Rolling in the deep: Sound Transit’s downtown Seattle tunnel would bring riders 145 feet below the street

If Sound Transit manages to build a second light-rail tunnel through downtown Seattle, passengers would use elevators, escalators or stairs to descend as deep as 145 feet to catch a train.

That new 3.3-mile central-city tunnel would be the core of the regional $54 billion ST3 program voters passed in 2016, to build 62 miles of light rail in three counties, plus commute-train and bus capacity, serving three-quarters of a million daily trips.

Natural and man-made obstacles would force transit contractors to drill into lower Queen Anne Hill, burrow under the Highway 99 tunnel portal and existing Westlake Station, then weave between skyscraper foundations before reaching daylight near the stadiums, based on preliminary designs the agency unveiled this month.

“We’re putting in the plumbing after the house has been built, to some degree,” said Jon Scholes, president and CEO of the Downtown Seattle Association.

The new tunnel’s mission is to boost capacity, so that combined with the existing tunnel, light rail could carry as many as 48,000 riders per hour per direction. Seattle was the nation’s fastest-growing transit market in the 2010s, before the COVID-19 pandemic.

But Seattle’s deep stations, currently proposed to open in 2037, create new challenges. Passengers would spend extra minutes catching a train. Taxpayers will spend more money, if costs soar to scoop vertical station entrances through wet glacial soils.

Sound Transit estimates riders entering Midtown Station, near the Columbia Center tower, would need two to three minutes for a standard trip that begins with an elevator trip down, then ends with a switch to an escalator, for the final leg from a lower concourse to the train level. That doesn’t include waiting time for elevators.

Cathal Ridge, the executive corridor director, said the promised 12 miles of trackways linking Ballard, Seattle Center, downtown and West Seattle remain affordable near last year’s $12.6 billion estimate. That’s already $5 billion above what the agency presented to voters during the ST3 campaign.

Downtown’s proposed station depths aren’t a complete shock, because during the campaign, officials assumed rotary tunnel-boring machines were needed to burrow past roads and utilities.

International studies by Eno Center for Transportation, and by New York University, encourage transit agencies to adopt low-cost construction methods as in Europe — so that U.S. cities will build more rail transit sooner for the funds available. Those include shallow “cut-and-cover” construction where possible, they say.

But leaders in the Puget Sound area seek to minimize long trenches that would block streets during construction. Elevated and surface routes in the city core weren’t seriously considered.

Sound Transit’s planning team left the public eye in May 2019 to study a preferred alignment of routes and stations that politicians sent to them plus roughly 25 possible combinations of map lines between stops. To date, upward of $113 million has been spent toward ST3 Seattle design.

This month, the agency displayed updated station shapes and entrance sites to new citizen-advisory panelists, and released the draft environmental impact statement. Designs are 10% complete.

Whether any stations can be made shallower or easier to use, remains an open question, before transit board members choose a final alignment in 2023.

Claudia Balducci, chair of the board’s System Expansion Committee, said she hasn’t been briefed yet, and hasn’t come to conclusions about the station plans.

“Very deep tunnels are a little less accessible, a little more difficult for transfers between trains,” said Balducci, a Metropolitan King County Council member from Bellevue. “I’m going to look at cost and feasibility.”

Some ST3 projects have already fallen behind schedule, and the board spent much of last year hashing out what’s called a realignment, to cope with inflation and a funding gap of $6.5 billion. Members issued new goals of 2037 for the downtown tunnel, and 2039 to reach Ballard, years beyond the voter-approved 2035 goal. West Seattle service begins by 2032 at the soonest.

The current “preferred alternative” calls for a 145-foot-deep passenger platform at Midtown Station, 135 feet for the second Westlake Station, 100 feet for a Denny station, 120 feet for South Lake Union, and 85 feet at Seattle Center.

Scholes compares a rider’s descent to University of Washington Station, which plunges 95 feet. Several new stops will be deeper, he said, making station circulation its own leg of the commute.

“At the end of the day, the system, the stations, how people access the trains, has got to be a really great experience for everybody,” Scholes said. “And frankly, our track record of managing and maintaining escalators is not a great one, and you can see it today in the current tunnel.”

Nine downtown escalators and three elevators were inoperable as of Friday. Managers say it could take seven years to replace all 58 of the worn-out machines they inherited from King County Metro.

Midtown caverns

Midtown Station, under Fifth Avenue, demands precision excavation. The south entrance would be on a corner lot surrounded by the 76-story Columbia Center tower, the Seattle Municipal Tower, the new F5 Tower, and the 800 Fifth Avenue tower.

Contractors can’t just dig a shoebox-shaped station near the surface, as at the off-street Roosevelt Station. Midtown and Westlake train platforms are simply too deep, said Ridge. Instead, they would be mined from within the earth, by machines lowered down a vault.

The early design drawings don’t make it clear how many passenger elevators are needed, or how they would coexist with escalators and stairs, to serve Midtown’s 15,500 daily boardings and equal crowds getting off the trains. Sound Transit is committing to all three methods at Midtown and Westlake, Ridge said. Agency policies now call for public stairs, in response to escalator failures that stranded travelers at UW Station in spring 2018.

Designers expect most people using Midtown and Westlake Stations would ride an elevator to a low-level concourse, then walk over to a train-level escalator, spokesperson Rachelle Cunningham said.

At Westlake, one drawing shows an option to ride eight escalators down. Entry times are estimated at three to six minutes, within a vast station providing the city’s biggest hub for transfers between crossing train tunnels.

Another Midtown version, below Sixth Avenue, requires an all-elevator drop of 170 feet, but that site makes walking easier to First Hill hospitals and apartments.

To improve circulation, Midtown Station’s underground pedestrian walkways might also connect to an existing pedestrian tunnel between Seattle Municipal Tower and Columbia Center, said Sloan Dawson, land-use planning manager. That addition probably requires contributions by tower owners, Ridge said.

Even the 145-foot Midtown depth assumes the transit board will choose a “shallow” 90-foot-deep version for neighboring International District/Chinatown Station, not a 180-foot-deep option which would force Midtown yet deeper, to keep the track slope climbable.

Despite serious flaws, the board, especially then-Mayor Jenny Durkan, were unwilling to scrap the International District deep-station options in 2019, noting the community was besieged by a century of highway, train and stadium projects. District advocates hope a deep station would cause less construction havoc than a Fifth Avenue South shallow dig, which would destroy some buildings and require wrapping the vermilion Chinatown Gate.

Citizen-advisers questioned whether sports crowds would clog a deep station after a game. While shallow options provide room to circulate, the 180-foot options are elevators-only, which “could lead to additional queuing in a post-event scenario,” said Cunningham. At normal times, a deep station there adds roughly 1½ minutes per trip, she said.

Unlike ultradeep Midtown and Westlake stations, the Denny, South Lake Union and Seattle Center stations would be cut from above, causing prolonged street closures over short distances.

The second Westlake Station would be conjoined with the existing hub by underground walkway. Imagine a bigger version of Capitol Hill Station’s passage under Broadway to Dick’s Drive-In.

Portland’s Tri-Met operates the deepest transit station in North America, some 259 feet below the crest of Washington Park in the west hills. Montreal is burrowing a 235-foot-deep station into a suburban mountainside. The Washington, D.C.-area’s Forest Glen Station goes 196 feet down, where engineers wanted to dig harder rock rather than the soil layers above.

Sound Transit’s Beacon Hill Station platform is 156 feet deep and reached by four high-speed elevators. Some 3,150 daily riders boarded trains there as of spring 2019, far fewer than downtown.

Is it doable?

The proposed ST3 tunnel isn’t as massive as the Highway 99 tunnel, passing 200 feet under Pike Place Market dug with a drill face seven times as large as a transit tube.

But the train stations need multiple passenger and ventilation shafts through a sundae of water, silt, and more-stable clay soil layers. New drawings show ST3’s tunnel boring machines must cross roughly 40 feet under the existing tunnel at Westlake Station, without causing sinkholes. That’s less daunting than London Crossrail, whose bore went 3 feet below live trains.

Three events illustrate the cost risk with deep stations.

In 2005 then-CEO Joni Earl and the board canceled a deep First Hill Station rather than risk cost overruns, such as weak soils collapsing midproject.

At the Beacon Hill site, engineers changed shaft positions shortly before digging, to find better ground, a technical paper says. Grout injections were needed to harden the soil, as contractors scooped soils from the hilltop to the platform 156 feet below.

During the Highway 99 tunnel dig, severe groundwater pressures threatened to make a 120-foot-deep vault of concrete pillars cave in, during the operation to extract and repair TBM Bertha at the waterfront. Soils are more cohesive uphill, but builders at north-end transit tunnels have needed to fight gushing water before.

Cut-and-cover work is no picnic either. Balducci recalls the late 1980s, when Metro built the existing transit tunnel, and Third Avenue merchants cried out while being blocked for months.

In recent years, Sound Transit bored tunnels on time and under budget through Capitol Hill, and from Northgate to Husky Stadium, though no station was deeper than 100 feet.

“The problem with going 140 feet deep, is getting to it,” said Enotrans policy director Paul Lewis. “There’s a huge advantage in having shallow tunnels.”

Spain has combined tunnel-boring with cut-and-cover stations, but used more efficient planning and designs than the U.S., Eno found. “In some cases, it’s cheaper to use a TBM rather than move utilities,” Lewis said.

What matters most, he said, is a mindset he called “rip off the bandage,” to keep rail affordable.

European authorities save time by inflicting brief and intense construction on neighborhoods, then move on, unlike customized U.S. projects that keep lanes open, or make other accommodations lasting months or years.

Eric Goldwyn, an NYU researcher, hasn’t studied Seattle’s geography, but said station costs tend to accelerate with each foot of depth, for ventilation shafts, emergency exits, and more concrete and steel in the walls, along with land to launch a drill. “To save money, you want shallower, smaller, less bespoke stations.” In other cities, a typical cut-and-cover station can be 30 feet deep, he said.

Other planning is underway for West Seattle and Ballard segments, where tunnels hold political momentum over elevated lines, but require so-called “third party funding” like tax increases, federal infrastructure grants, or fees on real estate developers.

Buoyed by arguments to “build ST3 faster,” new Senate Bill 5528 would empower cities or clusters of cities to collect higher local car-tab and parking taxes for Sound Transit, if their voters approve a future ballot measure.