Probabilistic and finite-element method hydro-mechanical analysis of rainfall-induced slope instability

The random field theory coupled with a finite-element method hydro-mechanical model was employed to analyse slope responses under rainfall, with a particular focus on post-rainfall behaviour. This study uniquely analysed the heterogeneity of soil strength and permeability through the application of random field theory, providing a novel perspective on its impact on slope stability. Numerical modelling under varying rainfall intensities revealed that the factor of safety (FoS) decreased with prolonged rainfall, particularly at higher intensities. Post-rainfall behaviour varied for different rainfall events: gentle and moderate rainfalls led to a gradual decline in FoS due to water infiltration, while heavy rainfall allowed recovery through drainage, and extremely heavy rainfall triggered immediate failure. A novel finding is that the post-rainfall water seepage can govern slope stability under extreme rainfall conditions. At failure, the surface layer of a fully saturated slope lost clay suction and exhibited concentrated plastic shear strain at its base. Weak zones with interconnected high-permeability channels significantly influenced slope stability by facilitating water infiltration. Probabilistic analysis further showed that the FoS at the end of simulations typically ranged from 1.1 to 1.4, with a recommended 5th percentile value of 1.1 for embankment slope designs.