Large Strain Analysis of Unsaturated Heterogeneous Slopes with MPM

Identifying stratigraphic heterogeneities, like weak cohesive layers in natural and artificial slopes, represents a key step in understanding instability mechanisms affecting slopes’ safety. These layers’ presence alters saturation and pore pressure distribution, in turn affecting soil strength. The quantification of their impact on failure triggering and subsequent propagation is often missing in current studies. This work investigates the problem with a recently developed Material Point Method (MPM) formulation for unsaturated soils, which can simulate slope failure from initiation to the entire collapse dynamics. A simple geometry accounting for the presence of a weak layer is considered, and two saturation “paths” are tested. One path reflects the effect of rainfall on the slope; the other mimics the rising of the water table induced by an upstream recharge. The evolution of failure captured by the MPM highlights several stages in the post-failure behavior, with initial motion which leads to an apparent dormant phase, followed by a significant reactivation. It is possible to fully quantify the soil masses in motion, thus allowing for a comparison between the two saturation paths effects. Lastly, the same analysis is carried out with the Limit Equilibrium Method (LEM). The critical review of the results allows for understanding the limitations of the technique in capturing the complex phenomenon under analysis, and the advantages introduced by the unsaturated MPM.