Washington Park Reservoir Replacement, Portland, Oregon
The City of Portland Water Bureau (PWB) recently replaced open water reservoirs No. 3 and No. 4 at Washington Park with 12 million gallons of buried storage. The project required designing a concrete tank located at the toe of a massive, slow-moving landslide such that the tank would not “feel” landslide loads during its 100-year design life. Seismic displacement of the landslide during a Cascadia Subduction Zone earthquake was a key factor for designing foundations, embankments, retaining walls, and the buried reservoir. As the Geotechnical Engineer of Record, CCI-LT worked closely PWB and the Project Delivery Team to complete the geotechnical design for this $205M project.

​Our team developed a 3D geologic model of the site using existing geologic and geotechnical data, as-built drawings from the 1890’s, and site-specific geologic structure mapping. The geologic model enabled the team to strategically locate borings for the subsurface exploration program and landslide investigation. The project required extensive coordination and permitting efforts with numerous City agencies and the Historic Landmarks Commission to obtain access for drilling. The subsurface investigation included extensive materials sampling and testing as well as instrumentation installation to characterize the landslide and foundation conditions. Downhole geophysical tests and joint set analysis were performed to measure in-situ properties of geologic units. Laboratory testing was completed to characterize shear strength and compressibility of overburden materials. 
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Aerial photo of Washington Park Reservoir, showcasing the ancient landslide and how it could affect the surrounding area.
Project Highlights
One key issue controlling the project design was the large, existing landslide uphill from the reservoirs. CCI-LT developed conceptual options ranging from a compressible inclusion that would “absorb” landslide movements, to large-diameter shear piles that would structurally stabilize the landslide. The compressible inclusion selected by PWB is a unique concept that saved the project millions of dollars compared to structural solutions. Seismic deformation analyses were completed to ensure the compressible inclusion could absorb normal landslide movement as well as movement resulting from a Cascadia Subduction Zone event. The 12-million-gallon reservoir is supported on 4- to 6-foot diameter drilled shafts, and the excavation for the tank is retained by 1,000 feet of 60-foot-tall tied-back walls. MSE retaining walls up to 55 feet tall were designed for other portions at the site. A series of four, small diameter tunnels were advanced below the existing concrete dam to accommodate outlet piping. 
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3D model showcasing the ground anchors and the innovative compressive technology to create a state of the art reservoir and reflecting pond.
Owner: City of Portland (PWB)
​More Information: Portland.gov - Washington Park Reservoir Project Overview