In this August 2, 1912 image, work is under way on the North Trunk Sewer siphon tunnel at Third Avenue West and Ewing Street, now the location of the West Ewing Mini Park.
In 1892, under the direction of Seattle City Engineer R. H. Thompson, construction began on a comprehensive system of sewers and sewer tunnels to divert sewage away from freshwater lakes and tidal areas and out into Puget Sound. Construction on the North Trunk Sewer began in 1911 to serve an area of about 30 square miles. Because the North Trunk system crosses the Lake Washington Ship Canal, tunnels were built beneath it to accommodate sewer pipes and other utilities. Shafts were dug 80 feet down on either side of the canal, and a brick-lined tunnel was constructed between them.
In the image below, a diver prepares to inspect the North Trunk Sewer’s outfall at Fort Lawton, in a photo taken by the engineering department on the same day.
When the city’s engineering department undertook building the sewer system that it has maintained, expanded and improved upon on ever since, the idea of treating sewage didn’t exist and the long-term impact of depositing it into Puget Sound was a known unknown. In the intervening years, treatment plants were introduced into the system; and more recently, building regulations requiring the inclusion of landscaping designed to absorb storm water runoff were put in place to mitigate the risk of raw sewage overflow into our water systems. Despite these efforts, incidents of overflow still occur during storms.
The same week 108 years later, construction is again under way at the Third Avenue West and Ewing Street site. In an effort to prevent future contamination during periods of high water input into the system, Seattle Public Utilities and King County Wastewater Treatment Division have begun work on the Ship Canal Water Quality Project. The project includes building a new 8-ft-diameter tunnel under the Ship Canal to convey polluted storm water and sewage to a new 29-million-gallon-capacity storage tunnel, building a drop shaft to direct flows into the new tunnel, and installing new pipes to convey storm water and sewage to the drop shaft.
More detailed information on the project can be found via the link below.