With increases in mining depth and intensity, disasters such as stress concentration, slab failure, and coal body dynamic outbursts at the coal–rock interface have become more serious. Therefore, it is important to analyze the stress–strain behavior of coal–rock combinations to explore the deterioration process and failure characteristics of coal–rock combinations. In this study, we used field survey, theoretical analysis, and numerical simulation methods to explore the microstructure characteristics of the coal–rock interface and the influence of interlayer thickness on the composite body. The results show that with the increase in interlayer thickness, the compressive strength of the composite body gradually decreases. This reduction is mainly due to the interlayer dividing the coal sample, resulting in a decrease in the equivalent elastic modulus of the composite body, weakening of the overall integrity, and a decrease in carrying capacity. In addition, the failure mode and mechanical properties of the coal–rock combination are influenced by the interlayer position. Different “soft layer” positions can lead to changes in the overall carrying and failure modes of the coal–rock composite. The position of the interlayer also has a significant influence on the failure mode and fracture propagation of the composite body. This study provides an important theoretical reference for the control of coal–rock deformation and instability and regional rock mass modification in underground engineering.

This study investigated the effects of borehole diameter and borehole interval on the stress relief performance of coal seams during mining. A discrete element numerical model was established based on the Voronoi and block models, and the stress relief performance was investigated in terms of axial stress, vertical displacement, and plastic zone development using the control variates method. The results showed that a decrease in the distance from the workface was accompanied by an increase in the effects of mining, the stress concentration, and the vertical displacement of the roof. Coal mining had positive effects on fracture propagation in front of the workface. As the borehole interval increased, the density of boreholes in the coal seams decreased, reducing the stress concentration and fracture propagation in front of the workface. The stress relief performance of the boreholes in the coal seams was improved at a borehole interval of 8 m. However, as the borehole diameter increased, the stress concentration near the boreholes decreased, reducing the effects of the workface mining on stress relief through the boreholes. Meanwhile, increasing the borehole diameter decreased the vertical displacement of the roof but exacerbated fracture propagation in front of the workface, which improved the stress relief performance of the boreholes in the coal seams. This study provides a reference for arranging boreholes in coal seams during mining.

Repeated mining under insufficiently collapsed gobs is a complex process in underground mining and is associated with safety hazards such as ground collapse and subsidence. The effect of layer spacing on the fracture network evolution and fluid transport mechanisms in rock strata during this process has not been systematically studied. In this work, the discrete element method was employed to analyze the fracture development and seepage evolution of surrounding rocks in the Nanliang coal mine across varying layer spacings (5, 20, 35, 50, and 65 m). A systematic evaluation of the rock mass integrity was conducted through damage coefficient quantification. The key findings revealed that an increase in the layer spacing progressively reduced the damage coefficients in both the overburden strata above the goaf and in the interlayer formations ahead of the working face, accompanied by reduced fracture propagation intensity. Shear failure mechanisms dominated throughout the mining process. Fractal characteristics of the fractures intensified with the advance of the working face, while the hydraulic conductivity and interstitial pressure in the interlayer strata exhibited declining trends with reduced attenuation rates. Our findings provide critical insights for ensuring the safety and improving the efficiency of repeated mining under insufficiently collapsed gobs.

Polyurea spraying is a new temporary support technology that can significantly enhance the mechanical properties of coal. However, the mechanism of interactions between the polymer coating and coal is unclear. In this study, the No. 4 non-stick coal from Mengcun Coal Mine and polyurea material were used to conduct experiments and numerical simulations. The tests and simulations were used to examine the role of the sprayed coating in the formation of residual strength and the unloading and rebound mechanism after brittle failure of the coal. The results showed that the presence of the polyurea coating had a significant impact on the mechanical behavior of the coal. The specimens sprayed with polyurea were affected by the confining pressure applied by the coating and the internal friction of the coal; consequently, the specimens exhibited certain plastic characteristics and maintained their residual strength after experiencing brittle failure. The polyurea coating not only effectively prevents the loosening and slippage of the coal but also improves the stability of the coal by altering its mechanical behavior during the loading process. This study lays the foundation for popularizing and applying polyurea spraying technology in coal mine support while providing rich data to support further theoretical research.

Water inrush disasters continue to plague the advancement of deep underground mining activities. A better understanding of the structural integrity of fragmented geological bodies is crucial to ensuring mining safety. The objective of this study was to accurately reflect the dynamic evolution of pore structure changes and seepage channels in fragmented coal and rock mass of actual goafs under the coupling effect of mining stress and seepage. The co-evolution laws of the axial strain, nonlinear porosity, and permeability of fragmented coal and rock mass under different particle sizes, gradation characteristics, and stress states were compared, and a stress-pore-water seepage coupling model of the fragmented coal and rock mass was constructed. When subjected to the same axial pressure, the saturated fragmented coal exhibited a higher water permeability than the saturated fragmented rock mass. The greater the particle size, the higher the water permeability of fragmented coal and rock mass. The higher the gradation index, the higher the water permeability of these masses. Their porosity and axial pressure satisfied an exponential attenuation function, whereas their water permeability and axial pressure satisfied the Boltzmann function. The research results can provide theoretical support for preventing and controlling water inrush in goaf areas.