The adoption of the United States Environmental Protection Agency's coal combustion residuals (CCR) regulations has stirred renewed interest in the stabilization/solidification (S/S) of fly ash residuals stored in landfills and surface impoundments. Soil mixing methods can be used to S/S a variety of materials to improve static slope stability, resistance to dynamic and static liquefaction, geotechnical properties for handling and movement, and to provide a stable surface upon which to construct a final cover for closure in place or to repurpose the site for other uses. While many geotechnical studies of fly ash reuse as a construction material have been conducted, studies of in situ S/S (ISS) of fly ash are limited. Further, most of the work to date has involved mixing lime or Portland cement with fly ash. While these conventional approaches can be successful, limited research has shown the potential for improved performance through the addition of sodium sulfate salt during mixing of fly ash with cement. The need for additional study of fly ash ISS is a direct result of the recently passed CCR regulations. In order to assess the feasibility of ISS, a study was designed and conducted to examine the use of ISS with lime and Portland cement to improve the ash properties in addition to assessing the potential benefits in ash improvement associated with using sodium sulfate salt in conjunction with Portland cement. Performance measures included unconfined compressive strength (UCS) and hydraulic conductivity. The results indicate that improved geotechnical performance can be achieved when sodium sulfate salt is a component of a cement based mix design for the ISS of fly ash using soil mixing methods.
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