Shear Strength Characteristics of Sliding Zone Soils and Mechanisms of Luanshibao Long Runout Landslide in Litang County, Sichuan Province, China

High-altitude and long-runout landslides exhibit typical characteristics including slip initiation at high altitudes, rapid sliding speeds, and intense dynamic shearing. These landslides exhibit differences in their long runout mechanisms due to variations in saturation characteristics of loose layers along their sliding path during high-speed sliding. This study examines the Luanshibao landslide in Litang County, Sichuan Province, which has a runout zone composed of saturated deposit. After a comprehensive investigation and analysis of the landslide characteristics, it conducted high-speed undrained ring shear tests at various normal stress levels of 200 kPa, 400 kPa, and 600 kPa using an ICL-2 high-speed ring shear apparatus. The test shearing speed was maintained at 50 cm/s, with a shearing distance of 300 m. The experimental findings indicate that the formation of the sliding zone can be divided into four stages: initial dilation, dilation-negative dilation, unreal dilation, and compression drainage. The pore water pressure increases nonlinearly and has a positive correlation with the normal displacement. When the shear distance reaches 100 m, the pore water pressure increases to more than 50% of the total normal stress. Sliding zone liquefaction caused by particle breakage is the main reason for the reduction in shear strength, leading to shear strength decreases by more than 50% and the pore water pressure increases to more than 70% of the total normal stress. The formation process of the Luanshibao landslide can be summarized as follows: seismic activity triggered high-speed sliding initiation, landslide body fragmentation formed a debris flow, sliding zone liquefaction facilitated long runout movement, and debris flow slowed and accumulated. The research findings could provide a reference for the analysis of the mechanisms of high-altitude and long-runout landslides with water saturated loose layers in their movement paths.