To master the overburden structure failure and fracture evolution law under multi-key stratum control during repeated mining in coal seams, this study takes the superimposed mining faces of 6^# and 7^# coal seams in Xinjiang mining area as the research background. Through comprehensive research methods including theoretical analysis, similarity simulation, numerical simulation, and field monitoring, it systematically reveals the evolution law of overburden fractures under repeated mining controlled by multiple key strata, and clarifies the crucial control effects of cumulative damage in the overburden and the superimposed effect of mining-induced stress fields on fracture morphology during repeated mining of close-distance coal seams. The research shows: (1) Based on key stratu theory, four key strata are identified above the working face, with the first sub-key stratum and the main key stratum located at 61.63 m and 174.63 m above the 7^# coal seam, respectively. (2) Similarity simulation displays that after mining the upper 6^#coal seam, the overburden failure exhibited an approximately “trapezoidal” distribution, with a fracture zone height of 40.2 m (height-to-mining ratio of 13.4); the fracture zone height in the area unaffected by repeated mining above the 7^# coal seam was 115 m (height-to-mining ratio of 12.8). Repeated mining intensifies cumulative damage in the overburden and superimposes mining-induced stress fields, leading to rapid fracture propagation. Ultimately, under the combined control of multiple key strata, the fracture zone and caving zone heights stabilizes at 139.68 m and 42.88 m, respectively. The overburden failure pattern evolves from the single “trapezoidal” structure of initial mining to a "double-trapezoidal" composite structure, with fracture evolution following the pattern of "slow expansion during initial disturbance-leap increase due to cumulative damage from repeated mining-gradual stabilization regulated by key strata". (3) Field monitoring using the double-end water plugging and borehole observation joint detection method measures the fracture zone height after 6^# coal seam mining as 38 m (height-to-mining ratio of 13.6 based on a measured seam thickness of 2.8 m), and the fracture zone height in the exclusively mined area of the 7^# coal seam as 100.33 m (height-to-mining ratio of 12.4 based on a measured seam thickness of 8.09 m). The predicted height-to-mining ratios under different coal thickness conditions show good agreement. The research results provide a theoretical basis for roof water hazard prevention and control and surface damage management in the Xinjiang mining area.

Based on the 7618 working face in Yaoqiao coal mine of Datun mining area, the activation mechanism of water-rich faults and the development characteristics of water-conducting fractures in overlying strata under the influence of faults are studied by theoretical analysis, numerical simulation and field measurement in this paper. The research results show that Anderson model and Mohr–Coulomb strength criterion are combined to establish the fault failure mechanical model, and the fault activation criterion under the influence of mining is obtained. FLAC3D numerical simulation results show that with the advance of the working face, the fault begins to be affected by the mining effect of the working face at the distance of 20 ~ 30 m from the fault. Meanwhile, with the advance of the working face, the overburden shear failure range also expands, and the fault fracture gradually expands from top to bottom. The failure zone of the working face roof is connected with the fault fracture zone. Then the fault is "activated" and causes the fault to become a water gushing channel, and finally the water gushing disaster occurs. Through numerical simulation and comparative analysis, the development height of water-conducting fracture is 73.2 m in the absence of fault, and 73.7 m in the presence of fault, indicating that the fault has little influence on the maximum development height of water-conducting fracture. The actual development height of the water-conducting fracture zone in the 7618 working face is 73.97 m and the fracture production ratio is 13.7. The research results can provide theoretical reference for the safe mining of similar working faces across faults.

In order to solve the water and gas discharge hazard caused by gob water and harmful gases (such as CO), the method of grouting overburden fractures is adopted to achieve the purpose of safe and efficient mining production in coal mines. This paper carries out the experimental research on the permeability reduction effect of grouting in fractured rock mass, expounds the relationship between gas flow rate and pressure gradient, seepage pressure and permeability, confining pressure and permeability, and analyzes the permeability change law of fractured rock mass before and after grouting. Besides, the grouting migration and permeability reduction model of fractured fine-grained sandstone is constructed by combining grouting test and numerical simulation, which reveals the dynamic evolution law of rock mass permeability in the grouting process. The results show that the permeability of the grouting rock sample decreases from 700–13,000 to 15–300 mD than that of the ungrouting rock sample, and the decrease is more than 95%, which indicates that the sealing performance of grouting slurry is better. Besides, numerical simulations show that the initial permeability of rock samples is 971.9 mD, and the permeability decreases to 45.79 mD after 1800s, and the permeability decreases to 95.3%, which is basically consistent with the experimental results after grouting. The greater the grouting pressure is, the better the grouting effect is. With the increase of the grouting pressure, the increase of the grouting effect is no longer obvious.

For the study of the layout of the roadway in the coal pillar and floor strata co-mining working face at the Zhaogu No.2 mine, a mechanical model of the segmental coal pillar within the working face was established through theoretical calculations. The analysis considered the stress state of the coal pillar area under different collapse conditions in the goaf after upper strata mining. Additionally, FLAC^3D numerical simulation software was used to simulate the stress distribution in the roadway for different layout positions during strata mining, thereby clarifying the impact of working face mining on the side roadway of the segmental coal pillar. The results show that the collapse of the goaf after upper strata mining significantly affects the stress distribution in the coal pillar area. To ensure safety during mining, roadway excavation and working face recovery should be conducted after the upper strata have fully collapsed during strata mining. The co-mining working face roadway should be positioned beneath the original upper strata goaf, avoiding stress concentration areas in the coal pillar location. Ultimately, it is determined that the side roadway for the lower strata working face should be arranged with an offset of 10 m outward. Practical on-site experience has demonstrated that under this offset, there is minimal deformation of the surrounding rock in the coal pillar side roadway, meeting the safety production requirements of the working face.