Physical model experiment of rainfall-induced instability of a two-layer slope: implications for early warning

Understanding the slope hydrology and failure processes of rainfall-induced landslides is key to landslide early warning; the heterogeneity of soil (e.g., grain-size distribution in different layers) can markedly affect rainfall infiltration and slope failure patterns. However, the hydrological and failure processes of heterogeneous slopes layered by different soil groups have received little attention. In this study, we use a typical landslide soil composition of rainfall-induced landslide in fault zones as a prototype and via flume experiments to simulate the hydrological evolution, failure processes, and patterns under rainfall conditions on material heterogeneity slopes with a combination of colluvial deposit and fault gouge. Our results showed that rainfall-induced slope settlement and rapid saturation of shallow layers of colluvial deposits led to the occurrence of layer-by-layer shallow flow-slides. The spatial variability of infiltration led to the generation of a relatively dry‒wet interface in deeper layers, causing differential changes in the mechanical properties of the fault gouge; this was conducive to the formation of a steep landslide back wall, perched water table in the shallow layer of the fault gouge, and a rapid increase in porewater pressure, which triggered deep sliding, with a change in the failure pattern to a retrogressive mode. There was a strong linear correlation between the displacement rate before slope instability and the Arias intensity (I_A) of the seismic signal; an abrupt change and rapid increase in I_A may indicate that the slope entered an accelerating creep stage before failure. The results of this study provide a physical basis for related numerical simulation research and a reference for landslide early warning based on seismic signals.