In situ stress constrains and redirects crack propagation during ultra-deep hole blasting of thick and hard roofs in coal mines

Abstract Ultra-deep hole blasting in thick and hard roofs is typically conducted under in situ stress and without a blasting free surface. Determining the extent of blast-induced crack propagation is therefore essential for blasting parameter design and performance evaluation. At the 73 upper 11 working face of Baodian Coal Mine, this study combines theoretical analysis, LS-DYNA numerical simulations and field borehole imaging to investigate the effects of in situ stress on the extent, direction and number of blast-induced cracks. In situ stress has little influence on near-blasthole pressure and particle-velocity responses but primarily restricts intermediate- and far-field crack propagation through the normal compressive stress acting on crack surfaces. As equal biaxial confining pressure increases from 0 to 30 MPa, the maximum crack length decreases from 231 to 43 cm and the number of major cracks from 18 to 0, whereas peak pressure at the blasthole wall and particle velocity change by less than 1%. Under unequal biaxial confining pressure, the mean normal stress primarily suppresses crack extent, whereas the principal stress difference mainly governs the directional concentration of crack propagation; the number of major cracks reflects the combined influence of these two stress components. To address the preferential crack orientation and spatially non-uniform distribution induced by in situ stress, an inclined-offset borehole observation method is proposed to acquire crack information at different distances and orientations. Major cracks were observed up to approximately 2.40 m from the effective charge segment, whereas only isolated peripheral cracks were observed at approximately 2.80 m. These findings provide a basis for evaluating the effective blasting range and designing blasting parameters for thick and hard roofs.