Ballard siphon replacement

Construction 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 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. 

South project site

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

View construction photos (flickr). 

North project 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 construction photos (flickr)