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Completed spring 2014

Project description

To protect public health and keep sewage out of Salmon Bay, King County built a new 7-foot wide sewer pipe under Salmon Bay between the Ballard and Interbay areas of Seattle. The new pipe is more than 120 feet underground.

The new pipe will partner with two 3-foot wide wooden pipes (PDF) built in the 1930’s to serve north Seattle through the end of the century. The wooden pipes were cleaned out and relined as part of this project.

On most days, sewage and polluted stormwater from north Seattle will flow through the older pipes. The new pipe will carry any extra stormwater and sewage during storms when the older pipes are full. All of north Seattle’s sewage and stormwater is sent to King County’s West Point Treatment Plant in Magnolia where it is cleaned and safely discharged to Puget Sound.

Ballard Siphon Project Map

Construction photos - South site

The tunnel boring machine launch shaft was located on the south site. The launch shaft was excavated using a Vertical Shaft Machine (VSM) to a depth of approximately 145 feet. Tunneling for the new siphon pipeline was completed in fall 2013.

View more photos (external link, Flickr).

Construction photos - North site

The tunnel boring machine retrieval shaft was located on the north site. The retrieval shaft was excavated to a depth of approximately 93 feet.

View more photos (external link, Flickr).


24-hour emergency and odor reporting:

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

Project location


More information

Seattle Municipal Archives - Photos of Ballard trunk sewer siphon, construction - wood stave pipe, 1935

Take a video tour of the tunnel! (WTD YouTube channel)

Ballard Siphon replacement tunnel into final phases, produced by TunnelTalkVideos, October 30, 2013


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