Detonating-Cord-Free Smooth Blasting in Peripheral Tunnel Holes: Mechanism and Technology

The quality of blasting in peripheral holes directly determines contour-forming quality in tunnels and the level of control over overbreaks and underbreaks. The aim of this study was to address the problems of uneven post-blasting contours, overbreaks at hole bottoms, and underbreaks at hole collars caused by a lack of detonating cord at construction sites. Focusing on a newly built tunnel in the Inner Mongolia Autonomous Region of China, this study uses Ansys/Ls-Dyna numerical simulation and field blasting tests to investigate the damage evolution and stress field distribution characteristics of smooth blasting without detonating cord in peripheral holes. The results indicate that the non-uniform distribution of explosive energy along the blast hole direction is the fundamental cause of overbreak and underbreak. The superposition of the stress field of later-fired holes and the residual stress field of earlier-fired holes contributes to crack coalescence, resulting in greater rock damage on the side of the later-fired holes, which causes overbreak at hole bottoms and underbreak at hole collars. Furthermore, this study proposes detonating-cord-free smooth-blasting parameters for Grade II, Grade III, and Grade IV surrounding rock sections of tunnels, as verified by field tests. For Grade II and Grade III surrounding rock sections of a tunnel, a combined long–short-hole smooth-blasting method for peripheral holes is recommended; meanwhile, for Grade IV surrounding rock sections, long-hole smooth blasting for peripheral holes is adopted. Blasting tests were conducted using the recommended smooth-blasting parameters. The measured average overbreak in the Grade II, Grade III, and Grade IV surrounding rock sections was less than 20 cm, with no underbreak at the hole collar, indicating satisfactory control performance. The conclusions of this study can provide a reference for similar engineering projects.