Alabama - United States

The U.S.Army Corps of Engineers is constructing one of the deepest secant walls in its history directly upstream from the Walter E George Lock and Dam, located on the Chattahoochee River near Fort Gaines, Georgia. The wall-which will take two years and $50 million to construct-is an attempt to eliminate seepage through a porous limestone formation beneath the dam.

With a minimum thickness of 2 ft (0.6 m), the wall will extend from a concrete projection at the base of the dam and pass through three distinct layers of limestone before reaching a depth of about 90 ft (27 m) below the lake bed. The wall will extend 1,830 ft (558 m) beyond the entire length of the concrete dam. The water in the lake  is more than 100 ft (30 m) deep.  Don Simpson, the resident engineer of  Mobile  district of the Corps, says the Corps has never before constructed a secant wall at such extreme depths.

The wall is being constructed as a series of overlapping drilled shafts, each 50 in. (1,250 mm) in diameter. In june crews began lowering a 54 in. (1,350 mm) diameter steel casing from a barge to depths of up to 140 ft (43 m) to points on the lake bed where the secant wall was to be constructed. At each point crews lower a drill bit to the bottom of the casing, then drill through the limestone until they reach a layer of sand approximately 90 ft (27 m) deep. During the drilling process the shaft is kept filled with water to maintain pressure on its interior. After the verticality of the shaft is checked with an inclinometer, crews fill the shaft with concrete. The concrete used to form this pile contains bentonite clay and has a strength of more than 1,600 psi (11,000 kPa).