Washington Bypass Construction Project

Turning a problem on its headConcern for the environment has led to a unique approach to road building in sensitive areas, as Jeff Daniels learns. When drivers traveling between the small towns of Washington and Chocowinity in North Carolina eventually gain access to the new Highway 17 bypass at the end of this year, their thoughts no doubt will simply revolve around the lower levels of congestion that the $192 million project has delivered. But this has not been just any old stretch of road.   One way or another, the Washington Bypass has chalked up quite a number of firsts.ThereÔÇÖs plenty of water around Washington, located as it is at the point where the Pamlico and Tar rivers meet the coastal estuary. If the North Carolina Department of Transportation had decided to take a more conventional approach on how the relief road was to be designed and built, instead of opting for a light touch, an innovative new construction technique might not have seen the light of day.But the Pamlico-Tar crossing, along with extensive neighboring wetlands, called for sensitive handling. Consequently, NCDoT commissioned the joint venture team of Flatiron and United Contractors to build the 6.8-mile stretch of road with the express aim of minimizing environmental impact.┬á By far the biggest design challenge was how to bridge the river and wetlandsÔÇöin all, 2.8 miles of road that in places required 45 feet of vertical clearance over the Tar River navigational channel. Extensive piling was obviously essential, but to pile in the conventional manner, using temporary access trestles at ground level, would have caused considerable intrusion into the habitat. So instead of working from the ground up, the problem was solved by working down from aboveÔÇöusing a pair of specialized launching gantries, one at each end of the water, and working independently toward the center. From these 750-ton movable platforms, every stage of the bridgeÔÇÖs construction is being carried out from the road deck level itself, with minimal disruption to the wetlands below. The bridge has two completely separate carriageways, for north and south travel. During construction these are being linked by concrete beams on which the 592-foot-long gantries sit. The head of the gantry is loaded up with pile-driving equipment and then cantilevered out 120 feet over the water in order to drive a row of five 30-inch-square, pre-stressed concrete piles into the river bed. Concrete piles weighing as much as 40 tons are pushed out horizontally along the gantry, then loaded and latched into the hammer. Hydraulic cylinders and winches rotate them vertically, ready to be driven down through sand into the bedrock.This patent-pending concept is the worldÔÇÖs first such incidence of piling from a gantry, but as if this wasnÔÇÖt achievement enough, the Italian designer Deal and its Canadian collaborator Berminghammer spent a year developing and testing the system, using 3D modeling, in order to be certain that it could withstand the 100-mph wind loads often encountered in this hurricane-prone area.Should the weather turn bad, construction can continue until winds reach 45 mph, at which time the gantry has to be secured and temporarily put out of action. If the winds strengthen to 64 mph, the gantry has to be retracted and anchored down until the storm passes.Once the piles are in place, the gantry reverts to its crane role, first building up the pile cap cross members and then erecting four rows of 72-inch-high pre-cast T girders that eventually carry the cast-in-place concrete decking. While the decking is curing on one carriageway, the gantry is slipped across to the adjacent carriageway and the process repeated. All these operations are being performed without the use of temporary access trestles. But the hard work put into designing this ÔÇ£top-downÔÇØ solution has paid dividends and received the full support of the many environmental agencies involved in the permit approval stage of the project.While intended primarily as a means of reducing environmental disturbances, the movable gantry has contributed to a shortened construction schedule in comparison with normal construction techniques. In fact, in order to minimize the projectÔÇÖs duration, NCDoT opted for its first design-build contract, whereby various stages are carried out in parallel rather than consecutively. In the first year, permit applications were made while design was progressing. Construction started in February 2007 and should be complete by the end of the year.  ÔÇô Editorial research by Steven Shah┬á