Sound Transit and WSDOT advise airport travelers of nighttime traffic impacts as crews build light rail bridge crossing SR518

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Starting Wednesday, June 13, commuters will see a new light rail overpass taking shape across SR518 near Sea-Tac International Airport. Sound Transit contractors will build the overpass over the highway just west of International Boulevard. Crews will employ a construction method and schedule that keeps the freeway open during the day, limiting lane closures and traffic impacts to nighttime hours.

Sound Transit contractors have successfully used the same construction method – balanced cantilever construction – for light rail overpasses crossing I-5 and the Duwamish River.

The work will begin with the nighttime closure of one westbound lane of SR518 between 11 p.m. and 5 a.m. on Wednesday, June 13 and intermittent nighttime closures will continue for approximately six weeks. All lane closures are expected to be complete by July 23 and there will be no closures between June 27 and July 9 as part of the holiday travel construction moratorium.

Detour routes and signage will be in place on SR518 throughout construction and the latest closure schedules will be posted on Sound Transit and WSDOT’s websites at: 

The work will require a total closure of SR518 in both directions for three nights and full one-way closures for 11 nights (five full westbound closures, six full eastbound closures). The schedule for the closures is subject to change. Please refer to the Sound Transit and WSDOT websites for the latest closure schedules.

The construction will utilize pre-cast concrete segments in a balanced cantilever design, a method first used in the Puget Sound region last summer when Sound Transit built a light rail bridge crossing the Duwamish River. For the SR518 overpass, two large bridge piers have been built, one on each side of the freeway just west of the existing International Boulevard overpass. Starting with the pier adjacent to westbound SR518, cranes will hang pre-cast concrete segments one at a time on alternating sides to keep the structure in balance. Steel tensioning cables running through the segments will keep them in place as the structure extends in each direction. After the structure is built over the westbound lanes of SR518, the structure over the eastbound lanes will follow, and the two structures will connect above the highway.

Link light rail construction between downtown Seattle and Sea-Tac International Airport is approximately 70 percent complete and scheduled to open for service by the end of 2009.


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Sound Transit’s regional network of express buses, commuter rail, light rail and transit facilities connects communities in King, Pierce and Snohomish counties.