Failure mechanism of surrounding rock and synergistic control of strong support-strong pressure relief for lower-seam roadways in close-distance coal seam groups

To address the problem of asymmetric dynamic failure and superposed stress concentration in lower-seam roadways during stratified mining of deeply buried close-distance coal seam groups in the Jiaoping mining area of Shaanxi Province, where strong mining disturbance, eccentric loading from residual coal pillars, and weak rock-burst effects are coupled, a surrounding rock stability control system based on a "strong support-strong pressure relief" synergistic strategy is proposed. By integrating theoretical analysis, numerical simulation, and field monitoring, the asymmetric failure mechanism of roadway surrounding rock induced by the coupling of overlying residual coal pillars and mining-induced stress is revealed. On the basis of an internally staggered roadway layout and a high-preload cable bolt support system, directional hydraulic fracturing pressure-relief technology is further introduced. The dynamic response characteristics of roadway surrounding rock under different impact locations are systematically investigated, and a method for determining hydraulic fracturing parameters for arbitrarily oriented boreholes is established based on elastic theory. Comparative analyses of roof impact and roof-rib impact loading conditions show that, under static loading, roadway stress and deformation exhibit pronounced asymmetry but remain controllable due to the high-preload cable bolt support. Under dynamic loading, roof impact mainly induces tensile failure of the roof, whereas roof-rib impact tends to produce a composite failure mode characterized by severe rib extrusion and overall roof subsidence. Directional hydraulic fracturing effectively weakens stress concentration in the surrounding rock and overlying coal pillars, thereby reducing the intensity of dynamic roadway response. Borehole transient electromagnetic detection results indicate significant fracture development after fracturing, confirming an evident pressure-relief effect. Field monitoring further demonstrates that bolt and cable forces remain within a safe range, and the asymmetric expansion of the loosening zone is effectively controlled. The results verify the applicability and effectiveness of the proposed "strong support-strong pressure relief" synergistic control technology for lower-seam roadways in close-distance coal seam groups, providing a reliable theoretical basis and engineering reference for surrounding rock stability control and safe, efficient mining under similar geological and mining conditions.