The Underwater Cut-Off at the Water F. George Dam-PDF

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The Walter F. George lock and dam on the Chattahoochee River experienced serious seepage soon after its construction in 1955–1963. Sinkholes, boils, and springs prompted decades of remedial work, including relief wells, grouting, filter trenches, and a cut-off wall beneath parts of the earth embankments. These measures were only partly effective: seepage became concentrated beneath the concrete dam, increasing erosion and threatening the dam’s stability and power generation.<br /><br />In 2001, the U.S. Army Corps of Engineers contracted for a deep cut-off wall in front of the concrete structure. The wall had to extend through water up to 90 feet deep, cross an existing lock and submerged retaining wall, and reach an impervious geological formation. Ground conditions included very hard rock, loose sand, cavities, fractures, and debris.<br /><br />The selected construction approach combined underwater secant piles with land-based slurry-wall panels. Before construction, crews drilled exploratory holes and grouted major cavities. They cleared debris from the lakebed and built a working apron, then used templates and temporary casings to guide the underwater piles. Reverse-circulation drilling used lake water as the drilling fluid, while spoil was managed with silt curtains. Hydromill equipment excavated the land sections and crossed the lock structure; specially planned excavation and concrete-replacement steps connected the slurry wall to the secant piles. A concrete cap beam tied the completed wall to the dam.<br /><br />The paper presents the project as the first recorded cut-off wall constructed under deep water, upstream of an existing dam and beneath an existing concrete lock. Its successful completion demonstrated a new approach to challenging underwater seepage-control and marine foundation projects.

Keywords

Walter F. George Dam

Chattahoochee River

seepage control

cut-off wall

secant piles

slurry wall

sinkholes

grouting

underwater construction

U.S. Army Corps of Engineers