Alternate Foundation and Structure Designs: Mississippi Backwater Construction Challenges

The Xcel Energy W3408 69kV overhead transmission line was originally constructed in 1934 and extends 2.7 miles (4.3 km) from the Nelson Substation in Wisconsin to the Wabasha Substation in Minnesota, crossing the Mississippi River and its backwaters through the Federal Upper Mississippi River National Wildlife Refuge. Xcel Energy identified 2.2 miles (3.5 km) of direct embed wood H-frame structures in Wisconsin through the backwaters area for a rebuild in 2018–2019 due to age and condition. Design and construction challenges include the following: the presence of 2 to 3ft (0.6 to 0.9 m) of standing water over a significant portion of the project; winter ice; environmental/wildlife concerns; a limited service interruption (outage) for the transmission line, and poor soils. The design team selected a high-strength conductor, minimizing sag and decreasing the total structure count from 27 to 17. The chosen principal foundation consists of groups of four (4) 3.5-inch (90 mm) outer diameter helical piles connected by a grillage at each structure leg. All required materials could be transported to the structure locations by airboat and installed using specialized marsh equipment without an extended, non-restorable outage of the transmission line. The grillage cap design consists of W-beams interconnecting grouted collars that accept the helical piles and transfer loads from the structure legs. This configuration develops a fixed head condition for the piles. The use of battered piles creates a stiffer load-deflection response thus improving capacity. Drifting and expanding ice loads were addressed by identification of risk-prone areas and development of a secondary foundation system consisting of vibratory steel caissons for the four (4) structures that could not be spotted in low-risk areas. These caissons consist of 0.563-inch (14.3 mm) thick galvanized steel in a dodecagonal shape with a 3.5-ft (1.05 m) outer diameter. They range in length from 56 to 62 feet (16.8 to 18.6 m) and are designed for up to 6.5 feet (1.95 m) of scour potential.