Drilling of Clearwater Dam to start soon as part of $90 million repair

PIEDMONT, Mo. -- Starting early next month, contractors will begin repairs to a massive dam under federal control in Southeast Missouri to prevent a possible breach.

Contractors will begin drilling and grouting the bedrock underneath the massive Clearwater Dam in Piedmont, Mo. By the time repairs are finished, about $90.3 million will be spent in two different phases to strengthen the nearly 60-year-old dam's earthen walls.

The need for repairs at Clearwater became apparent in 2003, when a sinkhole the size of a car appeared on the 154-foot-high, 4,225-foot-long dam. Clearwater falls under the jurisdiction of the U.S. Army Corps of Engineers Little Rock District, which is responsible for inspecting the safety of the dam and ordering any repairs.

Engineers determined the hole was caused by the seepage of water into the bedrock -- something the repairs will not eliminate but greatly reduce.

"All dams have some degree of seepage where water seeps through bedrock and comes out somewhere downstream," said P.J. Spaul, a spokesman with the corps' Little Rock District. "Clearwater has always had more than its fair share of seepage because of the nature of the porous bedrock underneath."

The corps believes the sinkhole formed after a deluge of rain in May 2002 that caused the Black River south of the dam to overflow its banks. Clearwater Dam was built to control flooding on the Black River, protecting towns like Poplar Bluff, Mo., and Pocahontas, Ark.

When the hole was discovered, crews immediately went to work exploring the space under the dam and filling in the hole with compacted clay. A study concluded that the optimal solution would be to build a concrete cutoff dam under the earth beneath the earthen dam. Engineers expect the concrete to minimize the seepage that runs into the bedrock under the dam.

But first phase one must be completed, which will consist of exploratory drilling and injecting grout into the bedrock to fill up the cracks.

The wall will be completed in sections when phase two of the repairs start. Clearwater resident engineer John Wedgeworth said phase two won't be needed in the best-case scenario, but that won't be determined until the phase one exploration.

Wedgeworth said engineers should have some idea what phase two will require about six months into the $16 million first phase.

Even during repairs, Clearwater Dam will continue to function and protect the land along the Black River, said Spaul.

Since the discovery of the hole and the initial patchwork the operating plan of the dam has stayed largely intact, controlling water the same way the dam has in the past, said Spaul.

-- Matt Sanders