Rainfall-Induced Loose Fill Slope Failure: New Insights from MPM Analysis

Abstract

Loose fill slopes are widespread globally, and their failures during rainstorms are frequently reported. While previous research has primarily concentrated on the initiation of such failures, the post-failure movement has received limited attention. This study presents an advanced Material Point Method (MPM) framework to analyze both initiation and post-failure behaviour of loose fill landslides. The model achieves hydro-mechanical coupling by incorporating non-linear strain-softening during undrained shearing through soil contractive behaviour, and accounting for soil density effects on hydraulic properties. The proposed MPM model was used to back-analyze the 1976 Sau Mau Ping landslide in Hong Kong and conduct parametric studies. Results show that contractive behaviour in loose coarse-grained soils generates positive excess pore water pressure, reducing Bishop’s stress and soil strength, which is critical to landslide initiation. The model’s predictions closely align with field observations. The failure process is divided into three phases: local slide, global slide, and flow-like slide, each with distinct stress paths. Parametric studies reveal that lower dilation angles decrease landslide extent and runout, while assuming a constant soil-water retention curve (SWRC) can overestimate these factors. These findings highlight the importance of considering soil contraction and its effects on hydro-mechanical behaviour in landslide analysis.