Engineering Inc. - July/August 2008 - (Page 28)

MULTI-PROJECT FEATURE The 58-year-old steel bridge over Montana’s Blackfoot River is soon to be replaced by a modern, 347-foot, three-span, prestressed concrete bridge. Milltown’s New River Crossing PROJECT: Scott Murphy Montana highway 200 Bridge Replacement, Milltown, Mont. FIRM: W Morrison-Maierle, Inc., helena, Mont. hen state officials in Montana decided to forge ahead with plans to demolish the historic Milltown Dam, part of the U.S. Environmental Protection Agency’s (EPA’s) Milltown Reservoir Sediments Superfund site cleanup, they knew the project would also require the demolition of the Montana Highway 200 Bridge. The 58-year-old bridge over the Blackfoot River was the main connector for motorists between the cities of Missoula and Great Falls. But with the dam coming down half a mile upstream, the bridge would no longer be safe for travel; they had to build a new bridge to account for the water flow that would result from the demolition of the dam. In search of a design-build firm with extensive understanding and capabilities, the EPA, Montana Department of Transportation (MDT) and Missoula-based general contractor Frontier West, LLC, turned to Helena-based Morrison-Maierle, Inc. (MMI). The project had several goals the design-build team needed to fulfill. The EPA’s demolition of the hydroelectric dam would restore the stream to its original state and rebuild the natural habitat in this environmentally conscious community. Building the new 347-foot, three-span, prestressed concrete bridge to replace the existing steel bridge would provide a modern structure for the community and ensure travelers’ safety. MMI prepared the technical proposal and design for the temporary detour bridge, the new bridge and the approach roadway. MMI also is responsible for construction phasing and traffic control plans, the river hydraulic analysis for the new bridge, geotechnical engineering, construction materials quality assurance and helping MDT coordinate with the EPA and the Montana Department of Environmental Quality. The $7.2 million project kicked off in August 2007 and is scheduled for completion by December 2008. “The aggressive time requirement is driven by the removal of the dam, as determined by the EPA, and the onset of a Montana winter,” explains Scott Murphy, vice president. MMI meets at least twice weekly with its design-build partners to discuss concerns and issues related to these challenges. “Design-build delivery provides MMI with Charlie Brisko John Pavsek more effective control over the schedule to ensure on-time completion,” says Charlie Brisko, project bridge engineer. In designing the new bridge, MMI had to account for substantial changes in the river hydraulics resulting from the demolition of the dam, including reduced water levels, increased current flow, pier scouring and ice jam forces. “To overcome these challenges, we used comprehensive hydraulic and scour modeling processes to determine the effect of the reservoir drawdown on the design of the bridge,” explains John Pavsek, MMI’s senior transportation engineer and project manager. n 28 ENGINEERING INC. JULY / AUGUST 2008

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Engineering Inc. - July/August 2008

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