Assessment of crack initiation and propagation in rock from explosion-induced stress waves and gas expansion by cross-hole seismometry and FEM-DEM method

Single-hole blast-induced damage in a granitic outcrop has been assessed through both controlled experiments and numerical simulations with a combined finite-discrete element method (FEM-DEM). Damage tomographies from decoupled short and long explosive charges in flooded boreholes were obtained through a high-resolution cross-hole system, by measuring pre- and post-blast seismic velocities around the blastholes. Damage was assessed through crack density, which was calculated by inversion of P-wave velocity measurements through an Effective Medium Theory (EMT) method. The resulting damage was found to be highly asymmetrical around each blast hole, both along the vertical and horizontal planes, despite the apparent isotropy and homogeneity of the granitic rock mass. Experimental results combined with numerical simulations carried out to assess damage from stress waves alone, showed that most damage from experiments was caused by the expansion of gases, while its magnitude and extension were strongly dependent on confining conditions along the blasthole. The difficulty of quantifying the relative contribution of stress waves and gas expansion and the problems inherent in prediction of blast-induced damage are described.