Blast-induced Crack Propagation Mechanism Between Tunnel Perimeter Holes Guiding Effect of Empty Holes

The quality of tunnel contour surfaces in drilling and blasting construction is fundamentally governed by the propagation efficiency and coalescence behavior of fractures generated between perimeter blast holes. This research employed physical modeling and computational analysis to investigate the influence of empty holes around a tunnel perimeter on the propagation law and coalescence effectiveness of cracks. Firstly, comparative model experiments were conducted to evaluate crack propagation and coalescence effectiveness between perimeter holes with and without relief-hole guidance, and to systematically analyze how blast-hole spacing (L) and the decoupling coefficient (K) affect these processes. Subsequently, numerical simulations were employed to study the stress wave propagation between holes under both configurations, thereby elucidating the fundamental mechanisms through which empty holes optimize fracture propagation paths and enhance coalescence effectiveness. Results indicate that effective crack coalescence between blast holes occurs under two parameter sets without empty hole guidance, K=2.0 with L=11D or K=1.25/1.5 with L=13D, where D denotes blast hole diameter. The empty hole guidance demonstrates optimal performance at K=1.5 and L=13D, producing a smoothly coalesced crack that aligns nearly perfectly with the inter-hole connecting line. The observed guiding phenomenon results from stress concentrations at the empty-hole walls, where superimposed reflected tensile waves and incident waves redirect fracture paths toward the inter-hole centerlines, thereby controlling crack propagation trajectories and coalescence patterns. Both experimental modeling and computational analysis revealed that the radius of the crack zone around the perimeter holes under empty-hole guidance was significantly smaller than under conventional blasting, demonstrating their dual functionality in simultaneously directing inter-hole crack coalescence while restricting undesirable radial fracture propagation.