Blast Induced Damage Due to Repeated Vibrations in Jointed Gneiss Rock Formation

Blasting is the most common method of rock excavation technique in mining and civil construction and infrastructure projects. Rock blasting produces seismic waves similar to those produced by earthquakes, but with relatively high frequency and low amplitude. General blast induced damage was extensively studied by researchers globally, but the studies on damage due to repeated blast vibrations is not yet reported, quantitatively, on underground openings. This paper deals with the research work carried on the effect of repeated dynamic loading imparted on the jointed rock mass from subsequent blasts in the vicinity, on the jointed rock mass at Lohari Nag Pala Hydroelectric Power Construction Project. The blast induced damage was monitored by borehole extensometers, borehole camera inspection surveys and triaxial geophones installed at three test sites of different joint orientations at the Main Access Tunnel of power house. The study reveals that there was extra damage of 60%, exclusively due to repeated blast vibrations. The results of the study indicate that repeated dynamic loading, resulted in damage even at 33% of the conventional damage threshold vibrations (Vc) in case of favorable joint orientations and 23% of Vc in case of unfavorable joints. The paper concludes in quantification of effect of repeated blast loading and the orientation of joints on the extension of damage zone in jointed rock mass of underground excavations.