Stability analysis of a large-scale landslide mass: a case study from Three Gorges Reservoir, China

Abstract

The landslide mass investigated in this study has a volume of about 20.6¡Á106 m3 and is located 17km upstream from the Three Gorges Dam. The tremendous volume and the short distance from the dam threaten the overall dam's safety as well as the daily traffic within the reservoir. The main objectives of this study were to determine: 1) how the stability of the landslide mass will be altered with changing boundary conditions, especially considering the rising water levels in the reservoir, which will also fluctuate seasonally substantially; and 2) how strongly the mechanical properties of the slip zone and the extent of rainfall infiltration into the landslide mass influence the factor of safety (FoS). During the systematic site investigation, both disturbed and undisturbed samples were collected from both the slip zone and the landslide mass and an integrated set of in-situ and laboratory tests were performed. Considering the progressive multi-stage construction of the dam, the corresponding water levels (present 156m and final 175m) and the predefined operating scenarios in flood seasons (e.g. rapid lowering of water level from 175m down to 145m), totally 12 boundary conditions were recognized for FoS calculations by employing transfer coefficient method for two representative profiles. The results of the analysis suggest that the landslide mass is stable with FoS values around 1.5, except for 0.98 for the scenario in which the water level is rapidly lowered from 175m to 145 with a speed of about 0.67m to 1.0m/day. Sensitivity analysis of the influential factors indicates that an increase of 5kPa in cohesion or 2o in friction angle cause FoS to increase by 0.01 and 0.10 respectively. FoS decreases from 1.10 down to 0.98 if the rainfall infiltration percentage is increased from 20% to 50% for the historical maximum daily rainfall of 358mm.