The mechanisms behind shallow failures in slopes comprised of landslide deposits

In areas hit by the 2008 Wenchuan earthquake that occurred in Sichuan Province, China, debris flows are often generated from landslide deposits during heavy rainstorms. The broadly graded and unconsolidated landslide deposits respond to rainfall in very complex mechanical and hydraulic manners. An artificial rainfall test was conducted to simulate the rainfall infiltration and surface runoff processes occurring on the landslide deposit slope at the Wenjiagou Gully, China, with heavy rainfall rates (140 mm/h) incorporated. An innovative flume was designed to collect the slope interflow and surface runoff separately. Sensors to monitor the pore water content (PWP) and volumetric water content (VWC) were deployed. The results indicated that there were four stages in the hydrological response of landslide deposits during the artificial rain event: infiltration, a slow increase in interflow (surface runoff begins), a rapid increase in interflow (surface runoff slowly increases) and a steady state. Bed gradient increase will lead to PWP rapidly ascending and regressive failure happening. Concomitant with the observed increases in PWP and VWC, the shear strength of the landslide deposits decreased and led to the occurrence of small-scale shallow failures. Surface runoff, interflow and fine particle migration effects are presented to interpret the process of shallow failure. And although shallow slope failure is the result of interaction with the above three factors induced by rainfall, the key underlying factor is the characteristically loose structure of landslide deposits.