Carolinas AGC Honors Best Highway Project: U.S. 17 Bypass

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The Carolinas AGC bestowed the 2010 Best Highway Project Pinnacle Award to the U.S. 17 Bypass in Washington, NC.  The design build project was a joint venture of Flatiron-United.  This 6.8-mile bypass on North Carolina’s coastal plain stretches over miles of environmentally sensitive wetlands, and features a 2.8-mile structure over the Tar River.  NC DOT had specified the requirement for minimizing the construction footprint into the proposal scoring method. The Flatiron-United joint venture team responded by developing an innovative “top-down” approach – a unique overhead gantry specially designed and built for this project. It resulted in both a minimal impact to the wetlands and an accelerated construction schedule compared to conventional techniques.

The gantry system essentially eliminated the need for equipment and temporary access trestles and ground work in the fragile wetlands, by constructing each new span from the newly-built permanent structure. The two self-launching 600-foot-long gantries at each end of the bridge weighed about 750 tons. The two gantries had to be capable of:  driving 124-foot-long precast piles;  erecting 50-ton bent caps; erecting 121-foot-long precast girders, and supporting deck pouring operations. 

The Washington Bypass extending over the Tar River In an assembly line progression, construction activities took place simultaneously in three of the four 120-foot long spans. As one span was completed, another’s deck was curing, and the gantry was launched ahead to begin the pile driving on the next span. Meanwhile material was delivered over the completed structure in the fourth span. The result was a complete span in as little as seven to nine days. 

The world’s first application of the pile driving operation from an erection gantry (approximately four bridge spans in length) is the most unique feature of this system. This essential element truly eliminates the need for equipment and temporary access trestles and ground work in the fragile wetlands, by constructing each new span from the newly- built permanent structure. 

The dramatic reduction in wetland disturbance offered by this top-down construction operation was welcomed by the U.S. Army Corps of Engineers, North Carolina Division of Water Quality, NC Department of Natural Resources, U.S. Coast Guard, and other environmental agencies. The gentry system has since been patented.

Flatiron’s business development manager Paul Newman, who served as project manager for the Washington Bypass project, said “the project represents what the design-build method of construction is all about. This success resulted from a contractor (Flatiron-United), designer(AECOM) and owner (NCDOT) collaborating to design and build the best possible solution.” He described the top-down construction technology, and specifically the patented pile-driving technology as being the cutting edge of what we can do to improve construction means and methods while reducing impacts to the environment and finishing well ahead of schedule. Flatiron is very proud to have been involved in this project and commend all partners on the team for a collaborative effort.”  Visit www.flatironcorp.com and uig.net for more information.

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