A bridge under construction on Sound Transit’s Federal Way Link Extension is a marvel of technology and engineering. Few will ever get to see it.
The bridge — also called a guideway — will span a stretch of unstable soil in a narrow space between Interstate 5 and wetlands. The unexpected construction came after a landslide in summer 2022 revealed the weak soil.
The 7.8-mile extension of Link light rail from Angle Lake, just south of of Seattle-Tacoma International Airport, to Federal Way at South 320th Street is 83 percent complete, according to Sound Transit. The bridge construction, located just south of South 259th Place, will push back the extension’s opening date to mid-2026.
The project broke ground in 2020 and had been slated to open in late 2024.
When it’s operational, the $2.5 billion project will whisk passengers from Federal Way to Sea-Tac in 15 minutes and downtown Seattle in 46 minutes.
While engineers can plan to the smallest detail, it’s nature that makes the final call. The 2022 slide just south of South 259th Place was a rejection, of sorts, of building an elevated guideway through the area using the standard 130-foot spacing between bridge columns. That would have placed several piers directly in the potential slide zone.
“We can’t have those foundations sitting in a zone that’s going to liquefy potentially in the future,” said Mike Jellison, a Sound Transit construction manager.
“If and when there’s a big earthquake, which will happen someday, it’s pretty much all gonna slide,” said Anna Bonjukian, Sound Transit’s acting principal construction manager. The choice was to engineer columns robust enough to withstand a slide or just span it. Engineers chose option B with a little bit of option A.
Now, two 300-foot-long and one whopping 500-foot-long span will be poured in place to get over the problem area. The beefy columns that will hold them are already there. The spans will grow out from each column, foot by foot, by using casting machines called travelers.
Called structure C, the guideway will contain the longest span in Sound Transit’s entire Link system.
All of structure C’s piers are now in place between its north and south abutments. The three most northerly spans only require the precast concrete girders that have been used through the system.
On a recent wet workday, crews removed the last forms from the piers and preparied the travelers to begin working north and south from the top of the structures. A stray black cat, informally raised by construction crews, roamed around the site’s trailers.
Although structure C has put the project back a year, Sound Transit is confident it is on the home stretch.
“I do believe we have the best builders in the country working on structure C,” Jellison said.
When the 28-feet-tall bridge is complete, soil will be built up around the piers and underneath the guideway, giving it the appearance of a ground level track.
The redesign and added construction costs for the new structure C added $72 million to the final cost.
Trains will only cross South 320th Street in Federal Way in order to switch tracks. But that guideway and adjoining station has changed the look of the area. A large lot next to the station will eventually become transit oriented development, according to Sound Transit.
The two other stations and parking garages in the extension are also assuming their final appearances. This week, crews were installing tile in the stations.
Flyover video from October shows most of the infrastructure — guideways, rails, stations and garages— in place. Rails still need to be installed south of South 288th Street to Federal Way. The system also needs overhead power lines, called a catenary, installed to energize the trains as well as telecommunication cables.
In 2024, the portion of the track between Angle Lake and the Kent Des Moines stations will be substantially complete, Bonjukian said. Portions of the new track south of Angle Lake are fully functional now. Sound Transit has been testing the tracks with trains (sorry, no passengers until 2026).
Drivers along I-5 and city streets should see trains on the entire Federal Way Link Extension in 2025 as contractor Kiewit and Sound Transit begin testing the system.