Effects of undrained shear strength profile on retrogressive landslides in sensitive clays

Abstract

The undrained shear strength of normally and lightly overconsolidated clays generally increases with depth, which could significantly influence the failure of a slope. Large-scale landslides in sensitive clays generally occur by a successive failure of soil blocks, and the maximum depth of the sliding plane of these failure blocks typically reaches the level of the toe or slightly below the toe. The traditional limit equilibrium (LE) methods cannot model the progressive failure, and the typical Lagrangian-based finite element (FE) modelling technique cannot simulate the complete process of sensitive clay landslides because of significant mesh distortion around the failure planes. Eulerian-based FE simulations are performed in this study to investigate whether the initial undrained shear strength profile and strain-softening could be the cause of failure observed in sensitive clay landslides. To investigate the effects of the initial shear strength profile, the analyses are performed first for linearly increasing shear strength with depth for elastic perfectly plastic soil (i.e., no softening) with a relatively stronger soil layer below the toe. It is found that the shear strength gradient and depth of the stronger soil layer below the toe affect the location of the critical failure plane, the factor of safety, and the strength reduction factor in finite element analysis. For sensitive clay landslides, in addition to the depth and strength increase in the stronger clay layer below the toe, the rate of softening affects the failure plane formation and landslide extent. Higher strength and slower softening rate could be the cause of shallower failure of the soil blocks and a larger retrogression distance.