Time-constrained demolition of a concrete cofferdam using controlled blasting

Abstract

A concrete cofferdam of a hydropower project in India had to be demolished within a limited time period due to impoundment of river water. The cofferdam was trapezoidal at bottom and middle, rectangular at the top and abutted to one of the spillway piers. Two more spillway piers, spillway gates and a dam were very close to it. The length and height of cofferdam were 62 m and 24 m, respectively. For demolition of cofferdam, a new controlled blasting technique was developed after conducting test blasts at different locations. Two test blasts with varying blast designs and charging patterns were conducted at the farthest end of cofferdam which was free from other structures. Two more test blasts were conducted near the abutted portion with a very light explosive charge per hole fired in delayed sequence using half-second electric detonators. Results of the test blasts, particularly ground vibrations, throw of blasted materials and breakage patterns were analysed critically. Based on the results, a cautious blasting zone was determined near the abutted portion and controlled blasting patterns were designed. The formation of a zone of disturbance from cautious blasts at the abutted end for every concrete lift significantly reduced the magnitudes of vibration waves for successive blasting operations in remaining portion of that lift. The use of half-second delay detonators helped in segregating the vibration waves from the holes fired in different delays. The demolition work completed safely within the time limit using cautious blasting technique and systematic sequencing of blasts.