When Kentucky’s Wolf Creek Dam was built in the 1940s, no one knew the extent of the troubles that lurked below the surface. Lacking the sophisticated tools and equipment that today allow for detailed evaluations of foundation conditions, Wolf Creek Dam’s original designers and builders felt certain that the cutoff trench that was installed into the bedrock foundation would be enough to prevent seepage below the dam. They were wrong.
About 15 years after construction of the Wolf Creek Dam was completed, serious reservoir seepage problems were discovered near the downstream toe of the embankment. The dam’s deteriorating karst limestone foundation kicked off a series of actions by the U.S. Army Corps of Engineers. The most recent project – which the USACE hopes to be a permanent solution – is a US$594 million (total cost) remediation project to cut off the seepage.
Dams on Karst Foundations
At Wolf Creek Dam, the limestone foundation has been dissolving over the years due to carbonic acid found naturally in underground water at a level well below the original trench. However, Wolf Creek Dam is far from the only dam affected by a pervious foundation and the problem isn’t just in the U.S. Dams throughout Canada are also built on troublesome karst limestone foundations, and have exhibited failures to varying degrees through the years.
The original Shikwamkwa Dam near Wawa in northern Ontario, for example, was built in 1958 on a highly pervious foundation. Soon after impoundment, seepage flows increased, sinkholes formed in the head pond and fine grained materials were discharged – all signs of a developing foundation problem. The dam was monitored and phased repairs planned, but it was eventually concluded that such fixes were no match for Mother Nature. The limestone foundation continued deteriorating, and in 2003, a decision was made to build a replacement dam downstream at a location where it was possible to seal the foundation adequately.
Other Canadian dams that have struggled with karstic foundations include the Stewartville Dam in Ontario, the Grand Rapids Dam in the Saskatchewan River Delta and the Arnprior Dam on the Madawaska River in Southern Ontario.
The current fix to Kentucky’s Wolf Creek Dam seeks to stop the seepage with the placement of a new 277-foot-deep, 3,800-foot-long barrier wall combined with a unique pile system of secant piles and rectangular elements and an unique quality control plan that will stop the seepage and greatly extend the life of the structure.
Sturdy Structure, Faulty Foundation
The Wolf Creek Dam structure itself has posed no problems. The western end of the 5,736-foot-long dam is a 1,796-foot-long gravity structure built across the Cumberland River. The rest of the structure is a compacted clay embankment. At ultimate capacity in flood conditions, it holds six million acre-ft of water. U.S. Highway 127 runs across the top of the dam.
Foundation troubles began in 1962 with the discovery of the downstream seepage. The issue worsened, and crews pumped in nearly 290,000 cubic yards of pressurized grout to temporarily stop the leaks as they studied more permanent solutions. It was found that the original foundation trench was designed to cut through the alluvial deposits above the limestone and 50 feet into the rock, but it didn’t reach far enough to do the job.
In 1976, the Corps recommended construction of a $50 million concrete cutoff wall. After significant debate, it was decided that a concrete diaphragm wall would be placed through the dam into underlying rock to about 250 feet, but only for two-thirds of the overall embankment length. The wall was built of 26-inch diametre, steel-encased primary piles and concrete secant piles, but their placement wasn’t precise enough to form a sufficient seal. Less than 20 years after the wall was complete, seepage once again appeared downstream of the dam. Lake Cumberland was lowered in 2007, and has since been maintained some 40 feet below its original 723 foot level to reduce the stress on the dam.
Check back next week for Pt. 2 of the Wolf Creek Dam Project