Planning & projects |
Rapid response to seismic risks
To address seismic risks at Priest Rapids Dam in the US, a downstream replacement RCC dam has been constructed
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. Public Utility District No. 2 of Grant County,
Above: Aerial view of the Priest Rapids Dam
Washington (Grant PUD) is the Federal Energy Regulatory Commission (FERC) licensee and owns and operates the dam. The Wanapum village, a permanent settlement of the Wanapum tribe of Native Americans, is located immediately downstream of the right embankment. A large section of the existing right embankment is founded on soils that could liquefy during a large earthquake. The Priest Rapids Right Embankment Improvement Project was undertaken as a seismic remediation measure, consisting of a new roller- compacted concrete (RCC) dam constructed directly downstream of the section of the original right embankment that lies on soils that could liquefy during a large earthquake. Beyond the liquefiable zone, the new RCC dam is connected back into the original embankment by a new zoned embankment with an internal plastic concrete secant pile cut-off wall.
Identifying the problem Recent seismic assessments indicated that the
Below: Drone shot of the project
Maximum Credible Earthquake (MCE) at the dam site is significantly larger than understood when the dam was designed in the 1950s. Following a Potential Failure Modes Analysis (PFMA) workshop that identified seismic stability concerns, Grant PUD engaged Stantec to conduct a multi-phase study to evaluate liquefaction potential, post-earthquake stability, and seismic deformation for the right embankment. The study began with analyses based on existing data and progressed to include phases of geotechnical investigation and more rigorous analyses, and ultimately determined that the foundation soils below a portion of the right embankment dam could liquefy if subjected to the MCE which could result in damage to or failure of the embankment dam. Since the Wanapum village is immediately downstream of the right embankment, Grant PUD decided to proceed with a remediation project to address the seismic risks, rather than further
28 | April 2024 |
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evaluating the dam using risk-informed decision- making methods.
The solution The new dam is designed to withstand an earthquake
with a moment magnitude of 7.1 and a peak horizontal bedrock acceleration of 0.64g, to retain the reservoir and prevent an uncontrolled release. The dam is 17m high and comprised of a 530m long RCC dam, along with a 53 m long zoned embankment between the upstream face of the new RCC dam and the downstream face of the original embankment. The plastic concrete secant pile cut-off wall in the connecting embankment is 18m deep and 81m long and constructed with 3m diameter piles socketed 1.5m into rock. The design of the RCC section utilised grout- enriched vibratable RCC (GEVR) for water-retaining barrier along the upstream face of the dam. Excess soil from the required excavation was used to create a buttress along the downstream side of the RCC dam. To save cost, an unformed sloping RCC face was used to construct the portions of the dam that would be covered by fill. Due to the buttress along the downstream side the dam, stability requirements were satisfied without needing to provide drains within the dam or its foundation. The connecting embankment between the downstream face of the existing right embankment and the upstream face of the new RCC dam was designed largely utilising soil from the required excavation and consists of an impervious core surrounded by filter zones and pervious shells. The plastic concrete secant pile was designed
to provide the water-retaining barrier between the RCC dam and the impervious core of the original embankment. This wall cuts off seepage that otherwise would flow through the pervious shell of the original embankment and its foundation soils. The plastic concrete mix for the secant piles was designed to have similar deformation properties as the embankment fills to minimize stress concentrations during seismic shaking – and thus reduce the possibility of the impervious core cracking – while also providing a durable non-erodible seepage barrier. The secant pile wall was constructed using 3m diameter piles with a centre-to-centre spacing of 2.2m to obtain a minimum wall thickness of 2m, and the wall was embedded at least 1.5m into the basalt bedrock.
By using the plastic concrete secant pile cut-off
wall to tie the new dam back into the impervious core of the original dam, the project avoided needing to either drawdown the reservoir or construct
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