Drilling to Begin on Center Hill Dam Repair

Drilling is set to begin around the Center Hill Dam in the next week as part of a massive rehabilitation project.

Water has been leaking around and under the 60 year-old dam through cracks in the limestone. The seepage has made those seams even bigger and put the entire dam at risk of failure, flooding cities and towns down stream on the Cumberland River.

The Corps of Engineers will drill more than a thousand holes the size of a tennis ball hundreds of feet down into the bedrock. A mix of water, sand and concrete – called grout – is forced down the holes to fill any caves and fissures.

Project manager Linda Adcock says the amount of grout it takes to fill the voids will tell the Corps how extensive the underground erosion has become.

“We do have an idea because of previous grouting programs, the condition of the rock. And the drilling and grouting will start to tell us. The foundation will start to talk.”

Grouting will take most of next year at a cost of 87-million dollars. The expensive part comes next. Engineers will build a concrete barrier inside the earthen embankments that flank the spillway of the dam.

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The largest part of this contract funds the installation of “grout curtains” that stretch 4,700 feet extending out from the dam. Along the same line, the Corps of Engineers put in grout lines 50 years ago, but not to this extent and not with today’s technology. Contractors have already cut through a hillside and laid a concrete driveway. The drilling rigs will put holes every 20 feet along the curtain line. Each hole takes at least 20 hours to drill, according to project managers. The rigs then go back and put holes every ten feet, then every five, and so on.

The grouting process is a science as well as an art form. Each hole is a bit different and requires a different viscosity in the grout mix. An entire office on the site houses the computers that control these machines. A hose first injects a thin grout so it goes in the smallest of cracks and gradually gets thicker. The process is complete when the pressurized grout stops moving. As a result, less water should find its way through the hillsides.