Puget Sound Energy completed ahead of schedule the $15 million rebuild of high-voltage electric transmission lines in Snohomish County’s Ebey Slough basin. Designed to enhance the safety and reliability of the regional high-voltage electric transmission system and the Snohomish County designated Ebey Slough wetland habitat, the project consisted of replacing 80 old wood transmission poles with 15 new steel poles and stringing 2.5 mi of new 115-kV and 230-kV transmission lines.
Transporting power between Canada and the southern border of PSE’s service area, the Ebey Slough transmission lines also provide year-round benefits to the regional electrical systems of Snohomish County PUD, Seattle City Light, and the Bonneville Power Administration.
“Replacing the Ebey Slough poles and transmission lines is not only a major accomplishment for improving the reliability of PSE’s transmission infrastructure, it’s critical to the stability of the region’s entire electric transmission system,” said Sue McLain, senior vice president of operations for PSE. “The project completion is a celebration of cooperation and hard work between the county, state, tribes, agencies and the local residents and supports work by Snohomish County to reclaim more than 230 acres of estuary for flood control and critical salmon and wildlife habitat.”
The project paves the way for Snohomish County to enhance Ebey Slough, providing habitat for eagles, blue heron, weasels, beavers, otters, egrets, and three threatened species – Chinook salmon, steelhead and bull trout. With 80-90 percent of historical estuary gone from the Snohomish River basin, this is significant preservation, noted McLain.
Driven by the changing environment of Ebey Slough and the December 2006 windstorm that knocked over poles weakened by the standing water that is now there, the new poles and transmission lines were constructed between May and October on water-resistant foundations along the same alignment using swamp buggies and Sky Crane helicopters, (see photo). Micropile foundations, consisting of approximately eight piles per tangent structure, were utilized in the unstable soil conditions. The old transmission lines, supported by wood H-frame structures built in the 1960s, were not designed to be in water. The resulting deterioration created line stability problems. In some cases, anchors and guy wires had pulled out of the soft, saturated soils resulting in unstable structures that were not anchored in load-bearing soil.
Construction of the transmission lines was completed ahead of schedule. For the project, PSE used a design/build team headed by Wilson Construction Co., a transmission line contractor. Other design/build team members included Crux Subsurface, Inc., Power Engineers, Landau Associates, SWP Environmental Services Co., PND Engineers, Inc, and DCI Engineers.