Hydraulic Fracturing Pressure Relief for Pillar Size Optimization in Ultra-Thick Coal Seam Longwall Panels: Design, Monitoring, and Field Verification

Entry stability in ultra-thick coal seam longwall mining is often challenged by high abutment pressures and the need for wide coal pillars. This study presents the design, implementation, and verification of a hydraulic fracturing pressure relief strategy to optimize pillar width and improve entry performance in the longwall panels of Buliangou Mine. A site-specific fracturing scheme was applied near the coal pillar, using staged multi-interval fracturing from angled boreholes in the roof strata. Field instrumentation, including borehole imaging, water pressure monitoring, and stress/strain sensors, confirmed successful fracture propagation and significant stress redistribution. Post-fracturing monitoring indicated a shift in peak pillar stress location and an expansion of the elastic core zone, with entry deformation (ribs and roof-floor convergence) reduced by up to 25%. Based on these results and comparative case studies, an optimized 26 m pillar width was proposed and subsequently implemented in a new longwall panel. Field verification demonstrated stable entry conditions, consistent support loading, and a notable increase in coal recovery. This study confirms that hydraulic fracturing, when combined with detailed field design and monitoring, provides a reliable solution for stress management and pillar size reduction in ultra-thick seam longwall mining.