Abstract: Landslide disasters frequently occur in the mountainous regions of Southwest China. In recent years, the extensive construction of transportation infrastructure has exacerbated risks associated with landslides. Excavation spoil accumulation and rainfall are primary triggers; however, the instability mechanism of accumulation landslides under rainfall and surcharge loading remains inadequately understood. Taking the Weicheng large-scale accumulation landslide along the Wu-Chang Expressway in Guiyang city as the prototype, this study conducted laboratory physical model tests with a geometric scale of 1:150 to investigate the water content, soil pressure, pore water pressure and displacement of the landslide under rainfall and surcharge loading, thereby revealing its deformation and instability mechanism. The results indicate that graded loading induces stress concentration beneath the loading zone, with the maximum increase in soil pressure reaching approximately 55.9%, and settlements exhibiting a stepwise incremental pattern. In the rainfall phase, soil pressure initially undergoes a slight increase followed by a significant decrease, with the maximum reduction recorded at 93.7%. Meanwhile displacement exhibits a growth trend that is initially sluggish but subsequently accelerates. Additionally, the water content and pore water pressure in shallow soils demonstrate fluctuations in response to rainfall cycles. The accumulation landslide exhibits a failure mode characterized by rearward pushing and front traction. Its instability process can be categorized into three distinct stages: initial stability, toe erosion, and progressive failure. The failure mechanism involves the following sequence: initially, loading induces stress concentration at the rear; subsequently, rainfall infiltration leads to an increase in pore water pressure and a reduction in effective stress; shear strength degradation at the slope toe then results in localized collapse; stress redistribution drives progressive crack propagation and coalescence, ultimately forming multi-level slip surfaces and triggering the progressive failure of the landslide. These findings offer a scientific foundation for hazard assessment, engineering optimization, and risk mitigation of loose accumulation landslides induced by the combined effects of rainfall and surcharge loading.

Key words: accumulation landslide, rainfall, surcharge loading, physical model test, instability mechanism