Fern Hill

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 continued rapid growth in the area, the Commission planned to increase the capacity of the plant by 20 million gallons per day. Cornforth Consultants was selected as part of a design team for the study of the expansion.

The plant expansion consisted of an additional surge basin, a new filter building, and an additional sedimentation basin. These new facilities were to be founded on loose, granular soil near the Tualatin River - material that could potentially liquefy during an earthquake.

Cornforth's investigation included drilling and sampling, assessing maximum credible earthquake and potential ground acceleration, cyclic triaxial tests of "undisturbed" samples (to simulate an earthquake), and engineering analyses to estimate potential ground movement. Cyclic shear strengths were compared with the seismic shear stresses estimated by the computer program SHAKE 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.