The Geotechnical Engineer Had Good News. He had taken the soil boring for the proposed structure and encountered no rock at the design depths, just sand and clay soil to depths of 60 ft (18 m). It was music to the ears of the design team, who wanted to get off to a fast start in Wisconsin with concrete pier foundation designs for more than 1500 structures along the 220-mile (354-km) Arrowhead-Weston 345-kV line, the biggest transmission line project in the state's history.

Imagine their surprise, then, when they later found karst rock below groundwater, while drilling the foundation for one of the most heavily loaded structures near Weston Substation. Karst consists of outcroppings that create unpredictable vertical seams, a designer's nightmare. It makes it difficult to install any type of foundation, let alone the concrete pier foundation planned for this structure. Although the soil borings had been drilled properly, the geotech had hit a seam and missed detecting the karst rock; a larger drill revealed otherwise. Still, the design team was ready. After all, they would have been crazy to expect everything to unfold exactly as planned; they knew to expect the unexpected.

DEMAND JUSTIFIES NEED

When the U.S. Department of Energy declared northwestern Wisconsin one of the four most-constrained transmission line areas in the country in the late 1990s, the need for a new, reliable line became clear. Because of limited power-transfer capabilities between Minnesota and Wisconsin, certain load conditions could cause blackouts in Wisconsin and surrounding areas. On top of this, the demand for electricity in the region was growing at the rate of 2% to 3% per year. The existing transmission grid was not equipped to reliably accommodate this growth. At the time of construction, Wisconsin only had four interstate high-voltage lines, compared to dozens in neighboring states. This was the impetus for the Arrowhead-Weston 345-kV transmission project.

Originally proposed and initiated by Wisconsin Public Service Corp. (WPSC; Green Bay, Wisconsin, U.S.) and Minnesota Power (Duluth, Minnesota, U.S.) in 1998, American Transmission Co. (ATC; Waukesha, Wisconsin), at the time a newly formed regional transmission operator, took over the project from the two utilities in 2002.

ATC immediately ordered a review of the original US$165 million cost estimate that had been previously approved by the Public Service Commission of Wisconsin (PSCW). In just five years, the project's estimated cost had ballooned in the face of rising material and construction prices, and growing opposition from environmentalists and concerned landowners. To account for more robust environmental impact mitigation, environmental inspection, farm disease mitigation, increased real-estate rights-of-way costs and increased public outreach efforts, ATC increased the project budget to $420 million, which was reapproved by the PSCW.

A BALANCED DESIGN

Because of the critical nature of the project, the design and construction team — comprised of ATC, WPSC, Minnesota Power, POWER Engineers (POWER; Hailey, Idaho, U.S.), M.J. Electric (Iron Mountain, Michigan, U.S.), Tri-State Drilling and others — set out to build a new line that would not only be highly reliable, but also highly flexible to meet each of the project's stringent requirements. The design was predicated on the idea that lines like this didn't get built every day. To do the project justice, it would have to be designed to the highest-possible standards.

The overarching goal was to find a design that balanced all of the project's elements, including environmental and landowner concerns, aesthetics, land use, extreme meteorological conditions, widely varying subsurface conditions, durability, constructability and more. The numerous stakeholders involved further complicated matters. Because the PSCW mandated that the new line use existing rights-of-way as much as possible, it became necessary to replace portions of 12 different existing lines with new double-circuit structures. The design required close coordination with various utilities to account for each of their unique design standards.

One of the early steps was to identify a basic structure design. The existing structures were primarily wood-pole H-frames that required large rights-of-way corridors. Adding additional wood structures was unrealistic, given the constraints placed on the design. Wood also had insufficient strength to carry more than one circuit. Because many of the structures would have to be double-circuit, wood was quickly ruled out. Although durable and able to accommodate several different tower configurations, lattice towers were deemed unsuitable because they, too, would create footprints that would exacerbate landowner and environmental impacts. That left single-pole tubular-steel structures, which best accommodated the host of line-design constraints. Steel poles offered engineers and designers the flexibility to create the most efficient designs with high levels of reliability.

DESIGN CHALLENGES

Because of the critical importance of this new line, ATC decided to design for high reliability. Although most lines are designed for a 50-year or 100-year return period event, ATC determined that this line needed to be able to withstand a 400-year return period weather event. (Designing for a 400-year return period means that one must design for a weather event that has a 1 in 400 probability of occurring in any given year.)

The decision proved complicated and required the design team to use two distinct loading criteria, one for the northernmost 20 miles (32 km) of the line (to accommodate the local weather effects from Lake Superior) and another for the remaining 200 miles (322 km) of the line (to accommodate the weather of north central Wisconsin).

The microclimate found on the northern portions of the line typically provides more severe icing conditions because of its proximity to Lake Superior. On this first section, the design team assumed that the conductor, supplied by Alcan Cable, could accumulate up to 1.85 inches (47 mm) of radial ice. In contrast, designs along the Wisconsin portion of the line took into account 0.925 inches (23.5 mm) of radial ice, about one-third as much when measured by volume. The more severe ice loadings near Lake Superior required heavier conductor, heavier structures and larger foundations. In both regions, the line was designed for an extreme wind of 108 mph (174 kmph).

Because the PSCW required that the line share corridors with as many existing utility easements as possible, cross-utility communication and coordination was essential. Seventy-five percent of the line was located within or along existing transmission lines, pipelines and railroad rights-of-way. This mandate required that double-circuit lines be constructed to replace portions of 12 existing transmission lines owned by four different utilities. So the design team had to simultaneously meet four different sets of design and material standards on the double-circuit lines. The existing lines were demolished and double-circuited with the new structures.

Steel poles were vital for narrower rights-of-way in both Minnesota and Wisconsin. Rights-of-way width was a key element in reducing the impact to the environment and agricultural land. It offered a smaller footprint and allowed designers to come up with custom designs ideal for multiple special-structure configurations required for entering and exiting double-circuit line sections. Drilled pier foundations provided flexibility to accommodate the different subsurface and geologic conditions along the route. The majority of the 1564 supporting structures were single-shaft steel poles with drilled pier foundations, which allowed construction crews to build lines along narrow rights-of-way, specifically 100 ft (30 m) in Minnesota and 120 ft (37 m) in Wisconsin. The average span length was about 750 ft (229 m). Most of the drilled pier foundations ranged in diameter from 6 ft to 8 ft (1.8 m to 2.4 m), although a few of the heavier angle and deadend structures required foundation diameters up to 12 ft (3.7 m).