Completed Seattle tunnel will be ‘one of the safer structures in town’

When it’s finished, the Seattle tunnel should not only be safe, it could be one of the safest structures in town. However, there may be one caveat.

University of Washington Professor John Vidale says he is under the impression that Seattle Tunnel Partners, which is the contractor responsible for the project, is building the tunnel to a high standard. Thanks to the firm soil — primarily clay right now — that helps ensure the sentiment.

“What’s clear is the tunnel will be much better than the [Alaskan Way Viaduct] that’s currently up, and it’s probably going to be one of the safer structures in town,” Vidale said.

Related: Bertha is now tunneling under buildings in downtown Seattle

There’s a forewarning that goes along with that message, though. Vidale, who is not involved in the engineering aspect of the tunnel, says a severe earthquake could be troublesome for the future viaduct replacement.

“It depends on the size and distance of the earthquake,” he said. “My understanding is that the tunnel should be safe unless the Seattle fault basically breaks along the tunnel.”

The east-west fault zone cuts across Puget Sound and actually goes under Seattle. It could stretch as far as Issaquah to Bremerton. Though the exact location of the fault is unknown, Vidale says it is near the south end of town, close to the port and stadiums. The zone shows indications of activity, but there’s a debate on where the most likely place is that the fault would break.

The likelihood of the fault breaking, however, is low. The most recent Seattle Fault quake was about 1,100 years ago, according to Seattle’ Office of Emergency Management. The fault has been active about three or four times in the past 3,000 years. A quake from the fault could be as large as magnitude 7.5, but will more likely be less than 7, according to the Office of Emergency Management.

That’s a small chance, but, “it would be pretty bad if it happened, so it’s a bit of a concern,” Vidale said.

The combination of loose soil in some areas of Seattle and buildings that aren’t up to current standards of earthquake resilience could make for a “dramatic effect on the city.” A recent report by The Seattle Times shows that an earthquake created from the Seattle fault could be worse than the dreaded “Big One.” During its investigation, the Times found there are at least 1,163 buildings in the city that would be significantly compromised during a major earthquake.

But a newly constructed tunnel could, and most likely will, be safer than the decades-old buildings. In fact, the Washington State Department of Transportation points out that all modern structures must be designed with earthquake safety in mind — just take a look at the new 520 bridge. Though “some folks” find it counter-intuitive that a tunnel is safe during an earthquake, WSDOT says underground structures are safe because they move with the soil, “while above-ground structures sway back and forth.”

The tunnel replacing the viaduct has been specifically designed to withstand a magnitude 9 earthquake, which has a small chance of happening in our lifetimes. The tunnel was designed to meet 18 different seismic codes, standards and specifications.

What is less clear for Vidale is how safe things are during the tunneling process and what would happen if a quake hit an incomplete tunnel.

“I’m just not qualified to say if it’s a little more dangerous or a lot more dangerous,” he said.

The contractor is building outer walls of the tunnel as it goes. Tunnel rings are installed as the machine pushes under Seattle.

Since diving under the viaduct, there have been no reports of negative impacts to downtown Seattle. The state continues to monitor the area to ensure nothing detrimental occurs.

Bertha most recently tunneled beneath the Columbia Street on-ramp to the Alaskan Way Viaduct, according to information from the state’s blog. The machine is in Zone 3, en route to First Avenue. As of May 26, Bertha has traveled 2,255 of the 9,270 feet needed to complete the job.

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