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Seattle's Viaduct Sinks An Inch Near Bertha Rescue Shaft

Flickr Photo/WSDOT
A 120-foot-deep shaft being dug to gain access to the tunnel machine's cutter head (shown here being installed before tunneling began) may have led the Alaskan Way Viaduct to sink an inch last month.

Transportation officials say a stretch of the Alaskan Way Viaduct settled an inch last month.

They told state legislators Friday that there is no risk to public safety from the newly discovered subsidence of the elevated highway. KUOW's John Ryan reports.


The viaduct sank an inch during a two-week span in November, right next to a giant shaft that's being dug near King Street and Yesler Way.

That access shaft is needed to dig up and repair the tunneling machine known as Bertha.

It's been a year and a day since Bertha hit a piece of pipe and forward progress all but stopped beneath the Seattle waterfront.

State officials say removing groundwater around the access shaft could have led the ground to subside.

The state's contract with Seattle Tunnel Partners allows up to two inches of settlement.

Matt Preedy is with the Washington State Department of Transportation.

Preedy: "There's no cause for concern currently that that access shaft work would need to stop."

WSDOT is sending inspection crews out this weekend to learn more.

Officials say the total amount of settling is less important than if one segment of the viaduct sinks a lot more than the segment next to it.

Other portions of the viaduct have sunk as much as six inches since the Nisqually earthquake in 2001.

Even without any additional delays, repairs to Bertha aren't expected to finish until March.

I'm John Ryan, KUOW News.

Year started with KUOW: 2009