Ballard Siphon Replacement Project

Project map. Click map to enlarge view.
Also available as PDF file (268KB)

Ballard Siphon Project Conceptual Profile. Click map to enlarge view.
Also available as PDF file.

The City of Seattle laid the 36-inch diameter sewer pipes in 1935. Currently, the pipes convey up to 60 million gallons a day of wastewater across the bay, on its way to the West Point Treatment Plant. Building wooden pipes may seem unusual today, but it was actually pretty common when the siphon was originally built. Construction is similar to a wooden barrel - in that metal compression bands hold the wood staves in place. More information on Ballard Siphon wood stave pipes can be found in the Seattle Municipal Archives.

Project Description

To protect public health and the environment while ensuring the system continues operating safely and reliably, the Wastewater Treatment Division is upgrading a 70-year-old wooden sewer pipe in the Ballard/Interbay area of Seattle. The project is needed to strengthen existing infrastructure while adding needed capacity to serve growth in the north Seattle area.

King County engineers and consultants are working to determine how to restore a pair of old wooden sewer pipes nested deep in the sediment of Salmon Bay. The active sewer pipeline between the end of 20th Avenue Northwest in Ballard and the former Marco Shipyard on the Magnolia side is called the Ballard Siphon. Wastewater from homes and businesses in Ballard now flows by gravity through the siphons and across the bay, moving from the higher elevated pipes on the north side of the canal to the lower elevation on the south side. The wastewater ultimately ends up at West Point Treatment Plant in Magnolia.

After design is completed, construction will begin in early 2009.

Design Options

Early analysis ruled out some possible replacement options - for example, simply laying new siphons on the bottom next to the existing ones. That method could involve a lengthy permitting process, affect tribal fishing rights, require working around salmon migration periods, and stir up sediments during construction.

The design team also looked at horizontal directional drilling and microtunneling methods to install replacement pipes beneath the floor of the Ship Canal. These tunneling methods would avoid construction within the waterway and decrease permitting and sediment issues. These methods were ruled out because of their risks and potential impacts on the neighborhood and businesses.

The current design focuses on slip-lining new plastic pipes inside the existing siphon barrels and constructing a deep shaft tunnel some 60 feet beneath the bottom of the canal, with new below grade connection structures at both ends of the tunnel. Design and construction of the Ballard Siphon Replacement Project will not be easy. Space is limited by underground utilities, railroad tracks and businesses on both sides of Salmon Bay. The project team will continue to work with business owners, community leaders and residents to identify concerns and develop plans to minimize impact.

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Project Schedule

Currently in design

Construction starts: Early 2009

Construction ends: Fall 2010

More information

DNS and Environmental Checklist (.PDF file, 1.3MB), issued March 18, 2008

Project Update, Winter 2007 (.PDF file, 296KB)

For more information about this project, contact Martha Tuttle at martha.tuttle@kingcounty.gov or 206-684-1207. Ask to be added to the project mailing list or e-mail list.