A Tale of Two Dams: Performance of Deep Cutoff Walls-Paper

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The paper reviews the construction and long-term performance of seepage cutoff walls at Beaver Dam in Arkansas and Walter F. George Lock and Dam in Georgia and Alabama.<br /><br />At Beaver Dam, seepage through karstic limestone began soon after the reservoir was filled in 1966. Grouting reduced but did not eliminate flows, which raised safety concerns. A 1992–94 remedial wall used overlapping secant piles drilled into hard, fractured limestone with large down-the-hole hammers, reaching depths of up to 185 feet. After completion, seepage fell from hundreds of gallons per minute to less than 4 gallons per minute. The wall has continued to perform well, including during record-high reservoir levels.<br /><br />At Walter F. George, seepage through limestone beneath the concrete powerhouse and spillway caused piping and substantial flows. A cutoff system completed in 2004 combined deep secant piles around the concrete structures with diaphragm walls through the embankments. Construction required work through water and rock, and included cutting slots through massive concrete structures to connect the walls. After completion, some piezometer levels dropped by more than 60 feet and have remained low. USACE later assessed the project as being in generally excellent condition.<br /><br />The authors conclude that both projects successfully controlled seepage over the long term. Their success depended on selecting construction methods suited to difficult site conditions and developing quality-control procedures for each project. The paper also notes that today’s more extensive quality-control requirements can make similar remediation work more complex, time-consuming, and costly.

Keywords

seepage cutoff walls

Beaver Dam

Walter F. George Lock and Dam

karstic limestone

secant piles

diaphragm walls

seepage control

grouting

piping

quality control