Pressure-relief mechanism and strata-pressure response of surface L-shaped well hydraulic fracturing for thick hard roofs in deep extra-thick coal seams

Rockburst hazards in deep extra-thick coal seams with thick hard roofs are closely related to high in situ stress, strong strata control, and energy accumulation in overlying key strata. This study investigates the pressure-relief mechanism and strata-pressure response of staged hydraulic fracturing through surface L-shaped horizontal wells, taking the 40,307 working face of Xiaozhuang Coal Mine as an engineering case. A comprehensive approach integrating in situ stress testing, key-stratum mechanical analysis, UDEC numerical simulation, microseismic monitoring, and strata-pressure observation was adopted to reveal the structural weakening mechanism of thick hard key strata and evaluate the effectiveness of hydraulic fracturing. The results show that the deep mining area is dominated by high horizontal stress, which promotes the formation of large-span suspended roofs and increases the potential for stress concentration and energy accumulation. Mechanical analysis indicates that surface L-shaped well hydraulic fracturing can weaken the integrity and continuous bearing capacity of thick hard key strata, promoting the transition from large-span integral failure to short-span segmented failure. Numerical results demonstrate that fracturing expands the overburden stress-relief range and reduces the periodic weighting interval from 59 to 82 m to 21–40 m. Field monitoring confirms that the four surface L-shaped horizontal wells generated continuous fracture networks in the target sandstone layer, and the average periodic weighting interval after fracturing decreased to 26.85 m. The integrated analysis indicates that surface L-shaped well hydraulic fracturing changes the roof failure behaviour from delayed concentrated failure to progressive failure and energy release, thereby achieving regional pressure relief and rockburst control. This study provides an effective approach for rockburst prevention in deep coal mines with high stress and thick hard roof conditions.