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.

To study the influence of cyclic stress on the nonlinear behavior of saturated sandstone, the residual strain properties and energy dissipation characteristics of the sandstone under tiered cyclic loading were experimentally investigated. The axial/radial residual deformation and energy dissipation characteristics of sandstone at different cyclic stress stages were analyzed in detail. By combining the mathematical statistics, fluctuation coefficients of the residual strain and energy dissipation, and correlation coefficients of axial/radial residual strain and energy dissipation were defined to describe the process. It was determined that these newly defined physical variables were closely related to the elastic-plastic state (or instability failure state) of the rock.

An underground coal gasification (UCG) process is strongly exothermic, which will cause thermal damage on rock cap. We proposed a new thermal damage numerical model based on a two dimension particle flow code (PFC2D) to analyze the inception and extension of cracks on pre-cracked red sandstone, which were thermally treated at a temperature of 25~1000 °C. The results indicated that: (1) a thermal damage value D_T obtained by extracting the thermal crack area of scanning electron microscope (SEM), which can be used as an indicator of the degree of thermal damage of the sandstone; (2) a thermal damage numerical model established by replacing the flat-joint model with the smooth-joint model based on the thermal damage value D_T, this approach can properly simulate the mechanical behavior and failure patterns of sandstone; (3) the critical temperature for strength reduction was 750 °C. The peak strength increased as pre-treatment temperature increased from 25 to 750 °C and then decreased. The elastic modulus E₁ decreased with the increasing thermal treatment temperature; (4) micro-scale cracks initiate from the tip of the prefabricated fissure, and expand along the direction of prefabricated fissure, finally developing into macroscopic fracture. This approach has the potential to enhance the predictive capability of modeling and presents a reliable model to simulate the mechanical behavior of thermally damaged sandstones, thereby offering a sound scientific basis for the utilization of space after UCG.