If Nashville's Grand Ole Opry House flooded with 20 feet of water, the best seats in the house would be in the balcony.

That could happen if Wolf Creek Dam, near Jamestown, Ky., had a critical failure. Grand Ole Opry performers 275 miles downstream would have to be evacuated, and the estimated damages could run up to $6 billion. The risk of the dam's failure makes a $594-million remediation a top priority for the U.S. Army Corps of Engineers.

Work is now 74% complete as contractors fight seepage that is dissolving—or "solutioning"—the karst-limestone foundation under the dam. Remediation consists of building a 275-ft-deep, 3,800-ft-long concrete wall composed of secant piles and rectangular panels installed through the clay embankment and into the rock of the dam within a 5-in. tolerance.

A similar smaller-scale fix was attempted in 1976. This time, the difference is in the barrier wall's greater mass and depth as well as the materials and methodology used.

Along the shores of Lake Cumberland, residents above the dam are eager to see the Corps and its primary contractor, a joint venture of Soletanche Bachy, France, and Treviicos, Italy, succeed.

They want to see the lake—lowered in 2007 to reduce stress on the dam—return to the normal 723-ft level to revive tourism. But even with the pressure of economic need, the Corps says it cannot rush construction. "Dam safety is our top priority," says Kathleen E. Lust, the site's resident engineer for the Corps.

Looks Deceive

Original construction of Wolf Creek Dam finished in 1951, impounding Lake Cumberland and creating the biggest reservoir east of the Mississippi River. It holds six million acre-ft of water at ultimate capacity in flood conditions. The dam is more than a mile long and is composed of two sections: a 1,796-ft-long concrete spillway and a 3,940-ft-long compacted- clay embankment.

The concrete section contains ten 37 x 50-ft tainter gates, two non-overflow sections at each end and six low-level, 4 x 6-ft sluices. A power intake section with six penstocks feeds now-idle turbines with a combined output of 270 MW—with the potential to generate $70 million in hydroelectric power revenue annually. U.S. highway 127 traverses the top of the dam.

"The dam itself is in top condition," says Tommy Haskins, the Corps' geologist and technical manager. "They did a superior job [in 1976] on the embankment. If they hadn't done that, it would likely be gone.

"The problem here is in the limestone foundation and the depth and construction of the core, or cutoff trench," says Haskins. The trench follows a solutioning feature in the rock, he says. "The cutoff trench was not only ineffective, it serves as a conduit of seepage," says Haskins.

Geological Consequences

Haskins says the embankment is unusually well built because, instead of the usual clay core burdened with coarser materials, the entire embankment is clay, which is plentiful in the area. The karst-limestone foundation is layered into what geologists call Leipers and Catheys, which are two similar limestones that can be dissolved by carbonic acid found naturally in underground water. When sandwiched together, "there is an even worse erosional surface between them," he says.

"The problem wasn't recognized when the dam was built," says Michael F. Zoccola, the Corps' chief of dam safety for the project. "The thinking was, 'As long as it's on stone, we're all right.' Most of the engineering at that time was done by looking at the embankment itself—what's above rock."

The original foundation trench was designed to go through the alluvial deposits above the limestone and 50 ft into the rock, but "they didn't go deep enough," says Haskins. "They didn't intercept all the features in the rock." Many other dams are built the same way, he adds. "You can usually get away with it," he says, "but not in the case of Wolf Creek Dam." There, a minimum 150-ft head of water is held above the foundation in which the karst limestone is solutioning. "It was terrible," Haskins says.