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The US Army Corps of Engineers (ACE) is tackling major seepage problems under an active mass concrete gravity dam by installing a bored pile cut-off wall. It claims this is the first time such an operation has been carried out in the US.

Seepage has been a problem at the Walter F George dam, which sits on the Chattahoochee river on the border of Georgia and Alabama, ever since it was completed in 1964.

Previous efforts to stem the flow using cut-off walls in front of associated earth embankments in the 1980s were largely successful. But grouting under the dam itself had failed to halt the 1,895 litres per second flow.

Under a design and build contract, main contractor for ACE Treviicos-Rodio joint venture is using two barge mounted Wirth reverse circulation drilling rigs to install 450 piles, making up a 610mm thick secant piled wall in front of the dam wall and through the existing lock walls.

Geology consists of an 1.2m to 2.1m thick stratum of earthy to sandy limestone, underlain by 1.5m of unconsolidated sand. The cut-off wall is being installed to a depth of -5m.

Casings are checked against adjacent piles to ensure that a positive cut off is achieved.

Concrete is then placed using a tremie pipe to a level with the lake bed before the casing is removed.

Completion is due in September 2004.

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