Variation in Residual Strength of the Large-Scale Landslides’ Slip Zones in the Three Gorges Reservoir of China

Since water impoundment of the Three Gorges reservoir, more than 200 large-scale existing landslides have reactivated in the reservoir area. It has been extensively recognized that seepage and uplift water pressure following rise and drawdown of the reservoir water played important roles in reactivation of these landslides, while variation of residual strength of landslides’ slip zones which were immersed by the reservoir water and its contribution to these landslides’ reactivation has been paid little attention. To explore this, residual strength of five large-scale landslides’ slip zones in the reservoir area, which were composed of clayey soils with significant amount of gravel, was experimentally investigated after being soaked for different periods. It was found that a significant reduction in residual strength of the landslides’ slip zones occurred after being soaked for around 30 days, and very minor reduction occurred after being soaked for a longer period. This indicates that reduction in residual strength of their slip zones may contribute to the landslides’ reactivation which occurred during early time of the reservoir’s operation. Particle size analysis and water chemical analysis of the slip zones’ materials after being soaked suggest that residual strength reduction of the landslides’ slip zones may be mainly attributed to slaking of coarse particles, followed by hydration of clay minerals, dissolution of calcite and hydrolysis of feldspar due to soaking.