Most of the natural and compacted fine-grained soil slopes that are in saturated or unsaturated condition undergo a large deformation prior to reaching failure conditions. Such slopes should be designed taking account of their strain-softening behavior using the residual shear strength (RSS) parameters. In this paper, the slope stability of a recently reactivated Outang landslide near the Three Gorges Dam in China is analyzed based on theRSSparameters of unsaturated soils. In addition, comparisons are provided in theFOSvalues of slope using both the peak shear strength (PSS) andRSSparameters. Firstly, a series of site investigations of the hydrologic and geologic conditions, ground surface displacements and cracks were described. ThePSSandRSSbehaviors of the sliding soils derived from a series of direct shear test results performed on saturated and unsaturated soil specimens are summarized. Secondly, a series of slope stability analysis were conducted considering the precipitation and Yangtze River water level variation within a representative period of 7 months, based on thePSSand theRSSproperties. In this study, three different scenarios were considered, which include: i) considering only the precipitation with a constant water level; ii) considering only the decrease in water level without rainfall; iii) considering the combination of precipitation and decrease in water level. In each scenario, four steps were included to calculate the values of factor of safety (FOS) at different times. 1) A steady-state seepage analysis was conducted with a constant total head at 525 m on the left boundary and 175 m on the slope surface below the Yangtze River water level. The initial pore water pressures were simulated in the slope under no precipitation and variation of water level. 2) A specific boundary condition was applied on the slope surface to model the precipitation and Yangtze River water level variation. A transient seepage analysis was conducted to calculate pore water pressures at different times based on the initial pore water pressures. 3) TheFOSvalues at different times were calculated by the Morgenstern-Price method taking account of the variation of pore water pressures at different times, using the peak shear strength (PSS) parameters. 4) The last step was repeated replacingPSSparameters withRSSparameters. TheRSSparameters were lower than the peak values from laboratory’s direct shear test results for the soils in the sliding zones. The reduction in shear strength from peak to residual state under unsaturated soil condition was greater than that for a saturated soil. TheFOSdecreased almost linearly with time for the scenario in which only the influence of rainfall infiltration was considered. However, the total reduction in theFOSwas relatively small. TheFOSdecreased rapidly at a linear rate with respect to time with a decrease in water level for the scenario in which Yangtze River water level decrease was considered. TheFOSreached to a relatively constant value after Yangtze River water level reached the lowest value. The decrease in Yangtze River water level was the dominant factor that contributed to a reduction in theFOS. TheFOSwas strongly dependent on the development of the phreatic line after the Yangtze River water level reached the lowest value. TheFOScalculated byRSS(i.e.FOSR) is less than unity; they were approximately 16% lower in comparison to that calculated byPSS(FOSP). IfPSSparameters were used, the slope would still be stable even under the combined influence of precipitation and Yangtze River water level decrease. These results are inconsistent with the field observations. For this reason, theRSSparameters should be taken into account to evaluate reliably the slope stability of the Outang landslide.
