Initiation of a large landslide triggered by Wenchuan earthquake based on ring shear tests

A great number of landslides were triggered during the 2008 Wenchuan earthquake. Among them, the Niumiangou landslide is the large-scale landslide in the epicenter area. The site investigation shows that the landslide mass collides to the NE side of the valley immediately after initiation. The moving velocity estimated is 19.8 m/s, suggesting the characteristics of high initial velocity. The materials are taken from the source area and a series of ring-shear tests are conducted. The results show that the materials have a high liquefaction potential under undrained condition. They are easy to liquefy and the apparent friction angle is only 9.4° under undrained cyclic loading condition. The liquefaction of the materials in sliding zone during earthquake may cause the initiation with high velocity of Niumiangou landslide. Through an energy approach, the dissipated energy in cyclic loading tests and the possible energy dissipated to the soil layer in the slope by the earthquake are estimated. The peak acceleration for triggering sample failure is 192 gal, and the energy for sample liquefaction is 2.3×10⁴ J/m². It is inferred that the possible seismic energy that can be dissipated to initiate the slope failure on the source area can be much greater than the value required for the initiation of liquefaction failure. The slope instability might have been occurring several seconds after the arrival of seismic motion.