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De-watering of Polavaram cofferdams begins after flood levels recede in Godavari

VIJAYAWADA: With flood levels receding in Godavari, water resource department engineers took up de-watering the space between two cofferdams to allow National Hydro Power Corporation for tests to determine how best is it to address the problem related to the diaphragm wall of the Polavaram project­—which was damaged during 2019-20 floods—and how best is it to fill the scoured part of the area between the two cofferdams, where ECRF is to be constructed.

The tests are expected to be completed by end of Jan 2023 and a report, as regards the structural stability of the diaphragm wall and suggestions as to what needs to be done, is likely to be out by Feb 2023.

Polavaram project chief engineer B Sudhakar Babu said at present, works pertaining to lower cofferdam are underway.

“Out of 53 panels, six have been completed and three are under construction. The 47 balance panels will be completed at the earliest. Simultaneously, with flood levels receding the area between the two cofferdams, where earth cum rock fill dam has to be constructed, is being de watered.

Once the water levels is reduced to a permissable levels, as per the suggestions of NHPC, holes will be drilled with diameter of 20 mm and depth of 45 cm to insert electrodes for every three meters, covering the total 900 meters of the diaphragm wall. In all, 2,100 electrodes will be used to take the reading,” said the chief.

“The process of installation will be completed by December 15 and further data will be recorded and submitted to NHPC, which will analyse the structural stability of the diaphragm wall and accordingly give its suggestions, which are expected by February,” he elaborated.

Besides, diaphragm wall suffering damage during 2019-20 floods, main dam area suffer scouring due to intensity of the flood. It is estimated that 40 lakh cubic meters has to be filled with sand.

As per the directions of NHPC, three types of tests — ECPT (Electric Cone Penetration Test), SPT (Standard Penetration Test), and vibro compression test will be carried out. Two trail pits of 50×50 meters at the scoured area will be being filled with compressed sand.

“The relative density has to be more than 85 percent, so any structure would not sink on the bed. Depending on the test results, it will be decided if the method of filling up the scoured areas of 40 lakh cubic meters with compressed sand has to be taken up or any other alternative method need to be adopted,” he said. If all goes as scheduled, the project works are likely to pick up pace by summer 2023.