Buildings sank when engineers investigated Bertha

by: Graham Johnson Updated:

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SEATTLE —

Documents obtained by KIRO 7 show buildings in Pioneer Square briefly settled when crews pumped out groundwater to investigate problems with Bertha.

In late December 2013, when problems with the tunnel boring machine left engineers stumped, crews pumped out at least 300,000 gallons of groundwater to prepare for a closer inspection. Previously, state officials told reporters that drawdown caused temporary, minor settlement of the Alaskan Way Viaduct, of between one tenth of an inch and two tenths of an inch.

 "It moved up and down a little bit based on the measurements we had seen," deputy project administrator Matt Preedy said on Feb. 28.

Documents obtained by KIRO 7 show sensors also detected settlement of nine buildings in Pioneer Square between Washington and King Streets, bordered by the viaduct on the west and an alley on the east.  A bar graph in a consultant's report shows when the groundwater level was drawn down, the buildings settled and then went up again when pumping stopped. The buildings settled evenly, so there was no damage. The Washington State Department of Transportation says at most, the buildings dropped a quarter inch before returning to within eight-one hundreths of an inch of where they began.

"Is this serious? No," Preedy said in an interview. "But the whole point of doing the monitoring is to ensure that the contractor and the state understand exactly what effect construction is having so if we do see anything concerning we can deal with it."

Preedy said this is the only building settlement he's aware of on the project so far, but engineers are always on the lookout for more.

 Along the nearly 2-mile tunnel route, nearly 200 buildings have monitors. More than 700 monitors are installed in streets and sidewalks.

Preedy said additional settlement is unlikely when crews dig a rescue shaft to access and fix the seals on the tunnel boring machine. That's because the only dewatering would happen within a wall crews will build around the access shaft, making it more controlled.

 Much of Pioneer Square is built on fill soil because sections of it were once part of Puget Sound. Tunnel project leaders said there are always minor tidal fluctuations in groundwater beneath the neighborhood.