BLASTING PROBLEMS IN UNDERGROUND CONSTRUCTIONS THROUGH DECCAN TRAP FORMATION - SOME EXPERIENCES AT KOYNA-HYDROELECTRIC-PROJECT, STAGE-IV

The underground excavations of Koyna Hydro-electric Project, Stage IV were carried out by drilling and blasting through Deccan trap formatio ns consisting mainly of basalts and volcanic braccia. Overbreak in, th e basaltic formation and underbeak in the volcanic braccia increased t he cost and time of construction considerably. Modifications in the se quence and method of excavation and implementation of smooth blasting practice improved the productivity substantially. The paper reports in details the rock mass condition - nd the blasting problems in the mac hine hall, the head race turnnelm and the surge shaft, the reasons beh ind such problems in retrospect of rock mass-explosive interaction, th e modifications made in the blasting practice and the improvements obt ained thereby. The observations and the results reported herein can he lp set guidelines for the planning and execution of future constructio n work in, the similar formations.