Time-constrained demolition of a concrete cofferdam using controlled blasting
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CASE STUDY
Time‑constrained demolition of a concrete cofferdam using controlled blasting Rakesh Kumar Singh1 · Chhangte Sawmliana1 · Panchanan Hembram1 Received: 10 August 2020 / Accepted: 30 September 2020 © Springer Nature Switzerland AG 2020
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. Keywords Hydropower project · Concrete cofferdam · Controlled blasting · Ground vibrations · Cautious blasting · Demolition
Introduction The National Hydroelectric Power Corporation (NHPC) Limited, a Public Sector Undertaking Company under Government of India, is having a 160 MW (4 × 40 MW) Hydroelectric Project designated as Teesta Low Dam Project-IV (TLDP-IV) on the River Teesta in Darjeeling district of West Bengal state in India. The project comprises of a 45-m-high dam with 4 penstocks of 45 m length and 7 m diameter each. The surface power house with installed capacity of 160 MW houses 4 units of 40 MW capacity each designed to operate * Rakesh Kumar Singh [email protected]; [email protected]
Chhangte Sawmliana [email protected]; [email protected]
Panchanan Hembram [email protected]; [email protected]
1
CSIR-Central Institute of Mining and Fuel Research (CSIRCIMFR), Dhanbad, India
under the net rated head of 25.05 m and designed to generate 720 million units in a 90% dependable year with 95% machine availability. During the construction phase of the project, a concrete cofferdam was constructed at the project in upstream side to facilitate the
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