Initiation and motion of rainfall-induced loose fill slope failure: New insights from the MPM

Loose fill slopes are prevalent worldwide, and their failure during rainstorms is frequently documented. While existing studies have primarily focused on the initiation of such failures, the post-failure motion of rainfall-induced loose fill slope failures has rarely been explored. This study addresses this knowledge gap by investigating both the initiation and subsequent motion of rainfall-induced loose fill slope failures. To achieve this goal, a hydro-mechanical coupled MPM model was utilized to back-analyze the catastrophic 1976 Sau Mau Ping landslide in Hong Kong and conduct parametric studies. From an engineering perspective, the contractive behaviour of loose coarse-grained soil, which induces positive excess pore water pressure and leads to Bishop's stress reduction and a drop in strength, is a major factor contributing to this landslide. The entire failure process can be classified into three phases with different failure modes: local slide, global slide, and flow-like slide, closely related to the soil stress path. The computed results closely match the field measurements on various aspects, including the landslide zone, mobilized volume, and runout distance. The parametric studies reveal that the landslide zone, mobilized soil volume, and final runout distance decrease with a lower value of dilation angle and a smaller critical state plastic deviatoric strain. Conversely, in the case of a constant SWRC, there tends to be an overestimation of these parameters. It is therefore important to consider soil contraction and its influence on hydro-mechanical behaviour.