Construction projects throughout the world have become more complicated and logistically challenging as environmental stipulations grow more demanding. Nowhere is this more evident than in the subarctic, in this case interior Alaska, where Golden Valley Electric Association Inc. (GVEA) recently constructed the 230-kV Northern Intertie transmission line project. The line originates in Healy at GVEA's mine-mouth coal power plant and terminates in Fairbanks, 96 miles (155 km) to the north.

Overall Design

The 96-mile 230-kV transmission line was constructed with four separate construction contracts dictated by distinct geological zones and restricted accessibility. Much of the alignment was underlain by warm discontinuous permafrost, prevalent throughout interior Alaska. The potential for change in the marginal permafrost over the life of the project had to be addressed at the design stage to provide the most economical project. During the life of a project, these areas have the potential to change from frozen to thawed states, resulting in a substantial change in the soils' ability to carry load. Ground temperatures in areas of marginal permafrost are typically greater than 30°F (-1°C) and often isothermal for some depth.

The foundation design had to recognize that engineering characteristics of the underlying permafrost would change over the life of the project. The load-carrying capacity of the soils and continuing maintenance costs as a result of the subsurface changes had to be addressed. All of the tower foundations consist of drilled and/or driven steel pipe piles varying in size from 10-inch to 66-inch (254-mm to 1676-mm) diameters and depths from 24 ft to 80 ft (7 m to 24 m). Dryden & LaRue was the design engineer and Golder Associates Inc. in Anchorage was the geotechnical consultant. All of the self-weathering tubular steel structures were provided by Valmont-Newmark.

Usibelli Section

The first 6 miles (10 km) of the project traversed the Usibelli Coal Mine and consisted of self-supporting tubular steel Y towers supported on a single deep steel piling. A small footprint was needed to fit the structures between the active coal mine haul road, adjacent river and 300-ft (91-m)-high gravel bluffs. The subsurface material consisted of glacial outwash and alluvial deposits, ranging in size from sands to large boulders; permafrost was not an issue. Deep pipe piles were selected because of the potential of erosion from the adjacent Nenana River.

Foothills Section

Moving from the active coal mine area into the Alaska Range foothills north of Healy, fore and aft guyed-X towers with a wide footprint were used. Subsurface conditions in the foothills consisted of densely packed alluvial gravels and uplifted lake beds. With the gravels being non-frost susceptible, the tower legs were supported on shallow drilled pipe piles grouted in place; anchors consisted of grouted anchor rods. Existing mining and exploration roads provided vehicle access to about half of the next 24 miles (39 km); however, the remainder was restricted to helicopter access. No disturbance of the root mat was allowed in these areas, which made for creative solutions to provide level work pads.

Tanana Flats Section

North of the Alaska Range foothills, the line crosses 60 miles (96 km) of the low-relief terrain of the Tanana River Flats. The structures used across this area consisted of fore and aft guyed-X towers for tangents, and light angles and unguyed four-legged swing-set towers for deadends and large angles. The tower foundations and anchors consisted of single driven steel pipe piles. Short-term loading from lateral and uplift forces caused by wind and ice controlled the design as sustained loads were relatively small for the X towers. However, the sustained loads were much higher for the swing-set towers.

The design of the foundations and anchors had to consider both the frozen and unfrozen state of the soils across the Tanana Flats. Geotechnical investigations for this portion of the project examined an existing GVEA 138-kV right-of-way approximately 3 miles (5 km) west of the Northern Intertie route. When constructed in 1966, the underlying soils consisted of an approximately 30-ft (9-m) layer of frozen silts and gravels. The original 100-ft (30-m)-wide right-of-way clearing removed all trees and shrubs, leaving the root mat intact. In the 31 years between 1966 and 1997, the permafrost degraded completely within the cleared right-of-way. This phenomenon is typical, and a well-known study by the U.S. Army Corps of Engineers has shown the clear link between the vegetation removal and the resulting degradation of permafrost.

The original foundations on the 138-kV line consisted of 30-ft wooden piling, and with the absence of the permafrost, the frost heave jacked the poles out of the ground 3 ft to 6 ft (1 m to 2 m). Resulting maintenance costs for this type of foundation failure can be substantial and was a major concern when designing the Northern Intertie. With the marginal permafrost across much of the area, it was not if it would thaw but when it would thaw that was of concern during the design.

Frost-heave forces will lift a pile somewhat during annual freeze-thaw cycles in what is known as frost jacking. This phenomenon was found to control the length of piles used along the route. Significant heave forces were expected because the soils were susceptible to frost, meaning the soils would wick the groundwater up to the freezing front. Within this active layer, the pore water freezes, forming ice lenses, and expands.

When permafrost is present, the soils provide a sustainable adfreeze resistance of between 1000 psf and 2000 psf (4882 kg/sq m and 9765 kg/sq m). When thawing occurs, that resistance is reduced to an average of about 500 psf (2441 kg/m), conditions that regularly allow frost-heave forces to jack foundation piles out of the ground.

The active layer in the Tanana Flats, or that layer subject to annual freeze-thaw cycles, varied between 2 ft and 8 ft (0.6 m and 2.4 m). The active-layer thickness and adfreeze strength were the most significant factors in determining the relatively high frost-heave forces expected across this area.

Adfreeze skin friction developed by the frozen soil in the active layer will cause the soil around the pile to heave, resulting in an upward load applied to the pile. During the initial design, a 4-ft to 5-ft (1.2-m to 1.5-m) active layer depth and 40-psi (2.8 kg/sq cm) adfreeze within the active layer were used. Estimated embedment depths up to 80 ft (24 m) were developed for the unstable soils within the Tanana Flats.

GVEA worked with the design engineer and geotechnical consultant to develop a business risk model using probability and statistics to estimate the variability in frost-heave potential along the alignment. As the owner, GVEA had to evaluate whether it was important to have no foundation movement through the installation of deep piles or would ongoing maintenance mitigate the significant frost heave (more than 8 inches [20 cm]) if shallower pilings were used. Although a probabilistic model was developed, it was ultimately thinking in terms of the risk of change over the life of the project that provided the design guideline. Whether it be in business operations or engineering design: “It is better to be roughly right than to be precisely wrong,” John Maynard Keynes once said.

GVEA decided to install piles to a minimum depth of 36 ft (11 m) in the flats. For those structures with higher loads, such as large-angle swing-set towers, the depth was 83 ft (25 m). The X tower has long been used in Alaska due, in part, to its ability to withstand relatively high differential movement between legs. To accommodate any differential movements of the foundation and provide adjustable connections, pile clamps were developed for ease of lowering or raising the tower leg as part of the ongoing maintenance. Pin connections to the tower leg also allowed the tower to be assembled flat on the ground and tipped upright into place.