Study On Deformation And Failure Mechanism Of Rainfall-induced Large Landslide

Rainfall is one of the major triggering factors that cause slope instability. Analyzing the infiltration mode of rainfall and understanding the relationship between precipitation, groundwater level and landslide deformation has great significance for landslide disaster mitigation. Xiashan landslide is a typical rainfall-induced landslide that has very complex structure and had historically slid several times owing to heavy rainfall. It will cause catastrophic consequence if the landslide mass slides into Shimen reservoir. The thesis takes Xiashan landslide as a typical case to 1) evaluate the stability of the landslide,2) study the mechanism of deformation and failure process caused by elevated groundwater,3) analyze the correlation between surface displacement and groundwater level and the correlation between groundwater level and precipitation in the landslide area through geological exploration, field monitoring and theoretical analysis. Several remarks are achieved as follow:(1) The surface displacement is monitored by GPS using double frequency receiver, TM30 measuring robot and laser rangefinder, while the deep deformation is monitored via vertical fixed inclinometer sensor. Precipitation and groundwater level data are obtained using tipping bucket self-recording udometer and vibrating osmometer respectively. The monitoring data provide a strong foundation for the research on deformation pattern and failure mechanism of the landslide etc.(2) The permeable and impermeable layers are interval distributed in the landslide, and thus both phreatic and artesian water exists in the landslide as a result of the complex stratum structure. There are two slip surfaces, the shallow one affected by phreatic water and the deep one affected by artesian water. The slow sliding along both of slip surfaces show intermittent feature.(3) The hysteresis between the peak value of displacement velocity and groundwater level is analyzed between the surface displacement and the groundwater data of Xiashan landslide. The lag time is 80-90 hours. The critical groundwater level of the landslide is confirmed when it uplift to L=7.5 m(the initial reading of osmometer buried at 9.7 m deep is taken as 0) and the buried depth of groundwater is 2.2 m. The deformation trend of Xiashan landslide is forecasted according to the groundwater monitoring data.(4) The influence coefficient of previous precipitation on groundwater level is solved via vector auto regression (VAR) model and 9 days previous precipitation has distinctly influence on groundwater level by analyzing the variation tendency of the influence coefficient. Using the 9 days previous precipitation and groundwater data as training samples, an optimization support machine model (GA-SVM) is build. The groundwater level is forecasted based on the previous precipitation using the model. Considering the influence of previous precipitation, precipitation threshold is calculated using the GA-SVM model on the base of the critical groundwater level. The sliding of the landslide is forecasted through the precipitation monitoring data.(5) A multiple property regression model is established based on several kinds of geological exploration and monitoring data. The factor of safety (K) of Xiashan landslide (the shallow slip surface) is calculated, the K is 0.965～1.078. The factors that affect landslide stability are evaluated using the model and groundwater is considered to be the major triggering factor that cause landslide instability.