Blasting construction influence zone and damping hole parameter optimization in large-section neighborhood tunnels with small-spacing

During the construction of small-spacing tunnels, blasting operations in the following tunnel may induce cracking and spalling in the lining structure of the preceding tunnel. This issue is particularly pronounced in large-section tunnels. Therefore, this study employs the peak particle velocity (PPV) as an evaluation index to investigate the influence zones and rational clear spacings for large-section tunnels with small clear distances under different surrounding rock conditions. Based on the findings, key design parameters for damping holes are proposed for scenarios where the required rational clear spacing cannot be achieved, and the effectiveness of these measures is validated through test sections. The main conclusions are as follows: For Grade III, IV, and V surrounding rock conditions, the rational clear spacings for large-section tunnels with small clear distances are 0.76D, 1.00D, and 1.29D, respectively. When the actual tunnel spacing is less than or falls below these values, the tunnel structure may become unstable. Furthermore, damping holes can effectively reduce vibration velocity. More effective vibration reduction can be achieved when the diameter of the damping holes exceeds 6 cm, the spacing between holes is less than 15 cm, the distance from the damping holes to the blasting source is greater than 1.5 m, and the number of rows of damping holes is more than two. The vibration characteristics of the preceding tunnel structure under the blasting construction of the following tunnel were analyzed. The reasonable spacing of blasting construction of small-spacing tunnel under the different conditions surrounding rock was proposed. The vibration reduction effect of the damping hole was quantitatively analyzed, and the design parameters were optimized.