Phreatic surface calculation and stability analysis of three-dimensional slopes under rainfall infiltration

Groundwater fluctuations are an important factor in the occurrence of landslides. Rainfall or changes in reservoir levels can cause fluctuations in groundwater within the slope, making the slope susceptible to landslides. This paper proposes a simple method that directly correlates groundwater level and rainfall, enabling the prediction of changes in the three-dimensional (3D) phreatic surface under rainfall infiltration. Based on the solved phreatic surface, we derive the seepage force and the normal stress expression on the sliding surface. A strict overall analysis method is proposed for solving the safety factor of the slope subjected to rainfall. The reliability of the proposed method is established through verification against various scenarios defined by a classic slope model. Furthermore, we investigate the variations in the phreatic surface and the evolution of safety conditions under rainfall infiltration for a project slope located in the Three Gorges Reservoir area. The results demonstrate that the rise in groundwater level due to rainfall infiltration is small and consistent relative to the entire landslide body. The magnitude of groundwater level rise is directly proportional to precipitation volume, while soil porosity above phreatic surface exerts a greater influence on safety factor than permeability coefficient and saturation.