Interpreting the Influence of Reservoir Water Level Fluctuation on the Seepage and Stability of an Ancient Landslide in the Three Gorges Reservoir Area: A Case Study of the Outang Landslide

The periodic reservoir water level (RWL) fluctuation on bank slopes during water impoundment in Three Gorges Reservoir (TGR) may lead to the reactivation or the failure of ancient landslides, which poses a serious threat to the geological safety of reservoir area. Thus, it is of great practical importance to study the reactivation mechanism of ancient landslides under water level fluctuation conditions. Based on the generalized engineering geological model of the Outang Landslide, the variation rules of the seepage field and the stability of the landslide under different fluctuation rates of RWL are investigated by numerical simulations. The results indicate that: (1) Due to the difference between the permeability of the sliding body and fluctuation rate of the RWL, the variation of the ground water level in the bank slope always lags behind that of the RWL, and the changes and weakening processes associated with water head differences between the inside and outside of the landslide have a significant time effect. (2) During the RWL drawdown periods, the seepage force, caused by the internal higher-water-head and external lower-water-head of the bank slope, decreases the stability of the landslide; in contrast, as the RWL increases, the seepage force, which originates from the external higher-water-head and internal lower-water-head of the bank slope, tends to maintain the stability of the landslide. (3) The safety factor of the landslide increases with the rising RWL and decreases with the drawdown in RWL; moreover, the variation rate and amplitude of the safety factor are positively correlated to the fluctuation rate of RWL.