Experimental and numerical study on the cumulative-deck charging structure based on explosive sympathetic detonation in tunnel smooth blasting

Smooth blasting technology usually applied in tunnel excavation to achieve high quality of contour and low depth of damage in surrounding rock. At present, the deck charging structure used in conventional smooth blasting techniques requires the use of detonating cords for explosive connection and auxiliary materials to fix the explosives, leading to complex and costly procedures. To address the issues, this paper proposes the cumulative-deck charging structure without the detonating cords based on the principle of emulsion explosive sympathetic detonation (EESD). Numerical simulations, theoretical analysis, and field experiments are employed to comparatively analyze the effects of EESD distance on rock breaking in columnar blastholes under different rock conditions. The results indicate that, in practical applications, the EESD distance should be less than 50 cm in hard rock conditions and less than 25 cm in fractured soft rock conditions. Combining with the charging structure design method of cumulative blasting, a comparative analysis was conducted on the rock-breaking effects and damage ranges of different shaped charge pipes. It was determined that halved PVC pipes should be used as auxiliary tools for fixing explosives in the cumulative-deck charging structure, replacing traditional bamboo pieces. In-situ experiments showed that this method facilitated the transmission of blasting energy from the peripheral blastholes towards the free face. After blasting, the half-hole marks around the blastholes were prominent, resulting in excellent tunnel cross-sectional shaping effects. Moreover, the cumulative-deck charging structure proposed in this study will provide a reference for the design of smooth blasting in tunnels with different rock conditions.