Sound Transit tunnel boring machine reaches U District station site

Mining of third light rail tunnel segment for Northgate Link extension completed, cross-passage and other tunneling work continue

The tunnel boring machine known as “Brenda” bored through the wall of Sound Transit’s future U District light rail station today, completing the third segment of the twin 3.5-mile tunnels that will serve light rail trains from Northgate Mall to the University of Washington starting in 2021.

See video of the tunnel boring machine finale at https://vimeo.com/144926809.

“The machine has now dug out 200,000 tons of dirt to reach the U-District Station,” said Sound Transit Board Chair and King County Executive Dow Constantine. “Three tunnel segments are done; three more to go to reach Husky Stadium.”

The boring today completed the third of six tunnel segments being mined by JCM Northlink LLC, Sound Transit’s tunneling contractor for the Northgate Link light rail extension. “Brenda” launched July 2014 from the Maple Leaf Portal at NE 92nd Street south of Northgate Mall. It will undergo maintenance at the U District station site before continuing south and connecting with the University of Washington Station, which opens with University Link first quarter next year.

A second machine named “Pamela” launched from the Maple Leaf Portal last November. It is expected to arrive at the U District site early next year. Tunneling with the tunnel boring machines is expected to be complete by the third quarter of 2016. Tunnel finishes and cross-passages, which connect the train tunnels and provide emergency exits, are scheduled to be complete by the first quarter of 2018. When light rail service on the Northgate Link extension begins in 2021, trains will enter and exit the tunnels at the Maple Leaf Portal.

To date, Sound Transit has bored nine tunnel segments for Link light rail construction. Mining of the twin tunnels serving the Beacon Hill Station were completed in 2008. Mining of parallel tunnels between downtown and the University Link Capitol Hill Station, and the Capitol Hill Station and University of Washington Station, finished in 2012. When the remaining three tunnels for Northgate Link are complete, Sound Transit will have mined a total of twelve tunnels for light rail construction.

Each tunnel boring machine weighs 600 tons and is more than 300 feet long including the trailing gear. The cutterheads are 21 ½ feet in diameter. By the time tunneling is finished, more than 500,000 cubic yards of soil will have been excavated and more than 7,200 concrete rings will have been installed to line the tunnels.

The $1.9 billion Northgate Link Extension will connect the University of Washington Station at Husky Stadium to underground stations in the U District and Roosevelt neighborhood, and to an elevated station at Northgate. The 4.3-mile extension from Northgate will provide 14-minute rides to downtown, seven-minute rides to Husky Stadium, and 47-minute rides to Sea-Tac Airport. Riders from Roosevelt Station will reach the airport in 44 minutes.

More information on the Northgate Link Extension Project can be found at soundtransit.org/northgatelink.