The Flathead Tunnel. A Geologic, Operations, and Ground Support Study, Burlington Northern Railroads, Salish Mountain, Montana.
The 7-mile Flathead Tunnel, located in the Central Salish Mountains of northwestern Montana, was constructed between 1966 and 1969. With the contract awarded April 19, 1966, drill and blast tunnel excavation began September 30, 1966, and was completed June 21, 1968, in 488 tunnel-driving days. Peak advance was 66 feet per day. On June 27, 1969, all tunnel concrete lining was completed. Average wall and arch concrete placing rates exceeded 1,700 cubic yards daily, with nearly half of the concrete placed during a severe winter season. Major cost savings in concrete tunnel lining were achieved through a value engineering clause in the contract. Achieving consistent results and high rates of placement in concrete lining ranked equal to ground control in overall tunnel problems. Ground control problems exceeded expectations and were of continual concern throughout excavation. One support collapse in the tunnel and one portal slide condition developed during construction. Critical surface geologic exposures were largely obscured; only a portion could be accurately projected underground. An instrumentation program to gather basic data on rock deformation near the heading face for support purposes was a pioneering effort. The flathead tunnel was placed in service on November 7, 1970.