The U.S. Army Corps of Engineers is moving into the final phase of construction of a replacement lock at Chickamauga Dam near Chattanooga, Tenn., where degrading concrete has caused decades of headaches. 

In an Aug. 19 tour hosted by the Waterways Council Inc., Corps officials detailed work to construct the new 110-ft by 600-ft lock, as well as the deficiencies of the aging 60-ft by 360-ft lock it will replace.

USACE Nashville District, which is responsible for navigable waterways in the Cumberland and Tennessee River basins, also provides support to the Tennessee Valley Authority (TVA) in major maintenance and operation of locks on the Tennessee, including Chickamauga. TVA owns the dam and its 119-MW hydroelectric plant.

Located about 7 miles above Chattanooga, the Chickamauga Lock and Dam were constructed in 1940, and soon after, the concrete was observed to be expanding due to alkali-aggregate reaction. Since its initial construction, Capt. Joseph Cotton, USACE Nashville District project manager, says the lock has expanded 1 ft in length and 4 in. in height. USACE has had its hands full managing the effects of that expansion, and even with ongoing maintenance, the aggregate reaction presents a continual threat to the facility’s structural integrity and limits its operational life, the Corps says.

Shimmick Construction Co. is the prime contractor for the $275-million project to construct the lock chamber. The project scope includes procurement and installation of the 150-cu-yds-per-hour onsite batch plant and concrete conveyor system needed to meet the 255,000-cu-yd requirement for structural and mass concrete.

Shimmick is at work excavating nearly 10,000 cu yds of rock, demolishing 3,600 cu yds of existing reinforced concrete spillway and installing 43 reinforced concrete drilled shafts. The firm will also construct a new operations building and gate control shelters and install new electrical and mechanical systems. 

Upstream approach walls are being constructed by C.J. Mahan Construction Co. as part of a $61-million contract awarded in September 2021, including emplacing 14 drilled shafts, four intermediate piers, two nose piers and eight approach wall beams.

The final phase, expected to run between $100 million and $250 million—and which could be awarded this month—will be a firm fixed-price contract, according to solicitation documents as previously reported by ENR. It includes wet commissioning and demolition of the spillway, construction of downstream approach walls extending underneath a nearby railroad bridge, as well as existing lock closure and backfill, with earliest completion dates set for November 2027 to have an operation lock and December 2029 for project completion. 

Gate work is set for November, when the team will be placing 14 metal gate segments weighing 150,000 lbs that will be welded together. Approach walls are fabricated off-site and will be floated intact down the river and installed. The longest of these walls will be 120-ft long, Cotton says, weighing 450 tons.

A Corps statement from June lists a total project cost estimate of $954.4 million, approximately $417 million of which had been spent to that date. The project has been in the works for decades, authorized by the Consolidated Appropriations Act of 2003 and reauthorized in America’s Water Infrastructure Act of 2018, per USACE. It’s funded via a 65/35 split between the federal general fund and the Inland Waterways Trust Fund, which itself is funded via marine fuel taxes on commercial customers.

A USACE barge prepares to enter the existing lock at Chickamauga Dam Aug. 19. 
Photo by Derek Lacey for ENR

It’s estimated that an average of 1 million tons of goods, valued at $175 million, pass through the existing lock annually, which also ranks as the most active lock on the Tennessee—and second-most active nationally for recreational vessels, with about 3,500 traversing the lock annually.

Maj. Jesse Davis, deputy commander for the USACE Nashville District, says the lock will improve efficiencies of the country’s inland navigation system, reducing times for traversing the lock by 80%, while enhancing conditions for workers at the lock and on the barges.

He also notes that the 3,500 recreational vehicles, which traverse the lock mainly on weekends during one-half of the year, pay nothing to traverse the lock, nor does other federal initiatives like TVA shipping electrical equipment or departments of transportation shipping aggregate for road construction.

Projected over the next 50 years, Cotton says for every dollar spent on the project, the Corps expects a return on investment of that dollar plus 7%, adding that the current lock has operated for longer than 50 years, “so we’re doing much better on that initial investment.”

Traffic is expected to pick up as well. During the COVID-19 pandemic, rates dropped to about 600,000 tons of goods through the lock annually, which increased to 1.3 million in 2023 and 1.1 million as of mid-August 2024.

“Our assumption was conservative,” he says. “It proved overly conservative. This is still an economically viable product.”

Tempering Reactions

An aggressive maintenance strategy over the decades has kept the dam in operation, though outages have occurred, blocking navigation of 318 upstream miles. 

At Chickamauga, the stone is the particular problem when it comes to the alkali aggregate reaction, as it reacts with silica in the concrete mix to create a gel-like substance on the outside of those billions of pieces of stone included in the dam, Cotton says.

“You’re getting a millimeter thin film, you have forces going all sorts of different directions,” he says. “Concrete is great in compression. It’s terrible with tension. And that’s what these unconventional forces are causing, is great tension across the dam, and ultimately we have to mitigate that through fairly aggressive measures.”

That includes slot cutting through the entire dam with diamond-tipped wire saws. There’s so much pressure in the dam, that by the time the cut reaches the bottom, the top has already closed back up, sometimes requiring the saw to be left in place, Cotton adds. 

“There’s some really unique mechanical work that was done to keep the gates actually operating,” Cotton says. “But that is how we keep the lock together. That’s why it needs a replacement.”

And as with any major civil works project, he says, the lock must be monitored. Normally that means 100-200 sensors, but at Chickamauga there are around 3,000, making it one of the most heavily monitored lock-and-dam systems on the planet.

The Corps also uses a tensioning system to mitigate impacts from the reaction, drilling into the bedrock and installing rebar, of which there are 300 instances across the lock and dam.

“I can confidently say that Chickamauga Lock and the dam are structurally sound,” Davis says. “Every action we’re taking considers this reaction and ensuring the final product is going to have long-term reliability and safety.”