• June 28 project update: New videos show crews at work inside the SR 99 tunneling machine

    Over the next several weeks, specialized crews will complete routine cutterhead maintenance at the front end of Bertha, the SR 99 tunneling machine. To safely do this, crews must stabilize the ground in front of the machine. They do this by injecting a type of clay, known as bentonite, into the front end of the machine. This creates a seal that prevents water and soil from entering – and air from escaping – their work space. 
    Next, crews over-pressurize the space by introducing compressed air, which pushes against the bentonite to counteract the ground and water pressure at the front end of the machine. This newly created "hyperbaric" work space has pressure levels that are higher than regular atmospheric pressure, similar to conditions found in an underwater dive. The graphic below illustrates the process.
    New videos
    Preparations for this work are underway now that Bertha has stopped for planned maintenance beneath Spring Street. Crews expect to make their first “hyperbaric intervention” next week. Seven, five-member crews will work around the clock to perform maintenance in the space behind the cutterhead. 
    Each crew member must spend several minutes in a special chamber to prepare for the greater pressures they’ll experience while working in hyperbaric conditions. The amount of time that crews can safely work in these conditions varies depending on the pressure of the hyperbaric work space. In previous interventions on this project, crews were able to spend up to an hour in these conditions before decompressing and returning to the surface. 
    The video below shows the chambers crews use to adjust to hyperbaric conditions and enter the space behind the cutterhead.
    The video below was provided by Ballard Marine Construction, the firm responsible for completing this work on the tunnel project. It shows crews at work behind Bertha’s cutterhead during a planned maintenance stop earlier this spring.
    The duration of the maintenance stop will depend on the extent of the work that’s needed. STP’s previous maintenance stop near Yesler Way lasted approximately six weeks. We’ll continue to provide updates as their work progresses.


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  • 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 planned 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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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