Ballard Siphon replacement

Project description

 
Project map. Click map to enlarge view. Also available as PDF (295KB)

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.

Project schedule

• Currently in design
• Construction starts: Early 2009
• Construction ends: Fall 2010

History of the pipeline

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.

Tests in late 2005 showed deformation and signs of fatigue in the lines, and King County launched an effort to determine how to replace them. The team has used soil samples, surveys, sonar inspections and closed-circuit television videos of the pipes to come up with the best replacement design.