Abstract
The underground excavations of Koyna Hydro-electric Project, Stage IV were carried out by drilling and blasting through Deccan trap formations consisting mainly of basalts and volcanic braccia. Overbreak in the basaltic formation and underbeak in the volcanic braccia increased the cost and time of construction considerably. Modifications in the sequence and method of excavation and implementation of smooth blasting practice improved the produotivity substantially. The paper reports in details the rock mass condition- nd the blastlng problems in the machine hall, the head race turnnelm and the surge shaft, the reasons behind such problems in retrospect of rock mass-explosive interaction, the modifications made in the blasting practice and the improvements obtained thereby. The observations and the results reported herein can help set guidelines for the planning and execution of future construction work in the similar formations.
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