Washington Bypass

The U.S. 17 project is one of the North Carolina Department of Transportation’s biggest design-build projects ever. With a significant portion of the new route traversing through a major wetlands area, the joint-venture contractor is employing a patent-pending method to minimize the impact of construction.

By Debra Wood and Aileen Cho

A joint-venture team is using a unique gantry system to protect wetlands during construction of its $192 million U.S. 17 Washington Bypass contract. The design-build project is a main part of the North Carolina Department of Transportation’s plan to convert 15.5 mi of U.S. Route 17 to a limited-access freeway.

“U.S. Route 17 is a strategic corridor in the state,” says Maria Rogerson, assistant resident engineer with NCDOT.

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A joint venture of Flatiron Constructors of Longmont, Colo., and United Contractors of Chester, S.C., received a $192 million design-build contract in February 2006 for the 6.8-mi, four-lane, “B” section of the bypass, and construction began in March 2007. Earth Tech of Long Beach, Calif., provided the design work for a 2.8-mi-long bridge spanning the Tar River, Kennedy Creek and associated wetlands; three land bridges; and 4 mi of roadway. The project is scheduled for completion by November 2010.

“Design-build moved the project along quicker because it’s the responsibility of the contractor to acquire the right of way,” Rogerson says. “The general public gets the product sooner.”

While the NCDOT project is noteworthy for its overall cost and scope alone, some of the most interesting work is taking place on the 2.8-mi bridge spanning the wetlands, where the design-build contractor team is using an innovative approach to greatly minimize construction’s impact.

Flatiron/United’s Gantry

To build in the wetlands area, Flatiron/United has employed a unique top-down methodology, working from north and south toward the middle of the Tar River and using two custom-built, 600-ft-long, overhead gantries. The system eliminates the need for an access trestle to drive piles, thus reducing impact on the wetlands. A trestle bridge would have required clearing about twice as much as was needed with the gantries.

The concept of the pile-driving launching truss came from Elie Homsi, Flatiron’s vice president of engineering. The Italian manufacturer Deal and Ontario-based Berminghammer, a division of Bermingham Foundation Solutions, came in to flesh it out.

“Cantilevering forward and driving piles has been done before” in marine projects, notes Patrick Berminghammer, CEO of the Ontario firm. But unlike traditional pile-driving, “this is a semiautomated, high-tech, low-crew operation.”

Adds his project manager, Stefano Gabaldo, “This is the only machine that can drive piles and create the bridge afterward.”

The team spent a year developing and testing the system, using 3D modeling from the start, Berminghammer says.

The pair of 750-ton launch gantry trusses were shipped in pieces from Italy last fall and assembled onsite.

The bridge will sit on 1,200 30-in.-square, prestressed concrete piles, driven to a depth of between 75 and 100 ft. The bottom 8 ft of the pile is solid, and the balance is hollow.

Coastal Precast Systems of Chesapeake, Va., is casting the piles and the 72-in. modified bulb-T girders, which are approximately 120 ft long. Flatiron casts the pile caps onsite to minimize the need for cast-in-place concrete and the risk of liquid entering the water.

The pile as heavy as 40 tons is pushed out horizontally along the truss, then loaded and latched into the hammer. Hydraulic cylinders and winches then tilt it vertically and drive it down through sands and clay.

“It’s like a fishing pole,” says Homsi of the pile driver extending from the edge of the gantry. A telescoping leg provides extra absorption against vibrations and helps realign the pile if necessary. The patent-pending system then sets the precast cap atop the pile and maneuvers onto it to set the girder. The deck section is then cast in place.

“The truss is so long that it spans three spans at a time,” says Mark Mallett, Flatiron’s project manager. Thus substructure and superstructure work can all happen in assembly-line fashion, with three spans under construction simultaneously. Then the gantry moves.

Switches and monitors throughout the truss system monitor pressure and alignment of operations. Deviances beyond a certain limit cause automatic shutoff, Gabaldo says.

“Every time it builds a span, it launches ahead a span, and there is a lot of load from the gantry that goes into the bridge itself,” Mallett says. Consequently, Earth Tech engineered the structure to withstand those loads.

Flatiron has filed for two patents: one for the truss with the pile-driver and the other for the related exterior girder system with a built-in curb that acts as a form for casting the deck, eliminating the need for overhang brackets, Mallett adds.

The team built a silt fence and turbidity curtain and adhered to precise clearing limits. No grubbing or work in the water can take place during peak fish-spawning season, between Feb. 15 and June 15 of each year.

“Once they finish driving piles, they can do strut work, because it’s above the water and not impacting anything in the water,” Rogerson says.

The contract includes a $100,000 erosion-control bonus, earned if Flatiron/United does not receive any violations throughout the duration of the job.

Other Work

Flatiron/United has initiated some changes to the project as a result of the design-build delivery system. The joint-venture team modified NCDOT’s original concept for the N.C. Route 33 interchange. Initial plans called for bringing the bypass over N.C. 33, but the design-builder determined it would be more cost effective to take N.C. 33 over the bypass, building a two-span, precast concrete beam bridge with conventional end bents.

Rogerson estimates the plan saved the contractor from hauling in 1 million cu yds of fill, and it reduced the effect on the wetlands.

“We saved the excavation and dirt work and hauling material onto the jobsite,” Mallett says. “It was a big component of our design at bid time that helped us acquire this job.”

The company also is constructing two single-span, precast concrete beam bridges over U.S. Route 264 and over 15th Street and an interchange at U.S. 264.

Flatiron/United subcontracted the asphalt roadwork to S.T. Wooten Corp. of Wilson, N.C., which will self-perform construction of the highway north of the bridge. Wooten has brought onboard PLT Construction Co., also of Wilson, to build the portion south of the bridge.

Mallett expects to have most of the paving work completed by the end of this year. Paving areas approaching the Tar River Bridge will be performed after the bridgework is complete.

Robertson Contracting began work in January on the $12.8 million, 4.2 mi “A” segment to the south of the Flatiron/United job. Also, NCDOT is currently acquiring right of way for the 4.2 mi section to the north of the Flatiron/United project. The section A contract also is scheduled for completion in fall 2010.

Portions of this story originally appeared in Engineering News-Record, also published by McGraw-Hill Construction.

Washington Bypass Project Team:

Owner: North Carolina Department of Transportation
Design -Build Contractor: Flatiron/United, a joint venture between Flatiron Constructors of Longmont, Colo., and United Contractors of Chester, S.C.
Engineer: Earth Tech of Long Beach, Calif.

Useful Sources:

North Carolina Department of Transportation:
http://www.ncdot.org/projects/us17bypass/