Aiming at the unclear mechanism behind the sudden instability of coal wall spalling, this study takes a fully-mechanized mining face with large mining height as an example. Through theoretical analysis and numerical simulation, the energy release mechanism governing the instability of the coal shell structure is revealed, and a theoretical failure model for spalling prediction is established. Based on the mechanical characteristics of coal shell failure and employing the theory of plastic strain localization in coal mass, a catastrophe model for coal wall instability was developed by analyzing the incremental relationship between coal shell deformation and the work done by deviatoric stress. The research outcomes derived quantitative expressions for energy release and proposed the expression for deviatoric stress strain energy, clarifying that energy exceeding a critical threshold is the fundamental cause of sudden spalling. Furthermore, expressions for the release of volumetric strain energy and gas internal energy were derived, revealing the coupling mechanism between internal and external stresses. The research outcomes can be directly applied to predict the risk of rib spalling by monitoring the degree of energy accumulation and to optimize support parameter design based on the energy release threshold, providing theoretical guidance for safe mining in high-stress coal seams. This study achieves a breakthrough from phenomenological description to mechanistic quantification, holding significant practical value for the prevention and control of rib spalling disasters in fully-mechanized mining faces.

Anchor quality is a critical factor influencing the anchoring performance after drilling and expanding anchor holes. Anchoring at the bottom of the hole is an effective technical measure to improve the anchoring quality of mudstone cemented soft rock tunnels. Through a comprehensive research approach including theoretical analysis, laboratory experiments, and numerical simulation, we conducted anchoring quality tests on a self-developed bottom-hole single-wing inverted wedge-shaped hole-expanding device. The anchoring quality under different hole-expanding parameters was analyzed to reveal the anchoring enhancement mechanism at the bottom of the anchor hole. We established an anchoring model for bottom-hole hole-expanding anchoring and derived the formula for the axial force on the anchor rod during hole-expanding anchoring. The experimental results show that when the anchoring agent is K2335 resin anchoring agent, the hole-expanding length is 100 mm, the hole-expanding diameter is 58 mm, and the inverted wedge angle is 9°, the anchoring effect is optimal with a solidification rate of 92.9%. Even after the anchor rod slips and loses anchor, it still maintains a high anchoring force. On-site application results indicate that inverted wedge-shaped hole-expanding anchoring can effectively increase the anchor rod support strength, reduce tunnel deformation, and ensure the stability of coal mine tunnels. This method has significant guiding significance for solving the anchor rod support of mudstone cemented soft rock tunnels.

The surface roughness of grout in fractured rock masses has a significant impact on the diffusion characteristics of grout, especially in millimeter-scale fractures. In this study, a self-constructed experimental system for grouting in rough fractured rock masses was used to conduct grout diffusion tests with varying fracture roughness, fracture aperture, and grouting pressure. A theoretical model was developed to account for the combined effects of fracture roughness, aperture, and grouting pressure on grout diffusion, and its validity was verified. The results showed that the theoretical calculations and experimental results had an error rate of around 12%, indicating the high reliability of the theory. Fracture aperture, grouting pressure, and fracture roughness all exhibited nonlinear relationships with the grout pressure distribution. With increasing diffusion distance, the grout pressure decreased, and the rate of decrease gradually slowed down. Fracture roughness and aperture had a relatively small impact on grout pressure, while grouting pressure had a significant influence on grout pressure distribution. The difference in grout pressure between the initial and final stages of diffusion was small, whereas in the middle stage of diffusion, the difference was more pronounced. This research provides a valuable reference for the selection of grouting techniques in the roadway surrounding rock projects.

To solve the whole cutting-off and large-area weighting of thick-hard roof led by the high-intensity mining in working face, the mechanical mechanism of the impact pressure of thick-hard top working face in the Shendong mining area is studied. The thick-hard roof in the Shendong mining area is characterized by large thickness, high strength, and difficulty in caving, which frequently triggers strong mine pressure, coal wall spalling, rock bursts, and mining-induced seismicity. Traditional thin-plate theory ignores transverse shear deformation and cannot satisfy the calculation accuracy requirements of thick-hard strata with a large thickness–width ratio, so it is necessary to establish a more accurate medium-thick plate mechanical model. By analyzing the boundary conditions of the thick-hard-roof working face in the Shendong mining area, the mechanical model of the medium-thick plate under four typical boundary conditions is established. Then, the finite integration method is used to solve the deflection, bending moment, and shear force of the thick-hard roof under the four boundary conditions. Through the programming operation, the influence laws of the deflection, bending moment, and shear force of the thick-hard roof are analyzed. The results show that under tensile failure, the cracks expand along the long side. Then, the cracks occur on the short side of the thick roof. Along the short side, the cracks occur in the middle and expand on the short side of the two sides with the increase in plate thickness, and the bending moment and shear values gradually decrease. For the medium plate of the same thickness, as the width-to-length ratio increases, the deflection value gradually increases, and the bending moment and shear value gradually decrease. The research results reveal the deformation and breaking rules of thick-hard roofs based on the mechanical model of thick plates, which lays a theoretical foundation for the study of the coal-wall plate impact-pressure mechanism induced by the instability of thick-hard roofs.