The U.S. Army Corps of Engineers Nashville District today (Wednesday) announced 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 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. Bauer and Corps employees and staff, and media attended the event.
The $106 million contract was awarded to Bauer in late 2011. "The main dam barrier wall is the major feature of the dam rehabilitation" said Project Manager Linda 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 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 acting as a barrier for potentially harmful seepage water moving beneath the earthen dam. Equipment will vary based on the material to be removed and depth of the excavation. 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 Bjoern Hoffman, Project Manager for Bauer Foundation Corporation. The wall construction will continue over the next two years.
Highway 96/141 over Center Hill Dam is now restricted to one lane. The restriction is necessary to enlarge work space on the dam to support the next construction phase of the Center Hill Dam Foundation Remediation Project, installation of the foundation barrier wall. During the two-year duration, one lane of Highway 96 will be closed and an automated traffic control system will safely manage vehicles across the dam. The maximum wait time is five minutes.
"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," said Adcock. In addition to lane closure, motorists can expect occasional traffic delays along Highway 96 as large equipment is transported to the site.
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," said Adcock.
Awarded in March 2008, the grouting filled voids and soil-filled openings in the rock foundation and left rim to southwest of the dam. More than one-and one-half 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 according to Adcock, and preparing the rock foundation for construction of the permanent concrete barrier wall.
The Corps identified the seepage problem through long-term dam monitoring, which stems from the type of karst limestone rock surrounding the foundation of the dam when constructed in the late 1940s. A study is underway to determine the optium 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.
Total cost of the project is estimated at $295 million, which 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.