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.