A Numerical Study on Extent of Crushed Zone Around Blasthole in Basalt Rock

Proper understanding of the rock breakage mechanism is important in tunnelling and mining activities. The excavation design uses complex energy distribution pattern when explosion and rock-blast occurs. This paper investigates the effect of a controlled-blast in rock mass using a numerical technique. The rock material properties were determined and fitted in an advance material damage model using the finite element code LS-DYNA. This material model had three governing surfaces, each of which was dependent upon the plastic strain accumulation. The damage zones around the blast hole were categorised into crushed, seismic and elastic zones. In all the cases, the material within the close vicinity of the explosion, failed in compression, whilst the material at distance away from the explosion, failed in tension. The influence of loading rate and its impact on the fracture pattern was also studied. It was shown that the higher loading rate led to the formation of short cracks and areas with large crack densities whereas longer and definite cracks occurred when the loading rate was slow. In both the cases, the rise time was found to be more significant than the decay time. This lead to the development of a relation between the critical rise time corresponding to the maximum damage. Crack velocities were higher for quick loading rates and achieved one-half the magnitude of P-wave velocity observed on basalt rock. The results of this model were validated using the case study of a tunnel excavation in Western Ghats of Peninsular India The numerically obtained ground vibrations in the old tunnel were observed to be under the permissible limits. Likewise, the vibrations deduced in the new tunnels indicated the initiation of cracks. Notwithstanding the above, a reasonably good comparison was obtained between the measured and predicted peak particle velocities.