Location: Ebey Slough Basin
Puget Sound Energy (PSE) commissioned the stringing of 2.5 miles of 115 kV and 230 kV transmission line, as well as the replacement of 80 deteriorating wood transmission poles with 15 new steel poles. The project was prompted by a 2006 windstorm that brought down wooden poles weakened by standing water. The area was dry in 1960 when original construction took place, and the foundations were not designed to be underwater.
The transmission lines cross through Ebey Slough Basin, which was being reclaimed for flood control and as an estuary for salmon and other wildlife. The lines transport power between Canada and PSE’s service area, and the replacement was expected to significantly improve PSE’s transmission infrastructure.
Puget Sound Energy, Wilson Construction Company, Crux Subsurface Inc., Quanta Subsurface
PSE contracted a design-build team of Wilson Construction, Crux Subsurface and Power Engineers to provide an innovative and constructible design to work within the wetland habitat.
Subsurface conditions along the alignment consisted of deep, soft, unstable sediments with organics. Due to these conditions, along with access restrictions, an alternative foundation design was developed by Crux, Landau & Associates and DCI Engineers. The selected design was a radial micropile foundation with a cast-in-place concrete cap base and plinth. The micropile design allowed for construction using lightweight equipment, and for flexibility to accommodate variable subsurface conditions.
Crux custom-built spin drills that set up on templates and drilled a radial array of battered micropiles within a sheet pile ring. Micropiles were constructed using 6-inch and 8-inch casing with reinforcing bars. Tangent pole foundations required 8 micropiles, while heavy angle pole foundations required 16 or 32. In total, 12,000 lineal feet were drilled and 132 micropiles were installed. Proof testing of micropiles was completed at each foundation prior to cap construction.
Concrete cap construction was completed using a combination of marsh buggy transport and line pumps. The upper plinth section incorporated the steel pole bolt cage and provided a pole base high enough to accommodate rising water elevations.
A key component of the project was minimizing the impact to wetland habitat and providing a final product that pleased the public, while still meeting agency requirements. The project was completed ahead of schedule and is recognized by PSE as a major accomplishment.
Difficult Access: Eleven steel pole foundations were located within the slough and did not permit conventional access. Crux used marsh buggies and buoyant tracked pontoon carriers to transport equipment, materials, and personnel to foundation locations. Booms on the marsh buggies were also utilized to set sheet pile rings, which were used as containment basins during drilling and as forms for the concrete caps.