The existing Fremont Siphon is a set of two sewer pipelines located in a tunnel under the Lake Washington Ship Canal, west of the Fremont Bridge. The siphon pipelines were installed in the early 1900’s and have provided wastewater conveyance service for the last 100 years. King County will install new pipelines in a separate crossing under the Ship Canal, just west of the existing siphon.
In early design phase, project teams conduct surveys and field investigations to refine plans for facility location, configuration, and construction. On the Fremont Siphon project, this work was carried out in 2011 and early 2012.
In the planning phase, the project team proposed to focus trenchless construction staging on King County’s property in Queen Anne. During early design, the team determined that constructability, technical and access considerations required trenchless construction staging to be located on the Fremont side. The Queen Anne site will be used for the microtunnel retrieval pit and staging for work to connect the new siphon to the existing sewer.
On the Fremont side, the possible locations of the new siphon facilities were limited to the Fremont Canal Park or the property located at 2nd Avenue Northwest and Northwest 36th Street due to hydraulic constraints. After reviewing the full range of community considerations and project requirements, King County Wastewater Treatment Division is moving forward with the project team’s proposal to site major construction activities and permanent structures on private property in Fremont. This recommendation followed careful consideration, including evaluation of a range of factors by a multidisciplinary team and management review. King County completed environmental review of the recommended proposal (refer to library page) under the State Environmental Policy Act (SEPA) in summer 2012.
Under the recommended proposal, some work will still occur in Fremont Canal Park to reconfigure the City of Seattle’s CSO outfall. Limited and temporary detours of the Burke-Gilman and Ship Canal Trails will be required for work in Fremont and Queen Anne, respectively.