After decades and delays, the U.S. Army Corps of Engineers (USACE) is moving into its final stages of construction on the $954 million Chickamauga Lock Replacement Project.
One of the most significant infrastructure initiatives for the region, the project is located on the Tennessee River, east of downtown Chattanooga, Tennessee. The new 110-foot by 600-foot lock is riverward of the existing 60-foot by 360-foot lock, downstream from the Chickamauga Dam.
The Chickamauga Lock and Dam is owned by the Tennessee Valley Authority (TVA), and the lock is operated and maintained by USACE.
Approximately 1.5 million tons of material passes through the essential Chickamauga Lock every year. The new, larger lock chamber is expected to increase efficiency by 80 percent for waterborne transportation, benefiting commercial and recreational industries.
Currently in the construction phase, the lock chamber contractor, California-based Shimmick Construction Company, Inc., has placed 95 percent of the concrete required to construct the chamber that will allow vessels to go up and downstream of the lock.
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“250,000 cubic yards is enough concrete to make a two-lane road from Chattanooga to Nashville,” said Joseph Cotton, USACE’s Nashville District Project Manager. “It's a significant amount of concrete that they've placed.”
“The miter gates — the doors that allow the boats to go in and out of the lock chamber — are being assembled right now,” he added. “Once assembled, the various pieces of the gates will be welded together on site by multiple welding crews.”
The Chickamauga Lock Replacement Project was created in response to structural deficiencies of the almost century-old lock. Physical expansion or “growth” of the concrete structure by a phenomenon known as alkali-aggregate reaction (AAR) or “concrete growth” was observed soon after initial construction.
“Essentially, the stone that was quarried back in 1940 when this lock was commissioned had a very high content of dolomite,” Cotton said.
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While dolomite is found all over the world, there is a substantial amount within the greater Chattanooga area. When dolomite reacts with water and Portland cement, over time a gel-like substance forms around the individual aggregates — in this case, the billions of aggregates that make up this lock and dam.
Alkali-aggregate reaction, a reaction between the alkali in the cement and the aggregate, causes concrete expansion, threatening the structural integrity of the existing lock and limiting its life, even with significant maintenance efforts.
“When they quarried back in 1940, no one knew what AAR was,” Cotton said. “We first started seeing issues associated with it back in 1942, just two years after the lock was operational. The first research into an aggregate reaction didn't start until the 1950s.”
“TVA has done a great job over the last 85 years keeping the dam structurally sound,” he said. “It is safe today, but through years of monitoring by the TVA, a decision was made that we couldn't just keep putting Band-Aids on this problem. We needed a long-term solution.”
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First authorized by Congress in 2003, replacement construction began in 2004 and continued through 2012. After a temporary suspension of construction activities due to funding constraints with the Inland Waterways Trust Fund, funding returned to the project in 2015.
The project’s current cost estimate is $954.4 million due to schedule delays, as well as inflation.
Cotton said that it is the team’s priority to “get this lock operational in 2028, thanks to the funding that we have received recently.”
“We've received over $236 million in fiscal year ‘24 as part of the Consolidated Appropriations Act,” he added. “That's allowing us to award the base of this next contract and allowing us to have our next contract move forward.”
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The current lock allows only one jumbo barge to pass through it at a time. The new lock will allow nine barges to pass through the Chickamauga Lock and Dam.
“If you're going to come up with a long-term structural stability solution, you might as well get the economic benefits of a more efficient, larger, newer lock while you're solving that problem,” Cotton said.
The new lock will maintain 318 miles of navigable channels upstream to support TVA; the Department of Energy at Oak Ridge, Tennessee; and numerous other businesses.
“It's typical to see 15 barges come through at a time. One boat moves 15 barges,” Cotton said. “So a nine-barge lock will allow that boat to come up with all 15 barges, put nine of them in the chamber back out.”
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“We're hoping to offer a lot more opportunities for the folks upstream of us, whether that's recreational or for the TVA operating their two nuclear plants, Watts Bar and Sequoia, that rely on us for transportation of heavy and large machinery, transformers, generators, turbines — what they need to produce nuclear power,” Cotton added.
Using a tow haulage system, barges are essentially lassoed with a rope and secured onto a rail that drags the barges out once the lock chamber is raised up to its final elevation.
“Then the water comes down, you bring the next six plus the boat in, hook it up, and away you go,” Cotton said. “That's about a two-and-a-half-hour process for a 15-barge tow. Our current lock would take about 15 to 18 hours to do that. It's incredibly time consuming. It's going to be a huge impact.”
According to Cotton, Chickamauga is the second busiest lock in the entire Corps of Engineers when it comes to recreational traffic.
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“Last year, we had over 3,000 small craft go through the lock,” Cotton said. “Whether it be the volunteer navy going up to Knoxville for football games [or] the Riverbend Fest in the city of Chattanooga, our lock operators are out until three or four in the morning sending 20 to 30 boats through the old lock at a time.”
Shimmick Construction’s work was awarded in 2017 and is expected to continue through 2026, completing the lock chamber itself, as well as the electrical mechanical systems, and ensuring the miter gates are put into place. A second contractor, Ohio’s CJ Mahan Construction Company, was brought on for the upstream approach walls. Navigational lock approach walls act as a funnel that will allow barges to safely enter and exit the lock chamber.
“Once we have an operational lock, and only once we're confident in that operational lock being able to perform, we will then start the decommissioning and structural stability portion of our work,” Cotton said.
The decommissioning contractor will remove the cofferdam, the dewatering structure around the existing work site. The team will also build the downstream approach walls, without impacting the Tennessee River Bridge immediately downstream.
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Cotton said they will be filling in a portion of the existing lock with concrete, in line with the spillway of the dam. The remainder of the lock will be filled in with materials that were excavated to allow the new lock to be constructed.
According to Cotton, the contractor has a conveyor belt system that goes from the concrete batch plant to the lock chamber itself, producing their own concrete on site. The conveyor belt can reach anywhere in the lock within two minutes. To ensure the new lock will not experience AAR concerns as the existing, the aggregate is thoroughly tested and verified that traces of dolomite are within acceptable standards prior to concrete production on site.
They will also build what he calls a “thrust block” that will be located on the north side of the existing lock, essentially acting as a massive door stop. Since only the lock is being replaced, the dam will continue to grow.
According to USACE, this thrust block will help hold the structure in and aid in preventing it from horizontally expanding further than its current state. In return, this will help mitigate future structural stability issues with the entire dam, providing TVA with a much simpler solution for maintaining the effects of AAR on the dam.
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“We are not going to stop AAR from happening, but we can control where it goes, and we can make the solution to deal with those forces much simpler for the TVA in the long term,” Cotton said.
The final contract went out to bid in early February. Proposals are expected on July 8, and USACE’s goal is to have the contract awarded by the end of the year.
“Folks from the Chattanooga area have seen this project underway for two decades now. We had to wait our turn for funding. We had to wait our turn in terms of capacity to bring on this last contractor,” Cotton said. “We don't want to see any slowdown of the work as we race towards our 2028 operational date. We are finally seeing the light at the end of this tunnel.”
“But we're thankful for all the work that the folks here are doing,” he added. “We're thankful for the congressional support that we've received and getting this project across the finish line in an efficient manner. ... We’re excited to be able to deliver an operational lock to keep the city of Knoxville connected to the greater inland navigation system to ensure there's opportunities both in upstream and downstream of Chattanooga.”
