Dewatering behavior of fly ash slurry with woven geotextiles

Mahmoud M. Khachan, Shobha K. Bhatia

In the proceedings of: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Geosynthetics - Drainage and Soil Filtration

RÉSUMÉ: L'utilisation de tubes géotextiles pour assécher le contenu élevé des eaux des boues de cendre volant s'est avéré être une alternative viable aux méthodes de disposition des cendres volants traditionnel telles que l'élimination sèche et les bassins de décantation. Les résultats d™études ont montré que les propriétés hydrauliques géotextiles ont un effet mineur sur la formation de gâteau de filtration et la performance globale de déshydratation. Cependant, l'effet du mécanisme de la formation du gâteau de filtration ainsi que le conditionnement de polymère sur la perte du sol et la turbidité effluente n'ont pas été pleinement explorés. Dans cette étude, six géotextiles tissés avec différentes propriétés hydrauliques ont été évalués pour leur capacité à retenir les classe C de boues à cendres volant. Des tests de filtration de pression (PFT) ont été menés afin d'évaluer l'effet du mécanisme de formation du gâteau de filtration sur la turbidité effluente, le montant de la perte de sol, le taux d'assèchement, et la force de gâteau de filtre. Les résultats des tests ont montré que la turbidité et la perte du sol augmentent avec l'augmentation de l'AOS, mais aucune perte excessive du sol a été enregistrée pour tous les échantillons climatisés et inconditionné. Il a également été constaté que le conditionnement floculant améliore significativement la force de gâteau de filtre. 1 INTRODUCTION The current practice of fly ash disposal has increased environmental concerns due to the limitation of pond capacity (Pandian 2004) and possible ground water contamination (Mehra et al. 1998). Studies have shown that geotextile tubes can be used successfully to dewater wet fly ash pumped from ponds. Geotextile tubes are made of high strength, woven geotextiles. For this application, geotextile tubes should be permeable enough to efficiently drain water, yet tight enough to retain solids within the tube. Geotextile tube dewatering performance is generally evaluated by its ability to retain the solids, and based on the clarity of the effluent. Since the soil loss amount and effluent clarity are function of compatibility between the dewatered solids and the hydraulic properties of the geotextile, the effect of the geotextiles™ hydraulic properties should be carefully considered (Kutay and Aydilek 2004). Several researchers have studied the use of woven and nonwoven geotextiles for dewatering fly ash. Kutay and Aydilek (2004) evaluated the retention of class F fly ash (d85 = 0.075 mm) with four woven (Two monofilament, one slit film and one fibrillated fiber) and four nonwoven geotextiles using filter press tests with external applied pressure of 27 kPa, and hanging bag tests. The geotextiles apparent opening sizes (AOS) ranged from 0.15 to 0.6 mm, with permittivity values ranging from 0.05/s to 0.4/s. Filter press tests were performed with 80%, 200%, and 500% water content slurries (45%, 33%, and 16% solids content respectively). While hanging bag tests were conducted at 1600% water content (6% solids content). The tests results indicated that the amount of piping (solids loss) increases with increasing geotextile permittivity and initial slurry water content. The loss of soil through the geotextile ranged from 0.1 to 10 % of the initial solids content. Tests results also showed that the dewatering performance is governed by the filter cake properties rather than the pore opening size of the geotextile. Additionally, Kutay and Aydilek studied the formation of the filter cake during the dewatering process; Tests results showed that the time for filter cake formation for all slurry/gotextiles combinations were similar (~3 minutes). On the other hand, for hanging bag tests, excessive amounts of piping through woven geotextiles were observed, being 45% of the initial poured slurry amount. However, the issue of high piping amount was reduced significantly to about 5% when woven and nonwoven geotextiles were used combined in the same hanging bad test. Kutay and Aydilek also compared the soil loss amounts to several the available soil filter criteria. They found that the existing filter criteria failed to predict

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