Fremont Siphon replacement project

Constructing the new siphon pipelines

King County is planning to replace the existing Fremont Siphon with a new crossing under the Lake Washington Ship Canal. Two microtunnels will be established just west of the existing siphon, and will be connected to the North Interceptor conveyance pipelines on the north and south sides of the canal. The new siphon pipes will be designed to operate with the same capacity as the existing system. Odor control options will be considered during the design phase. The County will still retain the existing space in the Fremont Tunnel for Wastewater Treatment Division uses.

Project areas in Fremont and Queen Anne. 


A slurry separation plant is used to separate tunnel soils from fluid used for excavation.


Pipe sections are inserted into the growing tunnel through the launch pit.


Microtunneling is a remote-colled pipe construction method.

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

Connections of the new siphon to the existing North Interceptor conveyance pipes will require work in the streets in both Fremont and Queen Anne. Both the Burke Gilman Trail and the Ship Canal Trail will be 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 are needed to launch the microtunnel machine at the beginning, and to retrieve it when tunneling is complete. The highest level of construction activity is at the launch pit, where pipe sections are added and soils are conveyed from the tunnel for processing. Truck traffic is heavier at this location than at the retrieval pit.