Over the last 30 years Seattle’s population had grown slightly, but steadily—just under 10 per cent every decade. However Census Bureau estimates show a recent jump, more than 20 per cent, from 2010. It’s one of the fastest growing cities nationwide.

With the influx of new jobs and residents, improving the city’s transportation networks, and in particular increasing its light rail system, has driven a whole host of tunnel work over the last decade. And there is still work to be done.

In 2016 Seattle voters approved the Sound Transit 3 (ST3) proposition, which called for major mass transit extensions over a 25-year period, with 62 new miles of light rail in the Puget Sound region. The plan was estimated to be worth about USD 54bn.

“In Seattle, we are building a city of the future with more access to reliable, affordable transit and more options for getting around,” says mayor Jenny Durkan.

It’s worth mentioning the notable Alaskan Way Viaduct replacement tunnel has completed. The 2-mile, doubledecker tunnel opened to traffic on February 4, 2019. Demolition of the viaduct started just over a week later, and finished in November. On the next page T&T showcases the city’s waterfront before and after.

In Construction

Tunnelling finished in autumn 2016 for Seattle’s Northgate Link LRT project. The 4.3-mile extension with three stations is largely underground. Construction started in 2012 and the line is set to open in 2021. Sound Transit reports the project is more than 90 per cent complete as of December.

Demand for affordable housing is also a significant consideration for policymakers in the region. Sound Transit, the regional transit authority, has committed to using surplus construction land, purchased for building stations or staging sites, for transit-oriented development.

At the new Roosevelt Station, there are three potential sites for transit-oriented development. One, Cedar Crossing, is already in planning and will see the construction of more than 250 apartments, many of which will have two or three bedrooms to accommodate families. Sound Transit says the development will only have 75 parking spots.

“Transit-oriented development serves as a significant opportunity for Sound Transit to contribute to the affordable housing inventory in a city and region experiencing a booming economy and rapid population growth,” said Sound Transit chief executive officer Peter Rogoff.

Also under construction is the East Link extension of 14 miles new light rail and 10 stations connecting Seattle, Mercer Island, Bellevue and Redmond (home to Microsoft). For this there is a 1,800ft tunnel excavated with SEM under downtown Bellevue. Tunnel construction is complete and the line is expected to open in 2023.

As for housing, land near one of the stations in Bellevue is also earmarked for transit-oriented development. Sound Transit acquired the property to build a new operations and maintenance facility, and once built, roughly seven acres will be available for development. The City of Bellevue envisions six buildings of office, retail and residential space.

There’s more than light rail work for the industry. The Seattle Public Utilities and King County Wastewater Treatment Division have started construction on a tunnel to significantly reduce the amount of polluted stormwater from rain and sewage that flows into nearby waterways.

In Planning

As part of the ST3 mandate in 2016 there are numerous projects in the design or planning stage for light rail and transit infrastructure. As far as underground work the project to watch is the West Seattle and Ballard Link Extensions, technically two separate light rail extensions. A major component of the approximately 12-mile-long project would be a tunnel in downtown Seattle of, potentially, about 1.8 miles.

The tunnel would pass through the densest part of downtown Seattle, under 5th Avenue according to the preferred alternatives. This is described as fairly narrow with a 60ft wide right-of-way, flanked on each side by skyscrapers with underground parking garages of at least 50ft depth, some up to 100ft, according to an NAT 2018 paper by Gordon Clark, HNTB, and Joseph Gildner, Sound Transit.

Contractors used temporary tie-back supports for the excavation, which will need to be removed should a TBM pass through the area. However, the authors report the right-of-way is thought to be too narrow for twin tunnelling, requiring a deeper tunnel, and have initiated a study for a single bore option (50ft [15m] od).

Last year the Sound Transit board identified its preferred alternatives for the tunnel and light rail alignments to be studied in a draft environmental impact statement, anticipated to be released in early 2021. The new extension could be operational as early as 2035.

In June 2019 a grassroots advocacy group launched a campaign for ST4, saying the city shouldn’t wait for the 2030s and the completion of ST3 projects to start thinking about what comes next.

They call for another ballot measure aimed at Seattle voters in 2024.