Climatic extremes have exacerbated rainfall-induced shallow landslide hazards in southeastern China. Using the granitic binary-structure slopes in Wuping, Fujian as a case study, this research develops a hydro-mechanical coupled finite element model in COMSOL Multiphysics, incorporating an equivalent continuum model for vegetation roots. This approach reveals the synergistic effects of residual layer thickness, slope angle, rainfall intensity, and vegetation type during sustained rainfall events.

The results demonstrate that geological structures and topographic features nonlinear regulatory effects on slope stability. Specifically, residual layer thickness dictates sliding surface location: thin layers promote penetration into the completely weathered layer, while thick layers exhibit internal sliding within the residual soil driven by intense "water storage" effects. Slope angle impacts are differential: steep slopes trigger easily, whereas gentle slopes harbor larger potential failure scales. Furthermore, as the dominant external trigger, rainfall intensity drives instability by inducing abrupt surges in pore water pressure and rapid declines in effective stress. Root architecture modulates infiltration: heart roots promote interfacial sliding, whereas plate roots drive surface saturation, both facilitating infiltration under extreme rainfall.

By elucidating slope responses and synergistic parameter evolution under multi-factor coupling, this research underpins a theoretical framework for targeted mitigation of regional landslides.

Rainfall-induced shallow landslides,Numerical simulation,Hydro-mechanical coupling,Granitic binary-structure slopes