Ground control strategies for longwall top-coal caving panel in extra-thick coal seams with thick-hard roof

Abstract Thick-hard roof fracturing presents a significant challenge for disaster prevention in longwall mining. This study investigates the 15,311 south working face and develops a mechanical model to analyze energy release during both initial and cyclic roof fracturing, along with the associated disaster mechanisms. Results indicate that total energy release increases with mining depth, roof thickness, and tensile strength, but decreases with the thickness of the immediate roof. The energy released during initial main roof fracturing is more than twice that of cyclic fracturing. Based on site geology, directional long-drilling fracturing parameters for the thick-hard fine sandstone roof were determined, with a single-point water injection of 20 m³ and a fracturing point spacing of 15 m. Following the “energy mitigation-structural weakening-stress regulation” principle, a “fixed-length drilling-hydraulic fracturing” model was implemented. On-site monitoring showed maximum movements in the tailgate of 236 mm in coal pillar gangs, 135 mm in solid coal, and 287 mm in roof-floor convergence. The initial pressure step decreased from 45 m to 18 m, and average cyclic pressure steps were reduced by 35% compared with a non-fractured face. The proposed scheme effectively facilitated safe and efficient mining under a thick-hard roof.