Pore structure characteristics of debris flow source material in the Wenchuan earthquake area

Wide-grading loose soils (WGLS) are a special slope deposit that often serve as a source for debris flows in the western mountainous area of China. Previous research indicated that rainfall infiltration induced the migration of fine particles within soil slopes and lead to the initiation of shallow failure and subsequent debris flows. Clearly, soil pore structure exerts a profound influence on seepage during rainfall infiltration, which is closely related to soil permeability and corresponding internal erosion. The current understanding of the mechanisms of fine particle migration is limited because of the complexity of WGLS micropore structure. The challenge lies in how to systematically characterize WGLS pore structure so that it provides a foundation for future numerical simulations of fine particle migration-induced debris flows. In this study, the characteristics of WGLS pore structure were analyzed quantitatively using scanning electron microscopy. The results revealed that WGLS predominantly contained micropores and mesopores. The pore size distribution exhibited a bimodal nature with one peak occurring at a pore diameter of 12 mu m and another at 72 mu m. The pore complexity of all samples exceeded 0.75, which indicates a low aspect ratio for each pore. The average diameter of the fine particles that migrated was then determined with a laboratory seepage test. A preliminary jamming probability analysis based on three-dimensional micropore reconstruction and average fine particle diameter indicate that jamming is most prevalent along the slope.