Overburden failure characteristics and fracture evolution rule under repeated mining with multiple key strata control

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.