Unsaturated Hydraulic–Mechanical Coupling and Stability Analysis under Combination of Rainfall and Reservoir Water Level: A Case Study of the Rongsong Deposit Slope at the Rumei Hydropower Station

The Rongsong deposit slope presents a significant risk of instability because of its large deposit volume and unfavorable inclination, threatening the normal operation of the Rumei hydropower station. To reveal the deformation mechanism, stability evolution, and dominant adverse factor of the landslide under rainfall and reservoir water level (RWL) fluctuation conditions, an unsaturated hydraulic–mechanical coupling and stability analysis method is developed by integrating the explicit finite-difference method and the finite-element method, and validated. The impacts of rainfall, RWL fluctuation, and their combination on the evolution of the landslide pore pressure, displacement, and stability are analyzed. The results indicate that rainwater infiltrates downward along the extension of the interlayer, primarily affecting the displacement of shallow layers and slightly reducing landslide stability. As the RWL declines, the settlement zone expands downward, and the maximum uplift magnitude within the zone increases, with the RWL decline presenting an increasing impact on landslide deformation. When the RWL drops to 2,832 m, the factor of safety of the landslide decreases to 1.02, with the sliding surface within the shallow interlayer exhibiting a high risk of connectivity. Under combined conditions, although infiltrated rainwater mitigates the reduction in pore pressure induced by the RWL decline, slope displacement is predominantly governed by the RWL decline and rainfall significantly intensifies shear sliding. Overall, the landslide stability is also primarily influenced by the RWL decline, with rainfall exerting only a limited impact.