Grabhorn Reservoir Replacement Project

CLIENT Tualatin Valley Water District

Location Beaverton, OR

Tualatin Valley Water District constructed a new replacement reservoir to increase the reliability of the water system in the service area. The existing reservoir, constructed in 1971, was a structurally deficient concrete reservoir. 

The seismically resilient 5-million-gallon Grabhorn Reservoir replacement is just north of the existing reservoir and is at the same overflow elevation. It is a 132-foot-diameter, 42-foot-deep concrete reservoir built adjacent to an Aquifer Storage and Recovery pump station, access roadways, and a detention pond on site. Since the construction of the original reservoir, new housing developments have been constructed on the adjacent properties. Significant grading was required to bury the new reservoir at the elevation of the existing reservoir and maintain the required pressure zones.

Shannon & Wilson provided geotechnical engineering services to inform TVWD on the subsurface conditions, the geophysical seismic profile, and geotechnical analysis to support the design and construction of the new reservoir in accordance with the 2014 Oregon Structural Specialty Code and 2015 International Building Code. 

Shannon & Wilson, along with the contractor, provided recommendations to support the project in using as much of the native soil and rock as possible for backfill, reducing the need to haul and dispose of materials offsite. Due to the slope of the property, construction of the new reservoir required a temporary shoring system, which was originally included in the project cost. After further evaluation, however, Shannon & Wilson determined that the team could use both the south side of the old reservoir as a shoring wall and then rely on the temporary stability of near vertical rock cuts to stabilize the slopes during construction. Based on the initial estimate, reusing parts of the existing reservoir structure and eliminating the need for temporary shoring allowed TVWD to net a substantial cost savings of approximately $550,000 for the project. Shannon & Wilson provided expedited reports and recommendations for this progressive design-build project to support TVWD in maintaining the tight schedule. 

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