Sound Transit constructing giant bridge in Kent for light rail

By Aaron Chau

Sound Transit

When you hop aboard new 1 line service between Angle Lake and Federal Way, slated to open in 2026, you’ll be traversing an engineering marvel — Sound Transit’s longest-spanning bridge to date.

When finished, this giant will stretch more than three football fields in length, threading the needle between the southbound lanes of Interstate 5 and a sensitive wetland area in Kent.

If you drive I-5 regularly, you’ve probably noticed the bright lights and major construction in this area, just south of the former Midway Landfill near South 259th Place.

So why bother with building such a beast? It wasn’t part of the initial Federal Way Link Extension plan, but it’s a creative solution to ensure this new light rail extension will withstand environmental elements for generations to come.

In July 2022, as Federal Way Link Extension construction was already well underway, a small landslide in this area revealed that soil conditions under this part of the new light rail path were more unstable than predicted.

In the event of a major earthquake, the previously planned track design in this area could have left our rail structure vulnerable, or even posed a risk to passenger safety.

So, Sound Transit quickly sent its contractors back to the drawing board. And after reviewing a range of creative solutions to work through the problem, the agency decided to go over it.

The innovative long-span bridge under construction today will be 28 feet tall and 1,100 long, consisting of two 300-foot-long spans and a 500-foot-long span between.

Creating single spans that long requires special machines called “travelers.” These massive, house-sized machines intricately inch across the divide, casting concrete as they go, until they meet in the middle.

The travelers and the concrete spans they’re building are anchored by equally colossal columns, which flare out at the top for support.

There are two columns on both sides of the 500-foot span, and structures called “pier tables” on top of these column sets. As two traveler machines work inward to connect in the middle of the 500-foot span, two others are working outward from the pier tables at the same time, creating a balancing act.

Down below, each pier table has two foundation shafts measuring more than 12 feet in diameter and plunging 110 feet deep into the ground, embedded in rocky sediment deposited by ancient glaciers.

This deeply anchored design will ensure stability regardless of the soil quality, making the bridge seismically resilient in the event of another landslide or a major earthquake.

While Sound Transit’s construction contractor is on target to finish the bridge in time for a 2026 opening, progress on the remainder of the 7.8-mile extension is much farther along; in fact, the project as a whole is 84.5% complete.

As an extension of the 1 Line, Federal Way Link will bring regional connections to more South Sound communities with three new stations: Kent Des Moines near Highline College, Star Lake in Kent, and Federal Way Downtown.

Editor’s note: This story first appeared in Sound Transit’s Spring Progress Report for 2024.

Sound Transit’s longest-spanning bridge to date will stretch more than three football fields in length, threading the needle between the southbound lanes of Interstate 5 (hidden in the background of this photo) and a wetland area in Kent. COURTESY PHOTO, Sound Transit

On both sides of the bridge’s 500-foot center span in Kent, double columns support angled “pier tables.” Below the surface, foundation shafts measuring more than 12 feet in diameter plunge 110 feet deep into the ground, embedded in rocky sediment deposited by ancient glaciers. COURTESY PHOTO, Sound Transit