GEOMECHANICAL MODELING OF THE STEINERNASE LANDSLIDE Alessio Ferrari, Lyesse Laloui and Christophe Bonnard

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1 GEOMECHANICAL MODELING OF THE STEINERNASE LANDSLIDE Alessio Ferrari, Lyesse Laloui and Christophe Bonnard Ecole Polytechnique Fédérale de Lausanne Steinernase, 13 May 8

2 CONTENTS Main features Analysis of the multisurface mechanism Assumptions of the geomechanical model Constitutive parameters of the involved materials Computation results: model calibration and validation Model response to sensitivity analysis Conclusions Steinernase, 13 May 8

3 Images by Google Earth

4 Main features Active area: ~1 km 2 Mean inclination: 2 Mean depth: 1 15 m Movement rates: 12 cm/y Images by Google Earth

5 Identification of active zones Analysis of displacements recorded in the period and observations in the field Kantonsstrasse Inclinometer Topographic control point 2662 Rhine River Highway E6 Railway y coordinate (m) 266? 32? 2658? Road Sliding mass limits 2656 Flue x coordinate (m) 4

6 Analysis of the multisurface mechanism Inclinometer profiles clearly show the existence of a multisurface failure mechanism. y coordinate (m) Kantonsstrasse Highway E6 Railway B3c B3e Inclinometer Topographic control point Rhine River 32 Depth (m) B3f B3e B3c Displacement (mm) Depth (m) Displacement (mm) Displacement (mm) Depth (m) 15 B3f Road? Flue x coordinate (m)? Inclinometer KB7 null measurement: measures from to

7 Analysis of the multisurface mechanism Inclinometer Topographic control point 4 Kantonsstrasse y coordinate (m) Railway Highway E6 C B Rhine River A 32 4 Elevation (m) 35 3 K.strasse Highway Railway B3d pr. B2/ B3/ B6/ B24/ Flue C? 5 B x coordinate (m) A? 4 4 Road Elevation (m) B3e B3f/95 B2/6 Section A 5 B3c B3/86 Distance (m) B7 B5/ Elevation (m) B1/ Section B B3b/95 Distance (m) B3a/95 The shape of the different failure surfaces has been identified in different 2D sections combining all available inclinometrer readings for the period Landslide body Section C Distance (m) Alluvium Bedrock

8 3 Elevation (m) 35 B7 B3 B3c B3e Kinematics of the multisurface mechanism 25 Section B Distance (m) 8 B3c, 3m Displacement (mm) B3e, 3m B3e, 9m B3c, 13m B3, 1m B7, 3m Displacement trends ( ) showing concentrated accelerations Displacement (mm) 2 1 Time (days) B3c B3e Relative displacements of the upper moving mass with respect to the central part of the landslide body

9 Hydromechanical coupling Total head (m) Total head at B3c, 1m Displacement at B3c, 3m Displacement (mm) Time (days) Total head and displacement registered at borehole B3c during the period 4/7/ /12/26

10 Hydromechanical coupling As pore water pressure variations within the landslide mass seem to be the main driving forces of the movement, a coupled hydromechanical approach has been performed. Hydrogeological modelling (Tacher et al.) Water boundary conditions FE : Hydromechanical modelling H H M Mechanical boundary conditions Constrained displacements Neglected dsr n β fsr + tp + div turf + Sr div tus = dp n = div cst u = t s ( : ) div D u + S grad p + ρ g= t s r

11 The hydrogeological model Geolep/EPFL Conceptual layout Cross section showing layers with different permeability values Planar view of the 3D hyrogeological model

12 1 st stage The analysis of the landslide morphology and of the displacement field leads to the conclusion that a 2D model is suitable for the study of the landslide behaviour The choice of the most representative 2D section has been performed according to the following criteria: Detailed topographic and geological information is available (see Geotest Bericht Nr. Z615.1/26) The section includes most of the zone where displacement rates are higher The section is well situated with regards to the available instrumentations The profile is close to the piezometer KB 3c/95 used for the calibration of the hydrological model 2 nd stage A 3D model has been also developed to test model results at a global landslide scale Kantonsstrasse Inclinometer Topographic control point B6/ y coordinate (m) 266 Railway Highway E6?? Rhine River 32 Elevation (m) 35 3 B7 B3/86 B3c B3e B3f/95 B2/6 Road Sliding mass limits? 2658 Flue x coordinate (m) Distance (m)

Mesh Different mesh solutions have been tested 1 Structured mesh with 958 isoparametric Q4 elements (168 nodes), no slip surface included 2 Structured mesh with 219 isoparametric Q4 elements (2354

13 Mesh Different mesh solutions have been tested 1 Structured mesh with 958 isoparametric Q4 elements (168 nodes), no slip surface included 2 Structured mesh with 219 isoparametric Q4 elements (2354 nodes); slip surfaces are included as an homogeneous material 3 Unstructured mesh with 1554 isoparametric Q4 elements (1694 nodes) three independent slip surfaces with different constitutive parameters smooth transition between finer and coarser mesh portions ratio between element sides always < 1/5 to keep satisfactory accuracy Landslide body + Alluvium + Bedrock + 3 Slip Surfaces = 6 Materials

14 Boundary conditions Pore water pressure imposition at each node at each computation instant Fixed total head for the nodes on the left edge (28 m, Rhine mean level) Fixed nodes for the domain bottom Seepage elements to prevent indefinite pore water pressure increasing by pounding Active state for the lateral pressure at the right limit (as a result of a parametric study)

15 3D Geomechanical model landslide body Topographic and bedrock surfaces deepest slip surface Alluvium Cross section of the 3D model 3D mesh (1 8 nodes, 7 28 elements)

16 Material properties A complete geomechanical characterization of the involved materials is not available. For the landslide body: index properties, 5 triaxial tests and 3 oedometric tests on specimens coming from boreholes S28, S26, S25 and S34, localized at the slope toe index properties and 3 series of direct shear tests on specimens coming from boreholes B3c, B3e and B3f In spite of some heterogeneities in the index properties at a local scale, the soil seems to present homogeneity at a landslide scale no characterization of the behaviour of the material in partial saturation conditions No mechanical analysis for the alluvium and the bedrock Values for uncharacterized model parameters were assessed in the calibration process Deviatoric stress, q (kg/cm 2 ) S28/1983 S28/1981/1 S28/1981/ Mean effective stress, p' (kg/cm 2 ) Vertical strain (%) S26/19228 S25/19355 S28/ Vertical stress (kg/cm 2 ) saturation degree rc from grain size distribution rc used in the analysis. 1.E2 1.E1 1.E+ 1.E+1 1.E+2 1.E+3 1.E+4 1.E+5 suction (kpa)

Constitutive law E E E CC CC CC E (kpa) 6 2 5 ν'.3.3.3 lambda.1.1.1 kappa.5.5.5 c' ϕ (M) 23 (.9) 24 (.

17 Constitutive parameters assumed at the end of the calibration process (93 different combinations have been tested) MATERIAL Landslide body Alluvium Bedrock Sliding surface 1 Sliding surface 2 Sliding surface 3 Constitutive law E E E CC CC CC E (kpa) ν' lambda kappa c' ϕ (M) 23 (.9) 24 (.95) 25.5 (1) ψ Assoc Assoc Assoc γ (kn/m 3 ) e K S res α (1/m)

18 Model calibration and validation Hydrological model calibration for the period 2 21 Pore water pressure distribution are obtained as an output of the calibration Pore water pressure are predicted for the year 26 Geomechanical model calibration for the period 221 Geomechanical model validation for the year 26 8 B3c, 3m Displacement (mm) B3e, 3m B3e, 9m B3c, 13m B3, 1m B7, 3m

19 The hydrogeological model Geolep/EPFL 5 45 Niederschlag Infiltration mm/tag Net infiltration computation Datum

20 The hydrogeological model Geolep/EPFL m.ü.m mm/tag Modell Messung Infiltration Datum Comparison between the measured and the computed total head at B3c (2 21)

21 Model calibration: Simulations of displacements for the years 2 21 Observed and predicted displacements in B3c at two different depths.4 Elevation (m) Distance (m) Displacement (mm) (m) Displacement (mm) (m) days Time (days) B3c,3m B3c,9m days Time (days)

22 Model calibration: Simulations of displacements for the years Jan 1 Mar 1 May 1 Jul 1 Sep 1 Nov 1 Jan 1 1 Mar 1 1 May 1 1 Jul 1 1 Sep 1 1 Nov 1 1 Jan 2 1 Jan 1 Mar 1 May 1 Jul 1 Sep 1 Nov 1 Jan 1 1 Mar 1 1 May 1 1 Jul 1 1 Sep 1 1 Nov 1 1 Jan B6/ B3f/95 B2/6 35 B3e B3c B3/86 B7 3 Elevation (m) Displacement (m) B3c B3e, 3m Pore water pressure (kpa) Time (days) Displacement (m) Pore water pressure (kpa) Compressive pore water pressure suction B3c B3e, 9m Compressive pore water pressure suction Time (days) Di ( )

23 Model calibration: Simulations of displacements for the years Jan 1 Mar 1 May 1 Jul 1 Sep 1 Nov 1 Jan 1 1 Mar 1 1 May 1 1 Jul 1 1 Sep 1 1 Nov 1 1 Jan 2 1 Jan 1 Mar 1 May 1 Jul 1 Sep 1 Nov 1 Jan 1 1 Mar 1 1 May 1 1 Jul 1 1 Sep 1 1 Nov 1 1 Jan B3c, 3m B6/ B3f/95 B2/6 35 B3e B3c B3/86 B7 3 Elevation (m) Displacement (m) 4 8 Pore water pressure (kpa) Displacement (m) B3e B3c, 13m B3e Time (days) Pore water pressure (kpa) Time (days) Di ( )

24 3D Geomechanical model Unit [m] Computed horizontal displacements (for the period 2 21) highlighting the concenration of displacements in the central part of the slope

25 Model validation: Simulations of displacements for the year 26.3 B2_6, 8m Jan 6 1 Mar 6 1 May 6 1 Jul 6 1 Sep 6 1 Nov 6 1 Jan 7 1 Jan 6 1 Mar 6 1 May 6 1 Jul 6 1 Sep 6 1 Nov 6 1 Jan 7 Displacement (m).3 B3c, 13m Pore water pressure (kpa) Displacement (m) Pore water pressure (kpa) Time (days) Time (days) B6/ B3f/95 B2/6 35 B3e B3c B3/86 B7 3 Elevation (m) Di ( )

26 Sensitivity analysis Model sensitivity on changes in pore water pressure distribution and shearing resistance parameters has been tested.4 1 Jan 1 Mar 1 May 1 Jul 1 Sep 1 Nov 1 Jan 1 1 Mar 1 1 May 1 1 Jul 1 1 Sep 1 1 Nov 1 1 Jan 2 Displacement (m) Displacement (m) original pwp pwp increased of 1% Time (days) Jan 1 Mar 1 May B3c, 13m 1 Jul 1 Sep 1 Nov 1 Jan 1 1 Mar 1 1 May 1 values used in the simulations 1 Jul 1 1 Sep 1 1 Nov 1 reduced shearing resistance parameters Time (days) 1 Jan 2 Response of B3e,3m to an increase of 1% of the pore water pressure distribution Changes in shearing resistance parameters for the sliding surfaces in the range (±1.5 ) do not affect significantly the displacement distribution

27 Possible solutions: subhorizontal drainage system 4 B6/ Elevation (m) 35 3 B7 B3/86 B3c B3e B3f/95 B2/ Distance (m) Decrease the pore water pressures at the slope toe below the deep seated sliding surface by means of a drain system: length 5m, inclination 3

28 Possible solutions: subhorizontal drainage system Drainage system in the hydrogeological model: reduction of the total head for the period 2 21 (, blue 16 m, red) (Geolep/EPFL)

29 Possible solutions: subhorizontal drainage system 4 B6/ Elevation (m) B7 B3/86 B3c Distance (m).4 1 Jan 1 Mar 1 May 1 Jul 1 Sep 1 Nov B3e 1 Jan 1 1 Mar 1 B3f/95 B2/6 1 May 1 1 Jul 1 1 Sep 1 1 Nov 1 1 Jan 2 Decrease the pore water pressures at the slope toe below the deep seated sliding surface by means of a drain system: length 5m, inclination 3 Displacement (m) original pwp with drainage system Time (days) Displacement trend in B3e,3m the with the drainage system

Conclusions Main features 3 infrastructures of relevant importance are involved a complex multisurface failure mechanism is present The geomechanical model proved to be able to reproduce in a

30 Conclusions Main features 3 infrastructures of relevant importance are involved a complex multisurface failure mechanism is present The geomechanical model proved to be able to reproduce in a satisfactory way the behaviour of the slope in terms of accumulated displacements with time acceleration phases relative displacements within the landslide body Displacement are clearly related to pore water pressure changes; in particular the unsaturated zones seem to play an important role for the landslide behaviour The model can constitute the base to investigate mitigation strategies

Numerical modelling of the hydro-mechanical behaviour of a large slope movement: The Triesenberg landslide

Numerical modelling of the hydro-mechanical behaviour of a large slope movement: The Triesenberg landslide B. François*, Ch. Bonnard*, L. Laloui*, V. Triguero* *Soil Mechanics Laboratory, Ecole Polytechnique