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Prevent Off-Site Migration of Contaminated Groundwater

Aurora, NC: Soil-Bentonite Slurry Wall, Cooling Pond


Geo-Con was contracted to install a groundwater cutoff wall, by the bentonite slurry method, to preclude the migration of phosphoric-acid/water from a cooling pond. The slurry wall was to tie into an existing slurry wall to create a continuous low permeability barrier around the pond.

The cutoff wall was installed along the southwest bank of the existing cooling pond and extends though several permeable zones in the soil strata, keying into the Upper Yorktown Clay Formation. Geo-Con installed 205,000 vertical square feet of slurry wall to an average depth of 78 feet, with a maximum depth of 80 feet below ground surface. The wall was excavated using a Komatsu PC1250 LC-8 excavator equipped with Geo-Con's custom long-reach boom and stick attachment.

Geo-Con completed a comprehensive sampling and mix design program to identify and optimize the slurry wall backfill mix. Full depth soil samples were obtained from six boring locations along the proposed slurry wall alignment. Geo-Con evaluated the soil samples and created several candidate design mixes to identify specific zones of soils that were beneficial or detrimental to the overall permeability of the mix, select the best borrow material and to minimize dry bentonite addition rates

To achieve the ultra-low permeability requirement of 1x10-7 cm/sec or less, without the use of "dry" bentonite addition, Geo-Con supplemented the soil-bentonite backfill with clay borrow. The clay was available nearby from overburden removal operations in the active mining area. Sandy and organic trench spoils were selectively segregated and replaced by the clay borrow to produce a low permeability backfill mix.

The new slurry wall was intended to tie directly into a previously installed slurry wall to provide a continuous barrier around the pond. Geo-Con developed means and methods to locate the existing wall and create a watertight tie-in with the new wall.

Two sets of high-voltage electric transmission lines paralleled the slurry wall alignment. The locations of the lines and the power poles created limited workspace and obstacles for spoils handling and backfill mixing. Geo-Con was able to maintain a safe 20-foot distance from the lines at all times during the work and complete the project on time, within budget, and without any Health and Safety incidents.


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