The project to replace an aging piece of Seattle’s sewer infrastructure is a major step closer to completion, after the tunneling machine tasked with boring under the Ship Canal was lifted by crane from the Queen Anne project area just off West Ewing Street last week.
For the last five months, “SWIZY,” the remotely controlled microtunneling machine — though many may not consider it small at 26-feett-long and 7-feet-wide — has been at work digging a pair of tunnels about 20 feet below the bottom of the canal. Those tunnels will soon carry wastewater from more than 100 square miles of north King County to the West Point Treatment Plant in Magnolia.
The existing Fremont Siphon was constructed during World War I, and is nearing the end of its useful life at almost 100 years old.
“We were real close to a failure on that,” said construction representative Marty Noble. “We’re still a few years away, but this is to make sure that doesn’t happen.”
That sentiment was echoed by acting Wastewater Treatment Division director Gunars Sreibers.
“It was clear to us when we looked at the age of the pipes and the condition they were in it was time to be replaced,” Sreibers said. “This is just a proactive move on our part to make sure our facilities are up to snuff.”
The current siphon pipes — installed in a tunnel constructed before the Ship Canal itself — have carried flows of up to 220 million gallons per day during storms, and is one of the most heavily used pipes in the regional sewer system. The Fremont Siphon is an inverted siphon, which means it uses gravity to push the sewage and stormwater downhill, with intake pipe at Fremont Canal Park at a higher elevation than where it exits in Queen Anne.
Preliminary design work on the replacement project started in 2011, followed by public comment in late 2012. The final design period stretched from that point into 2014, at which point the project went out to bid.
According to Noble, once the work began in April of last year, it took about six months to build the 85-foot deep shafts on either side of the canal, before tunneling began on the two shafts that sit about six feet apart.
Of course, as has been the case with much-maligned project on the Seattle waterfront, tunnel boring can present challenges.
“In this part of Seattle, the geotechnical conditions are very, very, very complex,” Noble said. “There’s a lot of aquifers, water tables, and a lot of potential for rocks and boulders and that kind of thing, so it is very challenging.”
Had something gone awry during the boring process, it could have spelled major trouble for the project’s timeline.
“What we have here is a navigable waterway, and the U.S. Corps of Engineers wouldn’t let us block the canal to rescue the machine,” Noble said, “so we had to get across the canal or we would have been in trouble.”
But, Noble said, through close collaboration with the contractors, they were able to make it through this portion of the work without any major issues, though there were contingency plans in place had something gone wrong either with the tunneling, or the existing siphon. Over the course of the tunneling process, approximately 650 cubic yards of dirt was dug out for each tunnel, at a rate of approximately one-to-four inches per minute.
“We went across once, and then we turned around and took the machine over and went back around for a second time,” Noble said.
Once the new pipes go into service, they’re expected to hold up for decades to come. Noble said the lifespan should be at least 75 years, “but they’re always a little conservative on those [estimates].”
While the removal of the boring machine marks a major step, there’s still work to be done before the two tunnels come on line. Sreibers said there are now two connections that need to be made to the existing pipelines, with working running through the fall and into the spring when the project is slated for completion.
Not that the moment SWIZY — named after one of the contractor’s family members — came out of the ground wasn’t one for celebration, though.
“It’s always a really good feeling when you get the most critical part done,” he said.
For more information on the Fremont Siphon project, visit www.kingcounty.gov/fremontsiphon, call the 24-hour project information line at (206) 205-5428, or email doug.marsano@kingcounty.gov.
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