Dynamics of the 2008 earthquake-triggered Wenjiagou Creek rock avalanche, Qingping, Sichuan, China

• We investigate dynamics and transport mechanism of Wenjiagou Creek rock avalanche.
• Dynamic course is divided into three segments: the failing, the falling and the flow.
• Structure of the deposit was determined by grain-size distribution analysis.
• Ring-shear tests were conducted to simulate the shear between grains within debris.
• Shear resistance between grains decreases due to pervasive fragmentation.

The Wenjiagou Creek rock avalanche was triggered by the 2008 Wenchuan earthquake. It traveled 4.6 km and left a granular deposit of 1.5 × 108 m3 in Wenjiagou Creek valley. The dynamics of the rock avalanche were studied through field and laboratory investigations and remote sensing. The dynamic course was divided into three segments: (1) Failing — the original slope failed due to the huge earthquake ground accelerations acting on a rock mass weakened by severe karstification; (2) Falling — the rock mass experienced several drops due to abrupt changes in slope, and broke into granular debris during collisions with the ground and mountain flanks; and (3) Flow — the broken granular mass flowed approximately 3 km down the valley of Wenjiagou Creek. The transport mechanism of the granular mass of debris was studied by analyzing the structure of the deposit and through ring-shear tests. A gully incised into the deposit by rainfall runoff and debris flows enabled the internal structure of the deposit to be examined and sampled. In cross-section, the exposed deposit was finest near the base, and gradually became coarser towards the surface (inverse grading); in longitudinal section, deposits near the base and on the surface both became finer with increasing travel distance. Near the base, the granular debris experienced the maximum normal and shear stresses, and fragmented into the finest grains. Both normal and shear stresses between the grains gradually decreased from near the base to the surface. Samples were sheared in a ring-shear apparatus to simulate the shearing within the granular debris. Shear resistance between grains was found to decrease as grains became finer due to pervasive fragmentation, which is one of the possible mechanisms by which the granular debris was able to travel a long distance at a high speed.