CHATTANOOGA, Tenn. (Sept. 27, 2023) – The Upstream Approach Walls for the Chickamauga Lock Replacement Project got a solid start when work crews placed 148 cubic yards of tremie concrete Sept. 20 into the drilled shaft on the bottom of the Tennessee River.
Allen Malcomb, contracting officer’s representative at the Chickamauga Lock Resident Engineer Office, said the U.S. Army Corps of Engineers Nashville District team worked closely with the contractor, C.J. Mahan, to ensure the smooth operation that required putting concrete trucks on a barge and into position to place concrete.
“I was there to monitor that the contractor was performing the work safely and address safety issues if they came up,” Malcomb said. “No major safety issues were observed due to the contractor’s communication and planning prior to making the placement.”
The contractor tested the concrete at the on-site batch plant to ensure fresh properties met contract requirements. After batching and testing, the three trucks were loaded onto a barge and pushed out to the placement site on the upstream side of Chickamauga Dam. At the placement location, the team tested the concrete again for quality, then each truck placed 10 cubic yards of concrete into a 91-foot-deep pipe into the shaft.
“As the concrete pumped, the contractor’s quality control team and the government’s quality assurance team monitored the concrete placement to verify the concrete rose as expected, embedment was maintained, and concrete quality was visually acceptable,” Malcomb added.
Matthew Curvin geologist in the Quality Assurance Section at the Chickamauga Lock Resident Office, said the first delivery of concrete had changed properties, so the contractor adjusted the concrete proportions and mixing time at the batch plant and improved concrete quality for a successful placement. The contractor began placing concrete at around 9:20 a.m. and completed the first of 14 shafts for the upstream approach wall piers by around 2 p.m., he said.
The amount of work performed on this drilled shaft is not visible at the surface of the lake. The size of the rebar cage in the shaft and its complexity in placing concrete is hidden by the water and casing. The contractor and Corps of Engineers worked closely to address safety concerns and placed concrete with no incidents or injuries.
“The Tennessee River is not an easy environment to work in due to the flow of the river and the geology of the bedrock that they are building on,” Curvin said. “C.J. Mahan leadership and staff worked in tandem with good communication and teamwork.”
To build the piers, the contractor must drill 14 eight-foot diameter shafts up to 35-feet into the bedrock, which is up to 135 feet below the surface of Chickamauga Lake. The shaft is drilled through a permanent steel casing. After the drilling is completed, a pre-constructed rebar cage is lowered into position. Up to 300 cubic yards of tremie concrete is placed in the shaft.
Capt. Joseph Cotton, Chickamauga Lock Replacement Project manager, said each shaft will anchor the approach wall beans in place that will be visible on the surface of the lake that will guide vessels into the new navigation lock. As shafts are completed, C.J. Mahan will begin work on the piers that will hold the approach wall beams in place, he explained.
“Approach walls act as a guide for barge operators when maneuvering their vessel towards the lock chamber,” Cotton said. “It provides a location to tie off barges that are waiting to move through and provides a water break to reduce the effects of current and wave action from the dam spillway or during inclement weather.”
Work on the approach walls is expected to continue through December 2024 and is integral in bringing the new Chickamauga Lock to an operational status. Once the new lock is commissioned, a tow system, commonly referred to as a mule, will be installed allowing barges to be moved in and out of the lock chamber without a boat.
“The new Chickamauga Lock will allow the passage of up to nine barges at a time compared to the single barge that the existing lock can pass. This greatly increases the speed that barges can move through the lock and helps create a safer work environment for mariners,” Cotton added.
