Petter Fornes

Effect of Strain-Softening in Design of Fills on Gently Inclined Areas with Soft Sensitive Clays

The effect of strain softening in geotechnical design of fills in areas with soft sensitive clays is studied by a large number of finite element analyses. The reduction in undrained shear strength with increasing shear strain after the peak value will reduce the maximum fill height before failure compared with a perfectly plastic material. The finite element program Plaxis together with the material model NGI-ADPSoft are used in this study. A non-local strain formulation is used in NGI-ADPSoft to overcome the crucial problem of mesh dependent results typical for this type of problems. The effect of brittleness is then fully controlled by input parameters. The material properties are taken from NGI’s database of undrained shear test results on high quality block samples. The effect of strain-softening is quantified by establishing a scaling factor Fsoftening that gives the ratio between the calculated capacity without and with the effect of softening. The purpose is then that the peak undrained shear strength of sensitive clays simply can be divided by this factor before used in conventional limit equilibrium analyses with a strain independent (perfectly plastic) assumption to indirectly account for the effect of brittleness.

Correction Factors for Undrained LE Analyses of Sensitive Clays

Correction factors to be used in conventional undrained stability calculations in order to account for post peak strain softening behaviour of sensitive clays, has been recommended based on an extensive sensitivity study with advanced finite element simulations. It is found that a correction of the material factor is preferred compared to a reduction of the shear strength. The input parameters to the sensitivity study that had the highest correlation with the required correction factor were the shear strength increase with depth and the brittleness, which is the rate of shear strength reduction with strain. In a large block sample database of sensitive Norwegian clays, there was no clear correlation between the brittleness and the sensitivity. Hence, classification of the clays based on the sensitivity is not recommended for evaluating the effect of strain softening on the capacity.

Effect of Strain Softening Behaviours on Run-Out Distance of a Sensitive Clay Landslide

Reliable prediction of landslide triggering threshold and landslide run-out distance is essential for hazard risk assessment. The paper focuses on studying slides in sensitive clays, which represent a major geohazard in many countries including Norway, Sweden and eastern Canada. Large deformation finite element (FE) analyses were performed using the Coupled Eulerian-Lagrangian (CEL) method in Abaqus, which allows for capturing of the full progressive failure mechanism (initiation, propagation and breakoff) involved in a sensitive clay slide. The 1984 slide in Vestfossen, Norway, was chosen as problem case of progressive failure in sensitive clay to be back-calculated by using the CEL FE-model. It is found that the failure mechanism predicted by the FE-analysis agrees reasonably well with the historical failure mode observed at Vestfossen. A parametric study has been performed on the remoulded shear strength as well as the rate of strain softening of the sensitive clay in order to evaluate their effects on the landslide run-out distance.