Evaluation of water and gas transport in layered soil covers for coal ash deposits: 2d-calculations and impact of a weak zone

Abstract

A methodology for evaluating the efficiency of a soil cover to reduce water infiltration and gas transport has been adapted for coal ash deposits and two dimensions. A cover consisting of two layers, a tight layer and a protecting layer, is studied. Studied design parameters are primarily the slope and the quality of the tight layer. Simulations show that the slope has a minor impact, while the hydraulic conductivity of the tight layer is shown to be of major importance. The calculations indicate that the hydraulic conductivity of the tight layer should be smaller than 10⁻⁸ m s⁻¹. However, for longer covers a lower hydraulic conductivity gives overflow indicating that a better lateral drainage must be provided for. This may be obtained by inserting a drainage layer. A negative side-effect, however, is that gas transport may increase due to the lower saturation of the cover. The impact of a high conductivity zone in the tight layer is illustrated with 3D calculations. For example, a weak zone covering 0.5% of the area with a hydraulic conductivity of 10⁻⁷ m s⁻¹ (10⁻⁹ m s⁻¹ for the rest of the layer) will increase the total water flow through the bottom by about 25%. The gas transport in such a heterogeneous cover will increase totally by a factor of 1.2 and locally by a factor of 5.