Study on the Prevention and Control of Hydraulic Fracturing Impact Ground Pressure of Hard Roofs During the Initial Mining Period of Thick Coal Seam Fully Mechanized Mining Faces

To address the rockburst hazard caused by overhanging hard roofs and difficult caving during the initial mining period of thick coal seam fully mechanized working faces, this study takes the N4202 fully mechanized top coal caving working face of the Santunzi Coal Mine as the field engineering background. The mined No. 4-1 coal seam has an average thickness of 9.46 m, and its overlying hard roof is composed of medium sandstone and siltstone. A total of 39 hydraulic fracturing boreholes, including type A, type B, type C1/C2, and fan-shaped holes, were deployed, with a designed fracturing depth of 19 m. Three testing means, including a CXK12(B) borehole imaging instrument, a KJ1222 microseismic monitoring system, and on-site roof caving observations, were adopted to comprehensively evaluate the field performance of roof hydraulic fracturing, and the rockburst prevention mechanism was analyzed. The field test results indicate that dense and well-connected fractures are formed after fracturing, with more than 8 fractures per single borehole and a fracture aperture of 0.8–2.2 mm, and the connectivity rate between adjacent fracturing boreholes reaches 92.3%. The initial mining top caving step distance of the working face is reduced to 13.2 m, while the theoretical calculated values are 10 m for the immediate roof and 15.6 m for the main roof. The roof gradually collapses, and the mining pressure is alleviated. During fracturing, the frequency and energy of microseismic events increase by 285% and 230%, respectively, compared to the state before fracturing. In the subsequent mining process, the maximum microseismic energy is only 4.56 kJ, which is far lower than the rockburst critical energy threshold (20 kJ) of this mine. Therefore, no rockburst hazard occurs in the working face. These research findings can provide a practical technical reference for rockburst prevention using hard roof hydraulic fracturing in similar thick coal seam fully mechanized mining faces.