Seattle’s huge sewage tunnel is halfway drilled. Take a look inside

Seattle has an underground rail line that starts in Ballard, with trains that run around the clock. But you won’t come across the route on any transit map, because the trains in question don’t move commuters. They move muck.

Tons of muck.

The custom-built rail line is part of a massive project by Seattle and King County to burrow a 2.7-mile-long, 19-foot-diameter tunnel that will store up to 30 million gallons of untreated sewage and stormwater during heavy rains.

The gigantic drill that began digging the deep-bore tunnel from Ballard to Wallingford in 2021 passed its halfway mark last month, chewing through soil that gets hauled away to a Ballard drop shaft (a vertical hole that reaches down to the tunneling site from street level) in 25-ton train carloads.

That was a major milestone for the costly project, which will keep polluted water from spilling into the Lake Washington Ship Canal, Lake Union and Salmon Bay when Seattle’s antiquated drainage system is overwhelmed.

“It’s really an investment in Puget Sound’s health and our region’s future,” Keith Ward, the project executive on the tunnel for Seattle Public Utilities, said while giving a tour of the Ballard construction site last Thursday.

But challenges remain. The price tag for the tunnel and related infrastructure, pegged at $423 million in a 2014 planning estimate and $570 million when the digging by contractor Lane Construction began, could increase another 8% to 14% before the job is done, according to Seattle Public Utilities, which is sharing the costs with King County’s Wastewater Treatment Division.

That’s partly because the COVID-19 pandemic snarled operations for some time and because the $15 million drill (dubbed “Mudhoney” in an online vote, after the Seattle grunge band) was delayed last spring when it had to crunch through a “mega boulder.” Measuring 12 feet tall, it was the largest rock ever encountered by tunnelers in North America, Ward said.

The costs are also expected to rise because the city and county have yet to solicit bids for the construction of an enormous pump station at the Ballard end of the tunnel, where the drop shaft is currently located, and prices for building materials like rebar and plywood have soared in the past year, Ward said. Previously estimated at almost $100 million, the cylindrical pump station, which will bewrapped in an 80-foot-tall steel lattice with shimmering light, could now cost as much as $125 million, the project executive said.

The good news, according to Ward, is that Seattle Public Utilities has committed to absorbing the project’s increases without raising customer rates higher than already approved as part of the agency’s current six-year plan. The plan calls for average annual rate increases of 4.2% between 2021 and 2026 for water, sewage, solid waste and drainage, with monthly costs for a typical house expected to reach about $275.

Soaking up the added costs for the tunnel could require Seattle Public Utilities to delay other, smaller projects, Ward said. But the tunnel is a priority for the agency, not only because of the sheer size of the undertaking, but also because Seattle and King County are obligated to complete the tunnel under consent decrees with federal and state environmental authorities aimed at reducing pollution related to “combined sewer overflows.”

In older Seattle neighborhoods, sewage from bathrooms and stormwater from street gutters drain through the same system. Today, when downpours flood that system, the contaminated mix spews through overflow pipes into public waterways, causing environmental and public health harm.

There are about 110 such overflow sites across the city, including six high-volume sites along the ship canal between Ballard and Wallingford that together average 100-plus overflows annually, Ward said. For example, 2-plus inches of rain on Jan. 2, 2022, led the six sites to spill 20 million gallons of sewage and stormwater (enough to fill 31 large swimming pools).

Those are the overflows that will be diverted into the tunnel during storms, reducing their number of discharges to six or fewer each year, Ward said. The liquid stored in the tunnel (about 10% sewage and 90% stormwater) will flow to Ballard, where the pump station will send it on its way to the West Point Treatment Plant in Magnolia. The entire project, which also includes connections to the tunnel at the various overflow sites, should reduce polluted spillage by more than 75 million gallons each year.

Tunnel work

First, the tunnel must be completed via a complicated process that involves Mudhoney, the narrow-gauge rail line (there are actually multiple tracks), gantry cranes and a small army of workers in orange vests and hard hats.

Manufactured in Germany and shipped through the Panama Canal, the machine crunches through soil, which is injected with a polymer to create a substance that workers call “muck” and that Ward compared to toothpaste.

The rocky dirt gets churned behind the machine’s cutter head, where a rotating screw and a conveyor belt are used to move the muck backward to the train cars. The soil along the Ship Canal is abrasive glacial till, Ward said.

The train cars, powered by three locomotives, are steered back to Ballard, into the base of the drop shaft, where their loads are tipped into a muck pile. The drop shaft is an 80-foot-diameter, 95-foot-deep hole in a construction site hidden behind plywood fences, near the Pacific Fisherman Shipyard.

A gantry crane above the drop shaft uses a giant claw to haul out the muck, which is moved by trucks at night to a gravel pit in Snohomish County. And a second crane lowers the tunnel’s concrete wall panels into the hole, onto train cars that carry the curved panels (also called segments) into the tunnel.

Each time Mudhoney bores 5 feet, workers install six panels in a ring to create a new section of tunnel, said Derek Dugan, a consultant on the project who was once involved with the Channel Tunnel between England and France.

“Right now, we’re on ring 1,805,” Dugan said last Thursday.

The wall panels fit together with dowels, sort of like Ikea furniture, Ward added. The project is like building a subway tunnel, but for dirty water.

Workers are hoisted into and out of the Ballard drop shaft by a metal elevator, and the project is monitored by Saint Barbara, the patron saint of miners and tunnelers. Her statuette, wreathed by a halo and wearing a crown, is positioned between vases of flowers in a bright blue shrine bolted to the wall of the tunnel. From there, she watches over crews of 17 to 20 workers each shift, including tunnel workers, train operators and crane operators.

They wear brass discs with numbers that would be used to identify their bodies were a disaster to occur underground. The project operates 24 hours per day, six days per week, with the time split into eight-hour shifts.

The crew’s daily target is to dig 65 feet, and time is money, so moving quickly is important, Ward said. On Thursday, the workers raced 95 feet.

But safety is important, too: Firefighters had to rescue a crew member last March who was pinned by a train in the tunnel. Protocols for the tunnel trains have been updated since then and the worker, who was seriously injured in the incident, has returned to work elsewhere, Ward said.

Mudhoney could reach an overflow site in Fremont as soon as this week, and the project’s managers are drawing up a revised budget and schedule, Ward said. That update should be ready by July, he said. The project, including the Ballard pump station, was supposed to be done by the end of 2025, according to the consent decrees, but will likely now stretch into 2026, Ward said.