The influence of horizontal borehole spacing on the interaction of two dynamic cracks propagating towards each other under unequal biaxial confining pressure

As resource extraction depth increases, deep rock engineering often encounters geostress environments where horizontal stress significantly exceeds vertical stress, sometimes by factors up to three. This distinctive stress state critically impacts the propagation paths and interaction mechanisms of blasting cracks. This study establishes an experimental system for blasting photoelasticity under unequal biaxial confining pressure (9 MPa horizontal, 3 MPa vertical). It investigates the propagation behavior of opposing cracks and the effects of explosive stress waves and crack-tip stress fields on nearby cracks at various horizontal spacings (8–14 cm). Results indicate that under unequal biaxial confining pressure, the shaped charge shows a marked directional effect, with the optimal horizontal spacing between boreholes being 11 cm. At this spacing, the main cracks between the two boreholes have longer propagation lengths and better connectivity. As horizontal spacing increases, both wave-crack and crack-crack interactions weaken, with crack-crack interactions attenuating faster than wave-crack interactions. When the spacing exceeds 11 cm, the influence of crack-crack interactions on the mode II stress intensity factor at the crack tip rapidly diminishes. Under dynamic-static loading, predictions of crack deflection angle prediction using the Maximum Tangential Stress (MTS) and Generalized Maximum Tangential Stress (GMTS) criteria align closely with experimental trends, particularly regarding the timing of inflection points. However, predictions are consistently larger than actual values, and T-stress has minimal effect on crack deflection. Thus, the simpler MTS criterion is preferable for practical applications. These findings provide key insights into the interaction of opposing dynamic cracks at varying horizontal spacings and strengthen the theoretical foundation for optimizing perimeter-hole blasting parameters in horizontal stress-dominated environments.