Bridges, Tunnels and other Transportation Structures
Oregon Inlet is one of the most dangerous inlets on the Atlantic coast. Treacherous currents, constantly shifting bathymetry, and violent storms are normal occurrences. But the surrounding North Carolina Outer Banks, a 200 mile long series of barrier islands, are also renowned for their scenic beauty, pristine beaches, and great fishing. Access along the Outer Banks is provided by NC Highway 12, which crosses Oregon Inlet on the Herbert C. Bonner Bridge, a critical link between Hatteras and Bodie Islands. Unfortunately, the Bonner Bridge has suffered from severe scour and deterioration in the harsh marine environment, and has required nearly continual maintenance, repair, and retrofit by the North Carolina Department of Transportation (NCDOT) since its completion in 1962. During a severe Nor’easter in 1990, the dredge Northerly Isle broke loose of its moorings and collided with the bridge, causing the collapse of six spans. An emergency repair project reconstructed the damaged bridge, but it was clear that the Bonner Bridge was in need of complete replacement by a more durable structure. So in 1993 NCDOT undertook a planning study to evaluate alternatives for replacement of the Bonner Bridge. The problem was more complicated than simply replacing the bridge; NC 12 south of the Bonner Bridge runs through the Pea Island National Wildlife Refuge, and Hatteras Island in this area has proven to be susceptible to frequent breaches and washouts during storm events. The entire NC 12 corridor needed study. A phased construction alternative was developed which addressed different segments of NC 12 over time.
In 2011, NCDOT awarded a design-build contract for the first phase of the overall NC 12 project – the replacement of the Bonner Bridge – to a design-build team lead by PCL Civil Constructors, with HDR as the lead designer. PCL’s winning bid was $215.8 million, approximately $64 million less than its nearest competitor. NCDOT’s contract specified a 100-year service life, design for up to 84’ of scour, and minimal environmental impacts. The associated design and construction challenges created opportunities for innovation and creativity. The design-build team divided the 2.8 mile long bridge into five design regions based on scour depth and subsurface conditions, required vertical and horizontal clearances, vessel collision requirements, and construction and environmental constraints. Simple, repetitive structural details took advantage of economies of scale in the long structure and facilitated the extensive use of precast concrete construction, contributing to high levels of construction quality, durability, economy, constructability, and environmental sensitivity. A first-of-its-kind driven pile foundation verification method addressed the unique difficulties associated with installing piles driven up to 130’ deep, but subject to future reduction of resistance when undermined by as much as 84’ of scour. Innovative, environmentally-sensitive construction approaches and extensive coordination and collaboration with permitting agencies alleviated concerns about large-scale construction activities in this highly sensitive site.