The second tunnel boring machine holed through the wall of Sound Transit’s future U District light rail station on March 24, completing the fourth segment of the twin 3.5-mile tunnels that will serve light rail trains from Northgate to the University of Washington starting in 2021.
“Just days after we celebrated the opening of the new light-rail station at Husky Stadium, we’ve reached an important milestone on a tunnel project that will connect the University District to high-capacity mass transit,” said Sound Transit Board Chair and King County Executive Dow Constantine. “With more tunneling work under way and construction of Northgate Station starting this year, we’re on target to extend fast, reliable light-rail service to Northgate in 2021.”
The hole through on March 24, completed the fourth of six tunnel segments being mined by JCM Northlink LLC, Sound Transit’s tunneling contractor for the Northgate Link light rail extension. The tunnel boring machine today, TBM #2, launched from the Maple Leaf Portal at Northeast 94th Street south of Northgate Mall November 2014. Another tunnel boring machine, TBM #1, launched from the Maple Leaf Portal July 2014 and reached the U District Station site last November. It is expected to reach Husky Stadium within the next two weeks.
Both machines will undergo a thorough inspection at their respective hole-through sites. Once inspections are complete, the contractor will determine which machine will be used to mine the final tunnel segment. Crews will then spend two to three months refurbishing the selected TBM to prepare for its launch to the University of Washington.
Tunneling with the tunnel boring machines is expected to be complete by the end of this year. 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.
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.
