Analysis of the correlation between fractal dimension of gravelly soil and debris-flow initiation through in-situ experiments

Particle size distribution of the gravelly soil in debris flow source area has a great impact on the formation mechanism of debris-flow. In order to explore the relationship between particle composition and debris flow initiation, we collected 182 soil samples from debris flow source areas. Sieving tests have been conducted in the laboratory to acquire the fundamental particle size distribution data. Subsequently, the fractal theory was employed to compute the fractal dimension of each soil sample, analyzing the results of calculation we found that the gravelly soil in debris flow source area is mainly concentrated in one fractal dimension, the value of which ranges from about 2.250 to 2.798. Comparing the results of calculation and sieving experimentation, the underlying relationship between particle composition and fractal dimension can be revealed: the particle compositions of the gravelly soil tend to remain uniform as the fractal dimension increases. On the basis of the calculation results, the in-situ artificial rainfall experiments can be designed and performed. Analyzing the phenomenon during the processes of the experiments, the initiation mechanism with different fractal dimensions can be preliminarily identified: The gravelly soil which can mobilise into debris flows exhibits significant criticality feature, and the gravelly soil with fractal dimension ranges from 2.4 to 2.6 are more susceptible to mobilise into debris flow, and the scale of the debris flow is relatively larger. For slopes with fractal dimension < 2.4, the possibility of mobilisation into debris flow is extremely low. When fractal dimension ranges from about 2.6 to 2.7, the density of the formed debris flow is relatively lower, under which the formation of debris flows exhibits significant intermittent characteristics.