isrm-eurock-1996-082_numerical modelling of rock slope deformations

8/12/2019 ISRM-EUROCK-1996-082_Numerical Modelling of Rock Slope Deformations

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Eurock '96, Barla (ed.) 1996 Balkema, Rolterdam. ISBN 90 5410 8 43 6

Numerical modelling of rock slope deformationsModelisation numerique des deformations d 'un versant rocheux

Numerische Modellierung von Deformationen einer Felsboschung

1.M. Vengeon, D. Hantz, A.Giraud & D. Raet -IRIGM-LGM, Universite Joseph Fourier de Grenoble,France

ABSTRACT: Near Grenoble (French Alps), a 100 million cubic meters slope movement threatens theRomanche valley and has been monitored since 1985. The deformation mechanisms are very difficult toidentity. A simplified geomechanical model of the rock mass has been used to simulate the excavation of thevalley, with finite and distinct element methods. The results obtained allow to explain some morphologicalfeatures of the slope and are compatible with the present movements.

RESUME. Pres de Grenoble (Alpes francaises), un mouvernent de versant de 100 millions de metres cube, quimenace la vallee de la Romanche, est ausculte depuis 1985. Les rnecanismes de deformation sont tres difficiles aidentifier. Un modele geornecanique simplifie du massif rocheux a ete utilise pour simuler I'excavation de lavallee, par les methodes des elements finis et des elements distincts. Les resultats obtenus fournissent uneexplication de certains traits morphologiques du versant et sont compatibles avec les mouvements actuels.

ZUSAMMENFASSUNG: Nahe Grenoble (franzosische Alpen) befindet sich ein seit 1985 unter Beobachtungstehendes Hangrutschungsgebiet mit einem Volumen von 100 Millionen Kubikmetern, welches das Tal der Romanche bedroht. Die Verformungsmechanismen sind sehr schwer identifizierbar. Ein vereinfachtesgeomechanisches Modell der Felsboschung mit finiten und distinkten Elementen wurde zur Simulation der Talentstehung genutzt Die erhaltenen Ergebnisse erklaren einige morphologische Besonderheiten dieser Felsboschung und entsprechen den momentan stattfindenden Bewegungen.

1 fNTRODUCTION

Deformation mechanisms of some large slopemovements in mountain areas are very difficult toidentity To be understood, they need to beconsidered as the result of a long term geological

process, and then to be analyzed in the appropriatespace-time scale. This is the case for the Sechilienneslope movement, which will be described later.

The geomechanical models presented in this paper have been elaborated to highlight the evolution of therock mass, due to the formation of the valley, and toanalyze the influence of different factors on its present behaviour. The models are not aimed at predicting the future behavior of the slope. We think that, for movements which are as complex as theSechilienne landslide, geomechanical simulations arenot sufficient for this purpose, and must be used together with more empirical approaches.

N

SECHILIENNE

SLOPE MOVEMENT

o 2 4kmL...l.....LL..J

Figure I Location map showing the site of theSechilienne instability

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2 DESCRIPTION OF THE SECHILIENNE SLOPEMOVEMENT

The Sechilienne slope movement takes place on thenorth flank of the Rornanche valley, 20 km upstreamfrom Grenoble (Figure 1). Rockfalls at the front of

the unstable mass were well known by the former generations of inhabitants who called the site "LesRuines" Their reactivation in 1985 proved dangerous for a major highway (RN 91) and led to acomplete geological survey of the slope and to theinstallation of a large monitoring net to measuredisplacements, by geodesy and extensometry. Thesestudies showed that the movement extends up to thecrest and, consequently, that the unstable mass ismuch larger than the 3 million cubic meters of thevery active frontal zone.

First volume estimation of the unstable mass was50 million cubic meters but recent geodesic and underground observations lead to an estimation of about 100 million cubic meters.

Different rockfall scenarios have been studied toestimate and prevent the damages wind effect,damming of the valley, flooding ... But the predictionof the probable rupture scenario is very difficult because of the hugeness of the mobile mass involved and because of the complexity of the slope structureand of its deformation mode (Giraud et al., 1990,Antoineetal.,1994)

2.1 Geomorphology

The slope angle is approximately 45 from the bottom of the valley (330 m) up to the altitude of 950 m and then around 20 up to the top of theMont-Sec at I 125 m (Figure 2). Near the crest, a 20meters high cliff reveals an old settlement. It isassumed that this morphology and the instabilityresult mainly from the glacial history of the valley.

II flO

1000

90.

800

70.

tillll

~11(1 [rhrlll'

'00' L ,.2 ( 1

JOO

U:S tWINES

~

I.A HOi\!ANnn:

IFigure 2 : Topographic profile of the unstable zone.

2.2 Geology and structure

From a geological point of view, the rock mass that builds up the slope is quite homogeneous and isconstituting of micaschists (hercynian metamorphicrocks) On the main part of the slope, the foliation is

almost vertical and displays a north-southorientation. The structure is rather complex with adense fracturing pattern and the presence of mylonitic zones.

The structural survey of the whole zone (Figure 3)shows five main joints sets playing a major role in thedeformability of the rock mass: the foliation (north-south to N20, vertical),

mobilized by huge shear zones (Fol); east-west subvertical fractures (Fl) ; north-east - south-west subvertical fractures (F2) ;

north-west - south east subvertical fractures (F3),mainly represented in the east part of the unstablezone;

east - west fractures dipping from 30 to 60toward the valley (F4), mostly discontinuous buthomogeneously distributed on the slopes.Contrarily to the other sets, the tectonic history of the area does not explain the genesis of this jointset.

Figure 3: Stereographic projection (lower hemisphere) of the main families of joints onSechilienne site.

2-3 Deformation of the slope

The east - west (F 1) and north-east - south-west(F2) fractures cut the rock mass into slices and

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are associated with a series of linear depressions and small ridges parallel to the valley. The movementsVectors are orthogonal to these depressions and lessinclined than the slope and than the F4 joint set(between 10 and 20) ..

The rate of opening of the main fractures between

adjacent slices is quite homogeneous (a fewcm/year). The monitoring data show a cyclicvariation in opening velocities connected with thehydraulic regime of the massif.

The foliation (F) has no direct effect on thestability of the rock mass but weakens themechanical characteristics of the micaschists and allows the individualization of cinematically distinct blocs.

1200

1000

800

600 RomancheRiver !

Altitude (rn)

Figure 4 : Cross section of the slope movement.

The association of the fractures dipping toward the valley with those determining depressions in theslope allows the development of a progressiveinternal failure with lateral dilatation of the mass(Figure 4)

The relationships between the structure and theslope deformation highlight many questions amongwhich the most important are: What is the influence of the slice-structure on the

global deformation of the rock mass?

What are the roles of the fractures dipping toward the valley and how have they been generated? Can a major slope failure happen, how and at

what depth?

3 MODELING AND SIMULATION

3.1 Geometry and structure

The valley is cylindrical, with a symmetrical U-shaped (rectangular) or V-shaped cross section. It is1000 m deep and has been excavated from anhorizontal surface in a semi-infinite homogenousmedium. Preliminary analyses have been made to

place the limits of the numerical model sufficiently far from the valley (Vengeon, 1994). According to thisanalysis, the depth of the model was fixed to 5000 mand its half-width to 6000 m (Figure 5). Vertical

joints, parallel to the valley, have been assumed.

A ~Y B x- ->0

I c D

E

Figure 5 : Geometry and boundary conditions of the

model for finite and distinct element methodssimulations.

3.2 Mechanical properties

Stress and displacement fields in a rock mass are theresult of complex processes, which are controlled bythe elastic, viscous and plastic properties of the rock and the discontinuities As a first approach, the rock material and the joints are assumed to beelastoplastic.

The mechanical parameters of the intact rock areestimated using the classification of Deere and Miller.The uniaxial compressive strength of a schist in thedirection of foliation lies between 75 and 150 l'v1Pa.A value of 100 l'v1Pa is chosen for the simulations,with a Young modulus of 50 GPa, a Poissoncoefficient of 0.33 and a specific weight of 25kN/m 3.

The Mohr-Coulomb failure criterion is adopted for the joints, with no dilatancy, zero cohesion and afriction angle of 20 or 30

3.3 Initial state of stress

The state of stress in the semi-infinite medium beforethe excavation of the valley has been simulated,assuming the plane strain hypothesis. Different boundary conditions have been introduced. For thecase with no tectonic stresses, a zero horizontaldisplacement is imposed on the vertical boundaries of the model. This leads to an horizontal/vertical stressratio of 0.5.

Two methods are used to simulate the effect of tectonic stresses. With the Cesar finite element code,

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::: < F3O

: 0 < F 2Oo)

)

FIgure 7 : Sliding zone on the vertical JOInts behind avertical slope (no tectonic stress). Depth and thickness of the zone increase when the friction angleOn the joints (cj decreases.

43 Influence of parameters.

Friction angle of the vertical jointsThe deformation is clearly larger with a value of thefriction angle of 20 than with 30 (Figure 7). If lj>==30,the sliding zone on vertical joints is 300 rn to50 m thick from the front to the rear of the model(first joint excepted). Its upper limit rises from thevalley corner up to the top of the last JOInt a tapproximately 45. If cj>=20, the thickness of t~esliding zone varies from 800 m to 300 rn and Itsupper limit is inclined by 30.

Tectonic stressThe introduction of an horizontal stress equivalent tothe weight of the excavated soil column (25 Mpa)alllplifies notably the toppling of the slices and

extends the sliding zone on the vertical joints.The of octahedral stress exceeds the limit only in

the first slice, close to the foot of the slope. Fromthis value of the tectonic stress and further up,tension appears on the rear face of the topplingslices.

Elastoplastic behaviour of the rock mass .An elastoplastic behaviour of the rock WIth theMohr-Coulomb criterion (tensile strength Rt= I0MFa) has been tested with a high tectonic stress

(k==O"jJO"v=2) Toppling is then limited to the twofront slices and the horizontal displacement ISreduced by a factor 0.7 in comparison with the

elastic case. In the same way, the accumulation of stress near the borders of the slices is lowered and the principal directions are hornogenised,

5 SIMULATING THE EXCA VAnON OF A

V-SHAPED VALLEY

This study has been realised with the distinct elementmethod (Udec code, Itasca), well adapted togeomechanical problems including sets of joints Theslopes studied are inclined at 39.7, 45or 63.4 and cut by ten vertical semi-infinite joints with anhorizontal spacing of 100 m.

100 m

1000m

Figure 8: Different slope morphologies studied:81=63.4 , 8 2=45and 8 3=39.7.

5.1 Deformation mode

This more realistic geometry confirms thedeformation mode previously observed, corrected from the exaggeration induced by the vertical slopeangle. The deformation is still affected by thecompetition between the toppling effect and the S shape. The final result is strongly influenced by all thetested parameters: slope angle, friction on the joints,tectonic stress, rhythm of excavation.

5.2 Stress distribution

The perturbation of the vertical stress distribution bysliding on the vertical joints (Figure 9) is much moreextended than for a vertical slope.

The extension and the depth of this perturbed zone depends on the slope angle and on the frictionon the vertical joints. The major oscillations indicatethe flexion zone caused by the toppling of the sliceabove. Figure 9 shows that the joints behind the topof the slope are open without toppling too much. Italso reveals that the depth of opening of the vertical

joints decreases quickly at the bottom of the slope.

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ALTITU/)E 1m)1000 Open vertical joint

HIIII

600

400

200

Figure 9 : Oscillations of the vertical stress contoursinduced by sliding on the vertical joints (0.=39 7,=20)

Applying a low tectonic stress (k=crl/crv= I), wenote the apparition of tension zones on the rear faceof the lower slices (Figure 10).

210

\

190 \

17." -

- ,

" - - ,


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of deformation which combines toppling withuplifting and d is to rtion due to the elastic valleyreb ou nd . A s a result, the fro nt slices ten d totopp le whereas the rear ones evolve to a S shape (see 4.1)

This s imple scheme of the s lope genes is does not

attempt to represent the monitored displacements . Nevertheless, it is comp atible with the mainobservations n ow here o n the slo pe is an y d iso rd er o bserved

that co uld b e in terp reted as t he emerg ence o f asliding surface;

the displacement vectors have small inclination, asif the front slices were still toppling today,

the depressions corresponding to the vertical joints are opening;

on to p o f th e slo pe, a 20 rn high cliff rev eals an

eventual old settlement.T his last point is t o be discussed because themodel indicates a competit ion between the uplift ingC ompo nen t o f th e elastic v alley reb oun d an d th eset tlement induced by the S shape deformation.However, a t the t ime scale of the valley excavation,the uplif ting may be considered as instantaneous anddoes not c rea te any typ ical morphology because thesurface o f the massi f is s ti ll s trong ly e roded at this phase. Toppling and the S shape deformationmay d evelo p more p ro gressively, lik ely after th eg laciers h ave melted an d th us erosion h as slo wed

down. Therefore , settlement due to the S shapemay explain the main morphological accidents on topof the present slope.

T he magn itud e o f the d efo rmation co mp uted isvery sensitive to the value of the frict ion angle on thevert ical jo in ts ( see 4 .3 and 53) In order to con tro lthe deformation of the massif, the verticaldiscontinuities must have low mechanicalcharacteristics, that is they m ust be crushed,wea thered, smooth o r wet: ... The value =20might be rep resentat ive of some major cru shed andweathered zones that have been observed in old minegalle ries and in the 240 m long tunne l bored duringthe years 1994 and 1995. The vert ical jo in ts mighthave been w eakened by the vertical shearingmovement and the water f lows induced by the glacialexcavation of the valley.

62 Origin o f the f ractures d ipping toward the valleyand their role.

The computed deformation mode induces a diagonal

zone with concentration of s tress on the front face of the s lices and deficit on the rear face. Under certainCondi tions (tectonic s tress) , tens ion may loca lly

appear (F igure 11). This s tate o f st ress is likely tocreate o r pro pag ate fractu res d ip ping tow ard th evalley (see 4.2 and 5.2). This can explain theoccurrence of such joints and their small extension (afew meters).

TRACTION(7,4 MPa)

(31 MPa) ,

COMl'RESSIO:'>J

Figure I I Maximum tens ile s tress computed, on therear face of the second slice with low tectonic s tress(a=45, =30,k=


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7 CONCLUSION

Far from trying to simulate the present movement of the Sechilienne slope, this numerical study highlightsthe difficulty of modelling such a complex natural phenomenon. Nevertheless, it clarifies the influence

of the vertical joints on the deformation and theorigin of the joints dipping toward the valley. It alsosuggests that a progressive failure could be more probable than a sudden global one. The model has proved very sensitive to the friction angle on thevertical joints and to the tectonic stress but these two pararneterss are very difficult to evaluate in situ.

For the future, important questions remain, suchas the interaction with the ice during the formation of the valley or the way the underground flows controlthe present deformation of the rock mass.

REFERENCES

Antoine, P. & A.Giraud & H.Evrard & L.Rochet1994. A huge slope movement at Sechilienne, (sere,France. Landslide News n08 pf5-f8.

Giraud, A. & L.Rochet & PAntoine 1990.Processes of failure in crystallophyllian formations Engineering Geology, 29, p2-lf-253.

Starfield, A.M & PA. Cundall 1988. Toward amethodology for rock mechanics modelling fill. J. Rock Mech. Mill. Sci. & Geeomech. Abstr. Vol 25, No3, pp99-106.

Matheson, OS & SThomson 1973. Geologicalimplications of valley rebound. Canadian Journal of Earth Sciences, 10, 961.

Ract, 0 1995. Modelisation du comportementmecanique des versants rocheux. Memoire de DEA,Universite Joseph Fourier, Grenoble.

Vengeon, 1.M 1994. Contribution a l'etude de ladeformation gravitaire du versant des Ruines deSechiliennc , Memoire de DEA, Uuivcrsite JosephFourier, Grenoble.

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isrm-eurock-1996-082_numerical modelling of rock slope deformations

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