ROADS/BRIDGES: Soil settling shuts down Seattle highway tunnel work

Dec. 10, 2014


Engineers presently evaluating whether planning needs to be adjusted in order to press forward

Beneath downtown Seattle, one of the world’s largest tunnel boring machines continues to sit dormant, as officials determine whether work can proceed further—and if so, when.

The Hwy. 99 tunnel that will run beneath downtown Seattle has had to shut down construction operations, due to the fact that recent tests have showed soil settling around an excavation pit site is uneven. The pit is being bored in order to reach a stalled digging machine, known commonly as Bertha. The settling near the pit is apparently the result of groundwater pumped out by Seattle Tunnel Partners as it digs a 120-ft access pit to reach and replace the damaged head of the boring machine, which overheated and stopped a year ago. At present, Bertha has drilled only about 10%  into the planned 2-mile tunnel to replace the Alaskan Way Viaduct.

The boring machine is stuck about 60 ft under a street not far from the Seattle waterfront. To remove the 57-ft diameter cutting head, the 80-ft diameter access pit needs to be 120 ft deep. At present, it is less than 70 ft deep.

While state engineers analyze settling data to ensure that the pit, the viaduct and nearby buildings remain secure, DOT has continued pumping that reduces water pressure on the pit, after a day’s pause in those operations.

This is not the first hiccup in the viaduct replacement project. The risk of the 61-year-old viaduct collapsing in an earthquake is one reason the state pushed for the tunnel replacement project to carry Hwy. 99 traffic through the city. The viaduct carries about 100,000 vehicles a day. Work on the access pit had previously been stopped for a stretch of weeks this fall after clam shells were uncovered, which experts determined were not archaeologically significant.

The opening was scheduled for December 2015, but as of this writing the completion date remains nebulous.

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