This work consists of design and construction of upstream and downstream Coffer Dam of Indira Sagar Project. This was the new concept of design used for first time in any country to construct a Coffer Dam using 5 Tons pre-cast blocks as shuttering on both faces u/s and d/s side of a dam, and filling the enclosure with boulders and stone-crete them under water. All work of coffer dam has been done underground and under water. The maximum height of u/s coffer dam was 24 meter, 12m under water and 12m above water. For under water portion 5 tons hollow pre-cast c.c. blocks were casted and placed on the u/s and d/s face of dam, with the help of divers. Before placing the blocks, river bed was leveled by using blasting under water and with special technique, PC blocks were placed. Bounders were filled in the enclosure of a 15m long cell and grout pipe with safety reinforcement were places. The cell of 15m was thoroughly caulked from outside, so that river flow does not have any effect in the cell. A colloidal grout, made out of sand, cement, water and super plasticizer was pumped through the grout pipe at bottom and level of grout slowly built up from down upward, thus colloidal grout (colcrete)replacing the water in the voids of the boulder and converting the bounder mass into concrete. About six pre-fabricated construction sluices of 2m*3m were placed to take care of post monsoon discharges up to 300 cumecs as the diversion tunnel was under construction and was not ready.
The entire work of u/s and d/s coffer dam was completed as per schedule in three working seasons, using a big contingent of cranes, under water divers, pontoons, winches and a big team of specialized skilled workers.
Stone masonry was constructed above water level over the under water works.