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  Evaluation of blast induced damage in rock mass using sonic velocity measurement

  In an underground excavation, damage to the surrounding rock mass not only endan-gers safety of structure but also increases cost of construction and time of completion. Drilling and blasting for excavation of any underground structure should strictly adhere into specialized controlled blasting pattern to minimize damage to peripheral in situ rock mass. Authors carried out experiments to measure blast induced damage in 99 MW L&T-Singoli-Bhatwari Hydro-electric Power Project, Rudraprayag. Repetitive NX size rock core up to 5.0 m depth were taken from side wall and face. Tests reveal that ultrasonic velocities reduce to 70 % of the average values in the initial 0.5 m from the excavation boundary. Microscopic analysis of the thin section reveals that intra-granular cracks are formed due to blasting upto 1.0 m from the excavation boundary. It is observed that the RQD and core recovery reduced by 30% in the vicinity of the excavation upto 1.0 m.

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