Fern Hill Water Treatment Plant Liquefaction Study
Forest Grove, Oregon
The 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 increased demand, the Commission planned to increase the capacity of the plant by 20 million gallons per day. Cornforth Consultants (CCI) were retained as part of a multi-disciplinary design team for the expansion study.
The plant expansion consisted of an additional surge basin, a new filter building, and an additional sedimentation basin. The new facilities would be founded on loose, cohesionless silt near the Tualatin River - material that could potentially liquefy during an earthquake.
Cornforth's investigation included drilling and sampling, assessing maximum credible earthquake magnitude and potential ground acceleration, cyclic triaxial tests of undisturbed silt samples and engineering analyses to estimate potential ground movement (lateral spread and settlement). Cyclic shear strengths were compared with estimated seismic shear stresses to calculate the factor of safety against liquefaction. Post-liquefaction slope stability analyses, lateral spreading evaluation, and estimation of settlement due to liquefaction were also performed.
Stone columns installed through the granular soil were the most cost-effective measure for improving the foundation conditions. The columns increase the strength of the foundation and create a path for groundwater to flow out of liquefied soil. CCI also provided construction inspection services during the installation of over 1,700 stone columns, as well as QA/QC during construction of other structure foundations.