CAMP LEJEUNE, N.C. — Crews working at the Onslow Beach Bridge, Marine Corps Base Camp Lejeune replacement site have been working 23 feet below the water line while staying nice and dry with the help of a millennia-old technology: the cofferdam.

In order to construct the pier which will support the bascule span and operator house for the brand new, single-leaf bascule bridge, crews working for the contractor of record, Archer Western of The Walsh Group, installed metal panel walls, or sheet pile, to frame out the cofferdam. Excavation work then began in preparation for 80 precast concrete piles to be driven into the ground. Crews worked roughly six weeks to dredge out the area with a long arm excavator and clamshell bucket, oftentimes rotating shifts to ensure work progressed 24 hours a day.

With the loose sand removed, a tremie seal was then placed to seal off the bottom of the cofferdam so that it could be drained. This non-reinforced concrete slab took 250 concrete trucks coming from three different batch plants to pour 2,498 cubic yards of concrete.

“Looking at the plans and specs, you can easily visualize how everything is supposed to look – what is amazing is how everything gets there and the means and methods that are used,” said LT Will Cornett, construction manager for the project. “Everything from driving 24” pile to placing concrete underwater has strict tolerances that must be adhered to and it’s impressive to watch them be achieved considering the sheer size of the materials being installed.”

Once the tremie was placed, crews then pumped water out of the cofferdam in order cut the concrete piles to a uniform height. This allows for pile cap formwork to begin, which will create a stable foundation for the bridge counterweight and the bridge tenders house.

The cofferdam is scheduled for removal in fall 2024, signifying below water level construction is complete.

The original bridge is a swing style built in the 1950s. Hurricane Florence distressed the already aging bridge, accelerating the need for a replacement. Beachgoers and Marines accessing training areas along the shoreline will be able to use the new bridge in fall 2024.