A view facing north through the Minnesota River crossing area shows the transition from three monopole structures with horizontal conductor configuration to a vertical quad circuit configuration in the background, where the utility colocated with the adjacent lines.
A migratory flyway, delineated wetlands, deer-hunting season and spring flooding presented challenges and restrictions to a transmission line expansion project in southern Minnesota.
The CapX2020 Brookings County-Hampton project is a 248-mile (399-km), 345-kV transmission line with seven new substations and two substation expansions. The transmission line is comprised of steel monopole structures, averaging 150 ft (46 m) tall, set atop drilled pier foundations — with a few exceptions. The line crosses the Minnesota River three times, and one crossing traverses a lowland floodplain near Belle Plaine, Minnesota, where the project team was faced with a series of unique challenges.
Flying, Hunting, Thawing
Because of the high concentration of eagles in the area, which also is a migratory flyway, the project was faced with a stringent permitting process that, among other things, restricted the height of structures to about 100 ft (30.5 m) above the floodplain. This resulted in relatively short structures with correspondingly short spans, which had to be set on foundations extended above grade by about 14 ft (4.3 m) because of the spring flooding that frequently occurs in this location. The phases had to be rolled from a double-circuit vertical configuration to a horizontal configuration suspended by three separate poles.
An old hunting lodge with an existing trail provided access for project crews. The project could not improve the trail because of restrictions on importing granular fill into the delineated wetlands, so conventional wood matting was used to provide access where underlying soils were not competent to support the traffic. Because of the deer hunting season that occurs in the area each fall, and the necessity to complete construction before the spring thaw, construction was limited to between December 1 and February 28.
The Belle Plaine crossing originally included six structures. Early in the design process, the team moved one of the structures about 100 ft to the south, far enough to get it out of the floodplain and on high ground so a conventional drilled pier foundation could be constructed.
One foundation was located on the north side of the river and had no extraordinary requirements, so it was constructed as a set of three drilled pier foundations with anchor bolts. A vibratory casing was used as a construction aid in poor soils as well as a concrete form for pouring the reveal, or extended height above the floodplain.
Four structure locations were adjacent to a municipal wastewater treatment facility, and there was great concern about damaging the integrity of the clay liner because of vibrations from heavy construction equipment traffic. Given this constraint, project managers and engineers decided to use steel caissons driven directly into the marginally competent soils rather than conventional drilled pier foundations. Drilled pier foundations would have required hauling many loads of spoils, rebar and concrete next to the facility. Vibration monitors were used throughout construction to continuously monitor and record vibrations. The treatment cells were tested before construction began and again after construction was completed. The tests confirmed the clay liner remained intact and the precautions taken by the team mitigated any possible damage.