Cofferdam not on engineers’ priority list

‘Can move forward only after completion of important structures such as spillway’

The State government’s plan to send water into Polavaram project canals by gravity using a cofferdam seem to be closer to being realised with the Dam Design Review Panel (DDRP) giving clearance to its designs.

Chief Minister N. Chandrababu Naidu even performed the groundbreaking ceremony for the upstream cofferdam on Monday. Engineers, however, say much work needs to be done even on the spillway before the construction of the cofferdam could be taken up.

The cofferdam is an important component of the project as it is needed to build the main Earth-cum-Rock Filled (ECRF) dam.

According to the design of the project, the spillway has been located on a diversion channel and not on the main river. It has been designed to manage a flood of 50 lakh cusecs. The cofferdam is temporary and is primarily meant to create a watertight compartment to facilitate the treatment of soil, mostly clay, on the bed of the river for construction of the ECRF Dam. It is designed to withstand a flood of only 28 lakh cusecs. The ECRF Dam will have the capacity to withstand a flood of 36 lakh cusecs.

Diaphragm wall

A diaphragm wall that goes as deep as 100 metres into the riverbed under the ECRF dam is being constructed by German company Bauer.

The cofferdam also requires a foundation. Another multinational company, Keller, has been hired specially for ‘jet grouting’ needed to develop a strong foundation for the cofferdam. It was for this that Mr. Naidu performed the puja.

Schedule goes haywire

While work on the foundation for the cofferdam, downstream the ECRF dam, began on December 1, the Centre, which felt that no cofferdam was needed, did not allow work on the upstream side disturbing the entire schedule.

While there is a lot of excitement about DDRP’s decision, engineers say the cofferdam should be built only after completion of the approach channel, the spillway, stilling basin and the spill channel so that there is no pressure on the ECRF Dam if there is a huge flood in the river. “Work can be taken up only on the foundation of the cofferdam this season. The embankment of the cofferdam has to be constructed in one season so that it is not washed away,” an engineer said. Work should be meticulously planned and executed if the cofferdam had to be completed by June 2019, he said.