By Luke Carothers
Just a few months ago, on October 2nd, Seattle’s Northgate Link Extension was completed. This 4.3 mile, three station extension builds on Sound Transit’s light rail network, connecting downtown Seattle with its northern neighborhoods. In addition, this project featured three miles of tunnel as well as 2,200 feet of aerial guideway, making the project a large undertaking from an engineering perspective.
The project was led by McMillen Jacobs Associates who enlisted the services of WSP USA as a subconsultant to handle track design as well as mechanical and ventilation, lighting, traffic design, and integration for the tunnels. WSP USA was also responsible for the structural design of the Northgate Station and design of the guideway. The WSP USA team was led by Project Manager Tara Olsen. WSP’s team was also led by Justin Clark who worked on the structural design team for the aerial structures. During the construction phase of the project, Clark led the structural work for design services during construction (DSDC).
When work on the Northgate Link Extension began, the project’s engineers knew they had to come up with creative solutions to solve some anticipated problems. According to Olsen, one of the biggest challenges was building, operating, and maintaining an underground rail network in close proximity to the laboratories on the University of Washington campus. With a large section of the new track being laid beneath the University’s campus, it was essential to mitigate the impact of vibrations on sensitive equipment. To solve this problem, the designers, led by Mike Abrahams, the Technical Director of Structures at WSP, designed. floating slabs to mitigate vibration through the tunnel. While typical floating slabs are engineered to 12 Hz, researchers at the University specified that these slabs had to be engineered to 5 Hz. To compound this difficulty, these slabs, which each weigh roughly 10,000 lbs each, had to be secured in place while contending with a 4 percent grade and several horizontal curves. The process of designing, testing, and installing these floating slabs took over two years. To test their designs, 300 ft of floating slab was installed south of the University of Washington light rail station, adjacent to Husky Stadium.
Another significant challenge for the project was digging two large excavation sites in urban areas for the U District and Roosevelt Underground stations. This necessitated temporary road closures in several neighborhoods. Additionally, much of the construction fronted against apartment buildings, so the project had to be altered to allow residents continuous access to their buildings. This also meant that the team had to provide temporary walkways for pedestrians and bicyclists.
There was also a focus on the Northgate Station and the structural challenges posed by the design, particularly surrounding the guideway portions of the project. Several geographic challenges–such as seismic activity, soil condition, and grade–made work on the elevated surfaces more difficult. According to Justin Clark, the team had to contend with the site’s proximity to a fault line and the liquefying effect that has on the soil resulting in the need for very deep drilled shaft foundations. The Northgate Station also straddles a road that has vastly different grades on either side of the street. To compensate for these issues, Clark and his team knew they had to balance the stiffness of the columns from one side of the road to the other. In turn, they isolated columns on the higher grade side using shaft silos that extended the length of the columns. Clark notes that this is just one of many innovative solutions the team used in this pursuit. Vertical post tensioning was also installed in a few columns to keep them in compression during seismic events.
This project has significant benefits for people living in Seattle. Prior to the completion of the extension, the terminus for the light rail network was an underground station south of the University of Washington campus. Now, thanks to this project, the northern terminus has moved 3 miles north to the Northgate area at Husky Station. This extends the light rail network into an area known as University District with the U District Station providing a second connection to the University campus. In addition, the Roosevelt Station is centrally located to provide access to students at a local high school.
The completion of this project also coincides with a substantial investment in the neighborhoods that will be serviced. To provide safe access to the Northgate Station over Interstate 5, the John Lewis Memorial Bridge was opened on October 2nd. This provides easy access to the Northgate Station from North Seattle Community College, the University of Washington Medical Center, and other communities on the west side of the freeway. The Northgate Station will also connect to the Northgate Mall site which has been rezoned as a mixed use development that includes the training facility for Seattle’s new NHL team, the Kraken. Additionally, according to Clark, both the Roosevelt and University Districts have been upzoned recently, marking a push to prepare for the project’s completion.
Opening ceremonies were held on October 2nd, marking a new era of accessibility for the people of Seattle. Sound Transit estimates daily ridership for the new section will be between 41,000 and 49,000 daily riders by 2022. The success of the Northgate Link Extension could indicate a further expansion of the light rail network in Seattle over the next decade.
Luke Carothers is the Editor for Civil + Structural Engineer Media. If you want us to cover your project or want to feature your own article, he can be reached at firstname.lastname@example.org.