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.