PRIEST RAPIDS DAM - A LARGE DIAMETER SECANT PILE CUTOFF WALL UTILIZING PLASTIC CONCRETE-PPT Presentation

PPT Presentation

Pdf Summary

The Priest Rapids Dam project addressed seepage and seismic-safety concerns in the dam’s right-bank earthen embankment. Studies found that native soils beneath parts of the embankment could liquefy during a larger-than-anticipated earthquake, creating a credible risk of failure and uncontrolled water release.<br /><br />Because the dam had to remain operational and a cofferdam was impractical, a new roller-compacted-concrete dam was built downstream. A connecting embankment with a large-diameter secant-pile core linked it to the existing dam. The wall used unreinforced, low-strength plastic concrete intended to provide a low-permeability cutoff while tolerating movement. It comprised 36 piles, approximately 9.84 feet in diameter, embedded into basalt.<br /><br />Construction began with test piles, which identified challenges seating casing in uneven basalt and cleaning shafts. The team revised procedures, including advancing casing into competent rock and using hydraulic dredge pumps to remove sediment. Production required three equipment spreads; dense, abrasive basalt caused substantial tooling wear.<br /><br />Developing a suitable plastic-concrete mix was a major challenge. Initial laboratory mixes did not meet the desired permeability and elastic properties. Trial batching refined the bentonite-water ratio and mix proportions. The resulting concrete met key fresh-mix targets, but had limited flowability and a working time of about three hours, requiring careful coordination during placement.<br /><br />Quality control included camera inspections of every pile, verticality measurements, and a live 3-D as-built model to verify wall overlap and thickness. Thirty-five of 36 piles met the project’s tighter 0.5% verticality target; the remaining pile measured 0.68%. Coring proved difficult: specified core methods yielded no recovery because the plastic concrete was weak and contained large aggregate. Despite repeated attempts and alternative methods, core quality remained poor, complicating confirmation of pile and joint integrity.

Keywords

Priest Rapids Dam

seepage

seismic safety

earthen embankment

liquefaction

roller-compacted concrete

secant-pile core

plastic concrete

basalt foundation

quality control