What seems to be a perpetually tenuous highway lifeline to Hatteras Island on North Carolina’s Outer Banks has frayed once again with the discovery of excess scour around underwater pilings supporting the Herbert C. Bonner Bridge over Oregon Inlet.
The conditions have forced the immediate closure of the bridge on Tuesday, December 3, so that the North Carolina Department of Transportation (NCDOT) can implement emergency repairs.
For more than half a century, the 2.5-mile, 260-span prestressed concrete girder structure has been the sole highway connection to Hatteras Island, whose 4,300 residents rely largely on summertime tourism for their livelihoods. Given the unique dynamics of Oregon Inlet’s currents, scour monitoring has been a priority for NCDOT long before plans to build a replacement structure sparked both controversy and a series of lawsuits by environmental groups.
According to NCDOT, inspections following routine sonar scanning of the bridge revealed that scour around a bent near the bridge’s southern end was accelerating, with nine of the bent’s10 piles below critical levels. The bent is located immediately adjacent to another bent that had required scour repairs two years ago.
Though the agency had been in the process of contracting for repairs, NCDOT engineers’ determination of an imminent safety risk led state Secretary of Transportation Tony Tata to order the bridge closed and declare a state of emergency, allowing the agency to suspend normal contract bidding procedures.
“This is the worst situation in regards of scour we’ve had on this bridge, and the first time we’ve had to close the bridge for because of this problem,” NCDOT Division Engineer Jerry Jennings told a press conference on Wednesday.
On Friday, December 6, Great Lakes Dredge and Dock Co., which was already conducting dredging operations in the area, is scheduled to begin transferring sand from Oregon Inlet’s main navigation channel to the excessively scoured areas. NCDOT has also awarded a $1.6 million contract to Carolina Bridge Company, Inc., of Orangeburg, S.C., to implement a repair strategy that calls for a ring of interconnected concrete armor units, called A-jacks, to be placed around the bent, with the interior then filled with sandbags.
An additional two layers of A-jacks and sandbags will be placed atop the base layer for a total of 10 to12 feet of protection. This structure halt further scour, and allow sand to collect over the sandbags and A-Jacks, providing additional support to the structure.
Carolina Bridge is the same contractor that erected a 662-ft long Maybe Universal Bridge across a 200-ft wide, 8-ft deep inlet created by Hurricane Irene on the island in 2010.
NCDOT has yet to announce a timeframe for the repairs, and under what circumstances the Bonner Bridge will be considered safe to reopen. An emergency ferry service has been established to preserve access to the island.
