Deformation and Failure Mechanism of Large Ancient Landslide Induced by Wenchuan Earthquake in Front of Zipingpu Hydraulic Dam

To make clear why nearly all large ancient landslides kept stable in Wenchuan earthquake, except for the collapse and deformation of steep slope areas and the boundary of the steep and gentle slope areas, a large ancient landslide was explored, which locates at the left riverbank in front of the dam of Zipingpu. Based on the monitoring data and numerical simulation, the deformation characteristics as well as the strain and stress distribution of the talus slope were analyzed. The results indicate that the maximum displacement of the slope under the earthquake reaches 150 mm, which locates on the shoulder surface of Dengzhanping plateau. The earthquake has a significant influence on shear stress in the ancient landslide, resulting in the stress concentration on the sliding surface in the vicinity of free surface in the front of the Dengzhanping plateau. The deformation and failure mode of the large ancient landslide largely depends on the surface condition between rock bed and overlying talus and the micro-landform. When the overall topographic gradient of the Dengzhanping ancient landslide is 20, and the inclination of rock-soil boundary is about 13, Wenchuan earthquake only led to collapse on the boundary of the steep and gentle slope areas, but not induced the overall instability of the ancient landslide.