Ebey Slough Transmission Line

(Everett, WA) Landau Associates provided design-phase geotechnical engineering and construction phase services related to the relocation of portions of Puget Sound Energy’s Horse Ranch-Bothell 230-kV and March Point-Beverly 115-kV transmission lines. The two parallel transmission lines cross approximately one mile of diked pasture land adjacent to Ebey Slough east of Everett, Washington, and were relocated as part of a Snohomish County project to breach the dike along Ebey Slough and create a new intertidal habitat area.

The relocation included replacing existing timber H-structures with steel monopole structures. The site is underlain by thick, very soft to soft, alluvial silt and clay deposits containing significant amounts of organic material, and is generally covered most of the year to shallow depths by water. Site conditions and the proposed use of the site precluded conventional, land-based construction methods. The foundation of the proposed structures consisted of 6- or 8-inch-diameter micropiles extending 80 to 100 ft below grade. The overriding geotechnical design issue is the behavior of micropiles under lateral wind loading and seismic loading. Because of the complexity of the soil-structure interaction under these loading conditions, the foundation analysis was completed using a three-dimensional numerical analysis (FLAC-3D). The FLAC-3D analysis was completed based in part on results of site-specific response analysis completed using EZ-FRISK and SHAKE2000 software. The results of the FLAC-3D analysis were used to optimize the micropile design that resulted in significant cost savings.

Landau Associates received an Engineering Excellence Award for this project from the American Council of Engineering Companies (ACEC) - Washington.

Photo 1: PSE