Stability analysis of Gongjiacun landslide in the three Gorges Reservoir area under the action of reservoir water level fluctuation and rainfall

To elucidate the formation mechanism of the Gongjiacun landslide in the Three Gorges Reservoir Area in China, the macroscopic deformation, matter, and structural characteristics, as well as slope stability, were studied via geological field investigations, indoor and outdoor tests, a back analysis, a finite element method, etc. The results show that the Gongjiacun landslide comprises three landslide bodies, with a total volume of 79.2 × 104 m3. The landslide is a medium-sized shallow-middle layer soil landslide. The three landslide bodies were not secondary landslides of a deep landslide. Based on laboratory tests and the values of similar landslides, the shear strengths of the three sliding-zone soils under natural and saturated states were obtained via direct in situ shear tests and back analysis. Using Geostudio software, the stabilities of the three landslides under different water levels and rainfall conditions were analyzed using the Bishop method. The results are as follows: The I landslide body is approximately stable in its natural state and under rainstorm conditions. However, considering the slope excavation and the wharf cargo load at the front edge of the I landslide body, many secondary landslides have occurred. The I landslide body has many potential safety hazards when there is a rainstorm or continuous rain. The II landslide body is stable in its natural state but unstable under different water levels and rainfall conditions. The III deformation body is stable in its natural state but unstable under different water levels and rainfall conditions. Following the impounding of the Three Gorges Reservoir Area, the front edge of the II landslide body and III landslide body would be submerged by 175 m of water. Fluctuations in the reservoir water level can easily induce landslides. During the rising process of the reservoir water level, the long-term immersion of rock and soil at the front edge of the slope causes the front to produce a plastic zone and continuously expand. As the reservoir water level declines, the soil experienced a large excess pore water pressure in a short time, reducing the safety factor of the slope. The influence of rainfall on slope stability was significantly smaller than that of water level change on slope stability. Some measures should be adopted to control landslides, such as anti-slide pile setting, slope-surface protection, surface draining, monitoring, and early warnings.