Activity law and hydraulics mechanism of landslides with different sliding surface and permeability in the Three Gorges Reservoir Area, China

The Three Gorges Hydropower Station is the largest hydropower station worldwide and the largest project ever built in China. There are about 2619 ancient landslides affected by reservoir level fluctuation in the Three Gorges Reservoir (TGR) area. More than 670 landslides have been reactivated and eight landslides have failed since the impoundment of the reservoir in June 2003, which seriously threatens people's safety and property. In this study, taking a large number of re-active and failed landslides as statistical samples, the activity and displacements of the reservoir landslide with special regard to different sliding surface and permeability are analyzed and discussed. It is noted that a significant correlation between the landslide movement and the reservoir filling process. The re-active landslides are mainly concentrated in the initial stage of water storage, and the frequency of activation as well as the degree of deformation decrease gradually after the third phase of water storage (175 m). Then the four dominant combinations of the inducing factors of re-active landslide are presented: (a) drawdown of water level, straight line sliding surface, 20–40% degree of immersion, and weak permeability; (b) drawdown of water level, arc sliding surface, 40–60% degree of immersion, and medium permeability; (c) raising of water level, fold line sliding surface, 20–40% degree of immersion, and good permeability; (d) raising of water level, chair shaped sliding surface, 0–20% degree of immersion, and good permeability. Furthermore, when the rate of the reservoir level fluctuation is greater than the permeability coefficient, the shape of sliding surface is from the straight line - the arc - the fold line - the chair shaped, and the hydraulics effect of reservoir water on landslide shifts from the seepage pressure effect to the buoyant effect. Meanwhile, the impact of the reservoir water filling on the landslide reactivation changes from favorable to worse, and the impact of the reservoir water drawdown on the landslide reactivation changes from worse to favorable.