• June 23 project update: Tunnel crews arrive at planned maintenance stop

    It’s a reality faced by all car owners: Every few thousand miles, it’s time for an oil change. The same principle applies to tunneling machines, which experience normal wear and tear as they grind their way through the earth.

    And so, after more than 1,500 feet of tunneling since their last pit stop, Seattle Tunnel Partners crews are set to begin regular maintenance on the massive machine. The front end of the cutterhead is now located approximately 120 feet beneath Spring Street, near Post Avenue. It will remain at that location for planed inspections and maintenance that are expected to last several weeks, though the duration of the stop will ultimately depend on the extent of the maintenance needs.

    Performing regular maintenance is a critical part of ensuring the tunneling machine remains in good working order. All tunneling machines, no matter how large or small, need routine maintenance. STP will assess the condition of various systems throughout the machine over the next several days. This assessment will help them determine how much maintenance needs to be completed.

    Crews are already preparing for their most challenging task: inspecting and replacing the tools that cut the ground in front of the machine. These tools wear down and may need replacement multiple times during the course of the tunnel drive. Inspecting and replacing the tools is challenging because it requires crews to work in hyperbaric conditions. For a detailed explanation of hyperbaric work, see Monday’s post.    

    Tracking progress

    To date, crews have tunneled 3,088 feet, or nearly one-third of the total bored tunnel length. Almost half of their overall progress was accomplished in the past eight weeks. Crews have now installed a total of 466 concrete tunnel rings.

    When all necessary machine maintenance is complete, crews will resume tunneling toward First Avenue. Our Follow Bertha page contains tunneling statistics and information about the conditions crews will face as they continue mining toward the north end of downtown. You can also follow progress on Twitter @BerthaDigsSR99.



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  • June 20 project update: Tunnel crews nearing planned maintenance stop

    After another week of good tunneling progress, Seattle Tunnel Partners is preparing Bertha, the SR 99 tunneling machine, for its next planned maintenance stop.
    STP crews have now mined a total of 2,971 feet and installed 449 concrete tunnel rings. The top of the tunneling machine is located approximately 120 feet below Post Avenue, between Madison and Spring streets.
    Crews have mined nearly 1,500 feet since Bertha’s previous planned maintenance stop near Yesler Way. STP plans to stop soon because the soils in the area are dense and stable, providing more suitable conditions for maintenance. The stop could begin this week and is expected to last several weeks, depending on the extent of maintenance needs. STP’s previous maintenance stop lasted six weeks.
    Over the course of the first week, STP will perform a thorough inspection of various systems throughout the machine. This inspection will help STP verify how much maintenance work is needed. Crews will then prepare to inspect the cutting tools that cut the ground in front of the machine, and replace these tools as needed. These tools wear down over time and must be replaced multiple times during the course of the tunnel drive. 
    Hyperbaric work
    Before crews can inspect and replace cutting tools, they must first stabilize the ground in front of the tunneling machine. They do this by using compressed air and a type of clay, known as bentonite, to create an air bubble. This air bubble allows them to safely work in the area behind the cutterhead, which would otherwise be filled by soil and water. 
    Before crews can safely work in this environment, they must first adjust their bodies to air pressure that is greater than the atmosphere we live and breathe in every day. It's the same process scuba divers go through during the course of an underwater dive, but STP’s workers won't need diving gear. Instead they will spend approximately an hour inside specialized pressure chambers within the machine that help their bodies adapt to these ‘hyperbaric’ conditions. The graphics below illustrate the process.
    (Click the image above for a larger view)
    When all necessary maintenance is complete, crews will resume tunneling toward First Avenue. Our next regular progress update is slated for Thursday, but we’ll post something sooner if crews reach the maintenance stop before then.


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  • New SR 99 off-ramp construction starts this summer

    Construction will soon begin on a new flyover off-ramp designed to connect the northbound lanes of State Route 99 to Seattle’s stadiums, Pioneer Square and downtown Seattle.

    The Washington State Department of Transportation awarded the $3.56 million contract to Interwest Construction, Inc. of Burlington, Wash., to build the ramp to South Dearborn Street, which will allow northbound traffic to exit in Seattle’s SODO neighborhood when the new SR 99 tunnel opens. The ramp will be built within the existing SR 99 work zone near the stadiums, so it should not have much effect on drivers in the area.

    WSDOT and Interwest expect to sign a contract giving notice to proceed in the coming weeks. Construction activities are expected to start in July and last approximately six months. The new off-ramp will open to motorists at the same time as the new SR 99 tunnel, currently scheduled for 2018.

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Program Spotlight

  • The story behind the north portal's signature tree

    The signs of summer are everywhere. There’s more sun in the sky and more boats in the water. There’s also more of something that most folks wouldn’t even know to look for: leaves in one of Seattle’s newest trees.

    Allow us to introduce you to the Garry Oak, the area’s only native oak species. Native Garry Oak prairies were once commonly found in Western Washington. This particular Garry Oak was planted this spring a few blocks east of the Space Needle, … more