Water Power Magazine Features Priest Rapids Dam Project

The Priest Rapids Dam is one of two dams on the Columbia River near Mattawa, Washington, which form the 1980MW Priest Rapids Hydroelectric Project. The Priest Rapids development consists of a 3080m long dam constructed between 1956 and 1961. The dam consists of left and right embankment sections, left and right concrete gravity dam sections, left and right fish passage structures with upstream fish ladders, a gated spillway section, and a powerhouse containing ten vertical Kaplan turbine-generator units with a total rated capacity of 855MW.

Article by Stantec's Hannah Reybrock and Thomas Andrews.

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"Sloping RCC placement:
The general contractor, IMCO Construction, used the sloping RCC placement method which reduced the length and size of the individual RCC placements. This reduced the area of cold joints that would need to be treated whenever RCC construction stopped – which needed to occur daily since work hours were limited to minimize disruptions to the Wanapum village. Such a reduction in cold joint treatment area increased the time available each day to place and compact RCC.

Drilling into an existing embankment:
The construction of the plastic concrete secant pile wall necessitated drilling multiple 3m diameter holes through the impervious core and filter zones of an existing embankment dam with an impounded reservoir. Due to the criticality of the pile drilling, the holes for the secant piles were fully cased and the casing was advanced ahead of the bottom of excavation to minimise disturbance. Water levels were monitored to maintain an outward gradient to ensure that the piles were installed without compromising the integrity of the embankment. Additionally, the secant pile specialty contractor, Malcom Drilling, prepared contingency measures – such as stockpiling material for emergency backfilling of a hole – to address events that could impact quality of the secant pile construction or dam safety.

The outcome
The Grant PUD took immediate action upon learning of the increased seismic risk to the Priest Rapids Dam and downstream areas, including the Wanapum village, and the Priest Rapids Right Embankment Improvement Project is designed to withstand a large earthquake and prevent an uncontrolled release of water, proactively addressing the seismic risks. Grant PUD also worked to mitigate impacts on the Wanapum village during construction."