The Marc Basnight Bridge has been constructed to replace the old Herbert C. Bonner Bridge, built in 1963 in the US state of North Carolina. The older bridge had been the lifeline for almost 60 years for the residents of the Barrier Islands in North Carolina. Serving as the only connecting route between the Bodie Island to the north and Hatteras Island to the south, it saw an annual tourist footfall of nearly two million. Additionally, this iconic structure provided highway access to communities located in the Outer Banks ─ including Buxton, Hatteras, Avon, and Rodanthe. The new Marc Basnight bridge, which was opened in 2019, carries North Carolina Highway 12 across the Oregon inlet between Bodie Island and Hatteras Island. The saltwater environment of the Oregon inlet is what had led to the corrosion and scouring effects on the Bonner Bridge, causing severe deterioration to the bridge’s substructure and superstructure.
As a replacement structure costing USD 252 million, the 2.8-mile-long Marc Basnight bridge is located in a sensitive marine environment. In order to overcome multiple issues ranging from scour effects, winds of hurricane level, and the impact of ships passing underneath, the new bridge required an extended lifespan. For the bridge to achieve a service life of 100 years, the client, North Carolina Department of Transportation (NCDOT), recommended robust concrete mix designs, which involved extensive use of ground granulated blast furnace slag, fly ash, a low water-cement ratio, silica fumes, and calcium nitrite corrosion inhibitor. The use of innovative approaches to construct the Marc Basnight bridge involved an all-precast pile bent system, which considerably eased the construction activities. Also, at sections where scour effects were prominent, two-column bents with multiple battered piles and a pile cap were used.
The Oregon inlet is frequently cited as one of the Atlantic Coast’s most complex and dangerous inlets. As tides and waves transfer the loosely packed sand and adjust the scale, shape, and position of the natural channel daily and even hourly, the inlet’s bathymetry changes constantly. The bridge features high quality and durable concrete elements (bent caps, box girder segments, piles, and columns) prepared in an off-site production yard under controlled conditions. The prefabricated box girder spans have variable depth, parabolic haunch and tapered overhangs. The centerpiece of the new bridge is an 11-span concrete box girder unit that is 3,550 feet long. The foundations of the bridge are designed and constructed to withstand:
Home to nearly 20 federal- or state-protected wildlife species, the marine environment in and around the Oregon inlet required innovative solutions to ensure a minimal environmental impact through construction activities. The Oregon inlet posed further challenges to the bridge structure via high lateral forces ─ including wind, vessel collision, and wave action ─ leading to concerns among the designers that the bridge foundations may overturn or collapse. This resulted in their making multiple models to capture different scour conditions for each bent.
|Owner||North Carolina Department of Transportation (NCDOT)|
|Construction Engineering Inspection||WSP USA, Inc.|
|Design/build team||PCL Civil Constructors, Inc.|
|Geotechnical engineering||HDR (design) and Froehling & Robertson (subsurface investigations)|
|Construction Engineering Services||Corven Engineering, Inc.|
|Hydraulic and scour analysis||INTERA Inc.|
The dynamic bathymetry of the Oregon inlet resulted in tailor-made designs, specific to the varying subsurface and scour conditions across the bridge length. The design and engineering process also involved 2D hydraulic modeling and scour analysis to ensure the long-term performance of the structure. The design team made use of a 3D LARSA model, which allowed a staged analysis of the superstructure and substructure. This analysis also helped in capturing the time-dependent effects of concrete physical properties like superstructure creep and shrinkage.
The critical location of the Marc Basnight bridge exposes the bridge structure to adverse environmental conditions. The bridge structure health was monitored as per the design models, using laser scanners. These mobile rapid-fire laser scanners were able to collect 750 million data points per second. Due to the remote site location, the data points collected through mobile scanning equipment were communicated to the instrument on the shore through GPS technology. The coordinates and elevation data will be used to construct a 3D model of the bridge, which will help in better analysis of the bridge performance.
Aside from creating a hurricane evacuation route, the project is expected to create 2,000 jobs, better trade, and economic benefits valued at USD 200 million to the Dare County where the bridge is located. Overall, the project will considerably improve healthcare, jobs, local tourism, and education.
With a robust preparedness plan, the project has been completed on schedule and was opened to traffic on February 25, 2019.
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