Abstract:
The Priest Rapids Right Embankment Improvement Project involved replacement of the existing right earthen dam embankment with a Roller Compacted Concrete (RCC) structure in order to improve seepage cutoff and seismic resiliency. A secant pile cutoff wall was required in order to connect the core of the existing earth embankment with the new RCC structure that was built downstream as the earthen embankment remained in service with a full reservoir pool in place during construction.
A 270 foot long secant pile cutoff wall was chosen as the solution by the project engineer, stipulating that the wall must provide a continuous minimum 5 foot wall thickness and a minimum of 5 feet embedment into intact underlying basalt formation with unconfined compressive strengths in excess of 30,000 psi. Full depth temporary casing was required during pile construction to protect the dam’s integrity. Plastic concrete composed of cement, hydrated bentonite slurry, fine, and coarse aggregates was batched on site and provided a low strength and impermeable matrix with properties similar to the existing embankment core.
Selection of pile sizing and spacing was left to the drilling subcontractor. Malcolm elected to utilize the oscillator method, placing 10 foot in diameter piles on center-to-center spacings of 7.5 feet in order to minimize the total amount of piles and joints, while staying within the targeted .5% verticality limit. Installing drilled shafts in an active dam embankment required continuous monitoring of excavated materials and water levels to ensure dam stability. Sonic Caliper equipment was used to verify the verticality of  each pile, with results modeled in REVIT 3-D to monitor overlap and wall thickness in real time. The Shaft Inspection Device (SID) was utilized to monitor shaft base cleanliness and inspection of in-situ rock conditions.