A multi-scale hydro-mechanical framework for assessing rainfall-induced instability in fractured slopes

Tunnel entrance slopes in mountainous regions are highly susceptible to rainfall-induced instability due to excavation-induced fractures, which accelerate infiltration and weaken slope materials. However, existing models often lack accuracy in capturing fracture-governed hydro-mechanical interactions. This study proposes an advanced coupled hydro-mechanical model integrating random fractures, surface runoff (shallow water equations), and subsurface infiltration (Richards' equation). A 2D random fracture model and a 3D explicit fracture model were developed to quantify the influence of fracture networks on pore water pressure and slope failure, while simulating surface–subsurface flow interactions under rainfall. A local factor of safety (LFS) method is used to assess rainfall–fracture–slope interactions. Sensitivity analyses show that increases in fracture width and depth markedly reduce the average LFS, while ponding has a more severe destabilizing effect than rainfall infiltration. When the mean fracture width is below 0.11 m, additional fracture depth has little effect on shallow instability. Results show that fractures intensify pore pressure buildup and stress redistribution, reducing LFS and aggravating instability. The 3D model reveals effective saturation above 90 % in fractured zones and over 30 % reduction in LFS, compared to unfractured conditions. Simulated failure zones (∼50 m long, 5.6 m deep) closely match observed landslides, demonstrating strong predictive accuracy and engineering relevance. Based on the model outcomes, mitigation strategies such as fracture sealing and targeted grouting were proposed to improve slope safety near tunnel entrances. These findings offer practical insights for disaster prevention during tunnel construction in fractured, rainfall-prone terrains.