Ship Canal Water Quality Project Storage Tunnel

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Project Update by Daniel Dreyfus, PE

The Lake Washington Ship Canal is a navigable channel that connects Lake Washington to the Puget Sound. Each year it receives over 75 million gallons (283.9 ML) of polluted stormwater and sewage due to combined sewer overflows (CSOs) from seven capacity-constrained outfalls, which represent over 80% of Seattle’s annual CSO volume. The Ship Canal Water Quality Project (SCWQP) is a US$570M program being undertaken jointly by Seattle Public Utilities and King County to significantly reduce the volume of CSOs entering the canal each year.

Ship Canal Water Quality Project Storage Tunnel

Reinforcing steel and blockout for the West Shaft final lining at the tunnel eye.
(Image courtesy of Seattle Public Utilities.)

The SCWQP will construct a storage tunnel providing 29.2 million gallons (110.5 ML) of offline storage for CSOs from these outfalls until they can be pumped to the existing West Point Wastewater Treatment Plant via a new effluent pump station. The facility must be operational by 2025 under a Consent Decree from the U.S. Environmental Protection Agency. In late 2015, McMillen Jacobs Associates was retained by Seattle Public Utilities to lead the final design of the CSO Storage Tunnel and the Effluent Pump Station.

As the prime consultant, McMillen Jacobs has led a team of over 20 subconsultant firms to deliver a final design that includes development of five access shaft sites with new conveyance and control structures to divert CSOs into the new storage tunnel. The shafts range from 11 to 87 feet (3.4–26.5 m) in diameter and 60- to 120-feet deep (18.3–36.6 m). They connect to a new 14,000-foot-long (4,267 m), 18.83-foot (5.74 m) internal diameter storage tunnel. The tunnel has an average depth of 80 feet (24 m) and is located almost entirely under city rights-of-way, winding through the Ballard, Fremont, and Wallingford neighborhoods. The entire alignment is within very dense to hard glacially overridden soils.

Final design of the Storage Tunnel was completed in 2019, and a US$255M contract was awarded to The Lane Construction Corporation (Lane) to construct the storage tunnel and conveyance facilities. Construction started in early 2020, right before COVID-19 restrictions were implemented. In April 2020, after safety precautions and procedures were put into place, specialty subcontractor Malcolm Drilling started construction of the West Shaft, which serves as the tunnel boring machine (TBM) launch shaft.

The West Shaft is an 87-foot-diameter (26.5 m) reinforced diaphragm wall with slurry panels extending to a depth of 200 feet (61 m) to provide groundwater cutoff for shaft excavation. Malcolm completed panel installation in 14 weeks, and Lane self-performed excavation of the shaft in 8 weeks. The monolithic pour for the 14-foot-thick (4.3 m) invert slab took 13 hours. Inclinometers installed in the diaphragm wall indicate only 0.25 inch (6.4 mm) of deflection thus far.

Ship Canal TBM

Crews assembling personnel locks on the TBM during a rare Seattle snowstorm.
(Image courtesy of Seattle Public Utilities.)

Malcolm is continuing with construction of the other four shafts, and Lane is preparing the West Shaft for installation of its pressurized steel bell TBM launch system, which will be used in lieu of ground improvement. A new Herrenknecht earth pressure balance TBM (named MudHoney) arrived from Germany in February, and launch is expected in June 2021. McMillen Jacobs participated in a live TBM factory acceptance test, via Zoom because of COVID travel restrictions.

Through the first full year of construction, McMillen Jacobs and our subconsultants reviewed and responded to nearly 700 submittals and 300 requests for information, and executed 21 design changes, all while adjusting to the unique constraints associated with a COVID work environment. The entire SCWQP team is excited to see tunneling get underway in a few months!

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