Surficial slope failures in residual soils are prevalent in tropical and subtropical regions due to rainfall infiltration and seepage. While numerous studies have explored rainfall-induced slope stability through the interaction between pore water pressure (PWP) and effective stress, the influence of water gravity on effective stress under infiltration and seepage boundary conditions has often been overlooked. This study introduces a novel coupled PWP and water gravity increase method (PWP–WGIM) model to comprehensively analyze slope stability and failure processes. The proposed model integrates PWP dynamics with the water gravity increase method (WGIM), taking variations in infiltration capacity and seepage boundaries into consideration. Using COMSOL Multiphysics, the governing partial differential equations are numerically solved based on the finite element method with appropriate hydrologic boundary conditions (rainfall infiltration and seepage face boundary conditions). Results indicate that unsaturated soil slopes experience more pronounced changes in effective saturation, PWP, effective stress, and deformation at both shallow and deeper regions when evaluated with the PWP–WGIM model compared to PWP-only models. Furthermore, the safety factor for the PWP–WGIM model is lower, revealing that gravitational effects coupled with PWP significantly reduce slope stability under varying rainfall intensities. These findings highlight the importance of incorporating water gravity in slope stability assessments, providing a more accurate predictive tool for managing risks in regions prone to landslides and erosion.
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