In preparing for the expiration of a land agreement between the state of Minnesota and a local mining company, set to expire in 2017, the Minnesota Department of Transportation (MnDOT) began plans for a new bridge. The new 1,100-ft weathering steel plate girder Rouchleau Mine Bridge sits 180 ft above the floor of the Rouchleau Mine pit and connects the mining towns of Virginia and Eveleth, Minn., on State Highway 53. Due to challenging site conditions, the three-span structure features a 480-ft main span and haunched girders with depths ranging from 7 ft, 9 in. at mid-span to 14 ft, 6 in. over the piers. Pier depth was limited to facilitate shipping to the rural area, and the girders were shipped horizontally. All steel was GR50 weathering steel.
To meet the project deadline, the bridge was designed and built under an accelerated schedule using the construction manager/general contractor (CMGC) delivery method. In addition to the fast-track schedule, the project was also challenged with near-vertical rock walls, mine waste rubble to depths of 120 ft below the pit floor, potential water elevation changes greater than 100 ft, the need to accommodate future mine blasting operations of the reactivated mine and construction over a lake that serves as the drinking water supply for the town of Virginia.
The design team worked collaboratively with MnDOT and the CMGC to validate the structure type and develop delivery strategies to directly address the project risks associated with schedule, the northern Minnesota weather and the unique terrain of the open pit mine. The project was delivered in two packages: an early steel package and a substructure/deck package. Structural engineer Parsons included detailer Tensor Engineering as a design team member to provide draft shop drawings as part of the bid package to minimize bid risks, facilitate mill orders and ultimately expedite fabrication. Within 52 days of Notice to Proceed, Parsons delivered the complete plans for the 5,200-ton superstructure. The remainder of the bridge package was delivered within the overall seven-month schedule, and the bridge opened this past summer.