PCL crews drive 36-inch-square precast concrete piles into the waters of Pamlico Sound.
Work on the replacement of the Herbert C. Bonner Bridge on North Carolina’s Outer Banks began in early December and is on hold pending the outcome of a judge’s ruling.
The Bonner Bridge is the only access route across the Oregon Inlet between Hatteras and Bodie islands. The current two-lane bridge, built in 1963, takes traffic across the inlet along N.C. Highway 12, the only major road on the narrow and wind-swept Outer Banks.
A replacement bridge became necessary due to the constant pounding the span has taken over the years from the climate of the Outer Banks and the Atlantic Ocean.
In addition, Atlantic storms, including several hurricanes, have passed through the area in the past 50 years. In October 1990, a storm swept a dredge into the bridge, damaging several of its spans.
The old bridge passed its projected lifespan 20 years ago, so damage from the elements make a new bridge an immediate necessity.
In July 2011, the North Carolina Department of Transportation (NCDOT) awarded a $215.8 million contract to replace the bridge to the design-build team of PCL Civil Constructors Inc. and HDR Engineering of the Carolinas. Both have offices in Raleigh.
Environmental groups quickly filed lawsuits to stop the project as the bridge crosses the eco-sensitive Pea Island National Wildlife Refuge. The refuge is part of the Cape Hatteras National Seashore.
“The same day we bid the job to replace the Bonner Bridge is the same day that the Southern Environmental Law Center filed a lawsuit against it,” said Matt Persing, senior project manager of PCL. “Since then, this has been ongoing and in the hands of the courts.
“The original contract gave us a cap of $17 million that allowed us to complete the design and do a load test program,” Persing said in early April. “Now, until such time as the judge or the courts rule and provide summary judgment, we are in a holding pattern. We are now wrapping up the load test program, then the design will be done and we will put our pencils down until the NCDOT calls and says ’Hey, we would like you to build the bridge now.’”
Beginning last December, PCL had a crew at the bridge site driving 36-inch-square precast concrete piles into the waters of Pamlico Sound in order to prove the designed pile lengths and capacities met the intent of the state’s Request for Proposal documents, said Persing.
They brought with them a HPSI 500 vibratory hammer and a Pileco D225 diesel impact hammer to do the job. Pile Equipment Inc., a distributor based in Green Cove Springs, Fla., just south of Jacksonville, supplied the two machines to PCL. The company has been in business for 26 years.
“We supplied the vibratory hammer to drive and extract the pipe piles which holds their template, or the guide, that will hold the pile when they are jetting it and then driving it a little ways,” said Mike Elliott, president of Pile Equipment. “Then the diesel pile hammer will actually drive the piles into the bedrock.”
Persing added that his crew actually drove two test piles, with each being 36-in.-square precast piles. Both are 130 ft. (39.6 m) long.
“These are very long, large and high-bearing capacity piles, meaning they have to able to support a lot of weight,” Elliott said. “To prove that PCL could do it, they needed the largest diesel hammer in North America and Pileco builds it. We brought this hammer in from China specifically for this job, where it is manufactured.”
Persing said that Elliott and Pile Equipment were on hand in the early part of the testing to offer their invaluable expertise to the effort.
“Whenever you talk to Mike it is easy to see that he is a pretty knowledgeable fellow,” Persing said. “He is really interested in the technical side of pile driving and this is really a fascinating load-test program due to the fact that we are jetting these piles into position with a custom-designed and fabricated jetting string, and then proofing the piles with the diesel hammer, which is pretty unique.”
PCL wanted to use a diesel impact hammer rather than a hydraulic impact hammer to drive in the piles.
The main difference in the two hammers, according to Elliott, is that the hydraulic hammer is a heavier ram, meaning the part that hits the pile employs a bigger weight but does so with a shorter stroke and, more importantly, has the capability to vary that stroke.
“A diesel hammer typically has to hit it with more velocity to get the same bearing or drive it the same depth,” said Elliott. “So what PCL did that was unusual was they talked to us and they said ’Can you lower and vary the stroke on that diesel hammer enough to where we can beat this pile in without destroying it?’ and we told them we could do that. So, in the end, it was quite unusual to use a diesel hammer rather than a hydraulic or air-steam hammer.”
If the courts rule in favor of PCL and the NCDOT Elliott said that PCL will then purchase three of the Pileco D225 hammers from his firm so they can immediately get to work.
Persing expected to have the load test program wrapped up in early April before beginning the waiting period to see when they would proceed on the bridge.
The new bridge, as envisioned by PCL, will employ two 12 ft. (3.7 m) wide lanes running north-south, with 6 ft. (1.8 m) shoulders on either side, making it much wider than the old bridge. The 2.7 mi. (4.3 km) bridge will have a maximum span length of 350 ft. (106.7 m) and a vertical clearance of 70 ft. (21.3 m).
“What is out there now is a bridge that is your typical pile, beam and cap system,” said Persing. “We are going to employ that for a majority of the new structure, but the main navigation span will be a pre-cast segmental with variable-depth box that will have a very exciting look.”
If the work gets the go-ahead, Persing expects more than 100 workers on site at the busiest time. He projects that the bridge will take 28 months to complete.
Even before environmentalists began their efforts to halt the project, PCL and the state of North Carolina agreed that due to the sensitivity of the marine environment around the bridge, 65 percent of the concrete would be pre-cast elsewhere and trucked in. By doing so, construction activities should be minimized, which should reduce the environmental impacts and the risks to marine life.