The U.S. Army Corps of Engineers Nashville District announced last week that its contractor, Bauer Foundation Corporation, is set to begin constructing a subsurface concrete wall to keep Center Hill Dam safe for generations to come.A "First Bite" ceremony was held at noon last Wednesday, with initial excavation into the earthen portion of the main dam.The much-anticipated work is the culmination of years of evaluation, design and preparatory efforts.The $106 million contract was awarded to Bauer in late 2011.Project Manager Linda Adcock said the excavation is a precursor to the construction of a concrete wall which will extend 100 feet underground.“The main dam barrier wall is the major feature of the dam rehabilitation" said Adcock. "A vertical concrete wall will extend down over 300 feet beneath the top of the dam, which includes over 100 feet into rock.”Bauer's specialty excavation equipment includes a BG-50 rotary drilling rig, which has an operating weight of more than 250 tons and a mast that can be extended to an overall height of well over 100 feet, is designed to drill more than 300 feet deep, with bore diameters as large as 14 feet.The machine will remove long columns of the earthen dam and rock foundation, which will be replaced with concrete.The concrete columns will overlap to form a long, continuous concrete wall, which will act as a barrier for potentially harmful seepage water moving beneath the earthen dam.Excavated columns are either steel cased or held open with a slurry liquid, which also transports cuttings to the surface for removal.“The equipment you will see on the dam supports, guides and operates the cutting tools working deep beneath the surface,” said Project Manager Bjoern Hoffman.The wall construction is expected to continue for the next two years.Total cost of the project is estimated at $295 million, with about $140 million spent to date on investigations, design and construction.The Corps currently manages Center Hill Lake levels targeting a summer high of 630 feet above mean sea level and a winter pool of about 620 feet; however, day to day lake levels are highly dependent on the weather.These target elevations are 10-15 feet lower than normal and are part of risk management until the repairs are complete in late 2015.Highway 96 will be restricted to one lane of traffic across the dam for the duration of the project, and an automated control system with a maximum wait time of 5 minutes will manage traffic.The restriction is necessary to enlarge work space on the dam to support the installation of the foundation barrier wall.Along with the lane closure, drivers can expect occasional traffic delays along Highway 96 as large equipment is transported to and from the dam.Adcock said that while temporarily inconvenient, the traffic restrictions are necessary for the completion of the project.“We regret the inconvenience to the public; however, this barrier wall, constructed by Bauer Foundation Corporation, is the major protective feature to keep the earthen portion of the dam safe for many years to come,” She shared.The seepage rehabilitation plan is a combination of grouting, completed between 2008 and 2010, and construction of a continuous concrete barrier wall for long-term stability, which is being installed from 2012 through 2014.“A vertical concrete wall, at least two-feet thick, will be constructed through the earthen dam and deep into the rock foundation below to prevent seepage from harming the earthen dam,” Adcock said.The grouting has filled voids and soil-filled openings in the rock foundation and left rim southwest of the dam.More than 1.5 million gallons of grout have been 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, preparing the rock foundation for construction of the permanent concrete barrier wall.The seepage stems from the type of karst limestone rock surrounding the foundation of the dam when it was constructed in the late 1940s.A study is underway to determine the optimum repair plan for the final phase, the earthen saddle dam, built to fill a low area about 1,500 feet east of the main dam. The study is expected to be concluded by the end of 2012.