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.

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.

This paper investigates the dynamic behavior of non-Darcy seepage systems in post-failure rock. A one-dimensional non-Darcy seepage evolution equation is established, and a 6-dimensional nonlinear ordinary differential system is derived via the spectral truncation method. Eigenvalue analysis is adopted to determine the instability and bifurcation conditions, with the bifurcation diagram plotted. The fourth-order Runge–Kutta method is used to obtain phase trajectory patterns under different initial values. The results confirm the existence of transcritical bifurcations and fold bifurcations. The dynamic response of the system is discontinuous with control parameters, and phase trajectory symmetry breaking occurs with the increase in nonlinear terms. The reduced-order model shows diverse phase trajectories including equilibrium, periodic, chaotic attractors and unstable states. The system is sensitive to initial values, which significantly affect phase trajectory behaviors. The system may lose stability and trigger water inrush hazards under critical conditions. The bifurcation diagram and critical parameters obtained can provide a theoretical basis for the early warning, risk assessment and prevention of coal mine water inrush hazards.