Study on the Mechanism of Landslide Promotion of Expansive Soil under the Combined Action of Rainfall and Reservoir Water

ObjectiveThe widespread expansive soil landslides poses a significant threat to the long-term stability and safe operation of the Danjiangkou reservoir. The expansive soil is rich in hydrophilic minerals such as montmorillonite and illite, which is very sensitive to the action of water. The structural characteristics, expansion and contraction characteristics and strength attenuation characteristics of expansive soil are the main reasons for the instability and failure of expansive soil landslide, and show the typical characteristics of shallowness, traction, gentleness, seasonality and repeatability. At present, most of the related research focuses on the mechanism of rainfall on the shallow failure of expansive soil landslide. For large expansive soil landslide affected by reservoir water, it is necessary to further clarify the combined effect of rainfall and reservoir water on the deformation and failure of landslide. Therefore,investigating the mechanisms of instability and destruction of expansive soil landslides under the effects of rainfall and reservoir water level fluctuations is crucial for implementing meteorological early warning, risk assessment, and engineering mitigation of landslides at the Danjiangkou Reservoir.MethodsLeveraging Jingang Village landslide, Yunyang District, Shiyan City, a typical cross-section in the main sliding direction was chosen to establish a two-dimensional slope experiment model based on comprehensive analysis of geological surveys, GNSS monitoring, and data on the rainfall and reservoir water levels. The model was vertically divided into the sliding mass, sliding zone, and sliding bed, whiling setting another three horizontal surfaces from the front to the rear edge. Each monitoring profile is equipped with moisture content sensors and pore water pressure sensors, and displacement sensors are placed at the surface of the landslide model. Based on the "step" deformation of the landslide corresponding to fluctuations in the reservoir water level and rainfall characteristics, and incorporating meteorological and hydrological data from the area around Jingang Village landslide, the reservoir experiences frequent heavy rainfall during high water level operations and rising water level phases. Three physical simulation experiments were designed: Condition one simulates the reservoir water level rising from low to high, remaining static for a period, then dropping back to low, repeated multiple times; Condition two simulates continuous heavy rainfall during high water level periods; Condition three builds on condition one, simulating continuous heavy rainfall as the water level rises from low to high. Real-time monitoring of the landslide destruction process, including pore water pressure, moisture content data, and slope deformation, facilitates the study of the mechanisms of expansive soil landslide instability under the influence of rainfall and reservoir water levels.Results and Discussions The main research content and results are as follows: (1) Model experiments show that the destruction process of Jingang Village landslide manifests as tractional failure, beginning with localized deformation at the toe of the slope, then progressively developing upwards, leading to overall failure in the mid-front part, while forming an open face and steep step at the landslide fracture, followed by stepwise collapse; (2) Reservoir water and rainfall cause different response patterns in the internal pore water pressure and moisture content of the landslide. Only during the fluctuation of the reservoir water level does the pore water pressure and volumetric water content at the front edge of the slope show cyclic changes, with no response at the middle and rear edges. After the application of rainfall, pore water pressure and volumetric water content outside the influence range of the reservoir water begin to respond, increasing with ongoing rainfall and eventually stabilizing. The increase in moisture content at the sliding zone is significantly higher than at the slope surface, indicating that water ultimately accumulates near the sliding zone; (3) In situ monitoring and physical model experiments show that a drop in reservoir water level produces sustained deformation at the front edge of the landslide. Rainfall causes overall deformation of the landslide, significantly greater than the deformation caused by the drop in reservoir water level. Landslides that have undergone cycles of rising and falling reservoir water levels are more prone to significant deformation under heavy rainfall conditions, potentially leading to instability; (4) The continuous deformation of expansive soil landslides affected by the combined action of rainfall and reservoir water is due to the dual effects of external added stress and the reduction in internal strength indicators. Persistent heavy rainfall rapidly infiltrates the slope body along fractures, raising the groundwater level, while the strength of the sliding zone soil continues to decrease due to prolonged soaking by groundwater, which is key to the ongoing deformation of large expansive soil landslides.ConclusionsBased on the analysis of the deformation mechanism of expansive soil landslide under the combined action of rainfall and reservoir water, combined with geological survey and in-situ monitoring data such as rainfall, reservoir water level and GNSS monitoring, based on the analysis of landslide deformation characteristics, a physical model was designed and manufactured. By setting sensors such as displacement, pore water pressure and water content, the changes of relevant parameters of landslide under different heavy rainfall and reservoir water conditions were monitored in real time, reflecting the deformation and failure mechanism of expansive soil landslide under the combined action of rainfall and reservoir water. It has reference significance for the study of expansive soil landslide failure theory and the actual disaster prevention and mitigation work.