Fracture Propagation Characteristics and Borehole Spacing Optimization of Bidirectional Shaped-Charge Blasting in the Hard Roof of a Deep-Buried Coal Seam

To address severe mining-roadway deformation induced by roof stress concentration resulting from delayed fracture and caving of deep-buried thick and hard roofs, the ZC1103 working face of Yadian Coal Mine was selected as the engineering background. A fracture criterion based on the crack-tip stress intensity factor and rock fracture toughness was introduced to analyze fracture propagation and arrest along the shaped-charge direction. Bidirectional shaped-charge blasting models with different blasthole spacings were established using ANSYS and LS-DYNA, and multi-hole synergistic fracturing was evaluated from fracture coalescence, inter-hole pressure response, and overlap of damaged zones, followed by field validation. Results show that the bidirectional shaped-charge structure significantly alters the spatial distribution of blast-induced pressure around the blasthole. At 100 and 200 mm from the blasthole center, peak pressures in the shaped-charge direction increased by 48.9% and 35.1%, respectively, relative to conventional blasting, whereas those in the non-shaped-charge direction decreased by 28.0% and 34.9%, respectively. Field tests showed that, at a blasthole spacing of 1.0 m, distinct fracture traces were observed on the borehole wall 0.5 m from the adjacent blasthole, while the borehole walls remained largely intact. Compared with the non-pressure-relief zone, the cumulative deformation of the solid-coal rib, gob-side rib, and roof in the roof-cutting pressure-relief zone was lower by 41.7%, 51.8%, and 38.9%, respectively. These results provide a reference for designing bidirectional shaped-charge blasting spacing under similar deep-buried thick-and-hard-roof conditions based on fracture propagation and inter-hole coalescence criteria.