The TBM, Bertha, tweeted: “I dug 2 feet today. This is a testing phase, so we’re taking it slow.” She would normally dig some 35 feet a day.
Bertha, is being used by Seattle Tunnel Partners - a joint venture of Dragados USA and Tutor Perini – to build a 3.2km tunnel to replace the SR 99 Alaskan Way Viaduct in Seattle, USA.
Pushing forward by 2 feet allows crews to build the next concrete ring of the tunnel. Workers are now testing systems and evaluating the machine to see what maintenance might be needed before they resume tunneling.
The JV and Washington State Department of Transportation (WSDOT) have been trying to determine the cause of the obstruction. The restart of tunnelling follows the completion of the last of 36 hyperbaric interventions, a total of 134 hours over 10 days. Crews created a protective bentonite membrane around the front of the machine and filled it with compressed air. When the ground and air pressure were stable, they could go inside, with each team of five spending three hours inspecting Bertha and working to get her moving again.
When Bertha’s crews experienced increasing resistance at the front of the machine, they stopped tunnelling. They started looking for obvious things that could have slowed the machine's progress and found clues, including part of a steel pipe inside the machine's excavation chamber. But obvious things, it turns out, aren't necessarily at the root of this issue. “While we understand the interest in knowing the reasons why mining was stopped in December, it will take time to review the results of the hyperbaric interventions and consult with tunneling experts advising WSDOT,” said WSDOT.
Once tunnelling resumes, the next critical milestone will be arriving at the third and final safe haven. This safe haven is approximately 500 feet ahead of the machine’s current location and was proposed by the contractor as a controlled environment for maintenance prior to mining under the Alaskan Way Viaduct and downtown Seattle. WSDOT plans to close the road while tunnelling occurs under the viaduct.
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