Oak Grove

The Oak Grove hydroelectric project stretches across 6 miles of mountainous terrain southeast of Estacada in the northern Oregon Cascade Mountains. Major structures that make up the project are Timothy Meadows Dam, Harriet Lake Dam, Frog Lake Dam, and a 49,000 kilowatt powerhouse.

As part of licensing requirements, Cornforth Consultants performed a Federal Energy Regulatory Commission (FERC) Part 12 Inspection for the owner, Portland General Electric. FERC also requested an evaluation of potential earthquake ground motions and their impact on the stability of the above mentioned structures

Along with visual inspection, large amounts of geotechnical information were reviewed including groundwater, embankment seepage and settlement history, and slope stability calculations. FERC's request for seismic evaluation necessitated the development of site specific ground motions for use in the analysis of the stability of the structures. For Timothy Meadows Dam, three exploratory borings were drilled and laboratory work was performed to determine if part of the dam was constructed with potentially liquefiable material. The stability analysis at Frog Lake Dam included an estimate of the possible movement of an extremely large ancient landslide due to an earthquake.