Herbert C. Bonner Bridge Replacement
Turbulent North Carolina Inlet Inspires Innovative Design Strategy
After more than two decades of planning, a revitalized connection for the residents and visitors to the Southern Outer Banks in North Carolina is becoming a reality. The existing Bonner Bridge carries North Carolina Highway 12 across the Oregon Inlet from Bodie Island to Hatteras Island, and lies within both the Cape Hatteras National Seashore and the Pea Island National Wildlife Refuge. The more than 50-year-old bridge serves as a critical hurricane evacuation route and is integral to the state's tourism industry. However, it suffers from severe deterioration and scour problems after five decades of use and exposure to harsh environmental conditions.
In 2011, the North Carolina Department of Transportation (NCDOT) selected a design-build team of PCL Civil Constructors as the contractor and HDR as the designer. The new 2.8-mile, $246 million replacement will provide a modern link to Hatteras Island 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 services, as well as environmental permitting services.
The centerpiece of the new bridge design is a 3,650-foot-long, 11-span, segmental concrete box girder bridge. This massive structure provides nine 350-foot spans, any of which can accommodate the shifting position of the navigation channel through the ever-changing Oregon Inlet.
Employing Proven Methods to Benefit 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." HDR tailored each region's design to fit its subsurface and scour circumstances. Each design utilizes time-tested methods and a specific assortment of simple but reliable structural elements — piles, pile caps, girders and bents — that facilitate delivery of a massive bridge at a practical cost. The project also involved complex 2-D hydraulic modeling, scour analysis and physical model scour testing, necessary to have confidence in the performance of the structure in this challenging environment. The final design capitalizes on the use of repetitive, precast concrete structural elements to improve constructability, quality and durability — key criteria in such a harsh marine environment. This approach led to maximum optimization of the design, allowing the contractor to develop an extremely competitive construction bid.
Structure Innovation Allows for 100-year Service Life
A bridge of this scale requires a solid foundation. Three substructure and foundation systems were tailored 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 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 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 challenges, the Bonner Bridge project also faces 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.
The project team's collaborative, multi-disciplinary approach addressed all of the design challenges, as reflected in their final bids that provided the best value solution to NCDOT for a challenging, large-scale and high-profile project.
Although the design was essentially completed in 2013, further progress was restricted due to litigation. The lawsuit was resolved in August 2015 and the project broke ground seven months later. Despite the many challenges of such a unique and complex site, the completed Herbert C. Bonner replacement bridge will provide NCDOT, local residents and innumerable vacationers with a safe, durable crossing for the next century.
The new bridge is scheduled to open to traffic in fall of 2018.