Southbound Interstate 5 will be closed three nights this weekend at Mountlake Terrace as Sound Transit works to build a concrete train bridge over the freeway.

Contractors will block all four lanes while they install steel frames that will temporarily support the new girders and concrete deck being created overhead.

The Lynnwood Link extension, which opens in 2024, will curve here from the west side of the freeway, across the median, and into a revamped Mountlake Terrace Station.

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Southbound freeway closures are planned from the 220th Street Southwest interchange to the Highway 104 junction on the King and Snohomish county line, during these times:

  • Friday at 11 p.m. until Saturday at 6 a.m.
  • Saturday at 11 p.m. until Sunday at 6:30 a.m.
  • Sunday at 11 p.m. until Monday at 4 a.m.

The marked detour route goes west to Highway 99, where drivers turn south and then return toward I-5 by using the Highway 104/Edmonds Way exit.

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Similar northbound closures will be needed in early 2021, transit spokesperson John Gallagher said. No daytime or two-way shutdowns are on the horizon at this time, he said.

Sound Transit has built a variety of overpasses, such as hollow pre-formed segments over I-5 at Tukwila, a giant poured-in-place bridge over I-405 in Bellevue, and I-beam girders for half of the Northgate-Lynnwood project.

In this case, a concrete pour makes sense because of the distinct curved shape, Gallagher explained. To span the Mountlake Terrace crossing using more I-beams, from supplier Concrete Technologies in Tacoma, would have required expensive custom designs, he said.

The $2.8 billion, 8.5-mile extension from Northgate Station to Lynnwood City Center Station is on pace to open in July 2024. The route, approved by voters in a 2008 sales-tax measure, is expected to serve 47,000 to 55,000 daily passengers.