Plans are underway to correct seepage problems around Center Hill Dam. It's a project that may take several years to complete and cost more than $220 million dollars.
Officials of the U.S. Army Corps of Engineers say Center Hill Dam was designed and built in the 1940's in a karst limestone foundation that is prone to seepage. Seepage is the movement of water through and under a dam. All dams have some seepage as the impounded water seeks paths of least resistance through the dam and its foundation. Seepage must, however, be controlled in both the velocity and quantity to keep the dam safe. Seepage can clearly be seen coming through the rock bluff formation near the dam.
Original designers recognized the nature of the rock and the potential for seepage through the relatively thin rim sections and acknowledged that maintenance would be required to keep the dam safe. The Corps has closely monitored seepage since the 1960's and has completed several limited grouting programs in an attempt to slow and control the seepage. Grout is a mixture of sand, cement, and water that can be placed into drilled holes into the foundation soil and rock to close any openings.
Foundation conditions continue to slowly worsen because clay filled joints are eroding in the rock within the rims and dam foundation. If untreated, this erosion would eventually jeopardize the two earthen embankments (main dam and saddle dam) and the integrity of the rims. The Nashville District of the Corps of Engineers has recently received Washington approval to begin a major rehabilitation project to ensure the long term safety of the dam.
The approved rehabilitation plan includes modern concrete cut off walls constructed within the entire length of the main dam and saddle dam embankments. These walls will extend deep into the rock foundation to effectively cut off seepage through the embankments and therefore protect the earthen portion of the dams from internal erosion. The approved plan also includes placing balance stabilized grout (durable and long-lasting grout) beneath the entire dam and along both sides of the dam.
Fish depend on continuous cold water, which the seepage has historically provided. The Corps plans to replace the cold seepage downstream by replacing a small undependable hydropower unit in the powerhouse. The existing 55 year unit was intended to provide back-up power to start the larger turbines. It will be replaced by a new 2 megawatt unit that will produce hydropower and provide the optimal minimum flow downstream.
The total cost of the rehabilitation plan will likely exceed $220 million. The Corps expects major work will begin in 2007 and the entire rehabilitation will take 5-8 years to complete.
Plans to accelerate the work are being considered. Ongoing work includes design and base mapping. Drilling into the dam foundation for rock information has begun. Initial grouting is planned to begin in the summer of 2007.
This grout will make the dam safer by filling voids within the foundation. Cut off wall construction will likely follow the grouting and is anticipated to begin in 2008.