Rainfall-induced stability of unsaturated tropical slopes in Pagar Alam, South Sumatra, Indonesia: Effects of slope gradient and soil layering
Rainfall-induced slope failure is a recurring geohazard in tropical regions, where intense and prolonged precipitation promotes infiltration, reduces matric suction, and increases pore-water pressure within near-surface soils. This study investigates the effects of slope gradient and soil layering on the stability of unsaturated natural slopes in Pagar Alam, South Sumatra, Indonesia, using soil properties obtained from laboratory testing. Three representative slope geometries derived from terrain data were analyzed under both homogeneous and layered subsurface conditions to assess the combined influence of topography and stratification. Transient rainfall infiltration was incorporated into a numerical framework, and stability was assessed using the Morgenstern-Price limit equilibrium method in GeoStudio. The results indicate that the factor of safety decreases rapidly due to rainfall infiltration. The greater the slope angle, the greater the reduction in the factor of safety. Layered slopes generally produced lower factors of safety than their homogeneous conditions, demonstrating that stratification can reduce slope stability even when the overall failure pattern remains similar. The results further suggest that hydraulic and mechanical contrasts between soil layers influence pore-water-pressure evolution and the distribution of shear resistance during rainfall. These findings highlight the importance of incorporating unsaturated soil behavior and realistic subsurface stratification into slope-stability assessment in tropical environments. The study provides a hydro-geologically relevant basis for improving landslide susceptibility evaluation and supports the development of site-specific monitoring and mitigation strategies for rainfall-prone slopes in Indonesia.