ROADS/BRIDGES: Seattle’s S.R. 99 tunnel rescue resumes

Following up on our story of Dec. 10, “Bertha” drill repair op is back online

December 17, 2014

On Dec. 10, Roads & Bridges reported on a halt in the operations to excavate a tunnel down to the boring machine known colloquially as “Bertha,” in order to perform repairs that would allow the machine to once more begin tunneling for the S.R. 99 project.

State engineers had voiced concerns that soil settling near the pit, apparently the result of groundwater pumped out by Seattle Tunnel Partners (STP) in the process of digging the 120-ft access pit needed to reach Bertha, was a potential hazard to adjacent buildings, as well as the viaduct itself.

The boring machine is stuck about 60 ft under a street not far from the Seattle waterfront. To remove the damaged 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. Bertha has drilled only about 10% into the planned 2-mile tunnel to replace the Alaskan Way Viaduct.

It is now announced that crews will be allowed to resume excavation at the tunnel access pit, according to the Washington State Department of Transportation. Seattle Tunnel Partners expects to resume digging on the evening of Dec. 18.

WSDOT stated that it had “suspended excavation to give our team time to gather more survey data and review STP’s contingency plan for turning off the dewatering wells, should that become necessary.”

WSDOT says that data now shows recently measured settlement has stabilized. 

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