Abstract:Pore diameters of wide grading soil are across several orders of magnitude.A preference flow which is a kind of unbalanced infiltration in space usually appears under rainfall infiltration condition.Based on the fractal structure of pores,quantitative demonstration of rainfall infiltration mechanism in wide grading soil was analyzed.According to the theory of fractal porous media and Hagen-Poiseulle equation,the orders of infiltration rate in different pore pipes were different.On this basis,soil pores were divided two regions—a matrix region and a preferential flow region.Their area ratio changed with rainfall intensity.In the matrix region,the infiltration rates of pore pipes were different and all of them reached their own maximum.However,in the preferential flow region,the infiltration rates of pore pipes were the same and they didn't reach their own maximum.The matrix region was composed of small diameter pore pipes and the preference flow region was composed of large diameter pore pipes.The magnitude of infiltration rate in the matrix region was far less than that in the preferential flow region.The expressions of infiltration amount and infiltration rate were developed in two regions.Infiltration features in different rainfall intensities and the relationship between area ratio and rainfall intensity were studied.The results showed that infiltration rate in the preferential flow region was much higher and it increased with the increase of rainfall intensity.The preferential flow phenomenon was produced by the multi-scale pore structure and free infiltration boundary without surface pond pressure.The value and distribution of infiltration rate were influenced by saturated permeability coefficient of soil and the rainfall intensity.The mechanism of a preferential flow was explained from the pores structure perspective.