Model test study on the hydrological mechanisms and early warning thresholds for loess fill slope failure induced by rainfall

Under rainfall conditions, the characteristics of hydraulic changes related to the failure evolution of fill slopes are critical for the early warning of slope failure. In this study, a series of model tests are conducted on loess-filled slopes with different slope types. Meanwhile, the matric suction, volumetric moisture content, pore water pressure and deformation of the slope are monitored in real time during the rainfall process. The development of cracks and failure modes of the slope are experimentally studied. An early warning strategy for fill slope failures induced by rainfall is proposed. The experimental results demonstrate that loess landslides induced by rainfall have the most unstable state or even failure when the matric suction is reduced to the lowest value and remains stable; this indicates a hysteresis phenomenon. In addition, a series of tension cracks are present on the slope before the landslide is induced by rainfall. The tension cracks not only provide the advantage of an infiltration channel to control the hydrological process, but also evolve into the trailing edge boundaries of each failure of the fill slope. Furthermore, combined with the infinite slope stability analysis of Mohr-Coulomb soil, the matric suction and volumetric water content thresholds of the fill slopes with different slope angles are determined. An early warning threshold model for rainfall induced slope instability is proposed and a new framework of early warning is provided that may become a reliable approach for failure forecast.