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18me Congrs Franais de Mcanique Grenoble, 27-31 aot 2007

A study of a slope failure case in unsaturated marly clay

M. Jamei*, H. Guiras, K. Ben Hamouda &M. Hatira

Civil Engineering Laboratory, National Engineering Scholl of Tunis (ENIT), B.P. 37, Le Belvdre Tunis,

Tunisia

[email protected]

ABSTRACT

The analysis of the some experimental field data on the unsaturated unstable slope constituted

with a marly clay soil showed that the swelling and collapse phenomena were the origin of the landslide

problem. In the studied case, field investigation data and laboratory tests based on the water retention

curve determination were analysed. The recorded pore-water pressure field helped us to identify the

hydrological conditions. In fact, the wetting and drying cycles involve the total suction and the degree of

saturation changes. To predict the shallow slope failure, a constitutive model taking into account the

suction effect as well as their dependence on degree of saturation is proposed. It is especially remarkable

that the collapse phenomenon is well reproduced when the model incorporates suction changes and

saturated preconsolidation stress. The analysis of the slope failure, basing on the field data and the

theoretical study, shows the hydraulic and mechanical effects.

RSUM

Lanalyse des rsultats issus dune exprimentation in situ dun talus instable constitu de

largile marneuse, montre que la nature gonflante de cette argile et son fort potentiel deffondrement sont

lorigine de cette rupture. Dans cette tude et en se basant sur des rsultats des essais in situ et de

laboratoire pour la dtermination dune part des caractristiques hydriques du site, et dautre part des

caractristiques physiques et mcaniques et de la courbe de rtention deau de largile constituant le talus,

on analyse leffet des cycles hydriques sur la variation de la succion, conduisant ainsi une rupturesuperficielle du talus. Un modle de prdiction de la variation de la rsistance au cisaillement incluant la

prdiction de leffondrement est propos. Par ailleurs, on montre que leffet de leffondrement sur la

diminution de la rsistance au cisaillement est bien pris en compte en introduisant la succion et la

contrainte de prconsolidation saturation.

Key-words:

slope failure; unsaturated marly clay; constitutive model

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1 Introduction

The landslide cases were more frequent in the Northern West of Tunisia. Every year the

Tunisian government invests large amounts of money to repair the damages caused by the

slopes failure along the national roads. This area belongs to a semi-arid region, with a mountain

relief, different climatic seasons and an important variation of rainfall intensity (from 6.5mm in

July to 80mm in January). The rainfall events lead to unstable slopes, often during the wet

season and particularly after the long dry season. Since one of the major geotechnical problems

is to design safe and economical dimensions for the slopes and to make well infrastructure roads

functional, interested has been mainly focused in hydraulic and mechanical origin of the

landslide kind frequently observed in the mentioned above region. The interaction of the

hydraulic and the mechanical aspects has been studied (water retention curve, permeability and

soil shear variation due to the suction changes). Conventional slope stability analyses using the

principles of saturated soil mechanics indicated the safe conditions for the slope (factor of safety

ranges respectively between 6.2 and 7.5 with the undrained shear parameters and between 1.17

and 1.43 with the effective shear parameters). These factors of safety values were corresponded

to an average 80 kPa of undrained cohesion and 4 for the undrained friction angle. The

effective failure parameters are c= 4kPa and ' = 10). Nevertheless, as the conventionally slope

stability is based on saturated shear parameters assuming the soil to be either dry or completely

saturated, this approach can lead to the worst case scenario of the slope behaviour (Kerrie

Fabious et al. 2004) and consequently leads to the conservatively analysis. Hence, in this case,

the conventional slope stability study appears not efficient to explain the observed slopes failure

or leads to an erroneous result (it is the case when the undrained shear parameters are taken into

account in the safety factor calculation). Besides, the contribution of matric suction towards an

increase in shear strength may be taken into account in the slope stability, particularly here for

the study case, where the ground water table is deep and the slip planes are shallow (e.g. the

shallow landslide: Figure 1). Mechanical effects associated with suction changes tend to be

more complex to study if the soil is like a marly clay, which presents a collapse or a swelling

phenomena (with a decrease of suction) and shrinkage (with an increase of suction). This is one

of the reasons for the present study in which we give an attention to found a theoretical simply

model which leads to more understanding of the landslides problems observed in the marly

clayey soils. This first part of the paper presents the site and field instrumentation data. These

results provide an overall picture of the landslide phenomenon and give an idea to the hydraulic

parameters variation effect on the slope failure mechanism. Summarized physical and

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mechanical parameters are also given. In the second part of the paper, interested is focused

firstly on the collapse response of the marly clay soil and secondly on an alternative model

combining the principle of the Barcelona model (BBM, 1990) and those proposed by other

authors (Vanappilli et al. 1996), Kerrie Fabius et al. (2004).

2 Description of the site and instrumentation

2.1 Soil profile and properties

The site is constituted by a success of slopes crossed by road (the average inclination of

the different slopes in this site is 22, Figure 1). Important observed shallow failures are with

the depth ranging 2m to 3m. The slope studied in the paper has an angle 15 and a transversal

length 150m (Figure 1).

FIG. 1 Slope profile and collapse of the marly clay after the wetting season

(at the saturation state)

The site is in semi-arid area with average annual rainfall intensity in this region is 600mm,

which the most party has been during september to march (Figure 2). For the economical

raisons, just two boreholes were drilled in the mid-slope of monitored area and used for

sampling and installing a piezometer. To describe the eventual movement of ground surface

some landmarks were installed across the profile and a topographic measurement was taken

each month. The physical and mechanical parameters from the boreholes around mid-slope are

shown in Table 1. Parameters in Table 1 indicate the fine soil character (97 % < 80m) and

relatively its high plasticity (the average value of plasticity indice Ip is 30%). The densities (wet,

dry and particle specific) are also given. The increase of the dry density is naturally related to

the earth weight effect. The shear tests which were performed in undrained conditions show the

low values of friction angles and a not important cohesion value. These tests correspond to a

degree of saturation Sraround 92%.

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0,0

50,0

100,0

150,0

200,0

250,0

SEPT

OCTO

NOVE

DECE

JAN

FEVR

MAR

AVPIL

MAY

JUIN

JUIL

AUG

rainfall

2000/01

2001/02

2002/03

2003/04

2004/05

FIG. 2Rainfall intensity during 2000-2005

Granulometry Atterbergs Limit Densitiesdepth

%>0,42mm %

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3 A model to describe the slope failure and the collapse strains

The model is based on the soil-water characteristic curve as tool to predict the shear

stress function for an unsaturated soil (Vanappilli et al. 1996). In this paper, using the fitting

parameters given by Jamei et al. (2007), the soil-water characteristic curve was obtained (Fig. 5-

b) across the particle size distribution (Fig. 5-a).

0

10

20

30

40

50

60

70

80

90

100

0,0001 0,001 0,01 0,1 1 10part icle size( mm )

Percentagea

ccruedbyweight(%

0

5

10

15

20

25

30

35

1 10 100 1000 10000 100000

Succion (kPa)

watercontent(%)

FIG. 5-a: particle size distribution curve FIG. 5-b: soil-water characteristic curve

The fundamental equation relating the suction and the diameter of equivalent particles (Jamei et

al. 2007) is:2 cos

s

i

w i

Ts

g r

= (1)

Where: s i = soil water pressure related to the meniscus at the equivalent particle (i); T s= surface

tension of water; =contact angle; w = density of water; g = acceleration due to gravity andri =

pore radius of particle (i). The model is based on the shear strength function (2-a) for

unsaturated soil proposed by Fredlund et al. (1996) which, in this paper, has been extended

including the apparent preconsolidation stress.

( ) ( )rwn ac u tg s = + + tg , wsat

w

w = (2-a) and (2-b)

Where: ua = air pressure related; n the total net normal stress; s the matric suction; w the not

dimensional water content defined by (2-b). r is a soil parameter which related to the plasticity

of soils and is the water content for saturated soil. In order to introduce the relation

between the collapse phenomenon and the preconsolidation stress, the equation (2-a) is

modified by (3):

satw

( )0

*0

0*

0

( ) ;( )

p

p

wn a

pc u tg s r

p = + + = (3)

The two stresses and were given by Alonso et al. (2003)0p*

0( )p

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0

max

n

cr c

sp p p

s

= + (4)

Equation (4) provides the current yield confining stress, , for a suction s and a current

(actual) saturation confining stress . As shown by Alonso et al. (2003) provides theintensity of the expected collapse. For a given initial (high) suction the collapse strains

can be predicted. In (4) the n is a soil parameter depending on the LC yield curve (BBM

model). The current confining stress can be related with the preconsolidation stress

(5). The Figures (6-a) and (6-b) show respectively the response of the model to predict the shear

stress evolution according to the suction changes (Fig. 6-a) and the dependence of the

constitutive model with the parameter r (Fig. 6-b).

0p

crp cp

maxs

crp*

0( )p

*

0( )or op p= + p (5)

0

20

40

60

80

100

120

140

160

180

200

0 100 200 300 400 500

Suction (kPa)

S

h

e

a

r

stren

th

k

P

a

Predictedcurve

effectif vertical stress = 20 kPa

0

20

40

60

80

100

120

140

160

180

200

0 100 200 300 400 500

Suction (kPa)

S

h

ea

r

str

en

th

k

P

a

r = 1

r = 2

r = 3

effectif vertical stress = 20 kPa

FIG. 6-a: shear strength-suction relationship FIG. 6-b: shear strength-suction evolution with r

4 Conclusions

The study has provided a case of unsaturated shallow failure slope due to the diminution of

suction and principally due the humidification and drainage cycles. The experimental site data

gives an idea on this phenomenon. The model takes into account the effect of the suction

changes on the collapse phenomenon, including the water retention curve data. It allows also to

a quantitative variation of the soil shear stress during the wetting process. Then, it may be leads

to the failure slope prediction across the safety factor calculation.

References

Alonso, E.E. & Romero E. 2003, Collapse behaviour of sand. In Proceeding Asian Conferenceon Unsaturated Soils, Japan, 325-333.

Jamei, M., Guiras, H. & Mokni N. 2007, A retention curve prediction for unsaturated clay soils,

2nd International Conference Mechanics of unsaturated soils, 7th- 9

thMarch , Bauhaus-

Universitt Weimar, Germany.

Kerrie Fabius & Thunder Bay 2004, Slope Hazard Management with the cautionary zone

approach: a case history. 57th

Canadian Geotechnical Conference. Quebec City, Canada.

Vanapalli, S.K., Fredlund, D.G, Pufahl, D.E., & Cliftron, A.W.1996 Model for prediction of

shear strength with respect to soil suction, Canad. Geot. J., 33: 379-392.

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