Study on the influence of coal thickness on the mechanics and acoustic emission characteristics of gas-bearing coal and rock mass

The thickness of coal seam has an important influence on the energy accumulation and release of coal–rock combination and the failure mode of coal–rock. The mechanical and acoustic emission (the manuscript is hereinafter referred to as AE) properties of coal–rock combination with different coal seam thickness are also very different. To study the influence of coal seam thickness on the AE characteristics of gas-bearing coal–rock combination during its fracture process, the coal–rock combination sample of a coal mine in Majiatan Town, Lingwu City, Ningxia Province, was taken as the research object. The self-developed gas-bearing coal–rock mechanical testing machine was used to carry out the mechanical and AE tests of gas-bearing coal–rock combination with different coal seam thicknesses. The stress–strain, failure mode, AE, damage, and energy characteristics of gas-bearing coal–rock combination with different coal seam thicknesses were compared and analyzed. The results show that: (1) With the increase in the thickness of the coal seam in the combination, its compressive strength and modulus of elasticity decrease by 85.77% and 83.05%, respectively; (2) the thickness of coal seam in gas-bearing coal–rock combination has a significant influence on the cumulative ringing count and cumulative energy of AE during the fracture process. The thickness of coal seam is positively correlated with the cumulative count of AE, but negatively correlated with the cumulative energy of AE, the cumulative AE energy decreases from 3.1 × 104 to 5.1 × 103 mv ms, a decrease in 83.72%; (3) based on the pre-peak damage variable of AE cumulative ring count, the damage evolution process of the sample is described. Under uniaxial compression, the damage evolution of the gas-bearing coal–rock combination has a good correspondence with the stress–strain curve; (4) the total input energy and total elastic energy decrease nonlinearly with the increase in coal seam thickness, and the elastic energy density decreases by 94.47%, and the proportion of elastic energy density in the input energy density is always high.