| Planning & projects
a cofferdam for the connection with the right embankment or, alternatively, needing to construct the new dam along the entire 995m length of right embankment. The existing embankment dam will remain in place.
To avoid collecting stagnant water between the two dams, drains through the RCC dam with slide gates were designed to allow for the discharge of water that otherwise could collect between the two dams.
Unique features The Priest Rapids Right Embankment Improvement
Project encountered unique challenges during the construction of a new dam alongside the existing embankment’s toe. Construction staging requirements were developed and assessed to confirm that the new dam’s construction – especially the excavations near the existing embankment – would not impact the stability of the existing dam or disrupt normal dam operations. Detailed stability analyses confirmed that the excavation maintained a safe setback from the existing dam’s toe.
Potential Failure Modes Analysis (PFMA): A project specific PFMA workshop was conducted prior to construction to evaluate the construction staging requirements, provisions to provide filtered exits for seepage entering the excavation, curtain grouting and foundation preparation measures, RCC and embankment construction, secant pile wall construction, and dam safety instrumentation.
Minimising impact on the Wanapum village: Given the project’s immediate proximity to the Wanapum village, early and regular consultation occurred during design and construction. Measures were implemented to minimise impacts such as noise, vibration, and dust. Access to and from the village was thoughtfully maintained during construction, with pathways provided to the reservoir and traditional gathering areas. To respect Wanapum religious and cultural practices, construction activities were limited to daytime hours. The use of roller compacted concrete (RCC) instead of an embankment dam reduced the excavation size and minimised construction impacts on the Wanapum village.
Excavation discoveries: During construction, the excavation for the new dam uncovered two shear zones associated with inactive strike-slip faults, along with several vesicular cylinders in the basalt foundation rock. The vesicular cylinders consisted of vertical concentrations of vesicles that were vapor induced when lava flows encountered surface water, and basalt within and immediately around the vesicular cylinders was typically highly weathered. The planned construction staging allowed sufficient time for investigation and treatment of these unexpected features without major disruptions to the project schedule.
Geologic mapping and foundation treatment: Geologic mapping of the dam foundation was performed using geographic information system (GIS) tools. Field data collection included rock descriptions and discontinuity information, along with annotated photographs. The GIS database facilitated remote
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review by FERC, streamlining foundation approvals. To address irregularities in the bedrock surface, leveling concrete was applied, creating a suitable area for the lower lifts of roller compacted concrete (RCC).
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.
Above: RCC placement
Below: Plastic concrete secant pile wall construction
Below: RCC construction
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