Building the Wilmington Bypass

GLF Construction Corp. is Erecting the $85.5 Million River Span

By Scott Judy

GLF Construction Corp. of Miami and its team of subcontractors are finally heading into the final stretch of its $85.5 million Wilmington Bypass project over the Northeast Cape Fear River in North Carolina.

It has been a long time coming. The project was slow going because it required building an extensive drilled-shaft foundation system for the structure located in the middle of a sensitive wetlands area.

Work began Jan. 15, 2001, and the target for completion is now June. Though the drilled-shaft work has taken twice as long as originally anticipated, Ed Thornton, contracts manager for GLF, said, "We're about where we thought we would be."

Despite the obstacles, officials with the North Carolina Department of Transportation are mostly positive."We're a little behind schedule, but overall I think everything's gone real well," said O.T. Anderson, NCDOT resident engineer. "GLF's done a good job. The subs have done a real good job. It's just a matter of getting it done."

NCDOT's positive opinion can be attributed to the project's minimal environmental impact.

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"Environmental was the major concern," Anderson said. "Getting in the drilled shafts, and making sure we didn't have problems with leakage of the slurry, has gone extremely well."

TreviIcos South of Tampa was the subcontractor for the drilled shafts, which numbered 500.

Project Basics

GLF and its team are constructing approximately 4.25 kilometers of four-lane divided highway, with the greatest focus on the 2.3-kilometer-long bridge structure spanning the Northeast Cape Fear River and adjoining wetlands. GLF's Thornton said the contractor is currently finishing the second half of the segmental portion of the channel span.

"We have finished the north-side approaches to the channel span and the north half of the segmental cast-in-place box girder bridge," he said. "We are currently about halfway finished with the south approach to the bridge."

GLF is self-performing the structure work and has subbed out the earthwork and associated roadway construction and landscaping to Triangle Grading & Paving Co. of Burlington, N.C.

The bridge utilizes a rather unique combination of two different construction methods. Approach bridges leading up to the main river span are being built with precast/prestressed bulb-tee girders and cast-in-place concrete decks. The 146-meter-long main span is then being built with cast-in-place cantilevered box girders. This section will have a vertical clearance of roughly 24 meters over the Northeast Cape Fear River.

T.Y. Lin International's Alexandria, Va., office handled the engineering of the river-span section. Reggie Holt, senior project engineer, said the cast-in-place box method won out over cable-stayed and arch alternatives.

"The primary force for the structure as a whole was vessel impact, which forced the piers to be outside the confines of the Northeast Cape Fear River," Holt said. "We were dictated span lengths in the realm of 500 ft., so once that decision was made, we were fairly limited on the type of structure that could make that span.

"We looked at (initial) costs, life-cycle costs and other performance issues. And the preferred alternative turned out to be the concrete box." Holt added that while the arch form had the highest initial cost, the cast-in-place box method beat out the cable-stayed alternative with lower life-cycle costs.

Precast box construction wasn't possible, either, due to the bridge's variable-depth box design and also the length of the span. Thornton said cast-in-place segmentals generally can achieve longer spans than precast segmentals.

Cast-in-place is "more difficult" than precast segmental erection and "a little slower," Thornton said. "With precast you may have the advantage of having your superstructure cast out by the time you get your substructure built."

Thornton explained that the balanced cantilever method of segment erection calls for casting and post-tensioning a segment, then launching the formwork system out from there, installing post-tensioning ducts and reinforcing steel for the new segment, and then pouring the concrete.

The segments are massive, measuring about 29 meters wide and featuring an interior height of about 35 ft.

GLF is also self-performing the deck work, which it expects to complete by March.

Drilled Shafts

Both bridge structures rest upon drilled shafts ranging from 1,220 mm to 2,439 mm in diameter installed to an average depth of approximately 24 meters. The shafts are drilled within permanent steel casings for the top 40-50 ft. of excavation. Below that, the drill rig excavates down to a depth of about 80 ft. through organic material such as peat.

Bud Khouri, vice president and regional manager for TreviIcos South, said the driving challenge was working in the wetlands environment. For starters, absolutely no bit of extraneous, excavated material, or spoils, could be allowed to fall back into the area surrounding the project.

"It's such an environmentally sensitive area that we took extra precautions and a lot of extra measures that we would not have taken had it not been in such an environmentally sensitive area," he added. "For instance, we made sure nothing dropped down - no dripping of anything of any sort, especially slurry, down into the swamps."

Excavation contractors use slurry in their drilling rigs to maintain an open hole in the area below the casings. Slurries are generally comprised of bentonite-type clay or polymer. TreviIcos proposed utilizing a polymer-based slurry that is more environmentally friendly.

After TreviIcos performed onsite load tests and proved to NCDOT that a polymer slurry would work just as well, the department approved the use of polymer. That was an exceptional occurrence; NCDOT and Khouri said they believe this is the first large DOT project to use a polymer slurry.

Overall, this part of the project has been difficult. TreviIcos started its work in August 2001 and now expects to wrap up by October. "That's the schedule right now," Khouri said recently, adding that the company was installing five shafts per week and had about 52 left to install.

Working in the Wetlands

Much of the slow-going can be attributed to the working environment. Contractors have to be careful how they work, and access to the site is limited.

To minimize the impact, NCDOT mandated that all construction be staged from a trestle bridge built over the wetlands. NCDOT's original plan was to have such a bridge on each side of the project. Instead, GLF proposed and eventually utilized a trestle on only one side. The company sought to enhance the trestle's productivity by incorporating a rail line that would deliver materials in and out of the job.

The wooden trestle, built on 24-in. pipe piles, has accommodated as much as seven American 9310 cranes at one time.

"Accessing your materials and personnel along a trestle, across a marshland, safely and productively and doing this in an environmentally friendly way - it's just challenging," Thornton said.

Still, he said he's pleased with the result of his company's diligence to the project's environmental issues.

"We've been particularly sensitive to the environment," he said. "If you take a walk down that trestle, you'll see that the area's cleaned up. We take the necessary precautions and we're sensitive to it."

Project Team:

Owner: North Carolina Department of Transportation
General Contractor: GLF Construction Corp., Miami
Bridge Designer: URS/Greiner and Woodward Clyde
Designer, Main Span: T.Y. Lin International, Alexandria, Va.
Drilled-Shaft Contractor: TreviIcos South, Tampa
Roadway and Landscaping Contractor: Triangle Grading & Paving Co., Burlington, N.C.