A physical model test on a seepage-initiation-braking-type (SIBT) landslide under the coupling of rainfall and water level fluctuation

Seepage-initiation-braking-type (SIBT) landslides are the majority of reservoir landslides in the Three Gorges Reservoir Area in China that involve gradual deformation in response to water level (WL) and rainfall rather than experiencing an abrupt failure and sliding directly into the river, highlighting the complex nature of this landslide. Here, a physical model test with the rainfall and the fluctuation of WL was conducted on a representative SIBT landslide, Huangtupo Linjiang No. 1 landslide (HTPLJ1). The changes in pore water pressure, earth pressure, and overall displacement of the landslide model were monitored by a monitoring system. The results revealed distinct stages in the landslide model: impoundment-induced deformation, preliminary sliding, stagnation and stability, re-initiation, short stability, and accelerated sliding to failure. Combined with monitoring system analysis, the rainfall infiltrations destabilized the shallow landslide by reducing the effective stress, while impoundment increased pore water pressure, leading to buoyancy-driven effects. However, the most notable deformations occurred during the WL drawdown stages, when seepage drag force induced by the low permeability of the sliding mass triggered more pronounced deformations. The deformation mechanism of HTPLJ1 with SIBT is attributed to a bulged slope toe induced by the seepage drag force, leading to increased effective stress along the resisting section and temporary stabilization. The site geological investigations and monitoring data indicated continuous buoyancy-driven effects and a higher sensitivity to seepage-driven effects in HTPLJ1. It can be inferred that the SIBT landslides undergo repetitive deformation characterized by “dragging and compression,” which leads to initiation and stagnation.