Rapid and long-runout movement of the Nixu rock avalanche in southern Tibet based on large-scale high-speed ring shear tests

The Nixu rock avalanche located in the southern Tibet of China has been described as one of the typical large, rapid and long-runout landslides, and its extraordinary mobility have attracted great attention of numerous researchers. In order to learn the movement mechanism of the Nixu rock avalanche, the large-scale high-speed ring shear tests under drained and undrained conditions were carried out on the saturated substrate materials taken from the accumulation zone of the rock avalanche. The results showed that excess pore-water pressure were generated during the shearing with 2.5 m/s velocity and the maximum excess pore pressure ratio increased up to 0.82 corresponding to the apparent friction angle of 2.15°. The observation and gain size analysis of the shear zone in the ring shear box showed that the high-speed shearing caused the pore-water pressure and fine particle content of the shear zone to increase notably, accompanied by a decrease of the shear stress and effective friction coefficient. The variations could be attributed to the grain crushing, the migration of fine particles and water to the shear zone during the high-speed shearing, resulting in the shear zone liquefaction. This study indicates that the “sliding-surface liquefaction” has played a significant role in promoting the rapid and long-runout movement of Nixu avalanche debris.