Experimental study on debris flow initiation

The paper investigates the initiation mechanism of debris flow through the survey with detailed analysis for several field debris flow events. The field investigation shows that the debris flow evolution includes distinct three stages: the soil structure changes and forms slide debris, slide debris mobilizes into debris flow, and the debris flow slips a long distance along the slope. In order to reappear and explain the aforementioned phenomenon existing in the initiation procedure of debris flow, the vane shear rheometer device is developed to test the shear rheology properties of debris source. Subsequently, the shear rheological tests with water drained out are performed utilizing the fine sand as tested material. A series of tests are performed and experimental data are obtained including the shear strain, negative pressure, water content, shear velocity, shear resistance, pore water pressure, and line displacements on the shear surface. Based on the changes of water content and negative pressure, the unsaturated permeability coefficients are obtained to explain the water-free infiltration process in the tests. According to the changes of shear velocity and experimental phenomena, the debris flow evolution is divided into three stages including the antecedent rainfall stage, accelerated initiation stage, and long-distance movement stage, which is similar to the three corresponding stages existing in the filed debris flow events. According to the analysis of test results, the mechanics of the debris flow initiation could be explained as the following: the debris source's shear strength decreased to the value of yield shear strength, the peak shear strength available during undrained loading of a saturated, contractive, sandy soil, after a long period of antecedent rainfall infiltration. The tiny cracks expended to form connecting crack surfaces in the debris source interior, and then the debris source moves rapidly under the continuous rainfall, indicating the debris flow initiation. During the long-distance movement stage, the particles of debris source on the shear surface rearrange and form a smooth slide surface as the values of water content and pore water pressure increase. Therefore, the increase of water content and the decrease of negative pressure caused the soil shear strength decaying, and that is the major reasons of debris flow initiation. In addition, the other key reason is the particles rearranging on the initiation surfaces. The vane shear rheometer apparatus and corresponding test results have provided a more concrete and rational mechanism interpretation for debris flow initiation.