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Large-diameter pressure relief boreholes are one of the primary measures for preventing coal mine rockburst. However, the implementation of these boreholes disrupts the original support structure of the roadway surrounding rock, leading to conflicts with surrounding rock control. Therefore, the pressure relief and energy dissipation behavior of variable-diameter boreholes in roadway surrounding rock was studied. Using a typical rockburst-prone coal mine as the engineering background. Based on elastic–plastic mechanics theory, the elastic solution for the stress distribution around the borehole and the extent of the pressure relief zone are analyzed. Numerical simulation software was used to study the effects of variable diameter drilling parameters (deep reaming diameter, deep reaming depth, and deep reaming spacing) on the pressure relief of roadway surrounding rock, energy dissipation in the roadway, and roadway deformation. The research results indicate that the distribution range of the pressure relief zone is influenced by the vertical stress, lateral pressure coefficient, cohesion, and internal friction angle of the coal body. The maximum radius of the pressure relief zone increases with the borehole diameter. As the deep reaming diameter increases and the borehole spacing decreases, the stress concentration in the surrounding rock of the roadway shifts more significantly toward the deeper region, making it easier to form a dual-peak stress zone. This enhances the pressure relief and stress transfer effect on the surrounding rock of the roadway, leading to greater energy dissipation. From the perspective of energy dissipation, it is concluded that the optimal location for the variable-diameter borehole should be within the peak vertical stress zone of the surrounding rock that has not been relieved. This study provides guidance for the prevention and control of dynamic disasters in deep coal and rock.

In the conventional radial uncoupled charge single free surface blasting, the bottom rock mass is difficult to be fully broken, which affects the blasting effect and restricts the tunneling efficiency. This difficulty adversely impacts the blasting outcome and limits the efficiency of excavation. To address this issue, this paper proposes a solution that involves modifying the charge structure to implement a variable diameter decoupled charge, and it analyzes the theoretical feasibility of this approach. The variably diameter decoupled charge and radial decoupled charge single-hole blasting model was established and compared using LS-DYNA. Furthermore, the effects of various parameters on the rock-breaking efficiency of variable diameter decoupled charges were analyzed. The results show that, in comparison to radially decoupled charges, variable diameter decoupled charges exhibit a greater explosive mass at their base. This enhancement leads to an increase in the effective stress on the surrounding rock, thereby effectively addressing the issue of inadequate fragmentation of the rock mass at the base of radially decoupled charges. Simultaneously, the directional effect of stress wave superposition and the balancing effect of the cavity on internal pressure contribute to an increase in the effective stress and reflected tensile stress of the overlying rock mass. This phenomenon ensures that effective fragmentation of the overlying rock mass can still be achieved, even with a relatively small amount of explosive charge. Under the condition of maintaining the same charge weight and borehole diameter, increasing the length and radius of the expanding section of the explosive significantly impacts rock fragmentation, whereas reducing the radius of the contracting section has a minimal effect. In engineering applications with a common borehole diameter of 4.2 cm, when the length of the expanding section of the explosive charge is half of the total charge length and the radius of the expanding section ranges from 1.65 cm to 1.70 cm, more effective rock fragmentation at the bottom can be achieved, resulting in an overall favorable fragmentation outcome.

In order to reveal the damage evolution and seepage enhancement mechanism of coal around staged variable diameter gas extraction drillings under mining stress environment, taking the N2302 working face of Gucheng Coal Mine as the engineering background, a systematic study was conducted on the stress distribution, plastic zone evolution characteristics, porous mutual interference effects and their impact on permeability evolution of coal around drillings using a combination of theoretical analysis, numerical simulation and physical similarity experiments. Based on elastic-plastic mechanics and Mohr Coulomb yield criterion, an analytical model for the stress and plastic zone of coal around the drilling under different mining location conditions was established, revealing the inherent mechanism of the plastic zone evolution from approximately symmetrical to asymmetrical under mining support pressure. Based on FLAC3D numerical simulation, a porous staged drilling model was constructed, and the reconstruction characteristics of the stress field around borehole under porous conditions were analyzed. The “shielding effect” caused by the superposition of group hole pressure relief and the secondary stress concentration phenomenon at the pressure relief boundary were clarified, and the key control role of the aperture mutation zone in stress disturbance and damage evolution was revealed. On this basis, the spatial coupling relationship between the plastic zone around the pore and the permeability field was systematically analyzed. The results showed that the plastic failure zone is the main control structure for the formation of high permeability channels. Porous disturbances can promote the originally localized high permeability zone to gradually penetrate in the horizontal and vertical directions, forming a high permeability connected zone with obvious anisotropy. Furthermore, by conducting uniaxial compression experiments on coal bodies through staged drilling, combined with digital image correlation (DIC) technology and acoustic emission monitoring, multi-scale verification was carried out on the initiation, propagation, and penetration processes of fractures around the drilling. The experimental results showed good consistency with numerical simulations in terms of fracture evolution paths, damage concentration zones, and seepage enhancement trends. The research results can provide theoretical basis and technical support for optimizing the drilling structure and efficient gas extraction under complex mining conditions.

Theoretical analysis and numerical simulation are used to study the influence of different parameters of variable diameter borehole pressure relief technology on the surrounding rock and support. A strain-softening model was established to analyze the intrinsic connection between the parameters of variable diameter boreholes and the evolution of surrounding rock stress, deformation law, and support strength. The results show that: (1) With the increase in shallow borehole diameter, it is easy to cause anchor de-anchoring phenomenon. (2) After the deep borehole diameter is more than 250 mm, it transfers the peak of the shallow vertical stress to the deep surrounding rock (about 16 m away from the coal wall). (3) If the position of the variable borehole aperture is set between the anchorage zone and the stress peak of the roadway, the stress transfer effect is better, and the influence and effective binding force on the surrounding rock is smaller. (4) When the spacing is 1.0 m~2.0 m, the vertical stress starts to transfer to the deep surrounding rock, the deformation of the surrounding rock is smaller, and the reduction in the effective binding force of the anchors is smaller. The result can provide a reference for similar production conditions.