ABI Mobilrams: The Right Choice For Winter Work In Alaska
Before setting out to install 64 miles of power transmission towers and lines throughout central Alaska, Anchorage-based Global Power & Communications determined that the ABI Mobilram system would be the best choice for the project. A winter of driving more than 1,200 piles through a variety of soil conditions proved that to be a wise decision. "The Mobilram was simply the right machine for the job," said Mike Gearhart, general manager of Global Power.
The project - Golden Valley Electrical Association's Northern Intertie Project - involved construction of a 230-kilovolt transmission line between Healy and Fairbanks. It was part of a project designed to meet increasing power demands, and to enhance power reliability, in that area. A total of 346 transmission towers, spaced about 1,000 feet apart, were erected.
Crews used two Mobilram systems, the TM14/17 and a TM16/20, both supplied by Hammer & Steel. The machines installed four piles at each tower location. "We chose the Mobilrams for their pre-augering and driving capabilities," Gearhart explained. "Our crews were driving into permafrost and working in a multitude of soil conditions, from condensed soil to very loose soils. About 50 percent of the piles required soil release drilling to allow for the successful installation of the piles."
Another reason for selecting the ABI rigs was their ability to install the piling accurately. Correct positioning and verticality of the piles is crucial, because prefabricated transmission tower frames will be lowered over the piling tops by helicopter. "The tolerances were very tight and the machines handled them extremely well," Gearhart said.
The 40 to 70 foot piles ranged in diameter from 10 inches to 30 inches, with wall thicknesses of 3/8 and 1/2-inches. To ensure that the piles were driven without deforming or damaging the casing, despite the thin walls, ABI made special jaw pads and jaw assemblies to fit inside the smaller casings. These jaw assemblies matched the contour of the casings to prevent deformation. The flexibility of ABI's high frequency variable moment driver allowed for the installation of the piling in tough driving conditions without exceeding the capacity of the piling, preventing the piles from being damaged.
The transmission line ran through an environmentally sensitive area, and construction was carried out under 47 stipulations designed to mitigate possible environmental impacts. For example, trees 10 feet tall and under remained standing in the right-of-way, and the towers were self-rusting so they would blend into the landscape. Both the Alaska Department of Natural Resources and the U.S. Bureau of Land Management frequently reviewed the construction operations.
The stipulations also prohibited construction of permanent access roads. Motorized access was only allowed when there is at least one foot of frost and one foot of snow on the ground. Global Power began work in early January 2002. Almost 90 percent of the piling was installed before the company had to stop in March, when it lost its ice road permits for the season. The rest only took a couple weeks to complete once the ice roads were available the next winter.
The Hammer & Steel technical staff prepared the Mobilrams for the harsh Alaska winter by modifying the hydraulic systems to accommodate typical daytime temperatures of minus 30 degrees Fahrenheit. At night, the rigs were kept under army surplus parachutes and warmed by large space heaters to maintain a toasty 10 degrees.
In addition, Hammer & Steel provided training for the Mobilram operators at the start of the project. Jim Fogarty from Hammer & Steel spent almost two weeks on site. "We had outstanding support on this project," Gearhart commented.
Gearhart was equally pleased with the ABI Mobilrams, referring to them as "excellent machines." He concluded, "They certainly proved their value and durability here in Alaska."