There are normally pre-existing cracks that can be observed in the coal seam and immediate roof that influences the stability of the rib spalling and the movement law of overlying strata. In this study, comprehensive research methods (e.g., theory analysis, experimental tests and numerical simulations) were adopted to reveal the mechanical characteristics, acoustic emission behaviors and failure modes of a coal–mudstone combined body with a single prefabricated non-penetrating crack. The results show that the influence of the crack angle on the elastic modulus of the coal–mudstone combined body samples was limited. With the increase in the crack angle, the unconfined compressive strength of samples decreased first and then increased in a V-shaped trend. In addition, the minimum unconfined compressive strength could be observed at a crack angle of 45°. Moreover, the number of acoustic emissions significantly increased with the process of continuous loading. In addition, the stress reduction zone could be observed in both ends of the prefabricated cracks at the initial stage of loading. The high- and low-stress zones were transformed with the process of continuous loading. Under an unconfined compression test, the failure models of the coal body part in the samples were mainly caused by shear failure, and only a few cracks occurred in the upper tip of the prefabricated cracks of the mudstone part. Therefore, airfoil cracks could be observed in the samples due to the strength difference of the coal mass and mudstone.

The double wedges sliding along the weak layer of the foundation can be observed on the slope of the waste dump and the sliding body is divided into the active wedge and passive wedge by the weak foundation and the failure surfaces of the waste dump. Because the conventional limit equilibrium slice method cannot reflect the polygonal slip surface of the slope of the waste dump with weak foundation, this study proposed a double wedge calculation method for the slope of the waste dump with weak foundation. The limit equilibrium analysis is performed on double wedges by considering the direction and values of the interaction force between double wedges to obtain the safety factor of the slope of the waste dump. Meanwhile, the quasi-3D double wedges stability analysis method of the waste dump slope with weak foundation is proposed by considering the influence of the geometry and sliding direction of the slope surface on the slope stability. The safety factor of the inverted dump slope is 0.82, the volume of the sliding body is 6.43 million m³, and the main sliding direction is 20° south by east. The shear strain rate cloud diagram of the section is ‘y’ type distribution, and the sliding body is divided into two independent blocks. The safety factor of the sliding body section obtained by the double wedge method is between 0.76 and 0.92, and the closer to the boundary of the sliding body, the greater the safety factor of the section. The quasi-three-dimensional safety factor obtained by theoretical analysis is 0.817. The results show that the calculation results of quasi-3D double wedge are basically consistent with the calculation results of strength reduction method, while the proposed method is simpler. It can be used as a quick method to evaluate slope stability in engineering practice.

Based on the complex hydrogeological conditions of the Chensilou mine, numerical simulations and field validation methods were used to study the mechanism of water inrush from the floor of the coal seam, which has faults and cracks, as well as the regional advanced grouting reinforcement technology during the coal mining process. The evolution laws of the roof stress field, displacement field, crack field, and plastic area are revealed at different mining distances. The coupling mechanism of floor water inrush channel formation under complex conditions is analyzed. Advanced grout filling reinforcement technology in the ground area is proposed, the slurry diffusion law of different grouting layers under different grouting pressures is revealed, and the grouting effect is evaluated, which provides a research basis for selecting a reasonable grouting pressure. Finally, the application of regional advanced grouting reinforcement technology was carried out at the site, and the grouting reconstruction effect was verified by the transient electromagnetic and three-dimensional DC resistivity method. The results show that the apparent resistivity of the floor after the grouting reinforcement is high, and the water yield of the verification borehole is less than 10 m³/h. The area where the three-dimensional direct current resistivity is less than 12 Ω·m only appears in the lower part of the middle of the working face, and there is no water in the verification borehole. Through our underground supplementary treatment and verification process, the initial water inflow meets the requirements of being less than 10 m³/h. It indicates that the ground regional advanced treatment project achieved significant results. The results of our research can also provide references for water hazard control in similar mines.

Taking a coal mine in the Weibei Mining Area of Shaanxi Province as the engineering background, and focusing on the floor water inrush problem encountered in the mining of narrow strip working faces above the Ordovician limestone confined aquifer, this study effectively overcomes the limitations of the traditional high-cost surface large-scale aquifer grouting reconstruction technology and the discontinuous underground short-hole aquifer grouting reconstruction technology, and proposes a continuous underground directional long-borehole grouting reinforcement technology targeting the aquifuge. A comprehensive research methodology integrating theoretical analysis, numerical simulation, and field measurement is adopted to systematically investigate the pore structure characteristics, mining-induced failure mechanism, water richness and grouting reinforcement effect of the coal floor. The results show that the permeability of quartz sandstone ranges from 0.033 to 0.059 mD, and its porosity varies from 1.926% to 7.776%. Notably, scanning electron microscope (SEM) observations reveal that abundant micropores, mesopores and distinct fractures are developed inside the rock, endowing it with certain seepage capacity. Under the condition of narrow strip mining, the maximum failure depth of the floor is approximately 5.74 m, with shear failure as the main failure mode. The rock strata within the range of -6.5 m to -15.5 m below the floor are basically unaffected by mining activities and exhibit favorable water-resisting performance. Therefore, based on the floor failure depth, water-resisting performance, and pore structure characteristics, the grouting reconstruction horizon is arranged within the quartz sandstone layer. Finally, grouting reconstruction of the target horizon was carried out using underground directional long boreholes, which significantly improved the apparent resistivity of the rock mass. This confirms that the continuous grouting technology can effectively seal fractures and pores and enhance the performance of the target aquifuge. The research results provide a new approach for the prevention and control of water hazards in mine floors under similar conditions.