In order to explore the influence of grain size on the mechanical properties of unbolted and bolted composite rock masses, uniaxial compression tests were carried out on unbolted and bolted composite rock masses of different grain sizes. The characteristics of the variation in the strength, elastic modulus, Poisson’s ratio and energy parameters of composite rock masses with grain size were analyzed. The evolution process of crack propagation in the composite rock masses was studied, and the influence mechanism of rock grain size on the mechanical properties of the anchorage bearing structure of the rock surrounding the roadway was revealed. The results show that with an increase in the grain size, the peak strength and elastic modulus of a composite rock mass decrease gradually, and the post-peak residual strength, Poisson’s ratio and total input strain energy increase gradually. The evolution of crack propagation is from tensile cracking to tensile to shear mixed to shear cracking. Prestressed anchor bolts can effectively improve the peak strength and post-peak residual strength of composite rock masses and have inhibitory effects on crack propagation in the anchorage zone, such as weakening, deflection and crack arrest. Compared with an unbolted composite rock mass, the bearing capacity of a bolted composite rock mass is stronger, and its elastic modulus is significantly improved.

To investigate the safety risks associated with gas tunnel coal uncovering, a physical and mechanical model of the critical safety rock pillar is proposed through a combination of theoretical analysis, numerical simulation, and field testing. Based on the principles of energy conservation and catastrophe theory, an expression for calculating the critical safety for rock pillar thickness is derived. The effects of tunnel radius, burial depth, axial stress, coal seam dip angle, and gas pressure on the critical thickness are systematically analyzed. The results indicate that the critical safety of rock pillar thickness increases with the tunnel radius, burial depth, gas pressure, and axial stress. Moreover, as the tunnel radius increases, the growth rate of the critical thickness also increases. Conversely, as the burial depth increases, the growth rate of the critical thickness decreases. For gas pressure and axial stress, the growth rate remains relatively constant. Using a tunnel project in Hunan as a case study, theoretical analysis yields a critical safety rock pillar thickness of 3.95 m. A corresponding numerical model is developed to simulate this scenario, and the simulation results align well with the theoretical predictions. Based on these findings, a combined treatment scheme of “advanced small-pipe grouting + gas drainage and pressure relief” is proposed for excavation upon reaching the critical rock pillar thickness. This scheme successfully ensures safe tunnel passage through the coal seam.

Tunneling through coal-bearing strata carries high risks of coal-gas dynamic disasters, with the determination of the safety pillar thickness constituting a critical engineering challenge for disaster prevention. This study establishes a theoretical model for tunnel safety pillars in coal measure strata under the influence of gas pressure, integrating the silo theory and the limit equilibrium methods. The research derives mechanical instability criteria for rock pillars, analyzes the multi-factor impacts on tunnel safety pillars, and investigates the plastic zone evolution processes under multi-factor conditions. Key findings indicate: (1) Under identical geological conditions, increasing the tunnel excavation radius, the coal seam dip angle, the lateral pressure coefficient, the surrounding rock classification, and the gas pressure all reduce pillar stability, requiring greater safety pillar thickness. (2) The excavation radius, the lateral pressure coefficient, and the surrounding rock classification demonstrate a significant influence on the required pillar thickness, with sensitivity ranking: surrounding rock classification > lateral pressure coefficient > coal seam dip angle > gas pressure. (3) The plastic zone evolution patterns remain consistent across different working conditions, showing intensified coal seam influence and increased outburst risks as the tunnels approach the coal seams. These findings provide crucial references for the stability analysis of reserved safety pillars in coal measure strata tunneling projects.