| Abstract: | Tunnels are required to be constructed for meeting different human needs such as power generation, transportation, underground storage, sewage etc. The predominant method of excavation, world over, is drilling and blasting owing to its capability to meet changing geo-technical conditions. Irrespective of the purpose for which the tunnels are driven, all are plagued by overbreak problems. Tunnels driven for water conveyance in hydroelectric power projects, in particular, need to be excavated with minimum overbreak to minimise cost of permanent concrete lining. Thus, predicting overbreak assumes significant importance to design site-specific blasts for minimizing rock damage. This paper presents a brief review of existing PPV (Peak Particle Velocity) based blast-induced rock damage estimation criteria and attempts to outline the ground vibration threshold levels for overbreak/rock damage in a tunnel driven through compact basalt. Rock damage manifested as overbreak is measured and correlated with the possible threshold levels of PPV. Also, the PPV levels for crack initiation and widening are proposed. The case pertains to a lake tap horizontal tunnel of Koyna Hydro-electric Power Project, India which is a water feeder tunnel for a fully underground hydroelectric power project. The tunnel was driven under a shallow rock cover of average depth ranging from 20 to 25m beneath a fully charged water body. The parting rock is mainly compact basalt. Blasting was carried out in two rounds in a controlled manner, i.e., by limiting the maximum charge per delay based on ground vibration monitoring. Ground vibration generated with free face (in second round) has been modeled and a new ground vibration propagation equation is proposed for tunnel blasting including the effect of an extra free face. The threshold limits of PPV for different degrees of overbreak/rock damage are proposed from extrapolated vibration predictor equation. The actual overbreak in the tunnel, measured using a Planimeter, varied from 2.45 per cent to 17.75 per cent of the finished tunnel area. The predicted overbreak from extrapolated PPV measurements is compared against the measured overbreak to validate the proposed blast-induced rock damage (BIRD) assessment model. The PPV level for overbreak was found to exceed 2050 mm/s in compact basalt. A linear relationship between the overbreak and maximum charge per delay is also established to design a tunnel blast in similar formations. |
