Abstract
The 7-mile long Flathead Tunnel in northwestern Montana was required in relocation of Great Northern Railway's main line track prior to impounding water by Libby Dam. Approximately 73% of the tunnel required structural steel tunnel sets for temporary support prior to the placement of concrete lining. A sliding floor, large special tunnel equipment, contractor initiated design changes to eliminate special trim and trench excavation, and laser survey alignment methods all contribute to a high speed excavation operation. Elimination of regularly spaced wall and arch contraction joints with waterstop through a value engineering proposal made a continuous lining operation possible. The conveyor placing system delivered concrete up to tunnel springline through form doors while the pump filled the tunnel arch.
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