Evaluation of Final Cover Systems and the Importance of a Geocomposite Layer on the Predicted Performance

Four candidate final cover systems were evaluated for closure of a Phosphogypsum stack. The desktop study was to evaluate the system that best meets the objectives of the final cover system, which is to isolate the gypsum material, support vegetative growth, control erosion, have good stability, and be constructible within fiscal restraints. The first candidate system was the base case that used a 24-inch thick layer of silty lean clay soil. The other three included different geosynthetic layers underneath the cover layer, including a textured high density polyethylene geomembrane (HDTGM) layer only, a geocomposite drainage layer overlying a HDTGM, and a geocomposite drainage layer only. The hydrologic performance was evaluated using the HELP model, which provided insight to the level of hydrostatic head building up in the outer cover soil and the quantity of moisture infiltrating into the underlying gypsum. The stability was then evaluated with an infinite slope model, including the level of hydrostatic head that was predicted by the HELP modeling. The two systems that had the best predicted performance were the two that included the geocomposite drainage layer and the preferred system was that having only a geocomposite drainage layer beneath the cover soil.