCES Chen Cheng-lung. 1987. Comprehensive reviews of debris flow modeling concepts in Japan. Rev. in Eng. Geol. 7:13-29. Chen, H. & Lee, C.F. 1999. Numerical simulation of debris flows. Can. Geotech. J (refereeing) Geotech. Eng. Office, HK Government, Dec. 1998. The Lai Ping Road Landslide of 2 July, 1997

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Slope Stabiky Engineering, Yagi, Yamagami& Jiang 0 1999Balkema, Rotterdam, ISBN 90 5809 079 5

Mechanism of soil deformations during the displacements of flow slides O.V. Zerkal Federal Centerfor GeologicalMonitoring, Moscow,Russia

V. N.Sokolov Moscow State University,Moscow, Russia

ABSTRACT: Microstructure transformation in clay soils during the formation and development of flow slides has been studied using the quantitative analysis of SEM images. Peculiarities of the landslide deformations mechanism have been discussed. Investigations confirmed that the soil mass movement (flow) during the development of flow slides in silty and clay soils with the moisture content close to the liquid limit occurs as a relative slipping of separate aggregates and large grains. The sudden changes in the distribution of the different micropores categories in the pore space of soils has been demonstrated at the initial stage of deformations during the preparation of the displacement. Such changes can be considered as one of the prognostic criteria of the slope displacement preparation for the evaluation of landslide hazard.

1 INTRODUCTION

A reliable prediction of landslide hazards for slopes composed of silty and clay soils is impossible without a consideration of the processes occurring at the development of flow slides. The displacement of a flow slide occurs as a viscous flow of disintegrated soil masses having an isotropic structure at the macro level. One of the way to study the character of landslide deformations is the investigation of the soils microstructure and its changes at the development of landslide (Tsaryova 1985, Zerkal&Sokolov 1995). It is also important to take into consideration a possible effect of microstructure on the slope preparation for landslide displacements. The data about the mechanism of deformations development during the displacements of flow slides are necessary for the validation of the schemes of physical or numerical modeling made for evaluation of the landslide hazard. As it has been demonstrated earlier in fhe studies of the microstructure changes in the landslide deposits from the section of “Ohli” seismic landslide, triggered at the Gissar 1989 earthquake and described in detail in (Zerkal 1994, 1996), the development of landslide deformations results in the complete transformation of the initial soil microstructure in the lower, mostly saturated part of the flow slide foot into the pseudohoneycomb microstructure (Zerkal&Sokolov 1995). It has been re-

vealed that the displacement in the lower part of landslide occurred as a movement of the saturated soil mass, while in the middle and upper parts - with the decrease of moisture content the displacement involved slipping of aggregates and large grains relative to each other. However, there are still no data giving the idea of the character of changes in the soil microstructure in the landslide movement direction. To study the changes in the structure of clay soils during the flow slides development the morphometric and geometric indexes of the microstructure of small flow slide deposits developed in spring 1996 in the cover loams at the large ravine edge in Kaluzhka river valley has been investigated. The ravine is 14 m deep and has steep - 25-30’ slopes. The cover loams with high moisture content underlying by the glacial loams of Moscow Mid-Pleistocene horizon were involved into the landslide deformations. The landslide has a drop-like shape typical for this type of displacement. It is up to 8 m broad and 0.5-0.7 m thick at the foot. The landslide changes its direction in the lower part of the slope - where it reaches the ravine bottom, and the development of the “tongue” along the bed is observed. The landslide consists of dark-brown water saturated fluid-like loams. The wall is made up of graybrown and brown cover loams, underlying by the unit of reddish-brown loams with inclusions of limestone boulders (up to 5%), poorly rounded 4-6 cm in diameter. The landslide is underlain by talus depo1403

sits consisted of gray, dense, moist loams overlying the horizon of Moscow glacial deposits. The clay soils composition and physical mechanics properties as well as microstructure have been studied by the series of samples selected along the axis of the landslide. A11 of this allowed to trace the changes of microstructure parameters with the landslide direction.

2 MICROSTRUCTURE TECHNIQUE

INVESTIGATION

The clay soils microstructure has been studied with the scanning electronic microscope (SEM) Hitachi ,54300. This type of SEM has a high resolution and allows to investigate the microstructure at the magnification from 20 to 300000 times. Microstructure of clay samples has been studies in the section perpendicular to the deformation direction. The freeze drying technique has been used to prevent the shrinkage of moist clay samples during the dehydration process. A qualitative microstructure analysis has been made with SEM microphotographs at the magnifications from 50 to 10000 times. Several typical segments were photographed for each sample. The quantitative computer microstructure analysis has been carried out with the STIMAN software (Sokolov et al. 1997). Since the studied soils were polydispersional systems the quantitative analysis was run with a special algorithm using SEM images of various scales including the full spectrum of occurring sizes. The samples were studied with magnifications 250, 500, 1000, 2000, 4000, 8000, 16000, 32000 times. This allowed to obtain the integral quantitative parameters of microstructure. The quantitative processing of the SEM images resulted in comprehensive characteristics of the soils microstructure and in the data providing the possibility to establish a relationship between these characteristics and the soils properties. All these allow to evaluate the development of the processes of flow slides formation.

3 MICROSTRUCTURE RESULTS

INVESTIGATION

The undisturbed soils, involved into displacements (the covered loams) and the soils underlying them (the glacial deposits of Moscow horizon), have a similar matrix-skeleton m i c r o s ~ c ~ rconsisting e, of unoriented clay-silty matrix including coarser sandysilty grains (fig. 1). Clay-silty matrix (fig. 1) consists of the uniform-

ly distributed microaggregates of clay particles 5- 10 pm in size and fine silty grains mostly isometric in shape and 10-15 pm in size. The larger structural elements - quartz grains up to 60-70 pm are covered with clay coatings (fig. 1). These grains are uniformly distributed in the clay-silty matrix and have no contacts with each other. In spite of sufficient similarity there are some differences in the microstructures of the cover loams and of the Moscow horizon glacial deposits. In the samples from the cover loams the destroyed grains are observed. Blocks of microaggregates occur instead of the grains (the size of the single microaggregates is 1550 pm), inheriting the shape of an original grain. Unlike the cover deposits the glacial soils of the Moscow horizon include quartz grains as well as the domain-like microblocks composed of the microaggregates of clay particles, appeared to result from the destruction of the unstable mineral grains. The pore space of the cover deposits (n=42%) is formed, mostly, by inte~icroaggregate-granular anisometric micropores, comprising up to 84% (large micropores with the equivalent diameter 1380 pm - 46%, small micropores with equivalent diameter 2-12 pm - 38%). Intramicroaggregate porosity (the equivalent diameter - 0.06-0.2 pm) comprise 9% and 7% respectively. Small intermicroaggregate anisometr~cmicropores (the equivalent diameter - 2-12 pm) comprising 44% of porosity prevail in the pore space of glacial soils from the Moscowhorizon. The portion of large micropores (the equivalent diameter - 12-41 pm) decreases in 1.7 times in comparison with the cover deposits up to 28%. The portion of intramicroaggregate porosity (the equivaIent diameter 0.15- 1.8 pm) increases more than 2 times reaching 22 %. The microstructure of the soils from the landslide foot has many similar features with that of the soils of intact part of the slope. The microstructure of the samples from the landslide deposits is of matrixskeleton type (fig. 2). Clay-silty matrix in these samples is built of unoriented and uniformly distributed in space isometric microaggregates of clay particles up to 15 pm in size and fine silty quartz grains of approximately the same size (fig. 2). The larger silty grains up to 25-35 pm in size (fig. 2) as a rule do not contact each other. At the same time, some changes in the structure of soils increasing with the distance from the landslide crown can be observed in the sample taken from the transit part. Despite of some densification of soil, the microstructure continuity gradually dis appears. There arise distinct large isometric aggregates (from 100-120 pm to 200 pm), consist of clay

1404

Fig. 1. Matrix-skeleton microstructure of the cover loams forming the intact part of the slope.

particles microaggregates and silty-sand grains, which are separated from each other by fissure-like pores up to 120-160 pm long and up to 20-25 pm wide as well as by the segments of finer loose matrix (fig. 2a). The more detailed investigation of these aggregates at the lager magnification (XI 000) demonstrates that inside they have a structure close to the original one. At the same time, in the vicinity of the aggregate edges numerous mineral grains with the traces of mechanical distortion are visible (fig. 3a, 3b). At the magnification of 3000 times the distortion of clay coating on the surface of mineral grains also becomes discernible in many places. The shape of the aggregates changes with the distance from the scarp along the direction of the landslide movement. They are less angular and their shape becomes similar to that of “rounded pebble” (fig. 2b-2d). At the side of the landslide where the velocity of its movement started to decrease, more rounded mineral grains up to 50 pin in diameter appeared in the microstructure of landslide deposits both within the aggregates (fig. 2d) and separately. The main changes in the microstructure of the studied samples have occurred due to the transformation of their pore space (fig. 4). In pore space of all the investigated specimens four categories of the micropores have been distinguished: D, - anisometric ultramicropores (Kf=0.46-0.57), which are also interparticle or intraultramicroaggregate ones, with sizes from 0.07 to 0.17 pm; D, - thin anisometric (Kf=0.47-0.54) interultramicroaggregate micropores (they also belong to the intramicroaggregate micropores) with sizes varying from 0.13 to 1.7 pm; D, - small isometric (Kf=0.48-0.5) intermicroaggregate micropores with sizes from 1.4 to 14.6 pm; D, - large anisometric (K~0.48-0.53)intermicro-

aggregate-granular micropores with sizes from 15.7 to 54.6 pm. A regular porosity decrease is observed along the landslide starting from 42% at the scarp to 33% - at the end of the foot. The figure 4 illustrates the alteration of the portion of the different pore categories in the total porosity of the soil. The main porosity changes occur due to redistribution (decrease) of the portion of large intermicroaggregate-granular micropores (DJ, decrease of their maximum sizes and increase of the portion of small intermicroaggregate micropores (D3). This is apparently caused by the process of mechanical destruction of mineral grains in the vicinity of aggregates edges, occurring during the landslide movement. In the soils of the landslide foot a more intensive increase (1.7 fold) of the portion of interultramicroaggregate micropores (D,) in comparison with intact deposits. Further, there are no sufficient changes of the portion of intramicroaggregate microporosity along the whole landslide foot, and its magnitude remains constant - 15-18%. The increase of the portion of the small intermicroaggregate micropores and decrease of the portion of large micropores is observed in the lower part of the landslide. Thus, the consolidation of the samples with the distance from the scarp occurs mostly due to the alterations of the character of pore space - redistribution of the porosity from large intermicroaggregate-granular micropores to small intermicroaggregate micropores. In side part of the landslide (fig. 4) - in the ravinebed some decrease of the portion of the small intermicroaggregate micropores is observed due to better spreading of large micropores under the additional saturation by the creek. The consolidation of the soils in the landslide is 1405

Fig.2. Separation of large aggregates in the soils, involved into the landslide “flow” deformations. In the front part of the foot aggregates have an angular shape (2a). The shape of aggregates is changed (2b, 2c) with the landslide movement and at the end of the foot they obtain a shape of “rounded pebble” (2d).

Fig.3. The internal structure of aggregates separating during landslide flow is similar to the soil structure in the intact part of the slope (3a). In the vicinity of the aggregate edges numerous mineral grains with the traces of mechanical distortion are noted (3b, 3c). The aggregates are separated from each other by narrow fissure-like pores (3c, 3d). 1406

Fig. 4. Transformation of the pore space of soils during flow slide displacement. practically not followed by sufficient changes of the degree of structural elements orientation (&=l.29.5%). The highest orientation of the structural elements is observed in the zone of landslide movement, and the lowest - in the zone of accumulation. As a whole the microstructure of all the samples can be classified as medium or low oriented types.

4 CONCLUSION The studied changes in the soil microstructure during the formation and development of flow slides confirmed, that the process of initiation and displacement of landslide is followed by a sufficient reconstruction of the soil microstructure. The start of deformation is marked by the more intensive alteration in the distribution of the different categories of rnicropores in pore space, which can be considered as one of the criteria of the beginning of slope displacements. The very displacement of a landslide appears to begin from the formation of the aggregates. The movement of a flow slide is a relative “rolling”, slipping of aggregates. One of the causes of the observed landslide deposits consolidation is the redistribution and denser “packing” of aggregates, having an angular shape in the foot of a landslide which changes to the “rounded particle” one with the landslide movement. The study of soil microstructure may improve the reliability and the quality of the landslide hazard forecast if included into the complex of investiga1407

tions for stability of landslide slopes evaluation.

5 REFERENCES Sokolov V.N., Kuzmin V.A. 1993 The application of the SEM images processing for estimation of the capacity and filtration properties of oil and gas contained rock. Bull. of RAS. Physics., 57( 8):94-98. Sokolov V.N., Yurkovetsh D.I., Razgulina O.V. & Melnik V.N. 1997. A method of qualitative analysis of the microstructure of solids on SEM images. Zavodskaya laboratoriya (materials diagnostics). 9: (in Russian). Tsaryova A.M. 1985. Classification of rock textures, forming in the landslide displacement zone. Investigation of the development mechanism of exogenic geological processes and their causing factors. VSEGINGEO: 45-52 (in Russian). Zerkal O.V. 1994. Seismic landslides caused by Gissar earthquake in 1989 (Tajikistan). Geological Bull., 49(2): 57-63. Zerkal O.V. & Sokolov V.N. 1995. Changes in the microstructure of silt loams during the formation of seismogenic liquefaction slides. Geological Bull., 50(6): 59-64. Zerkal O.V. 1996. Mechanism of formation and development of deep seismogenous landslides due sudden liquefaction of loessal soils. Landslides. Proc. of the Seventh Internat. Symp. on Landslides. v.2: 1055- 1060.

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Slope Stability Engineering, Yagi, Yamagami& Jiang 0 1999 Balkema, Rotterdam, ISBN 90 5809 079 5

Author index

Akai, T. 1027 Akesaka, N. 1349 Ali, EH. 687,1003,1287 Alias, S. 1291 Anishin, VIM. 859 Antoine, I? 1369 Ayalew, L.1181 Ayele, T. 107 1 Bao, T. 1105 Belabed, L. 1115 Bhandary, N.B. 701,1199 Bouazza, A. 863 Bromhead, E. N.1109 Cai, E 883 Calcaterra, D. 1361 Carrillo-Acevedo, A. 971 Carrillo-Gill, A. 97 1 Chan, L.-C. 1123 Chan, L.C.P. 1385 Chandler, R. J. 775 Chau, K.T. 1139,1355,1385 Chen, H. 1397 Chigira, M. 1145 Chlk, Z. 1003 Chowdhury, R. 1089,1309 Corominas, J. 1239 Couture, R. 1369 Dantas, B.T.1055 Deacu, D. 1297 Doi, M. 799 Donald, I. B.863 Doran, I.G. 769 Ehrlich, M. 1055 Erizal959 Evans, S.G. 1369

Farkas, J. 931 Farroni, A. 965 Faure, R.M. 1317 Fiener, Y.A.O. 1287 Flentje, I? 1309 Fredlund, D.G. 757 Fujii, A. 955,985 Fujii, K. 793 Fujimura, H. 1253 Fujita, H. 693,877 Fujiwara, T. 841 Fujun, N. 1275 Fukuda, M. 1135 Fukuoka, H. 1169 Funiya, G. 1169 Gibo, S. 727 Gili, J.A. 1239 Ghzewski, M. 925 Gotoh, K. 827 Gotoh, M. 895 Gottardi, G. 1211 Grebenets, VII. 859 Gudehus, G. 847 Gupta, A.S. 805 Hadjigeorgiou, J. 1369 Hamid, A. M. 901 Hanzawa, H. 787 Harada, T.83 1 Harris, A. J. 1 109 Hasegawa, T. 8 17 He, S.X. 1061 Herle, I. 847 Hirai, T. 997, 1033 Hirata, M. 763 Hiura, H. 1169 Hric, S. 1071 Hu, X. 1105

1409

Huang, C.C. 1049 Huang, Y. 75 1 Huat, L.T.687 Hussein, A.N. 943 Ibsen, M-L. 1109 Ichikawa, Y. 869 Iizuka, A. 763 Ikuta, T. 827 Inagaki, H. 1269 Ishihara, K. 675,75 1 Ishii, T.697, 1207 Ito, E. 1145 Ito, T. 1223 Izawa, E. 1165 Izawa, J. 991 Izumi, K. 1379 Jain, VI K. 919 Jamaludin, A. 943, 1291 Jayawardena, U.de S. 1165 Jiang, J.-C. 1043 Jiao, J. 1095 Kabai, 1. 1193 Kabir, M.H. 901 Kaino, T. 823 Kalotka, J. 925 Karnon, M. 1027 Kaneda, T. 83 1 Kasa, A. 1003 Kato, S. 709 Kawaguchi, T. 78 1 Kawahara, K. 913 Kawahara, S. 1343 Kawai, K. 709 Kawakami, H. 1379 Kawamura, I(.869 Kawamura, M. 1015

Kerimov, A.G.-o. 859 Keshav, K. 919 Khan, Y.A. 1043 Kimizu, T. 1229 Kimura, M. 877 Kinoshita, S. 877 Kishor, K. 949 Kitazono, Y. 1101 Kito, Y. 1229 KO,C. K. 1309 Kobayashi, A. 793 Kobayashi, I. 763 Koda, E. 937 Kohashi, H. 955 Kono, H. 1203 Kudella, l? 847 Kumano, K. 1207 Lashkaripour, G. R. 1259 Law, K.T. 1129 Lazanyi, I. 1193 Ledesma, A. 1239 Lee, C.E 1129,1355,1397 Lee, C. K.T. 1303 Lee, G.-S. 1123 Leopardi, M. 965 Liu, S.H. 681 Liu, Z. D. 1061 Lloret, A. 1239 Locat, J. 1369 Luan, M. 1095 Maeda, Y. 907 Mainalee, B. l? 1253 Mandolini, A. 1 151 Mariappan, S. 687 Marui, H. 1379 Matsui, T. 1021, 1039 Matsumoto, Y. 1349 Matsuoka, H. 681 Matys, M. 1071 Miki, H. 955,985 Mimura, M. 1027 Mitachi, T. 715,781 Miyata, Y. 73 1 Miyauchi, S. 959 Mochizuki, A. 83 1 Momiyama, Y. 1207 Monaco, I? 965 Mora, S. 1247 Morii, T. 8 17 Morishima, N. 1253 Moya, J. 1239 Muda, Z. 1291 Mukherjee, D. 949

Muraishi, H. 1263 Muro, T. 1343 Nabeshima, Y. 1039 Nagayoshi, T. 1009 Nakamura, S. 727 Nakano, J. 955 Nakano, M. 869 Nakasone, N. 1101 Nakayama, S. 675,1175 Narita, K. 837 Nattavut, T. 869 Nishida, K. 1065 Nishigata, T. 1065 Nishimura, E 1159 Nishimura, J. 1033 Nishimura, T. 757 Nishio, M. 977 Nishiyama, K. 877 Noguchi, T. 1263 Nomoto, K. 841 Noro, T. 1175 Nosaka, Y. 675 Noshimura, J. 997 Nurguzhin, M. R. 8 11 Ochiai, E 1175 Ochiai, H. 907 Ogata, K. 1009 Ogawa, N. 1039 Ogawa, T. 827 Ogita, N. 1229 Oh, K. 1233 Ohashi, T. 68 1 Ohne, Y. 837 Ohnishi, Y. 895 Ohno, M. 877 Ohta, H. 763 Ohtsuka, S. 73 1,799 Oka, K. 9 13 Okabayashi, K. 1015 Okada, K. 1263 Okawara, M. 715,781 Okuzono, S. 977 Omine, K. 907 Ouhadi. YR. 705 Palma, B. 1361 Parise, M. 1337, 1361 Park, B. 1233 Park, D. 1233 Paunescu, D. 1297 Pelella, L. 1361 Petley, D. J. 741,745 Petro, L. 1217 1410

Picarelli, L. 1151 Pokharel, G. 985 PolaSEinova, E. 1217 Porbaha, A. 1021 Pumjan, S. 1079,1085 Rao, K.S. 805 Rius, J. 1239 Russo, C. 1151 Saga, M. 693 Sakai, T. 959 Sakajo, S. 877 San, K.C. 1021 Sassa, K. 1169 Sato, 0. 1379 Schina, S. 775 Seiki, T. 869 Shi,D. 1095 Shibata, T. 1159 Shigematsu, H. 977 Shima, S. 1281 Shimada, K. 817 Shintani, N. 9 13 Sivakumar, Y 769 Sokobiki, H. 1175 Sokolov, V: N. 1403 Spena, A.R. 965 SU,M.-B. 1123 Suarez, J. 1187 Sugimoto, T. 841 Sugiyama, T. 1263 Suwa, H. 1379 Suwa, S. 1135 Suyama, K. 997,1033 Suzuki, A. 1101 Suzuki, K. 787 Suzuki, M. 721,735 Tada, M. 1009 Takahashi, A. 991 Takemura, Y. 991 Takeo, N. 1027 Tanabashi, Y. 9971033 Tanada, M. 715,1207 Tandjiria, V. 889 Tani, M. 1203 Tateyama, M. 1049 Tayama, S. 1009 Taylor, P 741,745 Terazono, T. 1101 Titkov, S.N. 859 Tochimoto, S. 877 Tonni, L. 1211 Totani, G. 965 Toyoto, H. 731

Tsai, C.C. 1049 Tsuji, K. 787 Tsukamoto, Y. 675 Tyagi, A. K. 805

Wong, C. K. M. 1303 Wong, R. H.C. 1355 WU,H.-C. 1105 Wu, J.J. 1355

Ueda, A. 9 13 Ugai, K. 877,883 Umezaki, T. 721,735 Ushiro. T. 1349

Xu, D. 1089

Vernier, A. 1181 Vinnichenko, S. 1331 Vizi, B. 1193 Wada, H. 907 Wagner, P. 1217 Wakuda, N.1033 Watanabe, K. 1165 Watanabe, Y. 823 Wehr, W. 847

Yadav, 0.€? 949 Yagi, N.697,701,1159,1199, 1203,1349,1391 Yamabe, S. 1043 Yamagami, T. 1043 Yamaguchi, M. 837 Yamamoto, K. 793 Yamamoto, T. 721,735,913 Yamanaka, M. 827 Yamashita, H. 693 Yamashita, Y. 1391 Yang, Q. 1095 Yang, X.Q. 1061

1411

Yashima, A. 977 Yasuhara, K. 1033 Yasuhara, K. 997 Yatabe, R. 693,697,701, 1159, 1199,1203,1229,1391 Yokota, K. 693,697,701, 1159, 1199,1203,1391 Yoshikuni, H. 1281 Young, D.S. 1079,1085 Yuhai, L. 1275 Yunohara, T. 1269 Yusof, N.M. 1291 Zerkal, 0.V.1403 Zhakulin, A. S. 8 1 1 Zhang, X.-B. 1105 Zhixin, C. 1275 Zhou, S.G. 1039 Zhusupbekov, A.Zh. 81 1 Ziehmann, G. 853

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