Research on Optimization Design and Blasting Parameters of Combined Cutting with Long and Short Straight Holes

To address the issues of insufficient free surface utilization, low crack penetration rate, and excessive rock residue at the bottom of deep holes in long and short straight hole cut blasting for small-section tunnels, an optimization scheme based on the empty hole effect theory is proposed. This scheme constructs a dynamic virtual large empty hole by moving shallow holes inward, thereby enhancing stress concentration and synergistic crack propagation for rock breaking. Key parameters such as the offset distance of inner holes and the spacing between deep and shallow holes are derived theoretically, and field comparative tests are conducted. The results show that the optimized scheme achieves a single-cycle advance of 2.7 m, a blast hole utilization rate of 92.5%, significantly reduced overbreak and specific charge, a crack penetration rate of 90%, and greatly thinned rock residue at the hole bottom. This scheme improves the rock breaking mechanism of cut blasting, enhances tunneling efficiency and economy, and exhibits promising engineering application prospects. Further numerical simulations are recommended to refine the dynamic evolution law of the virtual empty hole.