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Completed Spring 2017

Public involvement | Library

Project update

June 2017

Thank you to all the folks who came out on Sunday, June 18 to see the site and celebrate the completion of the Fremont Siphon Replacement Project. The project was completed on time and under budget. King County deeply appreciates the community’s patience and understanding during this important project.

Thanks again for your time and consideration throughout the project.  

June 18, 2017 Fremont Siphon Community Celebration

See how public art at KCWTD facilities connects community to our system and the environment.

In July 2016, a remote-controlled microtunnel boring machine completed two tunnels underneath the Lake Washington Ship Canal as part of a King County Fremont Siphon Replacement Project. Watch as the 26-foot-long, 7-foot-wide machine is hoisted from its exit pit in Fremont.

Project description

King County replaced the Fremont Siphon, a major sewer pipe running under the Ship Canal between Fremont and Queen Anne. The new siphon is located west of the existing siphon to reduce project risks and impacts to the community. Construction began in early 2015 and continued until spring 2017. The old siphon was decommissioned.

Fremont Siphon Pipeline Replacement, Project AreaFremont Siphon pipeline replacement, project area 

operations phase

24-hour emergency and odor reporting:

Contact West Point Treatment Plant at  206-263-3801.

Fremont Siphon service area

Building the Fremont Siphon Tunnel, 1913. Photo provided by Seattle Municipal Archives .

Learn more about the project

The Fremont Siphon has provided safe, reliable sewer service to north Seattle and other cities in north King County for decades. Sewage and stormwater from more than 100 square miles of pass through the Fremont Siphon every year to be cleaned and safely discharged at the county’s treatment plant in Magnolia. During storms, the pipe carries up to 220 million gallons per day, making it one of the most heavily used pipes in the regional sewer system.

The existing siphon is nearly 100 years old and has reached the end of its of service life. The new pipes will ensure north Seattle and northern King County continue to enjoy safe, reliable sewer service for decades.

Fremont facility site design – historic building with Pacific Northwest coastal plant palette

As part of the project, King County built a new odor control facility at the Fremont site. Community members, neighbors, and park users joined the King County project team at a January 2013 design workshop and March 26 open house to develop a design for this facility that fits with the community.

At the workshop, community members developed these common themes for the design:

  • Extend the park feel to the site
  • Fit with the historic nature of buildings in Fremont or develop a simple, modern feel
  • Balance opening the view to the Ship Canal with screening the facility
  • Minimize lighting onsite and minimize the pavement and fencing footprint

The front of the building at Northwest 36th Street and Leary Way Northwest will have a window to view into the facilityThe front of the building at Northwest 36th Street and Leary Way Northwest will have a window to view into the facility.


King County’s project team used the community’s input to develop a building style that evokes historic Fremont structures and a plant palette featuring Pacific Northwest coastal plants.

View more information on the design process  .

Other elements developed with community input include:

Design elements developed with community input

From the Burke-Gilman Trail, the facility will eventually be screened by trees and vegetationFrom the Burke-Gilman Trail, the facility will eventually be screened by trees and vegetation.


The corner of Northwest 36th Street and 2nd Avenue Northwest features a landscaped area and new benchesThe corner of Northwest 36th Street and 2nd Avenue Northwest features a landscaped area and new benches.

Thank you for contributing to the site design.Thank you for contributing to the site design.


The rain garden in the lower left corner will feature plants commonly found on the Pacific CoastThe rain garden in the lower left corner will feature plants commonly found on the Pacific coast.

The Fremont Siphon is an inverted siphon. Inverted siphons use gravity to push liquids downhill. In this case, sewage and stormwater from north Seattle and elsewhere is pushed through the Fremont Siphon by having sewage and stormwater enter the pipe in Fremont Canal Park at a higher elevation than where it exits the pipe in Queen Anne.  

To help keep the sewage and stormwater moving, inverted siphons in the sewer system are a smaller diameter pipe than those connecting to it on either side. The smaller diameter speeds up the passage of sewage and stormwater pass through the pipe to keep solids moving and avoid clogs.

Inverted siphon

In early design phase, project teams conducted 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.

Some work occurred 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 were required for work in Fremont and Queen Anne, respectively.

North (Fremont) 

South project area (Queen Anne)South (Queen Anne) 

Fremont project area  

New structures in Fremont

New above-ground and at-grade structures are needed for this project. During the planning phase, the community expressed concerns about odor from King County’s existing sewer system in Fremont. After monitoring and analysis, the project includes odor control for the existing and new system. A new odor control facility includes an above-ground structure.

Operations and maintenance crews will also need access to the new siphon pipelines. Concrete slabs and manholes at the surface are needed to provide access for inspections and maintenance.

Replacement and reconfiguration of the City of Seattle's CSO outfall

King County worked closely with the City of Seattle on replacement and reconfiguration of one of the city’s CSO outfalls. This outfall extended under the Fremont Canal Park to the Ship Canal and was in direct conflict with the proposed location of the new siphon. The possible locations for the siphon were limited due to hydraulic constraints. To eliminate pipeline conflicts, King County installed a new outfall adjacent to the existing outfall. The discharge point and capacity of the new outfall remained the same. The outfall installation required excavation in Fremont Canal Park and work in the Ship Canal, outside of the navigational channel. Learn more about the City of Seattle’s CSO system and CSO reduction program  .

Queen Ann project area Queen Anne project area  

New structures in Queen Anne

Operations and maintenance crews will need access to the new siphon pipelines. Concrete slabs and manholes at the surface are needed to provide access for inspections and maintenance to the new siphon and its connections to the existing pipeline. To limit impacts, King County located most new structures on the King County owned property, however, there may also be new manholes in the street right-of-way.

King County replacef the existing Fremont Siphon with a new crossing under the Lake Washington Ship Canal. Two microtunnels were constructed west of the existing siphon and connected to the North Interceptor conveyance pipelines on the north and south sides of the canal. The new siphon pipes were designed to operate with the same capacity as the existing system. A new odor control facility was built on the Fremont side of the Ship Canal. The county retained the existing space in the Fremont Tunnel for Wastewater Treatment Division uses.

King County constructed a microtunneling receiving pit on the county's property adjacent to the King County Environmental Laboratory at 322 West Ewing Street. A launch pit was constructed on the Fremont side of the Ship Canal.

Connections of the new siphon to the existing North Interceptor conveyance pipes required work in the streets in both Fremont and Queen Anne. Both the Burke Gilman Trail and the Ship Canal Trail were affected during construction.

How microtunneling works

Microtunneling is a remote-controlled pipe construction method in which pipe is pushed through the ground by hydraulic jacks, with a machine in front of the pipe that removes soil as the pipe moves forward. The excavated soil is mixed with a fluid and the resulting mixture is pumped up to the ground surface where the soil and fluid are separated and reused or recycled. This method allows installation of pipes without having to excavate an open trench in difficult locations like Ship Canal.

Two excavated shafts were needed to launch the microtunnel machine at the beginning, and to retrieve it when tunneling was complete. The highest level of construction activity was at the launch pit, where pipe sections were added and soils were conveyed from the tunnel for processing. Truck traffic was heavier at this location than at the retrieval pit.

King County Wastewater Treatment Division knows that communities are concerned about how their streets, trails, and parks will be restored after a construction project is completed. The Fremont Siphon Replacement Project was carried out in densely populated urban neighborhoods in Fremont and Queen Anne where parking, park space, and trail access are highly valued. As the project team conducted public outreach, we heard the community’s concerns about restoration.

The project team has completed assessments of existing site conditions, including property surveys, soil and groundwater conditions, roads and trails, existing utilities, and tree surveys within the project areas in both Queen Anne and Fremont.

Restoring green spaces

Project staff worked closely with agencies and jurisdictions on restoration conditions for the affected project areas (see map below). A tree survey was completed in the project areas and a certified arborist evaluated the trees for health and potential for transplantation.

Twenty‐five trees in Fremont and 21 trees in Queen Anne were removed for this project. Trees were replaced during site restoration and the team selected replacement tree species taking into consideration stormwater management functions, habitat, and aesthetics.

Environmental mitigation

As part of the project, King County improved vegetation along the Ship Canal in Queen Anne to mitigate the in-water work to replace the City’s CSO outfall. The project removed invasive ivy and replaced it with native vegetation that provides ground cover and overhangs the waterway (see “mitigation area” in map). The project team selected native vegetation that benefits local wildlife and provides erosion control.

Stormwater management

King County followed city requirements to mitigate impervious surfaces on site. Requirements were met by landscaping features that included bioretention swales, rain gardens, trees, or other measures.

Restoration and Mitigation AreasRestoration and mitigation areas 

Fremont Canal Park Fremont Canal Park

Burke Gilman TrailBurke-Gilman Trail

A rain garden in BallardA rain garden in Ballard

Native vegetation (salal)Native vegetation (salal)