Progressive Deformation Mechanism and Stability Analysis of a High-Fill Expansive Soil Slope

Background: To elucidate the hydraulic response, progressive deformation, and wetting-induced swelling effects of high-fill expansive soil slopes throughout the rainfall–cessation process. Methods: A high-fill expansive soil slope in Jianshui, Yunnan Province, China, was selected as the study case. A three-dimensional coupled saturated–unsaturated seepage and wetting-induced swelling model was established to simulate the evolution of pore-water pressure, displacement, maximum shear strain increment, and factor of safety during 24 h of continuous rainfall followed by 48 h without rainfall. Results: During rainfall, the shallow part of the slope exhibited a pronounced pore-water pressure response. Displacement and shear strain were mainly concentrated in the middle and lower portions of the slope, near the platform transitions, and around the slope toe. Throughout the rainfall–cessation process, the factor of safety generally exhibited a decreasing trend. Wetting-induced swelling further intensified slope deformation and shear strain concentration. At 72 h, the maximum total displacement increased from 92.5 mm without considering wetting-induced swelling to 139.0 mm when wetting-induced swelling was considered, representing an increase of approximately 50.3%. After rainfall ceased, pore-water pressure and displacement remained essentially stable when wetting-induced swelling was neglected. When wetting-induced swelling was considered, matric suction increased during approximately the first 0–9 h after rainfall cessation, whereas slope displacement continued to increase after the end of rainfall and gradually stabilized only after approximately 9 h. These results indicate that the hydraulic state and deformation of the slope continued to adjust after rainfall cessation. Conclusions: Wetting-induced swelling not only increased shear deformation and reduced slope stability but also altered the post-rainfall displacement response, causing deformation adjustment to persist after rainfall cessation. Neglecting wetting-induced swelling, or evaluating slope stability solely at the end of rainfall, may therefore underestimate the actual slope deformation and overlook the continued evolution of pore-water pressure and displacement, ultimately leading to an underestimation of the final deformation magnitude and potential instability risk.