Remote and virtually undisturbed, the Upper Mississippi River National Wildlife and Fish Refuge boasts a bounty of flora and fauna, both common and rare species that thrive in the largest delta floodplain in the Upper Midwest.
At the northern edge of this wilderness, near the confluence of the Chippewa and Mississippi rivers, is the Nelson-Trevino Bottoms State Natural Area, comprised of 3740 acres of forested floodplains, marshes, sloughs, and ponds.
Wisconsin State Route 25 bisects the area and is the only connection between Wisconsin and Minnesota for 30 miles. The highway is vital for area residents, as the nearest grocery store is four miles across the river in Wabasha, Minn.
To maintain that link, the Wisconsin Department of Transportation (WisDOT) in 2009 began a $10 million, 15-month project to replace three bridges and rehabilitate a fourth one in the area. WisDOT also received additional funding for the project through the federal Highways for Life program.
For this project, academia teamed with precast producers Spancrete, of Waukesha, Wis., and County Materials Corp., of Marathon, Wis. Michael Oliva, a University of Wisconsin professor of civil and environmental engineering, worked with WisDOT engineers and the two precast producers to develop standards for precast spans. Oliva's recent work has focussed on earthquake resistance of precast concrete systems. He currently is developing improved methods for highway bridge design and rapid construction to improve work zone safety and reduce impact on motorists.
In 2008, WisDOT used precast abutments for the first time on a bridge near Baldwin, Wis. These were developed with Oliva and his students.
Two of the seven-span, 544-foot Route 25 bridges are the first in the state to use precast concrete pier caps to support the bridge girders. “The seven-span bridges allowed span lengths where open-pile-bent piers could be used, minimizing the footprint of the piers in the Mississippi River slough and decreasing the time required to construct the piers,” says Michael Williams, a consultant bridge project engineer with the WisDOT Bureau of Structures.
The right connection
One of the most important aspects of using precast components is connecting them, says Oilva, who has studied connection methods worldwide and has developed several of his own. Because of the environmental sensitivity of this project, crews split the precast pier caps in half to lower their weight and reduce the size of the crane needed to handle them.
Placed atop the piles, the halves fit like a three-dimensional puzzle, with overlapping lap-ship joints skewered with steel pins. Steel dowels grouted in place also secure the pier caps to the piles.
Oliva sees precast concrete as the material of choice for future projects. “Our goal is to build a bridge in a week or two, rather than in three or four months,” he says.