As part of the largest transit expansion program in the country, Sound Transit has expanded into Snohomish County, the northernmost of the three counties that encompass the Seattle metropolitan area.

The transformative, 8.5-mile Lynnwood Link Extension brings reliable, cost-efficient light rail across the county line to serve three communities — Shoreline, Mountlake Terrace and Lynnwood — connecting them directly to Seattle’s urban core, the Seattle-Tacoma International Airport and beyond.

“This new service marks the first time Sound Transit’s Link light rail system has extended into Snohomish County, broadening the range of the entire line and connecting communities across the region,” said Snohomish County Executive and Sound Transit Vice Chair Dave Somers. “The Lynnwood Link Extension provides an efficient and sustainable transit option and brings numerous benefits to the region by enhancing daily commutes, reducing environmental impact and fostering economic development.”

HNTB served as project manager on all engineering and architectural aspects of the project design and provided construction support for the extensive, multiyear project, now called the “1 Line to Lynnwood.” It incorporates five new stations and three new parking garages. The culmination of innovative design and a collaborative partnership, the extension will improve regional mobility and significantly reduce commute times.

Achieving innovation through collaboration

The Lynnwood Link Extension delivers a fast, frequent and reliable connection for riders through some of the most significant traffic congestion in Washington state. The light rail line runs parallel to Interstate 5, nestling into hillsides, crossing bridge structures or passing through tunnels, never intersecting with traffic.

Tracing I-5, the Lynnwood Link alignment also minimized the line’s construction footprint and allowed gentle curvature on wide curves, permitting a maximum operating speed of 55 miles per hour.

By design, the project team worked to reduce costs and minimize traffic impacts during construction. Where the extension crosses I-5 north of Mountlake Terrace, for example, the original design called for a cast-in-place, balanced-cantilever long-span bridge that would be built over the interstate. It later became clear that costs would be higher than initially expected to build the structure and that it would create challenges with traffic maintenance during construction.

The general contractor construction management contract model chosen for the project — which brings the contractor to the table during design to provide feedback — enabled the designers, contractor, Sound Transit and Washington State Department of Transportation (WSDOT) to work together to identify a more cost-effective solution and better maintain traffic during construction.

Ultimately, the team opted to use falsework to facilitate construction of a more conventional cast-in-place, box structure. A section of the bridge was precast in Tacoma, Washington, while the other sections were poured in place due to the structure’s curved shape.

“It’s unusual to see falsework over I-5 in Washington,” said HNTB Project Manager Moein El-Aarag. “Making the new concept work effectively came from a thorough understanding of Washington Department of Transportation (WSDOT) requirements and what the contractor wanted to achieve and determining how to accommodate both. That revised approach resulted in cost savings without impacting the traveling public.”