Experimental and numerical investigation on rock fracturing in tunnel contour blasting under initial stress

In tunnel contour blasting, the rock fracturing is generally subjected to in-situ stress. In this investigation, the rock cracking and damage in tunnel contour blasting (smooth blasting and presplitting) under various initial stresses are experimentally and numerically studied. Eight model tests are conducted on tunnel contour blasting with 400 x 400 x 50 mm granite plates. High-speed digital image correlation is employed to identify blast induced rock fracturing, and image-processing with ImageJ is used to analyse the detailed fracture pattern and rock fragmentation. The variations in fracture networks, fragment size, fractal dimension of rock cracks and fractal damage to rock are thereby examined with increasing stress magnitude and stress dimension. Subsequently, the stress evolution and rock fracturing in model tests are simulated in LS-DYNA, and the energy dissipation during tunnel perimeter blasting is analysed. The investigation results demonstrate that initial stress results in the reduction of length and number of rock cracks under tunnel contour blasting, leading to smaller fractal dimension of crack pattern, less damage into rock and coarser rock fragmentation. Tunnel perimeter excavation performs well under low biaxial stress but tends to fail as the biaxial stress continually increases. Under uniaxial stress, smooth excavation peripheries can be achieved when the centreline of adjacent perimeter holes is parallel to the stress direction, while perimeter excavation is unsatisfactory when the connection line of adjoining boreholes is perpendicular to the initial stress. Comparing smooth blasting with presplitting, it is found that more explosion energy is consumed in rock fracturing during smooth blasting, especially in the excavation zone, leading to a better control of rock fracture and damage in tunnel contour excavation. Therefore, smooth blasting is preferentially recommended for deep tunnelling. Additionally, small burden is also recommended for deep tunnelling with smooth blasting to avoid potential oversize fragmentation in the excavation zone.