Abstract: The large-scale landslides, widely distributed in southwest mountains and canyons, are prone to produce instability due to rainfall. In Gushui hydropower station，not only is the extremely large volume of Zhenggang large landslide accumulation body up to more than 4 750×104 m3, but also the thicknesses of many ultra-deep landslide are over 50 m. Under the influences of the heavy rainfall in 2008, the landslide started to deform again and even more seriously. According to the results of field engineering geological investigation and analysis of geologic structure characteristics and hydrogeology conditions, the mechanism of rainfall infiltration and stability in the current situation was qualitatively analyzed first. Then the whole landslide was transfixed slipping surface in the state of creep deformation; so it should be treated such as excavating and reinforcing. Combined with the analysis result, quantitative study of the seepage properties of landslide showed the dynamic sliding mechanism that water infiltration caused deformation, evaluated the stability and treatment, thus proposed effective drainage measures before and after the treatment under the rainfall. The results also showed that the saturation line and backwater was formed due to saturated soil by infiltration. Before the treatment, the backwater was seriously persisted in local landslide and slip surface; and the worst stability came out 4 days later after the rain stopped; and the maximum thickness of backwater increased up to 6 m. However, with backwater reduced obviously, the stability was significantly increased by the treatment. The results truly reflected the situations and laws of the stability of landslide, which were consistent with the results of field exploration. The results were significantly improved by the treatment. Drainage measures proposed would keep the landslide’s stability efficiently. The research results will be useful and meaningful in the same projects as reference.

Key words: hydropower station, large-scale landslide accumulation body, rainfall, unsaturation, seepage properties, stability, drainage measure