The Walter F George lock and dam – constructed on the Chattahoochee River, between Georgia and Alabama between 1955 and 1963 – has suffered excessive seepage since it was first impounded.
The lock and dam consists of a 460m-long concrete structure housing the spillway, a non-overflow portion, the four generating units with a capacity of 150MW, and two 1800m-long earthen wing dams.
Previous efforts to stop the leakage have been only partially successful and in any case they did not address the main cause of the problem; that is seepage below the concrete structure, not through it.
The decision to construct a deep, “positive” cut-off, upstream of the main structure was taken when it became clear that this situation compromised the generating potential of the dam as well as potentially jeopardizing its stability.
The project is the first time the Corps of Engineers has commissioned a cut-off wall in front of an active dam. It is also unusually deep – up to 70m below lakebed level and with a water depth of more than 30m, the job is one of the most challenging underwater cut-offs ever attempted.
Furthermore the 130km-long lake created by the lock and dam harbours a 5000-hectare wildlife refuge, home to several bird species and a population of approximately 1,000 alligators. Contractors invited to tender for the cut-off had to propose construction methods that respected this delicate environment.