One of Chattanooga's biggest construction projects is being delayed this month after heavy rains filled in a temporary coffer dam erected to help build a new $758 million replacement lock at the Chickamauga Dam.

Within the new 110-foot-by-600-foot lock being built beneath the dam, a major excavation project was completed last month just before record February rainfall forced the Tennessee Valley Authority to open up most of the spillways at the Chickamauga Dam and raise the Tennessee River by 12 feet above its normal level.

The higher river and heavy rain complicated efforts to keep the water out of the coffer dam, where crews have removed more than 100,000 cubic yards of dirt and rock to dig down 35 feet to the riverbed.

Adam Walker, project manager for the Chickamauga Lock replacement project for the U.S. Army Corps of Engineers, said the excavation equipment was largely removed from the lock chamber early last month, but the rains a few weeks later delayed the start of the next phase of the lock construction.

"The decision was made last month that if we were going to be battling high waters and having to burn a lot of fuel trying to keep the dewatering pumps running to keep the water out of the coffer dam, it just made more sense to turn off the pumps and allow the coffer dam to fill until conditions return to normal," Walker said. "We're eager to get to dewatering within the coffer dam and get back working on the next phase of the project. But it doesn't look like the water levels and flow on the river will get back to a point where it is really effective to resume work for some time."

With more rain this weekend, TVA expects to keep the flow of water through the Chickamauga Dam above 100,000 cubic feet per second for at least another week, keeping the existing lock closed and barge traffic off the river due to safety concerns from the heavy flow and high water, especially through the Tennessee River gorge west of downtown Chattanooga on Nickajack Reservoir.

"It will probably be another week before the flow is down to a level at Chickamauga that barge traffic can resume [beyond the two weeks the Chickamauga Lock has already been closed]," said Cline Jones, executive director of the Tennessee River Valley Association, a trade group for the barge industry. "Most of the Tennessee River and the Tennessee-Tombigbee waterway is now open, but not Chickamauga. I've never seen anything this extended in my 10 years in this job."

The rains are the latest delay in the Chickamauga Lock replacement efforts by the Corps of Engineers, which began a decade and a half ago and, due to funding delays and construction complications, has more than doubled in cost. Walker said the Corps has already spent more than $250 million on site preparation, road rerouting and construction of the coffer dam.

With adequate funding, the new lock could be finished between 2023 and 2025, according to Corps estimates. Although no money for the Chickamauga Lock was included in President Donald Trump's original budget plan a year ago, Congress ultimately allocated $89.7 million for the Chickamauga lock project this year, and the Corps estimates efficient funding next year could be as much as $104.3 million.

Heeter GeoTechnical, which completed its excavation work on Feb. 1, dug down as much as 35 feet in the river bottom to prepare the lock site beneath the dam and work is now scheduled to begin on erecting the concrete walls of the new lock.

AECom is the contractor for the next $240 million phase of the project to build the new concrete walls of the lock. A concrete batch plant has been installed at the site and, by this summer, a conveyor system and tower crane to move the concrete to the lock site should be in place and in operation for the lock construction — "assuming it stops raining," Walker said.

Contact Dave Flessner at dflessner@timesfreepress.com or at 757-6340.