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January 27, 2020

National finalist: Gold award
Waste and stormwater

Carollo Engineers

Photo by Erik Holsather/SkySolutions NW
Geotechnical issues added complexity to Oak Harbor’s clean water facility project.

Project: Oak Harbor clean water facility
Client: City of Oak Harbor

In 2010, the city of Oak Harbor was facing big challenges. The 50-year-old plant where its wastewater was being treated had reached the end of its useful life. The outfall had failed, and all the city’s wastewater was being pumped to a lagoon treatment plant east of town that the city shared with the Navy. This plant’s lagoon treatment process did not accommodate future water quality standards or the city’s projected growth. Changes needed to be made.

Carollo Engineers was hired by the city to lead a design process that would result in a new clean water facility as well as significant improvements to the immediately adjacent Windjammer Park. The project team reviewed more than 11 locations, oversaw numerous design workshops and overcame a number of technical challenges. The team delivered the project with a level of complexity not typically tackled by communities the size of Oak Harbor.

The engineering team faced significant geotechnical issues and a saltwater challenge from Puget Sound. A slough that had been filled with dredged soils passed through the project site, creating significant groundwater issues.

The project’s location in a seismically active region complicated issues further. A significant portion of the secondary treatment and headworks buildings needed to be located in a deep excavation site that was subject to significant uplift forced from tidally influenced brackish groundwater.

The team responded with the use of micropiles to mitigate seismic-induced settlement and flotation forces. In addition, stone column ground improvements were recommended and installed for the slab-on-grade structures.

Still, the risk of liquefaction of unconsolidated materials in the event of an earthquake remained. A French drain piping system coupled with geotextile fabrics was installed to mitigate this, along with drainage blankets, gravel layers and pressure-relief valves to allow excess pore water to exit the areas without damaging equipment.

To cut off tidally influenced groundwater that could potentially threaten the lower gallery of the Secondary Treatment Building, the team researched and employed special concrete admixtures for leakage and corrosion in addition to the use of traditional PVC waterstop.

The clean water facility was delivered to the city of Oak Harbor in a dedication ceremony in November 2018, fulfilling a journey that had begun nine years earlier. Windjammer Park opened on June 29.


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