Effect of Wet–Dry Cycles and Cation Exchange on Gas Permeability of Geosynthetic Clay Liners Abdelmalek Bouazza 1 ; Thaveesak Vangpaisal 2 ; and Stephan Jefferis 3 Abstract: A series of gas permeability tests were performed on a partially hydrated needle punched geosynthetic clay liner GCL after exposure to wet–dry cycles and ion exchange. To be able to evaluate the effect of wetting and drying cycles combined with the effect of cation exchange, three sets of GCL samples were prepared with different types of hydrating liquid. The first set of GCL samples was hydrated with de-ionized water, which formed a baseline test series. The second and third sets were hydrated with 0.0125 and 0.125 M calcium chloride CaCl 2  solutions, respectively. All three sets of GCL samples were subjected to multiple wetting and drying cycles before undergoing gas permeability tests. Gas permeability of the GCL, hydrated with 0.0125 M calcium chloride solution, was found to be approximately one order of magnitude higher than that of the GCL hydrated with de-ionized water, whereas gas breakthrough flow was observed for all but the first wetting cycle on GCLs hydrated with the stronger 0.125 M calcium chloride solution. DOI: 10.1061/ASCE1090-02412006132:81011 CE Database subject headings: Gas; Permeability; Linings; Clays; Moisture; Geosynthetics. Introduction Covers over solid waste landfills are typically multicomponent systems that are constructed directly on top of the waste shortly after a specific cell has been filled to capacity. In this respect, a cover is a key engineered component of a landfill. A conventional approach to cover system design is to construct a “resistive bar- rier” that utilizes a liner with a low saturated hydraulic conduc- tivity to reduce the water ingress into the landfill and to control biogas escape to the atmosphere. Geosynthetic clay liners GCLs are now widely used in landfill covers as the resistive barrier as an alternative to soil barriers Bouazza 2002. GCLs are com- prised of a thin layer of sodium bentonite glued to a geomem- brane or sandwiched between two geotextiles. Geotextiles-based GCLs are bonded with an adhesive, needle punched, or stitch bonded, with the bentonite contained by the geotextiles on both sides. For the geomembrane-supported GCL, the bentonite is bonded to the geomembrane using a nonpolluting adhesive and a thin open weave spun-bound geotextile is adhered to the bentonite for protection purposes during installation. The resistive barrier is normally required to maintain a low hydraulic conductivity during the lifetime of the cover system. While this is a relatively simple approach, engineers may encoun- ter a number of practical problems with this barrier. For example, the integrity of the resistive barrier may be severely compromised if it undergoes cracking due to desiccation caused by environmen- tal drying. Seasonal variations in precipitation and evaporation can lead to severe desiccation of the resistive barrier, depending on the materials used, and may result in high gas flux rates through the cover soil. This can lead to potentially serious prob- lem by creating vegetation stresses or diebacks and contamination of surface waters, in addition to posing a safety and health risk. In the case of GCLs, it has been established that the sodium bento- nite component of GCLs provides an excellent self-healing ability to seal desiccation cracks when in contact with water containing low concentrations of salts, i.e., GCLs can undergo several wet– dry cycles and maintain their low hydraulic conductivity Shan and Daniel 1991; Boardman and Daniel 1996; Lin and Benson 2000. Although it is documented that the self-healing capacity of sodium bentonite GCLs is high, experimental evidence shows that this capacity can be impaired if the self-healing process is coupled with cation exchange. For example, if divalent cations such as calcium or magnesium Ca 2+ , Mg 2+  or trivalent cations are present in the infiltrating water, there can be an exchange of these cations for the monovalent sodium cation Na +  initially present on the bentonite of the GCL. This can cause irreversible damage to the bentonite resulting in a functional failure of the GCL Dobras and Elzeas 1993; James et al. 1997; Melchior 1997; Lin and Benson 2000. Studies where the effect of cation ex- change on hydraulic conductivity of GCLs was indirectly taken into account include investigations on the importance of the first hydrating liquid Didier and Comeaga 1997; Gleason et al. 1997; Petrov et al. 1997; Ruhl and Daniel 1997; Shackelford et al. 2000; Jo et al. 2001, 2004; Kolstad et al. 2004. A GCL hydrated with clean water low ionic concentration and then permeated with high ionic strength solutions for example CaCl 2 , NaCl, acid or base solutions tends to maintain a low hydraulic conductivity, whereas a nonprehydrated GCL usually has higher hydraulic con- 1 Associate Professor, Dept. of Civil Engineering, Monash Univ., Building 60, Melbourne, Vic. 3800, Australia corresponding author. E-mail: malek.bouazza@eng.monash.edu.au 2 Assistant Professor, Ubon Ratchathani Univ., P.O. Box 3, Warin Chamrap, Ubon Ratchathani 34190, Thailand. E-mail: thaveesak.v@ ubu.ac.th 3 Professor in Civil Engineering, School of Engineering, Univ. of Surrey, Guildford, Surrey GU2 7XH, U.K.; Director, Environmental Geotechnics Limited, Oxford, U.K. E-mail: egl@ environmentalgeotechnics.com Note. Discussion open until January 1, 2007. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and pos- sible publication on April 8, 2005; approved on March 7, 2006. This paper is part of the Journal of Geotechnical and Geoenvironmental Engineering, Vol. 132, No. 8, August 1, 2006. ©ASCE, ISSN 1090- 0241/2006/8-1011–1018/$25.00. JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / AUGUST 2006 / 1011 J. Geotech. Geoenviron. Eng. 2006.132:1011-1018. Downloaded from ascelibrary.org by Monash University on 11/19/14. Copyright ASCE. For personal use only; all rights reserved.