Deformation characteristics, mechanisms, and dominant factors involved in rainfall-hydrodynamic pressure landslides: case study of the Sanmendong landslide in the Three Gorges Reservoir

A 16-year surface macroscopic survey and GPS monitoring results from the Sanmendong landslide in the Three Gorges Reservoir Area have demonstrated that the deformation degree of the landslide is distinctly correlated with rainfall and fluctuations in reservoir levels. In this study, the seepage field and stability coefficient of Sanmendong landslide deformation under the conditions of rainfall were estimated, and fluctuations in the reservoir water level and their coupling effects were simulated and calculated, respectively. The simulation results revealed that as the water level rises, the seepage force is directed to the inside of the slope, thus creating conditions that bolster landslide stability. However, when the reservoir water level drops or rainfall occurs, the seepage force is directed to the outside of the slope, thereby hindering landslide stability. GPS monitoring data and the correlation between reservoir water level fluctuations, rainfall, and landslide deformation were further analyzed. The analysis revealed a positive correlation between rainfall and the displacement rate at monitoring point ZG361, with a high grey correlation degree (0.761). However, the correlation between the rate of fluctuating reservoir water levels and the displacement rate at monitoring point ZG361 was insignificant. The stability calculation demonstrated that under normal Three Gorges Reservoir operation, a decrease in water level only decreases the stability coefficient by  10%. The above results prove that the rainfall is the primary driver governing Sanmendong landslide deformation.