Influences of the Hard Rock Proportion Coefficient on the Evolution Pattern and Fractal Characteristics of Mining Fractures in a Composite Roof

The mining fractures in overlying strata are the channel of gas migration, which is closely related to the occurrence of coal mine gas disaster accidents. In this paper, eight groups of discrete element three-dimensional models considering the effect of the hard rock proportional coefficient (HRPC) are established for the rock structure characteristics of a composite roof, and corresponding numerical experiments are carried out to clarify the fracture evolution mechanism of different HRPC effects on the composite roof of the coal seam. Then, the multifractal figure processing program is applied to the analysis of the nonuniformity of the fracture's distribution and the size difference between fractures in the composite roof of the coal seam. Furthermore, the effects of HRPC on the multifractal characteristics of mining fractures in the composite roof are discussed, and the distribution characteristics and development degree of mining fractures of different HRPC models after mining are quantitatively characterized. According to the quantified values, the evolution rules of overburden fractures in each stage of coal seam mining can be clearly analyzed.