Jack Rabbit–Big Sky 161 kV National Forest Canyon Construction

The Jack Rabbit–Big Sky 161 kV Transmission Line is partially located in the Gallatin National Forest and parallels a busy two-lane road, US Highway 191, and the Gallatin River, all which fall within the confines of the steep narrow Gallatin Canyon. The 8.5 mile segment of transmission line that falls within the canyon has 22 concrete pier foundations. Access was not permitted before the special use permit was issued; therefore, subsurface investigation information was not available to aid in design of the pier foundations. An on-site geotechnical engineer performed the subsurface investigation during the excavation of the foundations. Once the soil stratum was known, the pier foundations could be modified accordingly. Limited access was available due to difficult terrain and requirements set by the United States Forest Service (USFS), requiring excavation of numerous holes by hand, some up to 8 feet in diameter and 15 to 20 feet deep. Helicopters were used to aid construction, such as flying in structures, equipment, aggregate backfill, as well as string conductor. This paper will cover in detail the design and construction challenges, solution options, and final design decisions.