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.

The dynamic compression mechanical characteristics of the surrounding rock mass of the shale formation tunnel in western Hubei province need detailed exploration. A Split Hopkinson Pressure Bar(SHPB) and a high-speed camera were employed to conduct impact tests on shale samples at five different bedding angles(the angle between the direction of impact loading and the normal of the bedding planes of the specimen, including 0°, 30°, 45°, 60°, and 90°). Meanwhile, the research team also studied the influence mechanism of bedding angles, impact pressure, and strain rate on the dynamic compression mechanical characteristics and failure mode of shale with different dynamic loading strain rates under different impact pressures. The research results indicate that the dynamic compressive strength of shale has an approximately U-shaped pattern with increasing bedding angles under different impact pressures and strain rates. Among them, the shale with bedding angles of 0° and 90° has relatively higher compressive strength, while the shale with a bedding angle of 60° has the most minor compressive strength. Furthermore, the dynamic compressive strength of shale with different bedding angles increases as the impact pressure and strain rate increase. The macroscopic failure modes of shale are mainly divided into tensile failure, shear failure, and mixed failure. Significantly, the macroscopic failure modes of samples with bedding angles of 0° and 90° under different strain rates are mainly tensile failure. The primary macroscopic failure mode of the sample shows a transition process of shear failure mixed failure tensile failure' as the strain rate increases when the bedding angle is 30°. The primary macroscopic failure mode of the specimen evolves from shear failure to mixed failure as the strain rate increases when the bedding angle is 45° and 60°. The energy absorption ratio of shale samples first increases and then decreases as the bedding angle increases under the same impact pressure. Additionally, the energy absorption ratio and the degree of sample damage are simultaneously maximum as the bedding angle is 60°. The degree of fragmentation of shale samples with different bedding angles increases, and the energy absorption ratio gradually tends to be consistent as the impact pressure and strain rate increase.