Timely roof breaking and pressure relief technology by hard roof staged fracturing

Hard roofs in longwall mining often generate large roof caving steps during initial weighting and form extended hanging roofs during periodic weighting. These phenomena can lead to sudden cave-ins, rock bursts, and mine seismicity. This study proposes a staged hydraulic fracturing technique, characterized by advanced hydraulic fracturing and delayed roof caving, to achieve effective roof collapse and pressure relief while mitigating secondary disasters associated with conventional blasting methods. A fracture mechanics model was developed to analyze the ultimate span, bending moment, and roof caving interval of hard roofs under varying stress conditions. The mechanical relationship between fixed-end beams and cantilever beams was established, revealing that the ultimate roof caving interval of a cantilever beam is 0.4082 times that of a fixed-end beam. Formulas were derived to determine the reasonable hanging roof length during periodic weighting, as well as the required fracture depth during both initial mining and periodic weighting. A comparative analysis was conducted between the proposed staged hydraulic fracturing technique and conventional timely blasting. The staged hydraulic fracturing approach reduced engineering workload by more than 28% compared to timely blasting while achieving equivalent roof-hydraulic fracturing and pressure-relief results. Field application at a longwall face with a hard sandstone roof demonstrated fracture distances of 24.04 m during initial mining and 20.73 m during periodic weighting from the working face, with optimal fracture depths of 9.3 and 5.1 m, respectively. The proposed staged hydraulic fracturing technology effectively enables controlled roof hydraulic fracturing and pressure relief under hard roof conditions. It significantly reduces project workload and provides theoretical and practical guidance for the safe and efficient management of hard roofs in longwall mining.