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Alaskan Way Viaduct sinks 1 inch in a few weeks; WSDOT says ‘it’s still safe’

The mission to rescue Bertha, the world's largest boring machine, which is still sitting quietly beneath the surface of Seattle, might be causing the Alaskan Way Viaduct to sink. (WSDOT Photo)

Seattle’s Alaskan Way Viaduct is sinking, dropping as much as one inch in just a few weeks.

The mission to rescue Bertha, the world’s largest boring machine, which is still sitting quietly beneath the surface of Seattle, might be the cause.

When all the parts of the viaduct sink evenly, at the same pace, it’s not a big deal. Tests over the weekend by the Washington State Department of Transportation show the settling is uneven, which is more risky than if the soil settled uniformly.

It has prompted the SR-99 tunnel contractors to stop pumping out the ground water, “dewatering” near the pit. Crews need to dig a 120-foot hole to reach and place the damaged head of the boring machine.

“When you remove water from underground, there’s a tendency for the ground and the mechanics of it to consolidate. The soils could be consolidating immediately adjacent to the access pit,” says David Sowers with the WSDOT.

According to Sowers, the viaduct is still safe for cars and construction.

“We don’t think the viaduct is in any risk to the traveling public right now. It’s still safe. We’re not taking any measures to close the viaduct based on one inch of movement.”

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