When August flood downed two lines, a threatened structure was moved back 200 ft until new towers, up to 35 ft taller with reinforced foundations, could replace them.
Alaska’s largest electric utility and one of its newest transmission contractors replaced six towers at a river crossing where floods threatened lines carrying half of the Anchorage metropolitan area’s power supply. The work was completed with no recordable safety incidents and 13 days ahead of schedule. The foundations of the new structures will keep them standing even if future floods leave the towers in the middle of the channel.
Reconfiguring the Susitna River crossing 30 miles from Anchorage became urgent last August, when the river flooded following heavy rains, toppling two of three 70-ft-tall aluminum towers supporting high-voltage lines owned by Chugach Electric Association Inc., Anchorage. The fallen lines were energized at 230 kV. The third line, energized at 138 kV, remained standing, but was threatened. “Our primary transmission line was operating in a somewhat compromised position,” says Shawn Wendling, Chugach project engineer.
The utility saved the 138-kV structure by moving it 200 ft back from the new channel cut by the floods. In late August, line crews and engineers prepared a temporary foundation using a pair of 12-ft-long culverts imbedded vertically in the soil. Workers centered a 16-ft steel H-beam in each and then filled them with a mixture of gravel and silt that hardened to form a strong base.
Linemen freed the two outside conductors and loosened the hardware on the center conductor. With guy wires holding the tower upright, crews jacked it up and reset it on wooden skids. In a 14-hour operation, a tracked mini-excavator pulled the tower in 4-ft increments to the new foundation.
PND Engineers In., Anchorage, designed the foundations for the six new towers bracketing the crossing. “They were designed for the river to erode away 35 ft of existing soil,” says Greg Huffman, principal, Dryden & LaRue Inc., Anchorage, prime design engineer for the project. The 48-in.-diameter steel-pipe piles were driven 80 ft deep in pairs, with a pile cap consisting of a metal box 17 x 2 ft, 7 ft high.
Northern Powerline Constructors Inc., Anchorage, won the construction contract with a low bid in a field of three. That astonished Jason Hodges, who with his wife April, had founded the company in April 2006. Northern erected the steel, Y-frame replacement towers in alignment with the existing line using cranes brought in over a 20-mile-long ice road built in December. The new towers ranged from 90 to 105 ft tall, and field-sleeving and sagging the new conductor was a delicate operation that “caused some consternation,” says Hodges.
Tower supplier Thomas & Betts Corp., Memphis, helped the schedule by delivering the towers in 15 weeks instead of a normal 28 weeks, says Joe Schnippert, T&B’s steel structures operations director. The final cost is about $6 million, say Chugach officials.
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