Two-layer material point method for modeling soil–water interaction in unsaturated soils and rainfall-induced slope failure

A two-layer hydro-mechanically coupled material point method has been formulated in the framework of unsaturated soil mechanics. The method employs two layers of material points to represent coupling effects between the solid and fluid phases. Several numerical examples demonstrate unique features of the method, including free water infiltration into the unsaturated soil, dynamic soil–water interaction in an example of underwater soil collapse and initially saturated soil column collapse. Furthermore, rainfall-induced failure in unsaturated soil slopes is analyzed. The proposed method can well capture change in matric suction, development of shallow and deep-seated shear bands, and finally, large-deformation post-failure behavior. Parametric studies also demonstrate soil cohesion, dilatancy and friction angle play significant roles on the slope failure mechanisms.