Marc Basnight Bridge
Marc Basnight Bridge
Turbulent North Carolina Inlet Inspires Innovative Design Strategy for the Replacement of the Herbert C. Bonner Bridge
After nearly three decades of planning, a fortified connection for the residents and visitors to the Outer Banks in North Carolina has become a reality. The new bridge, now named for former State Senator Marc Basnight, carries North Carolina Highway 12 across the Oregon Inlet between Bodie Island and Hatteras Island. It replaces the previous Herbert C. Bonner Bridge, which was completed in 1963 and has suffered from scour and deterioration problems for more than fifty years. The new Marc Basnight bridge serves as a critical hurricane evacuation route and is integral to the state’s tourism industry, and lies within both the Cape Hatteras National Seashore and the Pea Island National Wildlife Refuge. Most of the old Bonner Bridge will be dismantled and used to create new marine habitat at artificial reef sites offshore, but a portion will remain in place for pedestrian use and retain the Bonner name.
In 2011, the North Carolina Department of Transportation selected a design-build team with PCL Civil Constructors as the contractor and HDR as the designer to replace the Bonner Bridge. The new 2.8-mile, $254 million replacement provides a modern link between Bodie and Hatteras Islands with a 100-year service life. It will improve access to jobs, healthcare, education and recreation for the community, benefit local tourism and help feed a robust economy. As the lead design firm, HDR provided all roadway, geotechnical and bridge design as well as environmental permitting services.
The centerpiece of the new bridge is a 3,550-foot-long, 11-span, segmental concrete box girder unit. This enormous structure provides nine 350-foot spans, any of which can accommodate the shifting position of the navigation channel through the ever-changing Oregon Inlet, and is the third-longest continuous segmental concrete box girder unit in North America.
Inventive Use of Proven Methods Enhanced Durability and Constructability at a Reasonable Cost
The highly dynamic environment proved to be one of the most challenging aspects of the project for both the designers and the contractor. Varying conditions across the Oregon Inlet led our design team to divide the bridge into five “regions,” with each region’s design tailored to fit its distinctive subsurface and scour conditions, span length and height requirements, and load demands. Each design features an assembly of simple, but proven and reliable, structural elements — piles, pile caps, girders and bents — to facilitate delivery of the massive bridge at a practical cost. The engineering also involved complex 2-D hydraulic modeling, scour analysis and physical model scour testing to provide confidence in the long-term performance of the structure given the highly dynamic bathymetry of Oregon Inlet. The final design capitalizes on the extensive use of repetitive, precast concrete structural elements to improve constructability, quality and durability — key criteria in such a harsh marine environment. HDR’s comprehensive approach led to maximum optimization of the design, allowing the contractor to develop an extremely competitive construction bid.
Engineering Innovation Facilitated Long-Term Performance and Environmental Sensitivity
A bridge of this scale requires a solid foundation. Three substructure and foundation systems were customized specifically to the varying scour profiles and subsurface conditions. Using state-of-the-art analysis software, our structural and geotechnical engineers developed complex soil-structure interaction models of the foundations.
Domenic Coletti, HDR’s design manager, explained one of our unique solutions: “To our knowledge, no one has previously designed and built a foundation where piles had to be jetted and driven through nearly 140 feet of soil in a way that still provided adequate capacity after 84 feet of scour occurs. Our geotechnical engineers developed a completely new method for the rational calculation of required driving resistance for the piles. To vet our proposed method, we had international pile foundation experts peer-review and endorse the procedure.”
In addition to the design and construction complications, the replacement of the Bonner Bridge also faced a number of habitat-related hurdles — particularly for the environmentally sensitive Pea Island National Wildlife Refuge and the Cape Hatteras National Seashore. HDR’s permitting experts worked with the affected stakeholder agencies to ensure that the environmental effects are minimized.
Bringing it All Together
The project team’s collaborative, multidisciplinary approach addressed all of the design challenges, as reflected in their final bids that provided the best value solution to NCDOT for a demanding, large-scale and high-profile project.
Although the design was essentially completed in early 2013, further progress was restricted due to litigation until August 2015. The project broke ground seven months later, and the new bridge was open to traffic within three years on February 25, 2019, and officially dedicated at a ceremony on April 2, 2019.
Overcoming the many obstacles of a unique and complex site, the new Marc Basnight Bridge will provide NCDOT, local residents and innumerable vacationers with a safe, reliable crossing for the next century.