A Seattle sinkhole 35 feet long formed this week as contractors were using Bertha, the world's largest tunnel-boring machine, to work on the beleaguered Highway 99 project. The sinkhole forced Washington Gov. Jay Inslee to order a stop to construction until public safety can be assured.
The safety of the $1.35 billion Highway 99 project, helmed by Seattle Tunnel Partners, was called into question after the sinkhole formed atop an area where
Bertha was boring, The Seattle Times reported.
A project manager for STP told the newspaper that the company poured 250 cubic-yards of concrete and sand to fill the 35-foot long, 20-foot wide, and 15-foot deep sinkhole that developed Tuesday.
But the hole is continuing to sink,
according to a suspension letter from the Washington State Department of Transportation, and Inslee ordered a suspension of the drilling on Thursday.
A spokesperson for the WSDOT told the Times the sinkhole appears to only be affecting the area around Bertha and does not seem to have spread to the nearby Alaskan Way Viaduct.
"The contractor will not resume tunneling until they demonstrate that they can proceed in safety and proceed with their work and have plans in place to prevent this type of incident in the future,” the Times quoted Inslee from a Thursday news conference.
Highway 99 is an alternative-design project for replacing SR 99 Alaskan Way Viaduct that winds along
Seattle's waterfront, WSDOT said on its website.
A requirement of the Request for Proposal was that the project be substantially completed by November 2016, and that an open tunnel be created by
December 2015, WSDOT said. Challenges in the construction process have made that a problem.
The use of Bertha, considered state of the art, was promoted by WSDOT as a way to monitor the project.
"Belt measuring system would use radar to measure accurately the spoils and volume loss at the machine’s face. This would help identify any voids around the tunnel that should be filled, reducing the possibility of sinkholes at the surface," the WSDOT website said.
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