COVER STORY 2019 OPA WINNER Marc Basnight Bridge Widely acknowledged as the most dangerous channel on the Atlantic Coast, the Oregon Inlet routinely experiences treacherous currents, constantly shifting bathymetry and violent storms. However, it is also surrounded by North Carolina's Outer Banks, known for their scenic beauty, pristine beaches and great fishing. Opened in 1963, the Herbert C. Bonner Bridge on North Carolina Highway 12 provided a critical link across the inlet for decades, but it suffered from severe scour and deterioration in the harsh marine environment, resulting in nearly continual maintenance, repair and retrofitting by the North Carolina Department of Transportation (NCDOT). The $252 million replacement bridge, named in honor of State Senator Marc Basnight, is designed to provide a 100-year service life, resist unprecedented scour depths and minimize environmental impacts while offering a reliable, safe passageway for residents, visitors and tourists. The design and installation of large diameter concrete piles to support the new bridge represented the most challenging marine foundations project in North Carolina's history. Groundbreaking for the new bridge took place in March 2016, and it was opened to traffic on February 25, 2019. Challenges Oregon Inlet is not a site conducive to the construction and maintenance of large infrastructure. Often cited as one of the most dangerous inlets on the Atlantic Coast, Oregon Inlet is subject to frequent and heavy hurricane and Nor'easter storm activity. The inlet itself is extremely dynamic; the bathymetry is constantly changing as tides and storm action move the loosely deposited sand and shift the size, shape and location of the natural channel day to day and sometimes from hour to hour. To maintain navigation under the single navigation span of the old Bonner Bridge, the U.S. Army Corps of Engineers (USACE) was compelled to dredge a navigable channel virtually nonstop, year-round. Further, since the site is on the Atlantic Ocean, any structure in Oregon Inlet is subjected to a constant barrage of salt-spray wetting and drying cycles, leading to significant potential for corrosion and structural deterioration. These conditions contributed to unprecedented design criteria for the bridge. The foundations were to be designed and constructed to experience anywhere from 0 to 84 ft (0 to 25.6 m) of scour in some regions, combined with flow velocities of up to 12.4 ft/s (3.8 m/s), wind velocities of up to 105 mph (169 kmh), measured as “fastest mile of wind,” and vessel impact forces of up to 2,151 kips (9.57 MN). Due to the constantly shifting location of the natural channel, the USACE and the U.S. Coast Guard (USCG) requested that the bridge be designed to accommodate a “Navigation Zone” with a minimum width of 2,400 ft (731.5 m). All spans within this zone were to provide a minimum of 70 ft (21.3 m) of vertical and 200 ft (61.0 m) of horizontal navigation clearance so that the marked navigation channel could be easily relocated to minimize the need for dredging. In addition, NCDOT specified numerous prescriptive durability criteria associated with a general theme of achieving a 100-year service life. Construction was similarly constrained and challenged. High winds, rapid tidal flows and frequent storms hampered construction, particularly in late fall, winter and early spring. In 14 • DEEP FOUNDATIONS • SEPT/OCT 2019 AUTHORS Mike Batten, P.E., R. Dominick Amico, P.E., Domenic Coletti, P.E., and Elizabeth C. Howey, LG, P.E., HDR, Inc.
