The U.S. Army Corps of Engineers Nashville District has announced that the seepage rehabilitation project at Center Hill Dam will soon be transitioning from deep foundation grouting to constructing a foundation barrier wall in the earthen portion of the dam.
Awarded in March 2008, the grouting contract was the first major project of the seepage rehabilitation effort. The grouting filled voids and soil-filled openings in the rock foundation and prepared for the safe construction of a concrete barrier wall. More than 1.5 million gallons of grout have been successfully placed in the rock foundation along the 800-foot-long earthen dam, 2,700-foot-long left rim and 700 feet downstream of the earthen dam, making the dam safer. Now that the grouting project is complete, proposals are being evaluated for the formation of that barrier wall to ensure long-term stability of the earthen dam, according to Project Manager Linda Adcock. "The scope of the rehabilitation of the foundation is primarily two fold. The first step, which has been completed, is deep foundation grouting. Grout is a flowable concrete. It's a very thin concrete mix. The contract that we recently completed which took about three years from 2008 to 2011 pumped 1.5 million gallons of grout into the rock foundation of Center Hill. It was done through the earthen portion of the dam down into the rock foundation below and about 3,000 feet beyond the dam, if you're looking downstream to the left. That's why we've excavated the rock on the left. The large excavation that you see is actually a working platform from which we were able to move the grouting equipment along that left rim and pump the grout into the ground. This grout fills the holes in the rock and seals the foundation and reduces seepage. The dam is actually safer now because of the grouting," said Adcock.
The district anticipates awarding the 2.5-year-long contract by the end of May to construct the permanent seepage barrier for the earthen dam's foundation, according to Adcock. "The second step is the real permanent feature of the fix and that is a concrete cutoff wall. We have gone out for proposals from contractors. These are very specialty contractors that do this work building deep foundation cut off walls and we are in the process of evaluating proposals from contractors. We hope to have a contract awarded by the end of May. The cutoff wall primarily is a minimum two foot thick concrete wall which will be placed down through the earthen embankment and then down into the rock for the length of the earthen embankment which is about 800 feet long. Its that earthen embankment that is really the most vulnerable for seepage and the movement of material which is called piping. When seepage becomes piping it is a serious condition. We do know that we have seepage but we have not seen piping of material in the earthen embankment but we're being very pro-active with this rehabilitation program. The grouting has made the dam safer but it is a temporary fix. Grouting typically doesn't last much longer than ten to fifteen years so the concrete cutoff or barrier wall is the long term solution to make the earthen portion of the dam safe," said Adcock.
Seepage has been occurring at Center Hill dam since it was built but Adcock said measuring devices indicated an increase in the level of leakage a few years ago, prompting a study and a request by the Corps for funding to fix it. "The reason we're going to spend seven to eight years and hundreds of millions of dollars is because the dam has a history of foundation seepage problems. This is because of the type of geology that the dam is founded in and the way it was constructed back in the 1940's and early 1950's. The seepage problems could cause the dam to have problems. The earthen portion specifically. So we are rehabilitating the foundation to keep the earthen portion of the dam safe. This seepage has been going on since the dam was built in 1952. We have monitored it over the years. We have instruments throughout the dam. We have measuring devices that tell us if the water is moving through the dam. Some water moving through dams is normal. All dams leak but that leakage must be maintained at a low level. When it starts to increase then that's when we need to pay specific attention. We saw signs that the seepage was increasing. We completed a study in 2006. It got approved at our Washington level and we had funding for the rehabilitation starting in 2007," said Adcock
According to Adcock, the Corps has no plans to do any grouting underneath the concrete portion of the dam because the original grout curtain is still holding, "Our original plan was also to grout beneath the concrete dam but we have done additional investigations since we first started and there was a grout curtain put in during the 1940's as part of the original construction. We have evidence that this grout curtain is still doing its job. Its still working under the concrete portion of the dam. The concrete portion is not part of the dam that we are concerned with seepage and piping because it can't pipe or be pulled away by the water moving under it," said Adcock.
Original plans were also to stop the seepage around the right side of the dam, but Adcock said after further studies, the Corps has decided against trying to plug that leak. "Originally we talked at our public meeting about trying to shut that off. We have since done a lot of investigation of that rock and the seepage. That seepage moves quickly around the right end of the dam. We know that from some chemical analysis of the water. We do not believe that this rock can fail the dam catastrophically. We will monitor that seepage but we do not plan to shut it off. Its just normal seepage around the dam, through the rock and around the concrete. It's not dangerous. It does provide a good oxygenated flow into the Caney Fork River for the trout and other habitat. I know the fishermen like it. You often see fishermen around that waterfall," said Adcock
After the barrier wall is completed by 2013, Adcock said the third and final phase will begin. "It will likely be for a general contractor to do some cleanup work and regrading. We would like to leave Eisenhower Park, which we had to close due to all of this construction, and put in a public access there and some parking and restroom facilities so that the public can access the lake again at the dam.,"concluded Adcock
The total project cost is $295-million.