The detonation of explosive charges releases large quantities of energy that can produce deformations in the vicinity of blasting site. Extensive data are available on blasting in general and on the behavior of surface structures subjected to blast vibrations. However, only limited information is available on the effect of blast induced dynamic forces on underground structures like tunnels and caverns. This paper deals with the research work carried out at Koldam Hydroelectric Power Construction Project (KHEPP) on the effect of repeated blast vibrations on the jointed rock mass. Multiple rounds of blasts were conducted at the penstock tunnels and at the excavation site for powerhouse foundation. The damage caused by blast induced vibrations can be categorized into two types: i) near-field damage due to high frequency vibrations when the blast is occurring in the close proximity and ii) far-field damage due to low frequency vibrations when the blast is occurring relatively farther distances. The near-field damage was assessed by analytical damage models based on the ground vibrations. The far-field damage was assessed by measuring deformations of borehole extensometers and by borehole camera inspection surveys before and after the repeated blasting. Peak particle velocities generated by blast rounds were recorded by installing triaxial geophones near the borehole extensometers and borehole camera inspection holes. Damage assessment instrumentation was carried out at both the sides of penstock tunnel wall as another objective of the study was to compare the extent of rock mass damage with different joint orientations. The study reveals that repeated dynamic loading imparted on the jointed rock mass from subsequent blasts, in the vicinity, resulted in damage even at 23–26% of critical peak particle velocity. The far-field damage due to the repeated blast loading, after 56 rounds, was 77% of the near-field damage. It was also found that the far-field damage due to the repeated blast loading at the tunnel wall with 1500 joint orientation is 74% more than the damage at tunnel wall with 200 joint orientation. The results of the study indicate that repeated blast vibrations, even at less than critical vibration levels can cause damage problems to the structures in jointed rock mass. The paper stresses the need for consideration of the effect of repeated blast loading for comprehensive damage assessment as well as for fixing the threshold vibration limits to avoid the blast induced damage.
Blasting produces seismic waves similar to those produced by earthquakes, but with relatively high frequency and low amplitude and the degree of structural damage depends on the total energy of explosion, distance from the source, and the character of the medium. Blast induced damage weakens a rock mass, potentially leading to stability problems in the underground excavations. The blast damage problem is more severe and vulnerable for the jointed rock mass in underground excavations (Singh and Xavier, 2005). Unfortunately, there are no specific safety guidelines available for the blasted tunnels with regards to the threshold limits of vibrations caused by repeated blasting activity in the close proximity.