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The long-term filtration behavior of six nonwoven geotextiles with fly ash from two different power plants was investigated. Five specimen preparation techniques were evaluated: Dry Method, ASTM Method (D 5101-90), Modified ASTM Method, Slurry Method, and Vibro-Prep. ASTM Procedure was modified. Vibro-Prep is a new technique to prepare specimens at higher density to simulate filtration behavior of nonwoven geotextiles in the field. In addition, a method was developed for testing of geotextiles under high hydraulic gradient (HHG). HHG method is an expedient technique to assess the long-term filtration behavior of geotextiles. The results of this investigation indicated that method of sample placement, molding water content, and weight percentage of free lime (CaO) in fly ash were important parameters contributing to the filtration behavior of nonwoven geotextiles with fly ash. Clogging mechanisms were observed to be related to the flow patterns across the fly ash-geotextile systems. It was concluded that maximum change in filtration behavior of geotextiles takes place during initial stages of flow. Flow patterns across the fly ash-geotextile systems were related to the filtration mechanisms controlling the flow regimes. A new stability limit for fly ash-geotextile systems was proposed. Relevance of the existing soil-geotextile filtration criteria was evaluated for use with fly ash material. New filtration criteria were developed. A field project was completed at Fort Martin Power Plant, WV to demonstrate the applicability of this technology in field conditions. Geotextile used in the test cell retained fly ash and provided adequate permeability. A model was developed to simulate filtration behavior of geotextiles with fly ash. The results of the model compared well with the data obtained from the long-term filtration testing of geotextiles with fly ash in the laboratory.