Midway through the lock chamber construction contract for the Chickamauga Lock Replacement Project, the first permanent concrete has been laid, and the lock could be open as early as winter 2023.
The Chickamauga Lock and Dam, located seven miles upstream of Chattanooga, Tenn., at Tennessee River Mile 471, is owned by the Tennessee Valley Authority and operated and maintained by the Corps of Engineers. Alkali aggregate reactions present in the concrete since the existing lock’s original 1940 construction have resulted in concrete growth and cracking throughout the structure. As the reaction occurs, rocks throughout the lock form an expansive white gel that resembles the calcium build-up common to home showers and bathtubs, project manager Adam Walker said. Since construction, the existing lock is now 1 foot longer than when it was built and 4 inches taller.
Although the concrete growth can be mitigated, it cannot be halted, so the only solution is to construct a new lock. Because of the aggregate reaction, the existing lock has a finite life and will eventually have to be closed due to stability concerns, according to the Nashville Engineer District. Instrumentation all over the lock monitors for any sign of growing structural fatigue.
Construction funding was first received in fiscal year 2004 to begin road relocations for the lock replacement project, now estimated to cost $792 million. Lock design was completed in 2009; however, the project did not receive consistent annual funding, leading to delays. The project has not been included in the president’s budget since 2010, although since 2015 funding has been received through the Corps’ Work Plans from funding pots created by Congress, Walker said.
In 2012, a cofferdam was completed and tested, but without any federal funding in the budget, water was allowed to fill back in. Finally, after a three-year suspension of construction, a contract to place grout along the perimeter of the cofferdam was awarded in September 2015 to help stabilize the structure. Then the $35 million lock excavation contract was awarded in September 2016. The $240 million lock chamber construction contract followed in September 2017, which will continue on-site construction until at least June 2023.
In the past few years, work has been steady, with the excavation project substantially completed in January 2019, Walker said. In 2019, the Corps received an allocation slightly above the efficient funding amount requested for the project.
“After all the years of constrained funding and construction, we have a lot of momentum right now,” Walker said.
The excavation contractor removed 102,000 cubic yards of rock, which would be 46 feet tall if laid out in the size of a football field, he said. Although the riverbed is karst limestone, the excavation contractors did not encounter a significant number of unanticipated voids in the rock, Walker said.
High water has also led to minimal delays. A high-water event in February 2019 happened to coincide with the period between the excavation and lock construction contracts.
“It was fortunate the contractor had not moved any equipment in,” Walker said.
For about a month, the construction contractor turned off dewatering pumps, saving fuel costs as the cofferdam was again surrounded by water, before bringing in the construction equipment after the delay. This year, high water impacts have delayed the contract for 11 days, but no equipment had to be removed, Walker said.
The outbreak of COVID-19, the novel coronavirus, has provided different types of challenges to the construction industry.
“We deemed this contract essential, so they have kept moving throughout this pandemic,” Walker said.
Although no critical path activities have been impacted, Walker said the project has weathered some supply chain disruptions, especially with pieces coming out of other parts of the country. The contractor has also been staggering crews so that the project doesn’t have a large number of people at one time going through the elevator system to get into the cofferdam. Additionally, crews are limiting the sharing of tools between workers and separating crews to provide more physical distance between them as they work on the project’s entire footprint.
The lock chamber construction contract was awarded to AECOM Energy and Construction, but company’s name changed recently to Shimmick Construction Company Inc. The contract was awarded as a base with 13 options, and through December 2019, the Corps had applied about $165 million toward the contract. For fiscal year 2020, the project received $101.7 million. With that, the Corps will exercise all remaining options on the contract, Walker said.
“What this contract does is it will fully complete the lock structure within the cofferdam,” Walker said.
All concrete for the new lock is mixed on-site and delivered by an elevated conveyor system. The first permanent concrete placement was in October.
“We’re basically placing concrete at the lowest elevations right now,” Walker said.
Several components of the project were completed in advance, using 2009 stimulus funding. Those included miter gates, culvert valves and culvert bulkheads, all of which were completed in 2012 and stored at a TVA facility in Muscle Shoals, Ala., until they are ready to be brought back to Chickamauga as needed. Approach wall beams were also completed in 2012 and stored at the TVA-owned Watts Bar Dam in east Tennessee.
The existing Chickamauga lock is 360 feet by 60 feet and was built to handle four standard barges. Only one jumbo barge can lock through at a time, which means it can take nine hours or more to lock through a standard tow of nine jumbo barges. The new lock will be 110 feet by 600 feet, a size common to other downstream locks on the Tennessee.
Although the lock chamber construction project is currently expected to last through June 2023, the new lock won’t be open until the following contract breaches both the cofferdam and the existing dam. Because the new lock is located just downstream from the dam, four spillways must be cut out to allow boats to transit into the new lock.
Additionally, the downstream approach wall cannot be constructed in advance because it would impede navigation using the existing lock, Walker said. He said the Corps anticipates the likelihood of needing helper boats in place as necessary to aid transit through the new lock as the approach walls are being built.
The contract to construct the approach walls and decommission the old lock should immediately follow the existing contract.
“So we’ll, hopefully, have a seamless transition,” Walker said.
If all goes as planned, the new lock could open as soon as the end of 2023. The goal is to have the new lock operational before the alkali aggregate reaction necessitates the closure of the existing lock due to stability concerns. The existing lock is analyzed in two- to three-year increments to plan maintenance. Beyond that it is difficult to predict how quickly the concrete will erode, Walker said.
“It’s coming at some point, so our goal is to build the new lock as fast as possible,” he said.
Site restoration work will continue once the new lock is open, with the entire project completed as early as December 2024 with efficient annual funding.
Caption for photo: Contractors work February 20, 2020, inside of the cofferdam at the Chickamauga Lock Replacement Project in Chattanooga, Tenn., on foundation preparations, installation of reinforced concrete drilled shafts that will support future lock walls, installation of concrete formwork and reinforcing steel and concrete placement using an elevated conveyor system that is supported by two tower cranes. The Nashville Engineer District is constructing a new 600- by 110-foot navigation lock at the Tennessee Valley Authority project. (Photo by Ryan Cleary/U.S. Army Corps of Engineers)