The Antimony to Burke project involved the rebuild of an approximately 13-mile 115 kV transmission line between Western Montana and Eastern Idaho. The line traversed a mountain pass and featured steep, rugged terrain. Structures on the Idaho side had recently been replaced, and the 8 miles of aging wooden poles on the Montana side were in need of replacing as well.
NorthWestern Energy, Crux Subsurface
Crux designed and installed steel cap micropile foundations for 15 structures, including 12 steel monopoles and 3 steel H-Frame structures. The majority of these structures were located on extremely steep slopes and required a unique construction set up. Materials and equipment were flown to the site by helicopter, and micropile drills were set on custom leveling platforms capable of providing a flat, stable workspace in the extreme conditions.
The challenging terrain resulted in limited, site-specific geotechnical information available during the design phase. Drilling of the first pile at each structure doubled as a geotechnical boring, enabling the installation team to characterize the subsurface conditions during construction. This information determined the number and length of piles to be installed as well as other foundation specifics. In total, Crux installed 70 micropiles ranging in depth from 35 to 64 feet.
A computing error was introduced at one H-Frame structure when determining top-of-foundation elevation with respect to ground level. The error was discovered after pile installation and before the cap was set, requiring an on-the-fly solution to ensure the structure legs would match up as designed. Demonstrating the value of integrated design and construction, a riser assembly was successfully designed and installed on top of the original pile cap without delaying the project.
Additional project challenges included working with helicopters at high elevations, between 3,500 and 5,700 feet. Extra planning was required to ensure that heavy loads did not exceed capacities as temperatures rose throughout the day. All foundation work was successfully completed within schedule and budget.