Sinking soil near tunnel dig may have damaged water pipes, could cost millions

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A view of the Alaskan Way Viaduct from ground level. (Photo: KCPQ-TV)

SEATTLE — The Big Bertha boring machine that is supposed to be digging Seattle’s waterfront tunnel to replace the Alaskan Way Viaduct is about a year behind schedule. But the timeline is taking a backseat to public safety concerns.

The soil around the project near the viaduct is sinking. On Monday, Seattle Public Utilities told the City Council that the water pipes underneath First Avenue South near Pioneer Square may be damaged because of it.

“The concern is these are old; they have sunk, causing deflection of the joints,” SPU expert Gavin Patterson said.

There could be flooding issues if the pipes are not replaced. Experts say the replacement could cost millions.

SPU says Washington State Department of Transportation data so far points to possible damage to the water pipes. They say the soil has dipped past the safe threshold. It’s a threshold that was agreed upon between SPU and WSDOT before the tunnel project started.

Pioneer Square businesses owners are bracing for the worst. Several stores say they are noticing cracks that were not there a month ago.

“It’s outrageous,” Ars Obscura owner Joel Radcliffe said.

Radcliffe is worried about the soil opening up right underneath him.

“It should be priority number one,” Radcliffe said.

“If a water main breaks, there will be damage across the board, structurally or otherwise,” Radcliffe said.

State Transportation Secretary Lynn Peterson assured the City Council that public safety is the priority. She promised to keep the contractor, Seattle Tunnel Partners, accountable through the process. She said the project was 70% complete but could not give a completion date.

“The governor and I are still committed to this project. However, we cannot commit to a specific deadline,” Peterson said.

Bertha is broken and stuck beneath Seattle.

The tunnel contractor has been pumping groundwater at Bertha’s access pit in hopes of getting to the machine to fix its problem. Experts say it’s too early to tell if that pumping process is the reason why the soil started to sink last month.

“This was not anticipated at all, so something went wrong and that no one talked about, which gives me a lot of concern,” City Councilman Mike O’Brien said.

The main concern for city leaders continues to be the safety of the viaduct.

“It’s still not clear to me on how the call will be made to shut down the viaduct, to stop dewatering to stop excavating,” O’Brien said.

The Seattle Department of Transportation said the agency is working on a new plan to mitigate the traffic mess in case of a viaduct closure.

SDOT is also keeping a close eye on the intersection of King Street and First Avenue South, where cracks in the street have widened and lengthened recently. Signs are up at the intersection blocking commercial trucks from passing through until engineers can get to the bottom of the soil settlement.

WSDOT says the soil is stabilizing and that all the cracks are cosmetic in nature and not structural.

WSDOT says they learned of the compromised water pipes on Monday and will be following up with SPU.

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1 Comment

  • EJ

    Just wait until the big 300-500 year earthquake hits. The last time that happened 300+ years ago just off the U.S. (then just Native people lands), that earthquake caused tsunami tidal waves that hit the shores of Japan. They estimate that because the entire West coast of the U.S. just buckles and holds back plates underwater land near fault lines, when those give way, they give way in a HUGE way. The U.S. has not ever seen the 300-500 year breaking of the fault lines. The Seattle underground tunnel just might be royally f@#% some day when that happens. Hopefully we are inland enough that we’ll be protected.