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Evaluation and Design Considerations of Geocomposite Leachate Collection Layers for Coal Ash Landfills

The paper presents results of laboratory and field tests evaluating the application of specialty filters and drainage geonet composites as leachate collection layers in landfills containing coal combustion residuals (CCRs). Different types of CCR materials (stabilized FGD, gypsum and fly ash) from coal-fired power facilities were used. Laboratory tests included gradient ratio tests (ASTM D5101) and hydraulic conductivity ratio tests (ASTM D5567). The paper also presents results of field test pads constructed to compare the performance of leachate collection system alternatives; systems including a geocomposite with the specialty filter to the systems using bottom ash as the leachate collection drainage layer. Measurements included field flow rates, leachate total suspended solids (TSS), total dissolved solids (TDS), and PH values. Results from these Laboratory and field performance tests provided the basis for the use of the drainage geocomposites in actual projects. The paper then presents design considerations for a geocomposite leachate collection layer and the proper use of US EPA HELP model for the leachate quantity and hydraulic head calculations. The paper concludes with a recent project case history.