Characterizing Hydraulic Properties of Coal Combustion Products (CCP) in Sluiced CCP Basins

Burning coal for electricity produces coal combustion products (CCPs) that are disposed in coal ash basins. To comply with current federal regulations, basins are being permanently closed in the United States by excavation or close-in place methods. Characterizing the hydraulic properties of CCPs is necessary to determine the rate of pore water drainage from the CCPs once the flow into the basin ceases, also known as drain down. Understanding how these basins behave is necessary to predict flow and chemical transport in CCP disposal facilities. Undisturbed and disturbed CCP samples from sluiced coal ash basins in the southeastern United States were analyzed in this study. The influence of sampling disturbance, sample preparation technique, and effective stress on the saturated hydraulic conductivity were evaluated. ASTM D5084 procedures were followed for saturated hydraulic conductivity testing for effective stresses ranging from 20 to 500 kPa ( 1 to 25 m below ground surface). Hydraulic conductivities of the CCPs ranged from 10-9 to 10-5 m/s, with the hydraulic conductivity decreasing approximately a factor of two as the effective stress was increased from 20 to 500 kPa. Particle size distribution, loss on ignition (LOI), and mineralogy were also determined for each CCP. Relationships between chemical and physical properties and hydraulic conductivity were analyzed. The CCPs were comprised predominantly of Al and Si, with slight variation between basins. Sample preparation methods affected the saturated hydraulic conductivity, with slurry consolidated and compacted CCPs being less permeable than intact undisturbed samples. Testing was conducted in oxic and anoxic environments to compare conditions commonly existing in the field (anoxic) to conditions commonly encountered in the laboratory (oxic, or oxygen present). The presence of oxygen (or not) had no influence on the saturated hydraulic conductivity.