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Look inside this giant new tunnel that will improve water quality in Seattle’s Ship Canal

By June 22, 2023No Comments

The famed “MudHoney” drill, Seattle’s latest celebrity boring machine, has finished its 100-foot-deep tunnel-boring job. It broke through its destination at a shaft in Wallingford on June 15 at 10:18 a.m., completing a 2.7-mile journey from Ballard along the Lake Washington Ship Canal. The work took 21 months at a pace of about 50-feet a day.

The actual breakthrough was not visible at the time because the shaft was filled with water. But once the last few tunnel segments are installed the shaft will be emptied and crews will start taking apart the machine and hauling it out in segments.

This is a major milestone in a multi-year project to construct a tunnel and pump system that will improve water quality for the benefit of people and wildlife. A partnership between Seattle Public Utilities and King County Wastewater Treatment Division, the Ship Canal Water Quality Project is the largest current joint investment in a regional effort to protect our waterways through investments in wastewater infrastructure.

Into the depths

Shortly before breakthrough, as MudHoney was hitting the homestretch, this is what the tunnel — and this massive, subterranean construction site — looked like.

Shaft with equipment

The Ballard shaft is the entry point into the tunnel. It will one day contain the massive pumps that will send the polluted water in the tunnel to West Point Treatment Plant to be cleaned.

To get to the tunnel being built by the Ship Canal Water Quality Project, you first have to take the Alimak down into the Ballard shaft. It’s a steel cage elevator that begins and ends the day for 60 miners who have been building the tunnel.

A train supports the tunneling operations, bringing people and supplies deep into the depths, and “muck” from the tunnel boring back out. At the very end of the line is MudHoney, the 14,256-foot drill aptly named after Seattle’s famed grunge-celeb rock band.


Train and train tracks going into the tunnel

The train hauls in equipment and construction infrastructure and hauls out the muck from the tunnel boring.

A massive storage tank

The 18-foot, 10-inch diameter tunnel runs more or less along the Lake Washington Ship Canal, past Salmon Bay to Lake Union. Its sole purpose is to be a massive storage tank during the winter rainy season that can hold, out of sight, more than 29 million gallons of polluted stormwater and sewage. That’s enough to fill 44 Olympic-sized swimming pools!

All this effort is needed. Parts of Seattle’s sewer system were built a century ago when it seemed like a good idea to combine sewage and stormwater into one set of pipes. The economizing had a huge drawback though in rainy Seattle. The pipes can get overwhelmed during storms and the only way to avoid backups into homes and businesses is to send the combined stormwater-sewage mixture into the nearest water body – in this case Lake Union, Salmon Bay, and the Lake Washington Ship Canal – through a series of outfalls.

The new tunnel will temporarily hold the excess flow until it can be pumped to King County’s West Point Treatment Plant – keeping about 75 million gallons of polluted water from entering the waterway every year. It’s a tremendous gain for water quality, and is needed now more than ever because of climate change. The storage tunnel is sized to meet the more intense storms of the future.

Miner’s station

At 100 feet below ground, the tunnel is damp and cold, and there are long stretches of track void of life. At the midway point where trains can pass each other, the miners have set up a modest break station hosting a coffee pot, a microwave, and decorated with the flags of their home countries – Mexico, Italy, Romania, Spain and The United States. A Virgin Mary statute placed at the tunnel entrance is a reminder of the constant dangers inherent in mining work.

Table with microwave, a clock above, and flags from the home countries of the miners.

The miner’s break station is midway through the tunnel and holds a few creature comforts for those who work the long shifts underground

MudHoney at work

A couple of turns in the track later is MudHoney, chewing away at the compacted, glacial soils of gravel, sand, and silt. An accompanying machine follows, placing huge, curved slabs of pre-made concrete along the rim of the hole, effectively creating the tunnel. Following that machine comes a sequence of work to lay down the train tracks, the electrical lines, and the venting that keeps the tunnel breathable. It’s hard to tell exactly what’s going on from the rear of the giant operation, but suffice to say the machines are massive, complex, and are so loud you need ear plugs.

Giant machine and man in safety gear

MudHoney is hard at work boring through the last section of tunnel before reaching its destination in Wallingford.


Man in yellow safety gear and hardhat outside a control room

Armando Torres is one of the MudHoney operators.

Giant slabs of concrete are stacked while men in safety gear stand beside.

Giant slabs of concrete are in place to line and will be positioned to create the tunnel walls.

It must come out

Surprisingly, all the infrastructure that’s placed – except for the tunnel walls – is all temporary. When the tunnel is done, everything must come out. MudHoney will be soon be disassembled and raised up in several pieces through a shaft in Wallingford. Then the next phase of the project kicks in to construct a 12-million gallon per day pump station in the Ballard shaft. The massive tunnel will remain, dark and empty, until the first storm when it begins to hold polluted stormwater and sewage from entering our water bodies.