In this study, the failure behaviors of debris flows were studied by flume model tests with artificial rainfall and numerical simulations (PFC3D). Model tests revealed that grain sizes distribution had profound effects on failure mode, and the failure in slope of medium sand started with cracks at crest and took the form of retrogressive toe sliding failure. With the increase of fine particles in soil, the failure mode of the slopes changed to fluidized flow. The discrete element method PFC3D can overcome the hypothesis of the traditional continuous medium mechanic and consider the simple characteristics of particle. Thus, a numerical simulations model considering liquid-solid coupled method has been developed to simulate the debris flow. Comparing the experimental results, the numerical simulation result indicated that the failure mode of the failure of medium sand slope was retrogressive toe sliding, and the failure of fine sand slope was fluidized sliding. The simulation result is consistent with the model test and theoretical analysis, and grain sizes distribution caused different failure behavior of granular debris flows. This research should be a guide to explore the theory of debris flow and to improve the prevention and reduction of debris flow.

Abstract. The failure mode of fine-grain debris flow is investigated by using self-designed flume model device. Artificial rainfall is applied onto grain slopes in a flume model. Full-field displacement contour of fine-grain rainfallinduced debris flow during failure process is analyzed by using Digital Photogrammetry Deformation Measurement (GEODOG). The studies indicate that the particle movement of fine grains' experimental results respectively concentrated on the anterior and rear of sand slope of debris flow. The pore water pressure test results show that in the fine grain sample it is higher than it in coarse sand.