Abstract:  ring-shear test; residual strength; slide zone soil; Huangtupo landslide Slide zone soil of Huangtupo landslide is studied in detail as a typical example in the Three Gorges Reservoir. Three types of shear methods, including single-stage shearing, pre-shearing and multi-stage shearing, are conducted to investigate the residual strength of slide zone soil by employing ring-shear apparatus. It is conducted that the formation of shear band of slide zone soil is relevant with the shear displacement; the residual strength increases with the increasing of effective normal stress. For slip soil sample with a pre-existing shear band, residual shear strength can be soon and easily reached; initial dilatancy phase, contraction phase due to grain migration and constant contraction phase make up shear settlement curves of different ring-shear tests and the changing trend of shear stress in each stage is different. The residual strengths obtained from pre-shearing test and multi-stage test are larger than the actual value; the single stage shearing should be the preferred approach to acquire residual strength of slide zone soil.

Key words: ring-shear test, residual strength, slide zone soil, Huangtupo landslide