Experimental study on internal material migration and failure mechanism of multi-layered sandy slope induced by rainfall

Western China’s multi-layered sandy slopes are highly vulnerable to rainfall-induced failure， posing significant risks to human life and infrastructure. However， the internal material migration and failure mechanisms of these slopes are not yet fully understood. This study conducted flume model experiments to quantify internal migration processes and elucidate failure characteristics and mechanisms. The results indicate that rainfall infiltration induced the migration of fine particles (< 2 mm) from the slope crest to the toe， with a measured transport rate of 19% and the subsequent formation of an accumulation zone. Analysis of the initial response time of the volumetric water content revealed that the wetting front progressively converged towards the bedrock interface， facilitating slope saturation and the accumulation of shallow water. The rainfall-induced failure mode exhibited a progressive， multi-stage characteristics， involving the sequential development of surface rills， rill coalescence， local sliding at the slope toe， and ultimately， the formation of slope debris flows. A criterion termed the Migration-Hydration Ratio (k) was proposed，and the fitted k-D curve demonstrated that shallow slope failure occurs when the k value reaches a critical threshold of 0.43. Fine particle migration and moisture accumulation are key driving factors for the instability of multi-layered sandy slopes.