Crack propagation mechanism of smooth blasting holes for tunnel excavation under high in-situ stress

Smooth blasting is an important excavation method in tunnel construction, creating a smooth and even excavation profile during the excavation process. In this study, the influence of in-situ stress, hole spacing, and radial decoupling coefficient on the superposition of blasting stress and the cracking characteristic between holes were investigated by using integrated analytical and numerical analyses (FDEM). Based on those results, the design of smooth blasting parameters considering the influence of in-situ stress were suggested, and verified against the data collected during the blasting excavation of diversion tunnel at Jinping II hydropower station. The results indicated that the propagation of blast-induced fractures are significantly influenced by the insitu stress conditions, tending to extend towards the direction of the maximum principal stress thus the holes should be arranged along the direction of the maximum principal stress to ensure the formation of cracks between holes and smooth excavation of the wall.