Originally built in 1958, the Priest Rapids Dam is a composite structure with a center-channel spillway and powerhouse and a left and right embankment. Through proactive hazard identification modeling, the client discovered that the right abutment embankment dam was susceptible to failure under seismic events. This $60.9-million project aimed to provide a backup dam in the event the existing earthen embankment dam fails, ensuring the safety of the neighboring Wanapum Indian Tribal Village located immediately downstream and protecting lands used for traditional cultural practices. To mitigate impacts to the village during construction, the entire project was also subject to strict working hours.

Scope of work included a 55-ft-high by 1,785-ft-long roller-compacted concrete (RCC) dam constructed downstream of the existing right embankment. Beyond the liquefiable zone, the RCC dam is connected back into the existing embankment by a new 250-ft-long earthen embankment, further reinforced by a bentonite-infused plastic concrete secant pile cutoff wall drilled through the new and existing embankments. The wall is 60 ft deep and 300 ft long and composed of 10-ft-dia piles with 5-ft to 10-ft rock sockets.