Stability of Shallow Soil Slopes Driven by Concurrent Wetting Front Migration and Surface Runoff

Heavy rainfall infiltration driving the downward movement of the wetting front, combined with surface runoff, often induces large-scale shallow soil slope instability. Consequently, a stability analysis model for shallow soil slopes considering the combined effects of rainfall infiltration wetting front movement and surface runoff is proposed. This model constructs a limit equilibrium analysis framework that accounts for the interactions among wetting front depth, soil saturation, and runoff shear stress. It assumes that shallow soil slope instability follows a “top arc–straight line–bottom arc” combined sliding surface, with soil shear strength adhering to the Mohr–Coulomb criterion. An explicit expression for the stability coefficient of shallow soil slopes is derived using the moment equilibrium equation of the combined sliding surface, and its validity is tested through physical model experiments. The results indicate that: (1) The combined sliding mode of shallow soil slopes can degenerate into classical circular arc and infinite slope sliding modes; (2) As soil saturation increases, the critical sliding depth of shallow soil slopes decreases exponentially; (3) Runoff shear stress significantly impacts shallow soil slope instability. This method provides a reference for early warning and resilient design of rainfall-induced shallow soil slope instability.