Abstract: According to slope structure analysis and simulation using finite difference method, changes of stress and strain in the Tanggudong slope during the down-cutting of Yalong River valley in the history were studied. The deformation mechanism of slope with structural planes dipping outside in the slope foreside can be summarized into three stages (unloading tension crack, sliding tension pressed crack, and tension pressed-shearing). According to the mechanism analysis and simulation, the locked section of rock mass in the upstream residual deformation slope will show strong stress concentration and severe plastic failure under the impounding of downstream dam site of Lenggu hydropower project and earthquake condition. That can lead to high-speed landslide of the whole slope.

Key words: Lenggu hydropower project, Tanggudong landslide, slope structure, deformation mechanism, structural planes dipping outside slope, locked section