The 6.6-m (21.8-ft) diameter earth pressure balance tunnel boring machine named Mudhoney launched in Seattle, WA to begin creation of the 4.3-km (2.7-mile) long tunnel that will store combined sewer overflow as part of the city’s consent decree with the U.S. Environmental Protection Agency.

The $225-million underground storage tunnel is part of the larger $570 million King County Ship Canal Water Quality Project.

Lane Construction, contractor on the tunnel portion of the project, launched the TBM on April 19 and plans to have the overflow tunnel ready by 2025. When completed, the tunnel can, on average, prevent 75 million gallons of polluted stormwater and sewage from entering the neighboring waterways. The tunnel will capture and temporarily hold more than 29.6 million gallons during heavy rains.

Engineering News Record reported the storage tunnel is part of the lager Ship Canal project from Seattle Public Utilities and the King County wastewater treatment division. Along the tunnel path, five vertical shafts in Ballard, East Ballard, Fremont, Queen Anne and Wallingford can collect stormwater and sewage flows from each basin and send them 64 km to 128 km (40 to 80 ft) below ground to the tunnel. To bring flows from Queen Anne into the storage tunnel, an additional 12-km (8-ft) diameter conveyance tunnel underneath the Ship Canal connects the vertical shafts in Queen Anne and Fremont. New pipes will connect the existing sewer system to the shafts.

In some parts of Seattle, sewage and stormwater share a set of pipes in a combined sewer and stormwater overflow system that can send untreated co-mingled water into the environment. The region is under the federal order to prevent the sewage releases.

Parts for the TBM were shipped to Seattle from Germany and assembled in Ballard..

The machine, which requires 10 to 15 operators, was built new in 2020 and is more than 14,200-ft long. The machine includes 18 double-disc cutters, 48 scrapers and 16 bucket cutters, with the ability to remove about 400,000 tons of dirt during the dig, including 105 ft under the Aurora Bridge.

Smaller tunneling machines will create conveyance tunnels under the Ship Canal between Fremont and Queen Anne and in Ballard