Finite Element Simulation of the 2010 Saint-Jude Landslide in Quebec

Chen Wang

Dans les comptes rendus d’articles de la conférence: GeoVancouver 2016: 69th Canadian Geotechnical Conference

Session: GEOHAZARDS - VI Climate Change Floods & Landslides

ABSTRACT: Spread is a type oflarge-scale landslidesthat can occurin sensitive claydeposits. One of the unique features ofaspreadis that a long quasi-horizontal shear band first forms because of local instability such as toe erosion. Thesoil mass abovethe shear band is then displacedover a large distanceand can be dislocated intoa number of blocks. The mechanismsand kinematics of spread cannot be explained usingthelimit equilibrium (LE) methods commonly used for slope stabilityanalysis. Moreover, because of extremely large deformation of the failed soil mass, spreads cannot be simulated using thetraditional finite element (FE) modeling techniques developed in Lagrangian framework. In the present study, FE simulationof spreads isperformed using a large deformationFEmethod. Numerical analysis is performed using the Coupled EulerianLagrangian (CEL) approach available in Abaqus FE software. A modelforpost-peak degradation of undrained shearstrength of sensitive clayisimplemented in FE analysis. The Saint-Jude landslide, whichoccurred in 2010 in Quebec,Canada,is simulated. It is shown that many features reported from post-slide investigations could be explained using thepresent FE simulation.The initiation of failure, development ofquasi-horizontal shear band, formation of a rotational slide,dislocation of failed soil blocks, retrogressive distance and the morphology of the debris after landslidearesuccessfullysimulated.R•SUM•Un †talement est un grand glissement deterrain pouvant se produire dans des d†p‡ts d…argiles sensibles. L'une descaract†ristiques de ce type de glissement est qu'une longue bande de cisaillement quasi-horizontale peut se former suite— une instabilit† locale, provoqu†e par exemple par de l…†rosion en pied de talus. La masse de sols au-dessus de la bandede cisaillement peut se d†placer sur de grandes distances et peut se disloquer en plusieurs blocs. Les m†canismes de larupture et la cin†matique des †talements ne peuvent pas –tre expliqu†s par les m†thodes d…analyse — l…†quilibre limiteutilis†s couramment pour †valuer la stabilit† d…une pente. De plus, les †talements ne peuvent pas –tre simul†s par lesm†thodes classiques en †l†ments finis (EF) d†velopp†es selon une formulation Lagrangienne,en raison des trƒs grandesd†formations impliqu†es. Dans cette †tude, des simulations d…†talements sont effectu†es en utilisant une m†thode en EF— grande d†formation. Les analyses num†riques sont conduites en utilisant une approche coupl†e Eul†rienne-Lagrangienne disponible dans le logiciel Abaqus. Un modƒle de la d†gradation post-pic dela r†sistance au cisaillementnon drain† d…une argile sensible est impl†ment† dans le modƒle. Nous simulons l…†talement de Saint-Jude qui s…est produiten 2010 au Qu†bec,Canada. On montre que les simulations expliquent raisonnablement bien les principalescaract†ristiques de l…†talement. L…initiation de la rupture, le d†veloppement d…une bande de cisaillement quasi-horizontale,la formation d…un glissement rotationnel, ladislocation de la masse de sols, la distance de r†trogression, ainsi que lamorphologie des d†bris, sont correctement simul†s.1INTRODUCTIONManylargelandslidesalongriver bank slopesinsensitiveclays have been reported in the literature. Among thedifferent types of large landslides (slide area >1 ha)thatoccurred in the province of Quebecsince 1870, spreadrepresents more than 37% (Demers et al. 2013). Spreadshave some unique failure patterns compare to other typesoflandslides. The traditional limit equilibrium (LE) methodscommonly used for slope stability analysis cannot explainthe mechanisms of this type of failure.Based on post-slideobservation, previous researchersrecognized that a horizontal weak layer/surface is requiredfor this type of large-scale landslide (Odenstad 1951;Carson 1977).The mechanicsinvolvedin formation of thisweak layerarestilldebated. Examiningslope failures insensitive clays,recent studiessuggest that a weak layermight form due to erosion, excavation or small slides nearthetoe of the slope(Locat et al.2008,2011,2013; Quinnet al. 2011). This type of small local failure could create along horizontalshear band (zone of strain concentration)where the shear strength reduces because of strainsoftening behaviour of sensitive clays. The soil mass abovethe horizontal shear band slides above this rupture surfaceand breaks into several blocks at large displacements.The strain softening behaviour of sensitive clay cannotbe incorporated in traditional LE methods.Griffiths andLane(1999)suggested that finite element (FE)method isa powerful alternative approachfor modeling of slopesbecause, unlikeLEmethods, fewer a priori assumptionsare required and the failure occurs naturally through thezones where shear stress reaches the shear strength of thesoil.However,the FEprogramsdevelopedinpurelyLagrangian frameworkcannot handle such largedeformation problems becausesignificant mesh distortionoccurs around the failure planesthat results innumericalinstabilities and lack of convergence.A comprehensivediscussion on FE modeling of large-scale failure of

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