Could a tunnel be the best replacement for the West Seattle Bridge?

Jun 15, 2020, 12:13 PM

West Seattle Bridge...

The West Seattle Bridge. (SDOT, Flickr Creative Commons)

(SDOT, Flickr Creative Commons)

The future of the West Seattle Bridge continues to be uncertain. That being so, could a tunnel be the best plan for a replacement should that scenario come into play?

City preps for potential West Seattle Bridge replacement

Retired civil engineer Bob Ortblad thinks so, after publishing a lengthy op-ed detailing his proposal for a tunnel rather than a new bridge. And while the memories of the problems created during the construction SR-99 tunnel are still fresh in the minds of many, Ortblad doesn’t see a West Seattle tunnel encountering the same problems.

“We’ve had a bad, bad taste around the last tunnel, I’m sure, but this is totally different type of tunnel — the crossing’s only about 500 feet,” he told KIRO Radio’s Dave Ross. “And this has been done all over the world. There’s probably 150 of what they call immersed tube tunnels.”

In terms of the cost, he claims it will actually be cheaper than it would be to build new bridges across the Duwamish to carry transit and vehicle traffic.

Beyond that, there’s also the question of longevity when it comes to a tunnel versus a bridge.

“You’re going to wind up with a tunnel that’s going to last probably twice as long, and be several times more earthquake resistant,” Ortblad pointed out.

His proposed tunnel would consist of two 250-foot concrete boxes, accommodating six lanes of traffic and two lanes for light rail.

Those boxes would be cast in Tacoma or Aberdeen, and then floated down the Duwamish River, before being sunk in a dredged trench. The river itself isn’t particularly deep either, totaling just 30 feet down to its floor.

West Seattle seeing ‘massive’ increase in traffic without bridge

Ortblad says he has “quite a bit” of support from West Seattle residents, although there’s less momentum on the technical advisory committee formed by the city to look into solutions for the West Seattle crossing.

“The technical advisory committee (has) seven members and I think five are bridge engineers; the other two are geo-technical engineers,” he noted. “So there’s no immersed tube tunnel experience on the committee right now.”

Ortblad still plans to push the city to at least consider his tunnel proposal, though.

“It’s just a project that makes sense,” he said. “I’m just trying to be a good citizen and make sure we don’t waste our money.”

Listen to Seattle’s Morning News weekday mornings from 5 – 9 a.m. on KIRO Radio, 97.3 FM. Subscribe to the podcast here.

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Could a tunnel be the best replacement for the West Seattle Bridge?