Cross passages & tunnels

 

Mode: Link light rail

Construction complete: 2021

Project phase: Tunnel construction and design of tunnel support facilities and screening walls

What's up with the drills and closed streets?

Crews are stabilizing the soil before they dig "cross passages" between the two light rail tunnels. 

Cross passages are emergency exits that connect the two separated light rail tunnels. As the passages are built, extra precautions are needed to keep water from destabilizing the work area. Crews either pump water out or, in some cases, freeze the ground to keep it stable.

Ground freezing is done to keep ground water from destabilizing the ground excavation. Crews dig directly above the cross passage location and insert 6' diameter pipes down 130 feet below the surface. Crews then pump a brine of calcium chloride and water in pipes through a chiller plant located on the surface. This chills the soils to 10 degrees Fahrenheit. The chiller plant and generators remain running throughout the cross passage construction. Once the cross passage construction is complete road surface is repaired and the chiller plant is removed.

If you would like to see a video of the ground freezing process please go to: http://www.soundtransit.org/video-ground-freezing

Construction of the Northgate Link Extension began in fall of 2012.

Crews are currently:

  • The project's two tunnel boring machines have completed their work to build a north and southbound tunnel. The final TBM hole-through happened for the southbound tunnel at the University of Washington Station in September 2016.
  • Cross Passage and ground freezing construction is under way.
    • Cross passages are emergency exits that connect the two separated light rail tunnels. As the passages are built, extra precautions are needed to keep water from destabilizing the work area. Crews either pump water out or, in some cases, freeze the ground to keep it stable.
    • Ground freezing is done to keep ground water from destabilizing the ground excavation. Crews dig directly above the cross passage location and insert 6” diameter pipes down 130 feet below the surface.  Crews then pump a brine of calcium chloride and water in pipes through a chiller plant located on the surface. This chills the soils to 10 degrees Fahrenheit. The chiller plant and generators remain running throughout the cross passage construction. Once the cross passage construction is complete road surface is repaired and the chiller plant is removed.
  • Continuing to survey along the alignment.

Traffic and parking:

  • Street parking along the alignment will be affected as surveying and installation of monitors continues.
  • Street parking on First Ave NE between NE 95th and NE 100th streets is closed until further notice.

Tunnel construction, mid-2014 - late 2017

  • Two tunnel boring machines dug the tunnels from the portal south to the University of Washington Station at Husky Stadium, passing through the Roosevelt Station and the U District Station along the way.
  • Dig cross-passages for safety. Cross passages are small person-sized tunnels that connect the north and southbound train tunnels.
  • Nighttime noise variance applies during the tunneling phase of the project - July 2014 through 2017.

If you would like to see a video of the ground freezing process please go to: www.soundtransit.org/video-ground-freezing

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