Rainfall-induced instabilities in highly permeable earthen slopes typically originate at the slope toe; however, the triggering mechanism remains unclear. In this study, we captured the initial microscopic deformations and the overall macroscopic progressive damage of slope instability, extracted the stress paths and contact force chains of soil particles in different parts of the slope before and after rainfall, and revealed the triggering mechanism of soil slope instability induced by rainfall by conducting model tests and utilizing CFD-DEM (computational fluid dynamics-discrete element method) fluid-structure coupling numerical simulations. Our findings revealed that the slope toe exhibits stress concentration prior to rainfall and is a sensitive area of the entire slope before rainfall. After rainfall, rainwater infiltrates, and the seepage rate is the highest near the slope toe. The force-chain arch formed by the large particles at the slope toe, which play the role of the skeleton, is gradually weakened. The essence of rainfall-induced soil slope failure lies in the gradual erosion of the stable contact force chains between soil particles at the slope toe by seepage forces, leading to a progressive weakening, fracture, and disappearance from the outside inward in a collective movement. Once the failure of the slope toe is triggered, the damage area of the inter-granular contact force chains is significantly larger than the displacement plastic zone (or shear band), and the stress in the soil near the slope rapidly transitions from high to low. Subsequently, as the soil particles continue to slip and roll, the soil stress fluctuates and gradually increases, forming a stress-concentrated force chain arch at the rear edge of the slip surface highlighting the slope's certain self-stabilizing capability after failure. Throughout the process, the stress path at the foot of the slope is the longest.

Conventional CFD (Computational Fluid Dynamics)-DEM (Discrete Element Method) coupling methods encounter apparent difficulties in addressing the large deformation exhibited by soils with arbitrarily shaped fluid domains for undrained triaxial shear tests with flexible membranes. Herein, a novel CFD-DEM coupling method is proposed to address the main challenges of dynamically reproducing complex external boundaries and mapping for fluid fields. The workflow of surface mesh construction, mesh coarsening, and internal volume division is proposed to generate required meshes. The mapping of fluid information between updated and original meshes is implemented by a distance-weighted interpolation strategy. The coupling method is subsequently applied to investigate the effect of flexible membranes with and without clamped ends on undrained triaxial shear characteristics of soils after its comparison to the constant volume method for validation. The flexible membranes without clamped ends are proven to delay the shear dilation and weaken the inter-particle contact force. Moreover, they enable the free development of the shear band and induce significant octahedral shear strain at both ends of the band. The fluid pressure distributions of both boundary types are uniform and a vortex-shaped velocity field for the fluid is obtained due to the effect of the particle-fluid interaction.