Microstructure Features of Powdery Coal-Bearing Soil Based on the Digital Image Measurement Technology and Fractal Theory

With the rapid development of infrastructure, more and more coal-bearing soil are encountered in the southern mountain area of China, the property of coal-bearing soil has a great impact on the safety of the projects. Taking powdery coal-bearing soil along the Guangdong distract of Wushen Expressway as research object in this paper, the images of powdery coal-bearing soil microstructure were obtained by using Environmental Scanning Electron Microscopy (ESEM) under different water content (10%, 15%, 20% and 25%) after direct shear test. The microstructural morphology characterization of shear plane are analyzed by MATLAB, and the microstructure parameters of coal-bearing soil are measured by Image Pro-Plus (IPP) software. Based on the fractal theory, the fractal model of coal-bearing soil microstructure is established, and the fractal dimensions of pore profile and number and size distribution of pores of coal-bearing soil are obtained in two-dimensional space. The fractal dimension of pore profile and porosity changing with pore size are analyzed. The results show that the basic unit of the microstructure of coal-bearing soil is schistose aggregated particles. The contact relationship is mainly surface-surface contact and face-side contact. The shape of the flaky granule aggregated body is irregular and the orientation is not obvious. The shape of pores is anisometric and slotted. With the increase of water content, the roughness and fluctuation degree of the shear plane first increase and then decrease. The connectivity between the pores is improved and the shearing strength parameters also increases first and then decreases. The turning point is near the optimum moisture content (between 10 and 15%). The microstructure of coal-bearing soil is more complex and has obvious fractal characteristics. It is described by the fractal models of the equivalent area and equivalent perimeter of pores and number and size distribution of pores, and the fractal dimension is between 1 and 2 in two dimensional space. The fractal model of the microstructure morphology of shear plane is established, and the height and length of micro-convex bodies can be calculated by mathematical equation.