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June 29, 2016

Workers take special care when working deep underground

  • Bertha is now about a third of the way done on the 9,270-foot tunnel.
  • Photo by Benjamin Minnick [enlarge]
    A pressurized shuttle, with the four upright tanks to the left, can be used to transport workers from Bertha to the medical lock (inside the bottom right cargo container).

    Bertha, the machine digging the replacement tunnel for the Alaskan Way Viaduct, is getting a facelift of sorts.

    After digging about a third of the 9,270-foot tunnel route, Seattle Tunnel Partners has stopped the machine for maintenance about 120 feet below Spring Street, near Post Avenue.

    Besides checking out the machine, crews will replace any worn cutter tools on the cutterhead.

    STP project manager Chris Dixon said the work will begin after the July 4 weekend. He said he didn't know how long it would take, but the machine has been digging in favorable clay-based soils.

    “It's a good place for us to stop and do interventions,” he said.

    Earlier this year, crews spent about six weeks performing maintenance in a zone called Safe Haven 3 before tunneling below the viaduct. Dixon said they changed 11 cutting tools, which wasn't that many considering the machine has about 800.

    Inspecting and replacing the tools is challenging because it requires crews to work in hyperbaric conditions, similar to a deep underwater dive.

    To do the work, crews must first stabilize the ground in front of Bertha. This is done by using compressed air and injecting a type of clay called bentonite to create an air bubble. The air pocket allows crews to safely work in the area behind the cutterhead, which would otherwise be filled by soil and water.

    To work in that environment, crews have to adjust their bodies by spending about an hour inside specialized pressure chambers within Bertha.

    Ballard Marine Construction has been working with STP on the machine's hyperbaric maintenance since the project began. Ballard Marine is providing workers and equipment, including a pressurized chamber called a medical lock.

    Justin Costello, hyperbaric division manager at Ballard Marine, said workers can spend up to 28 days in the medical lock, if needed.

    Costello said Ballard Marine's equipment includes a pressurized shuttle that can be lowered into the tunnel to pick up injured workers or to return workers living in the medical lock that are taking breaks between an extended work schedule.

    Seven crews of five workers will be doing this round of maintenance. Each crew consists of three STP workers and two Ballard Marine workers.

    Dixon said the tunneling schedule is being recalculated and is dependent upon how much time is spent on maintenance, among other things. He said there could be eight months of tunneling left if STP can build five tunnel rings a day, or seven months if it can build six rings a day. He said they built an average of six rings a day in May and seven in June.



    
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