The new $252 million, 2.8-mile-long Marc Basnight Bridge spans Oregon Inlet, one of the most dangerous channels on the Atlantic Coast due to its treacherous currents, constantly shifting depths, and high winds.
The conditions meant the original 130-ft-wide steel girder span had to be dredged continuously to preserve the required 14-ft channel depth, and it was rapidly reaching its end of life. That bridge acted as a lifeline for residents of North Carolina’s barrier islands, which meant its replacement had to be reliable and extremely durable.
“Providing the required 100-year service life for a bridge subject to that extremely harsh saltwater environment was a major challenge,” says Domenic Coletti, principal bridge engineer for HDR, the lead design firm for the project. His team tackled this requirement through the extensive use of precast concrete, providing a high quality, economical, resilient and low-maintenance structure that can resist 84 ft of erosion, hurricane-level winds, and impacts by the occasional passing ship.
The decision to use precast concrete occurred early in the prebid engineering phase, when the design-build team recognized the many benefits this material leant to the project design. Using precast concrete improved quality and durability while reducing costs and shortening project timelines, Coletti says. It also minimized disruption to the environmentally vulnerable barrier islands, and it lessened the risk of impact of construction on the area’s 20 legally protected species, which include manatees, bald eagles, Peregrine falcons, and five species of turtles.
The precast concrete design also helped to address one of the biggest challenges faced on this project: location. “The project site is remote, with limited access for material delivery,” Coletti says. “And the working conditions in the Oregon Inlet were subject to very fast currents, high winds and surf, storms, nor’easters, and hurricanes—making fieldwork difficult.”
By choosing precast concrete over fresh concrete, the precaster was able to reliably deliver structural elements to the project site in a carefully timed sequence. Furthermore, using precast concrete elements facilitated much faster, safer, and higher-quality construction in the harsh marine environment, he says.
140 feet Beneath the Sea
The 2.8-mile-long replacement bridge features the third-longest continuous segmental box-girder superstructure in North America, with 11 spans at lengths up to 350 ft. The 44 approach spans use virtually identical precast concrete bent caps and cylinder piles, and the bridge’s foundation consists of 669 precast concrete piles, totaling over 12 miles.
To ensure durability in the extremely harsh environment, the piles’ final depths are up to 140 ft below the seabed. This depth accommodates anticipated scour (seafloor erosion), which is expected to remove as much as 84 ft of the seabed, while still providing sufficient embedment into the seafloor to maintain stability and strength.
Throughout construction, the team took extensive measures to minimize consequences for the animals and plants throughout the project area. This included using the minimum number of driven precast, prestressed concrete cylinder piles in the approach structures to reduce impact on submerged aquatic vegetation beds. They also leveraged a “leapfrog” approach to limit the length of their work trestle, pulling spans from the trailing end of the trestle and moving them to the leading end as the construction of the bridge’s north-approach spans progressed from north to south. “This greatly minimized temporary environmental impacts,” Coletti says.
Adding further environment benefit, the original mostly precast concrete bridge was broken down and deposited at several offshore artificial reef sites to provide new habitats for fish.
The project met the owner’s requirements and created a beautiful and sustainable structure that the community loves. “Throughout the project, the local year-round residents of the Outer Banks were very excited and supportive of the project,” Coletti says. They even threw a Community Day party to celebrate the grand opening in February 2019. “Despite 30° temperatures with 30 mile-per-hour winds, over 2000 local residents showed up.”