The historic Pioneer Square district—the front porch to Seattle’s downtown waterfront—is the City’s first neighborhood. The area is shaped by its proximity to the waterfront, active seismology, and over 150 years of urban development, infill, and industry– including housing one of Seattle’s first true startups, the Yesler Lumber Mill.
Built in the early 1850s, the Yesler sawmill was a steam-powered sawmill on the shores of the growing downtown Seattle.
Photo Credit: www.historylink.org
These confluence of factors also trigger specific criteria for building in the neighborhood today – taking historical preservation compliance, seismic codes, and a tricky subsurface into account.
In the heart of the neighborhood, at 165 South Washington Street, Aspect is helping Johnson & Carr, LLC guide the development of an eight-story workforce housing project. The site – currently a vacant pit – formerly housed an apartment building bearing directly on weak urban fill and sawdust. That building was damaged in two separate earthquakes – the 1949 (Olympia) earthquake and 2001 (Nisqually) earthquake, leading to it being condemned and demolished.
The project site, currently a vacant pit that’s sat unused for years, awaits design and construction of a new 8-story workforce housing building.
The Complexity of Seattle’s Original Sawmill Dumpsite
This project site is directly influenced by the past in several ways. It was originally a tidal marsh, used over a century ago, among other things, as a dumping ground for sawdust from the Yesler Mill. This means current project design must grapple with up to 25 feet of sawdust fill in the subsurface. As white settlers filled the surrounding waterfront during the Klondike Gold Rush era, the former tidelands were swallowed up by all manner of new buildings and roads in the haste to infill the neighborhood (as seen on the figure below).In addition, several seismic sources contribute to the seismic risk at the site, including the Seattle Fault Zone which is less than 2 miles away, and the Cascadia Subduction Zone, which can trigger a magnitude 9 earthquake. The seismology, historical uses of the project area, and decades and decades of man-made development have only added to the complexity of building here.
The project site was the dumping ground for sawdust from the nearby Yesler mill. The map on the right shows how much the downtown Seattle shoreline has changed in the last 150 years.
Next Steps: Geotechnical, Seismic, and Engineering Problem Solving
To set the stage for building design and construction in this complicated subsurface, Aspect is conducting several geotechnical and environmental evaluations to develop design recommendations to solve the challenges posed by the historical fill and sawdust at the site. Our detailed evaluations include:
Conducting a site-specific seismic response analysis which includes determining dynamic properties of the sawdust to model its behavior under seismic loads
Conducting deep foundation design to transfer building loads to underlying competent soils
Evaluating and mitigating risk associated with environmental issues which are ubiquitous with the historic fill throughout the Pioneer Square area.
The Vision: Realizing More Affordable Workforce Housing for the Community
These evaluations will be critical to shepherd the development through the City of Seattle’s permitting process and help create more workforce housing in the area. Workforce housing aims to provide a more affordable rental option within high real estate cost areas for workers essential to the local economy, such as service workers, police officers, fire fighters, teachers, nurses, and medical personnel.
When completed, Pioneer Square will gain a new eight-story building with street-level commercial space below seven stories of residential workforce housing.