• Aug. 22 project update: Bertha passes the 4,000-foot mark

    Seattle Tunnel Partners crews have now mined more than 4,000 feet as they continue their push toward the SR 99 tunnel’s future north portal. As of this morning, Bertha, the SR 99 tunneling machine, had traveled approximately 4,000 feet of the 9,270-foot-long tunnel route, and installed 607 of the concrete rings that form the tunnel’s exterior walls. 
    The top of the cutterhead is approximately 165 feet below the centerline of First Avenue, approaching Union Street. Crews will continue mining beneath First Avenue for several blocks. Then, at Stewart Street, where First Avenue veers to the west, tunneling crews will continue mining north en route to the spot near Seattle Center where Bertha will emerge
    As we’ve highlighted before (links to YouTube), the program has an extensive network of instruments in place to monitor ground movement. Crews continue to see little to no movement as they tunnel through a mixture of clay, sand and gravel beneath downtown.
    The entire tunnel route, including descriptions of each of the 10 zones through which Bertha is mining, can be found on our Follow Bertha page. Tunneling statistics are updated on that page on Mondays and Thursdays. You can also track Bertha’s progress on Twitter @BerthaDigsSR99.
    The SR 99 tunnel under First Avenue in Seattle
    A look at the SR 99 tunnel as it curves toward First Avenue. Other recent project photos can be seen here.
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  • Aug. 4 project update: New video highlights tunnel segments as Bertha nears First Avenue

    Bertha, the SR 99 tunneling machine, is approaching the intersection of First Avenue and University Street as crews with Seattle Tunnel Partners continue mining north beneath downtown.
    As of this morning, the tunneling machine had traveled a total of 3,518 feet and installed 533 concrete tunnel rings. The top of the machine is nearly 130 feet below street level. Earlier this week, crews successfully tunneled beneath the off-ramp that connects northbound SR 99 to Seneca Street. The machine passed approximately 90 feet below the the piles that support the ramp.
    Crews will continue tunneling north beneath First Avenue for several blocks. Their ultimate destination is a receiving pit near Seattle Center, where Bertha will end her 9,270-foot-long journey beneath downtown.
    The building blocks of the SR 99 tunnel
    Crews have now installed more than 5,000 curved concrete segments in the tunnel – each one weighing an average of 36,000 pounds. Bertha takes 10 of these segments and creates a five-story-tall tunnel ring, as shown below. 

    Steps of ring building

    A look at how each tunnel ring is pieced together by Bertha, the SR 99 tunneling machine.

    It will take 1,426 of these rings – or 14,260 concrete segments – to create the nearly two-mile-long tunnel. All the precast segments were built locally, at the Encon Washington plant in Pierce County. Segment production wrapped up in fall 2014. Every segment is checked multiple times for quality before being brought into the tunnel for installation. This video shows you exactly what the tunnel is made of – building blocks of steel and concrete.


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  • Crews break ground on new off-ramp at the tunnel’s south portal

    South-end travelers will soon see their future access point from northbound SR 99 to downtown Seattle and SODO take shape near the stadiums. 
    Crews from Interwest Construction broke ground this week on a new bridge that will connect northbound SR 99 to South Dearborn Street when the SR 99 tunnel opens to traffic. Construction of the $3.56 million ramp is expected to last approximately nine months. 
    This work won't significantly affect the public because it will take place inside the existing south portal work zone, although it will require a few overnight closures on SR 99. We will keep the public informed as work progresses.
    South Dearborn Street Off-ramp Bridge   
    The above rendering shows the future off-ramp from northbound SR 99 to South Dearborn Street, near the stadiums. Crews from Interwest Construction broke ground on the new ramp this week (below).
    Ramp construction begins
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Program Spotlight

  • Kids discover engineering at Milepost 31

    What do you get when you combine an engineer, a group of curious young students and a table full of craft supplies? The answer, we hope, is inspiration.

    For the past two years, Milepost 31, the SR 99 Tunnel Project’s information center in Pioneer Square, has hosted engineering activity days for numerous youth groups. These events are designed to teach the kids in attendance about different fields of engineering through a variety of hands-on activities. In addition to having fun, we … more