Internal Erosion Controls Failure and Runout of Loose Granular Deposits: Evidence From Flume Tests and Implications for Postseismic Slope Healing

Landslides in granular soils can be highly hazardous when exhibiting flow-like behavior. The extensive mass wasting associated with the 2008 M-w = 7.9 Wenchuan earthquake (China) left several cubic kilometers of loose granular material deposited along steep slopes and in low-order channels. Rainfall-triggered remobilization of these deposits evolved often into catastrophic flow-like landslides. Ten years after the earthquake, most of the deposits are still in place but landslide rates have decreased significantly. Internal erosion-induced grain coarsening is one possible process producing this decrease. Through experiments on loose artificial slopes we demonstrate the major role of the internally erodible small grains in triggering failure and fluidization and producing grain coarsening. Under the same hydraulic boundary, if the erodible fraction is removed or reduced, the loose deposits remain stable or fail without fluidizing. Our results provide an experimental evidence to the patterns of sediment export and debris flows observed in nature after a strong earthquake. Plain Language Summary Landslides exhibit a variety of behaviors, from imperceptible creep strains to catastrophic failures. Flow-like landslides are among the most hazardous, as they can happen suddenly and cover long distances at high speed. Strong earthquakes, such as the 2008 M-w = 7.9 Wenchuan earthquake (China), can cause extensive landsliding, feeding hillslopes and drainage channels with abundant debris. Rainfalls can remobilize these deposits, and the movement can evolve into catastrophic and deadly debris flows. Ten years after the Wenchuan earthquake, most of debris is still in place within the orogen, but landslide rates have decreased significantly. Various processes can strengthen the deposits of debris, reducing their susceptibility to failure. Among them, we show the role of the small granular fraction-that can be eroded and transported within the large soil pores-in triggering instability, failure, and fluidization of the deposits. If these small particles are fewer or absent, the deposits remain stable and water drains easily thanks to the higher hydraulic conductivity. In the light of our experiments, we suggest that a progressive removal of the small particles in absence of failure, which was observed in nature indirectly, can be one realistic process leading to the stabilization of the deposits.