Progressive deformation mechanism of colluvial landslides induced by rainfall: insights from long-term field monitoring and numerical study

Colluvial landslides develop in loose Quaternary deposits, with deformation generally being progressive and crack development dominant in the sliding mass surface layer. With the Tanjiawan landslide in the Three Gorges Reservoir (China) as a case study, field investigations, deformation monitoring, and groundwater level monitoring data were integrated to analyze the landslide deformation characteristics and elucidate the influence of cracks on its deformation. We used numerical simulations, including the finite element and discrete element methods, for investigating the progressive deformation mechanism of rainfall-triggered landslides in the accumulation layer and predicting the failure process. The results indicated that crack formation instigated a preferential seepage channel in the shallow layer of the sliding mass, rainfall infiltration along cracks generated water pressure, and the landslide gradually morphed from a stable into a “step-like” progressive deformation state. Preferential flow inside the cracks effectively elevated the groundwater level within the landslide, and either the number or depth of cracks significantly affected the groundwater seepage field, thereby influencing slide stability. Geological conditions controlled the deformation and failure processes of each landslide section. The uplifted bedrock on the right side blocked the sliding process of the rear sliding mass, and the middle and front sliding masses moved faster but the sliding distance was shorter. The deformation trend is deformation, crack formation, preferential flow occurrence, crack extension, and deformation. The ultimate cause of failure was a steep rise in groundwater level following short duration heavy rainfall or long duration light rainfall.