Machine is done digging Seattle’s huge sewage and stormwater tunnel

SEATTLE – The 900,000-pound machine that started boring a gargantuan sewage and stormwater tunnel under Seattle in 2021 at last burst into the open this month, completing its rocky 2.7-mile route in just under two years.

Nicknamed “MudHoney” after the Seattle grunge band, the drill started digging in Ballard and emerged into a Wallingford drop shaft near Stone Way.

When operational, the tunnel will store up to 30 million gallons of untreated sewage and polluted stormwater. It’s part of a $615 million-plus effort by Seattle and King County to keep the liquid from spilling into the Lake Washington Ship Canal, Lake Union and Salmon Bay when the antiquated drainage system in nearby neighborhoods is overwhelmed by heavy rains.

There are other aspects of the Ship Canal Water Quality Project that still need to be built, including an enormous pump station at the Ballard end. But digging the 19-foot-diameter tunnel was the most challenging and expensive job, so the project has passed a crucial milestone.

“Most of this project is underground,” SPU project executive Keith Ward said Monday near the Wallingford drop shaft, peeking into the 85-foot-deep hole as hard-hatted workers climbed down a yellow ladder and cleaned the machine.

Manufactured in Germany and shipped through the Panama Canal, MudHoney bored through North Seattle’s glacial till just like the machines that dig subway tunnels, crunching past rocks with a gaptoothed cutter head and moving the soil backward with a rotating screw.

Train cars on the tunnel’s custom-built rail line carried the muck to a drop shaft in Ballard, where a crane with a clamshell bucket grabbed the goop. Trucks hauled the material to a Snohomish County gravel pit, and another crane lowered concrete wall panels into the shaft. Each time the drill churned ahead, workers installed a ring of panels to create a section of tunnel.

MudHoney and the workers operated 24 hours per day and six days per week, splitting the time into eight-hour shifts. The digging took about 10 months longer than scheduled, Ward said Monday, citing COVID-19 complications, a labor shortage and a “megaboulder” the machine had to chew through last year.

“This was one of the largest boulders ever encountered” during a tunnel project, dropped by a glacier 15,000 years ago, Ward said.

Before MudHoney broke through on June 15 at the end of its journey, the project’s tunnel contractor filled the Wallingford drop shaft with water to counteract pressure that could have otherwise destabilized the tunnel. Then workers made sure the tunnel was sealed and began removing the water. Next, the tunnel machine will be disassembled and hoisted out in pieces.

“They’re actually going to send the machine back to the manufacturer in Germany to reuse as much of it as possible,” Ward said.

When MudHoney launched in 2021, its steel case was painted bright red, yellow and blue. Resting in the sun Monday, almost all of that color had been scraped away, though Ward pointed out a hint of red residue

The price tag for the entire ship canal project, pegged at $423 million in a 2014 planning estimate and $570 million when the tunneling began, is now estimated at $615 million to $650 million. SPU promised last year to absorb the increase without raising customer rates higher than already approved.

The city and county have yet to select a bidder to build the Ballard pump station, which will be wrapped above ground in an 80-foot-tall steel lattice and shimmering lights. Previously estimated at $100 million, the pump station could end up costing as much as $125 million, Ward said last year.

Seattle and King County were supposed to complete the whole project by the end of 2025, per consent decrees with environmental authorities that are aimed at reducing “combined sewer overflows.” The work is now expected to continue into 2026, according to SPU.

In older neighborhoods, sewage from bathrooms and stormwater from street gutters drain through the same system. Today, when rains flood that system, the mix spews through overflow pipes into public waterways.

There are about 110 such overflow sites across the city, including six high-volume sites between Ballard and Wallingford that together average 100-plus spills annually. Those are the flows that will be diverted into the new tunnel, theoretically reducing their number of discharges to six or fewer each year.

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 overall project, which also includes connections to the tunnel from the various overflow pipes, should reduce polluted spillage by more than 75 million gallons each year.

This story contains information from The Seattle Times archives.