Activity response of a landslide with complex structure to fluctuation of reservoir water level:a case study of the Chaiwan landslide in the Three Gorges Reservoir

The Chaiwan landslide is a large and complex landslide in the Three Gorges Reservoir area. The landslide is composed of three kinds of materials and develops three slip zones along the contacts among the materials and along the contact between the landslide and the bed rock. The materials of the landslide from top to bottom are very dense coarse soil with some silt and clay with low hydraulic conductivity, dense and coarse soil with higher hydraulic conductivity and very loose cataclastic rocks with the highest hydraulic conductivity, respectively. Reactivation of the landslide was observed since the operations of the Three Gorges Reservoir. Response of the Chaiwan landslide to water level fluctuation of the Three Gorges Reservoir is explored in this study based on the structure of the landslide and field monitoring data from 2007 to 2015 Variation in pore water pressure within the landslide during filling and drawdown of the reservoir was simulated using the Seep/W code. The results show that the landslide is under a slow movement with about constant rate and a retrogressive movement pattern and along the upper and middle slip zones during both the filling and drawdown periods of the reservoir, while the movement rate of the landslide suddenly increases when the drawdown rate of the reservoir is greater than 0 8 m/d. The upper and lower parts of the landslide are the most sensitive to the reservoir water fluctuation. The movement pattern of the landslide is due to itself complex structure and great difference in hydraulic conductivity of the materials. The continuously slow movement of the landslide during both the filling and drawdown periods of the reservoir is attributed to the uplift pressure during the filling period occurred within the middle and lower materials of the landslide where the hydraulic conductivity and seepage pressure is high to very high during the drawdown period generated within the upper and middle materials where hydraulic is lower than that of the lower material.