Fern Hill Water Treatment PlantThe Fern Hill Water Treatment Plant provides drinking water to the cities of Hillsboro, Beaverton, Forest Grove and the Tualatin Valley Water District (Joint Water Commission). Due to regional growth, the Commission increased the capacity of the plant by 20 million gallons per day. Cornforth Consultants provided geotechnical design for the expansion.
The plant expansion consisted of an additional surge basin, a new filter building, and an additional sedimentation basin. These new facilities were founded on loose, granular soil near the Tualatin River - material that could potentially liquefy during an earthquake.
Cornforth's investigation included drilling, sampling and testing of loose cohesionless silt. Analyses were performed to evaluate liquefaction-induced settlement and lateral spread. Recommendations were provided for dry, bottom-feed stone columns to improve ground conditions and minimize settlement. Design recommendations were also provided for allowable bearing capacity, site grading, temporary shoring and dewatering, and lateral earth-pressures. Cornforth also provided full-time construction observation during installation of the stone columns, and part-time Quality Assurance/Quality Control for foundation excavation and preparation.