Cement-Bentonite Slurry Walls for Seismic Containment of the Kingston Coal Ash Landfill

TVA recently capped a coal ash landfill at the Kingston Power Plant. Cement-bentonite slurry walls were built around the two-mile (three-kilometer) circumference of the facility. The landfill contains roughly 18 million cubic yards (14 million cubic meters) of coal fly ash, including material recovered after the 2008 dike failure at the site. Constructed on the footprint of the failed facility, the new landfill must survive a 2,500-year seismic event. The subsurface, perimeter retaining wall system was designed to stabilize the landfill slopes and contain the stored ash in an earthquake that triggers soil liquefaction. The wall layout consists of evenly spaced shear walls, oriented perpendicular to the landfill perimeter, plus circumferential walls in critical segments. Stantec designed the walls using complex, 2D dynamic computer simulations and 3D structural stress analyses. Deep mixing methods were assumed in the bid package, but prospective contractors were encouraged to propose alternate construction technologies. The winning contractor (Geo-Con, now Geo-Solutions) successfully built the walls using cement-bentonite, slurry trench methods. Over 11 miles (18 kilometers) of wall were constructed, requiring over 520,000 cubic yards (400,000 cubic meters) of slurry. Challenges during construction included characterization of achieved wall strength, mitigation of soil inclusions in the slurry walls, treatment of cold construction joints, soft working conditions on top of the old ash deposits, and collapse of trenches in areas with high groundwater levels.