Northgate Link tunnel

The Northgate Link tunnel is a light rail tunnel in Seattle, Washington, United States. The twin-bore Link light rail tunnel, built as part of the Northgate Link extension (formerly known as "North Link"),[1] carries a section of the 1 Line and connects the University District to Northgate.

Northgate Link tunnel
Viewed from Roosevelt station in 2019
Overview
Line 1 Line
LocationSeattle, Washington, U.S.
StatusActive
StartMaple Leaf Portal (near Northgate station)
EndUniversity of Washington station
Operation
Constructed2012–2021
OpenedOctober 2, 2021 (2021-10-02)
OwnerSound Transit
TrafficLink light rail
Technical
Length3.4 mi (5.5 km)
No. of tracksDouble
Track gauge4 ft 8+1⁄2 in (1,435 mm)
Tunnel clearance21 ft (6.4 m)
Width21 ft (6.4 m)
Route map
to Lynnwood Link extension
 Northgate
Maple Leaf portal
 Roosevelt
 U District
 University of Washington
to 1 Line (University Link tunnel)

The tunnel begins at University of Washington station (the north end of the University Link tunnel) and travels northwest across the University of Washington campus. It serves intermediate stations at U District and Roosevelt before emerging on the east side of Interstate 5 at a portal in Maple Leaf. Light rail trains then continue on an elevated guideway to Northgate station, where the extension ends. The extension was approved in 2008 by voters in the Sound Transit 2 (ST2) package, began construction in 2012, and entered service on October 2, 2021.[2]

Routing

For the 4.3-mile (6.9 km) Northgate Link extension project, the paired tunnels constructed for University Link continue north to the University District and the Roosevelt neighborhoods before emerging at the surface along I-5 near NE 94th Street to serve the elevated Northgate station. The tunneled portion is 3.5 miles (5.6 km) long, while the elevated section is 0.8 miles (1.3 km).[3]

An extension north to Lynnwood was approved as part of the Sound Transit 2 ballot measure in 2008, along with funding to continue planning future service all the way to Everett. The Lynnwood Link extension is scheduled to open in 2024.[3]

The section that passes approximately 140 feet (43 m) under the University of Washington campus has a capped speed of 35 miles per hour (56 km/h) to prevent interference with sensitive equipment used for university research. The tunnel also uses 7,500 feet (2,300 m) of rubber supports for "floating slabs" that cushion and absorb energy from the trains.[4][5]

Stations
Name City/Neighborhood Location
University Link Tunnel to Capitol Hill and Westlake
University of WashingtonUniversity District, SeattleMontlake Blvd. NE and NE Pacific St.
North end of University Link
U DistrictUniversity District, SeattleNE 45th St. and Brooklyn Ave. NE
RooseveltRoosevelt, SeattleNE 65th St. and 12th Ave. NE
North end of tunnel (Maple Leaf Portal); south end of elevated section
NorthgateNorthgate, Seattle1st Ave. NE and NE 103rd St.
End of line; future extension to Lynnwood

Construction
Cutting head of tunnel boring machine "Brenda" (right) at Roosevelt Station pit after completing tunnel from Northgate

Construction on the Northgate Link extension began in 2012 and opened for service on October 2, 2021.[3][6] Train testing began in January 2021.[7]

Tunnels

Each 3.4-mile (5.5 km) tunnel is dug southbound, starting from the Maple Leaf Portal, in three segments separated by stations, using a 21.5-foot (6.6 m) diameter tunnel boring machine. The machine digging the tunnel for northbound trains was named "Brenda" and the sister machine for the southbound trains was named "Pamela", but the names were dropped in March 2016 to reduce associations with the troubled "Bertha" machine used on the Alaskan Way Viaduct replacement tunnel project.[8] Brenda was previously used on the University Link Tunnel from 2011 to 2012, digging both tunnels between Capitol Hill station and the Downtown Seattle Transit Tunnel.[9]

TBM #1 (formerly Brenda) began excavation of the northbound tunnel on July 9, 2014,[10] and reached Roosevelt station on March 17, 2015.[11] It later reached the U District station on November 6, 2015,[12] and completed the entire northbound tunnel to University of Washington station on March 30, 2016, less than two weeks after light rail service to that station from Downtown Seattle had begun.[13] TBM #1 was disassembled and removed from the University of Washington station. Due to damage sustained by TBM #2, TBM #1 was refurbished and excavated the last (southbound) tunnel segment between June and September 2016.[14][15]

TBM #2 (formerly Pamela) began excavation of the southbound tunnel on November 20, 2014,[16] and reached Roosevelt station on July 13, 2015.[17] For six weeks from late December 2015 to early February 2016, the machine was stopped at a point 650 feet (200 m) north of U District station after encountering hard soil that damaged five motors and the main gear of the machine. Repairs were made and the segment was completed on March 24, 2016.[18] Additional repairs to TBM #2 needed to complete the final (southbound) tunnel segment would have potentially delayed the project's opening if the five months of float time were exceeded,[19] leading JCM Northlink to finish the final segment with TBM #1.[14]

The tunnel has 23 cross-passages between the bores that contain emergency exits, which were excavated using ground freezing.[20]

References
  1. "New Project Name". Sound Transit. Retrieved July 20, 2012.
  2. Craighead, Callie (October 2, 2021). "Seattle's Northgate Link light rail extension officially opens for riders with 3 new stations". Seattle Post-Intelligencer. Retrieved October 2, 2021.
  3. "Northgate Link light rail extension will open October 2" (Press release). Sound Transit. April 9, 2021. Retrieved September 13, 2021.
  4. Gray, Bruce (September 22, 2021). "World-class engineering under ground keeps trains quiet on UW campus above". The Platform. Sound Transit. Retrieved September 30, 2021.
  5. Newcomb, Natalie (September 29, 2021). "A smooth ride between UW and U-District light rail stations: This is why". KUOW. Retrieved September 30, 2021.
  6. "Project phases – Northgate Link Extension". Sound Transit. Retrieved May 5, 2016.
  7. "Light rail vehicle testing begins on Northgate Link extension". Sound Transit. January 18, 2021. Retrieved February 2, 2021.
  8. Bush, Evan (March 28, 2016). "Thanks to Bertha, Sound Transit nixes nicknames for its own tunnel machines". The Seattle Times. Retrieved April 4, 2016.
  9. Stiles, Marc (April 28, 2014). "Bertha's stalled, but Brenda's ready to roll once again". Puget Sound Business Journal. Retrieved May 5, 2016.
  10. Miller, Ben (July 10, 2014). "Boring! Sound Transit's Northgate tunnel machine begins digging". Puget Sound Business Journal. Retrieved May 5, 2016.
  11. Lindblom, Mike (March 17, 2015). "Transit-tunnel machine reaches daylight at Roosevelt Station". The Seattle Times. Retrieved May 5, 2016.
  12. "Sound Transit tunnel boring machine reaches U District station site" (Press release). Sound Transit. November 6, 2015. Retrieved May 5, 2016.
  13. "TBM 1 has tunneled to the UW Station". Seattle Daily Journal of Commerce. March 31, 2016. Retrieved May 5, 2016.
  14. "Northgate Link Tunnel Boring Is In The Final Stretch". The Urbanist. Retrieved July 1, 2016.
  15. "Sound Transit contractor completes mining last tunnel segment for Northgate Link Extension" (Press release). Sound Transit. September 2, 2016. Retrieved September 2, 2016.
  16. "Sound Transit launches second Northgate Link tunnel boring machine" (Press release). Sound Transit. November 20, 2014. Retrieved May 5, 2016.
  17. "Sound Transit finishes second light rail tunnel from Northgate to Roosevelt" (Press release). Sound Transit. July 13, 2015. Retrieved May 5, 2016.
  18. Banner, Ellen (March 24, 2016). "Tunnel machine breaks through in the U District". The Seattle Times. Retrieved March 28, 2016.
  19. Lindblom, Mike (February 11, 2016). "Another Seattle tunnel drill (not Bertha) damaged, needs repair". The Seattle Times. Retrieved February 11, 2016.
  20. "Northgate Link tunnel wins national award". Seattle Daily Journal of Commerce. September 13, 2018. Retrieved October 9, 2021.
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