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SR 99 TBM Reaches Half-Mile Mark

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The interior of the SR 99 TBM.

Seattle Tunnel Partners has now bored more than one-half mile of the SR 99 tunnel. As of June 13, Bertha, the SR 99 tunneling machine, had traveled a total of 2,724 ft and installed 411 concrete rings. Crews have tunneled more than 1,100 ft since leaving the machine’s most recent planned maintenance stop near Yesler Way on April 29. The completed bored tunnel will be 9,270 ft.

The top of the machine’s cutterhead is approximately 115 ft below Madison Street, east of Western Avenue. It’s approaching the southern edge of Zone 4, which continues the tunnel’s path toward First Avenue.

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STP has begun preparing for its next planned maintenance stop. The details of the stop are still being developed, but it will likely occur sometime in July. During the stop, STP crews will inspect the machine and perform cutterhead maintenance, a process that could take several weeks.
Bertha now climbing
After steadily descending over the first part of the tunnel drive, Bertha has now begun climbing toward her eventual exit point near the Space Needle. Interestingly, the lowest spot in the tunnel isn’t its deepest point underground. Due to the city’s topography, the top of the tunnel will be more than 200 ft below the surface near Virginia Street.

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