Coupled influence of reservoir water and rainfall on the response mechanism of accumulated landslides with chair-like interfaces between sliding masses and bedrock

The Three Gorges Reservoir (TGR) Dam is currently the largest hydroelectric power project in the world.TGR has a daily power output of 9.81×10⁹ kWh and a reservoir capacity of 3.92×10¹⁰ m³.It was completed in Southwest China in 2009 and the reservoir water was impounded to 175 m.Consequently, a long and narrow hydro-fluctuation belt (over 600 km, from 145 m to 175 m), which extends from Yichang City in Hubei Province to Maoer Gorge in Chongqing Municipality, China, appeared along the TGR.The completion of the TGR Project and the subsequent impoundment of the 660 km-long reservoir reactivated and induced over 5 000 landslides along the banks of the reservoir.These landslides have posed great threats to residences, shipping along the Yangtze River, and dam stability.Thus, gaining insights into the behavior, triggering and conditioning factors, and evolutionary mechanism of these reservoir-induced landslides is crucial for landslide control, management, and decision-making.Water, including fluctuations in reservoir water levels and periodic rainfall, is one of the most common triggering factors for bank slope failures.Variations in reservoir water levels can affect slope stability during the drawdown and impoundment stages.Furthermore, rainfall can trigger slope failure events, including shallow and deep-seated landslides on slopes along the reservoir.Although the individual effects of rainfall or reservoir water on bank slope stability have been widely studied, the combined effects of rainfall and reservoir water on landslide deformation have been rarely discussed.Moreover, many reservoir-induced landslides have been triggered by the coupled effects of rainfall and reservoir-level fluctuations.This study aims to study the coupled influence of reservoir water and rainfall on the response mechanism of accumulated landslides with chair-like interfaces between sliding masses and bedrock.

The Outang landslide, a typical landslide with a chair-like interface between the sliding mass and bedrock, was selected as the research object.Monitoring data analysis, correlation theory, and discrete element method were adopted to reveal the deformation characteristics and possible failure evolution mechanism of landslides.

Research results revealed that from a regional perspective, the landslide deformation rate increased with the increase in elevation, whereas from a temporal perspective, landslide deformation showed a step-like increase.Reservoir water and rainfall were considered the main triggering factors of landslide deformation.Reservoir water affected the front of the landslide with a corresponding seepage-induced deformation mechanism, whereas rainfall affected the remaining part of the landslide, and its corresponding movement mechanism transformed from water swelling during the initial period of rainfall to the seepage-induced mechanism.

The landslide exhibited composite push-retrogression-type failure evolution with fluctuating reservoir levels destabilizing rock masses toward the front section and precipitation mainly mobilizing materials in the upper section of slopes.This research will further elucidate the reactivated ancient landslide issues, such as deformation characteristics and failure evolution, and provide practical experience and insights regarding analogous old landslides in the reservoir area.