Tension–compression transition mechanism and fractal characteristics of fracture networks during synchronous dual-borehole blasting

Stress-wave interaction in the inter-borehole rock bridge is a key factor controlling crack initiation and fracture-network evolution during dual-borehole blasting. To clarify the coupled effects of the initial stress ratio and borehole arrangement angle on the failure mechanism of the rock bridge, this study combines theoretical analysis with numerical simulation. Based on elastic wave theory and Laplace inversion, the attenuation and tension–compression transition of radial and hoop stresses under single-borehole blasting are analyzed. A stress superposition framework for synchronous dual-borehole blasting reveals dynamic stress evolution in the rock bridge. Numerical models with different initial stress ratios and borehole arrangement angles are developed to investigate stress redistribution, fracture morphology, and fractal characteristics. The radial compressive wave propagates farther, while hoop tensile stress concentrates near the borehole wall, highlighting unloading-induced tension for crack initiation. Synchronous blasting generates strong stress superposition at the rock bridge center, creating compression along the connecting line and tension perpendicular to it, promoting crack coalescence and vertical splitting. As the initial stress ratio decreases, stress redistribution shifts from near-symmetry to anisotropic zoning, enhancing shear concentration and crack directionality. The arrangement angle adjusts the alignment between geometric guidance and stress constraint, controlling crack deflection, branching, and coalescence. Fractal and multifractal analyses show fracture-network complexity varies non-monotonically with stress ratio and depends on its coupling with the angle. Network heterogeneity is dominated by sparse branch cracks and weak damage zones, while main cracks and crushed zones remain localized. These findings guide directional blasting and borehole-pattern optimization in deep rock engineering.