Seattle Sewage Tunnel Reaches Halfway Mark


The tunnel boring machine digging a 2.7-mile-long, 19-foot-diameter sewer tunnel in Seattle reportedly reached the halfway point last month, marking a major milestone for the Ship Canal Water Quality Project. As of Jan. 12, the machine has tunneled 9,100 of 13,946 feet.

The underground storage tunnel is expected to significantly reduce the amount of polluted stormwater and sewage that flows into the Lake Washington Ship Canal, Salmon Bay and Lake Union from the city’s sewer system.

It will also improve water quality regionally by keeping more than 75 million gallons of polluted water from flowing into these areas on average each year.

Project Background

According to the project’s website, 150 tunnels spanning over 70 miles have been constructed in Seattle for sewers, utility corridors, and transportation needs since the 1800s. In some parts of the city, sewage and stormwater share a set of pipes in a combined sewer, with heavy rains often exceeding the pipes’ capacity and sending water into the canal.

The $570 million Ship Canal Water Quality Project includes a 29-million-gallon offline storage tunnel, six diversion structures for diverting influent combined sewage away from existing Combined Sewer Overflow outfalls to the tunnel, five drop structures to move combined sewage into the storage tunnel, and odor control systems.

During a heavy storm, the new $255 million tunnel being built by the Seattle Public Utilities and King County Wastewater Treatment Division is expected to capture and temporarily store more than 29 million gallons of untreated stormwater and sewage until the treatment plant is ready for it.

Along the tunnel path, there will be five vertical shafts in Ballard, East Ballard, Fremont, Queen Anne and Wallingford. Each will collect storm water and sewage flows from each basin and send them approximately 40 to 80 feet below ground into the new storage tunnel.

In April 2021, Seattle Public Utilities unveiled the new name for the tunnel boring machine after receiving 35,000 votes from the public. “Mudhoney” shares a name with the Seattle-based ‘80s rock band, and its name was painted on the front of the TBM by muralist Devin Finley later that month.

That same month, the machine arrived and was lowered by crane into a 107-feet deep, 80-foot diameter shaft in Ballard. 

Ballard will be home to the western end of the tunnel and above-ground facilities supporting the tunnel. Tunnel boring started at this site in August 2021 by contractor Lane Construction Corporation and moved toward Fremont and Wallingford.

The new pump station at Ballard will be a 70-foot-tall, illuminated tower that will house above- and below-ground mechanical and odor control equipment. It will pump the flows captured in the storage tunnel to the West Point Wastewater Treatment Plant.

Project designers created a cylindrical shape that mirrors the pump station and below-ground equipment space. The frame around the building echoes the industrial feel and scaffolding of Ballard’s shipyards.

Latest Milestone

According to reports, the drill reached its halfway mark in Fremont last month, marking a major milestone for the project. However, officials report that there are new challenges they are facing, including cost increases and delays.

Seattle Public Utilities said that the price tag for the tunnel and its related infrastructure was originally estimated to cost $423 million in the planning face, increasing to $570 million when digging by Lane Construction began.

However, this could reportedly increase another 8% to 14% before the project is completed. This is partially due to COVID-19 delays, as well as Mudhoney’s encounter with a “mega boulder” last spring. The rock measured 12 feet tall and was the “largest rock ever encountered by tunnelers in North America.”

Costs are also reportedly anticipated to rise because bids have not yet been solicited for the construction of the Ballard pump station at the end of the tunnel. A project executive noted that prices for building materials have “soared” in the past year, with previous estimates at almost $100 million now potentially reaching $125 million.

Seattle Public Utilities has, however, 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.

Currently, the crew’s target is to dig 65 feet per day. Mudhoney was anticipated to reach an overflow site in Fremont this month.

Project managers are reportedly working on a revised budget and schedule, with an update anticipated to be ready by July. The project, which was originally scheduled for completing by the end of 2025, will likely stretch into 2026, officials say.

Seattle SR-99 Tunnel

Several years ago, Seattle had another infamous giant tunnel-borer, “Bertha,” who bored through 9,270-feet of earth starting in 2013 for the Alaskan Way Viaduct Replacement Program. In April 2017, Beth broke through to the other side of the 2-mile tunnel to connect the south end with a 1-mile stretch of highway and an overpass that will guide vehicles over train traffic south of the tunnel.

The project replaced the Alaskan Way Viaduct—a stretch of SR 99 in the city that is subject to risk from seismic activity—with a new route that includes a nearly 2-mile-long tunnel. The project began after the 1950s-era viaduct structure sustained damage from a 6.8-magnitude earthquake in 2001.

In October 2017, the Washington State Department of Transportation released a new fly-through video of the tunnel bored Bertha, threading through the tunnel’s north end then out the south end, located near the Seattle stadiums.

Bertha, with a diameter of 57 feet and dubbed at the time the world’s largest tunneling machine, was designed specifically for the ground conditions under Seattle, according to WSDOT. As the borer moved, it pulled pieces of precast concrete into place behind it, creating the tunnel structure. Crews worked in the areas further back, creating the double-deck roadway inside it.

Some maintenance work involving the removal of cutting tools from the head of the borer had to be done in hyperbaric conditions, requiring crews to seal up the ground in front of Bertha with bentonite clay, then overpressurize the work area with compressed air.

Now that its mission is complete, the Seattle Times reported at the time that Bertha will be almost entirely scrapped and melted down for reuse. The only parts eligible for recycling as-is are “generic parts such as hoses, belts, wires.”


Tagged categories: Government; Government contracts; NA; North America; Ongoing projects; Program/Project Management; Stormwater; Tunnel; Utilities; Water/Wastewater

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