Directional long borehole hydraulic fracturing for pressure relief in shallow buried coal seams longwall faces beneath gullies

To address the problem of severe strata pressure behavior and hydraulic support crushing accidents during the uphill mining stage of the 50,211 shallow buried coal seam working face beneath a gully, a directional long borehole hydraulic fracturing pressure relief technology for hard roof strata was investigated through a combination of theoretical analysis, numerical simulation, and field engineering practice. A load model and mechanical relationship model of the main roof cantilever beam in the uphill section beneath the gully were established to reveal the asymmetric roof loading characteristics and the mechanism of abnormal strata pressure behavior under gully terrain conditions. Numerical simulations were conducted to analyze the effects of hydraulic fracturing on roof stress evolution and overburden structural stability, thereby clarifying the pressure-relief mechanism of hydraulic fracturing in hard roof strata. Based on the gully distribution characteristics and borehole lithological data of the 50,211 working face, eight directional long boreholes were drilled perpendicular to the gully direction within the uphill section beneath the gully, and large-flow staged hydraulic fracturing was carried out. Fracture propagation and pressure-relief performance were comprehensively evaluated through pressure-flow monitoring and strata pressure monitoring. The results indicate that the directional long borehole hydraulic fracturing technology can achieve continuous and uniform weakening of the hard roof in the gully-affected area. After fracturing, the average periodic weighting interval of the working face was reduced by approximately 5 m. Safety valves were activated on only 12 hydraulic supports, corresponding to an activation rate of 1.71%. In addition, the coal wall remained intact, and no support crushing accidents or abnormal strata pressure events occurred. The study demonstrates that the proposed technology can effectively control severe strata pressure behavior during the uphill mining stage of shallow-buried coal seam working faces beneath gullies, providing a valuable technical reference for safe extraction under similar complex terrain conditions.