Watch Those Pressures! Closing Cutoff Walls at Normal Pool

This paper presents two case histories of cutoff wall installations with a reservoir in place. During installation, pressures can increase significantly near the diminishing gap in the cutoff wall as the cutoff elements are installed. Potential mitigation techniques include temporarily lowering the pool or placing a berm against the downstream slope to offset the higher pressures.
The first case history is the installation of a secant-type, drilled shaft cutoff wall at Wolf Creek Dam, in southeast Kentucky. This project is owned and operated by the Nashville District of the USACE and has a long history of foundation issues and seepage problems. In 1975, a diaphragm wall was installed to control seepage. However, subsequent instrumentation readings and seepage observations indicated additional seepage controls were needed. In 2009, the Corps installed a secant pile, barrier wall that extended deeper into the foundation rock than the 1975 wall. The secant wall consisted of 50-inch diameter drilled shafts installed at a spacing of 35 inches. As the barrier wall construction neared the spillway, downstream piezometers indicated higher pressures. However, once the gap in the secant wall closed, the higher downstream pressures stabilized.
The other case history was the rehabilitation to an ODNR dams at Portage Lakes State Park. East Reservoir Dam exhibited severe spillway deficiencies. Overtopping solutions were selected for the dam and Tetra Tech designed an innovative soil-cement gravity overtopping structure to be used at the project. The soil-cement gravity structure was designed to both withstand an overtopping event as well as remain stable at full pool with the downstream portions of the existing embankments washed away. A critical piece to the design was a cutoff wall at the heel of the gravity structure. This cutoff wall consisted of overlapping 48-inch diameter soil-cement shafts at 24-inch centers that extended 30 feet below the heel of the gravity section.
At East Reservoir, the cutoff wall closed at a location where a downstream roadway embankment buttressed the downstream slope of the dam. However, significant seepage was observed near the toe of the roadway embankment and a sandbag ring was employed to offset the seepage pressures.