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GEOMEMBRANES OVER GCLS: THE OPTIMAL COMBINATION FOR BARRIERS AGAINST CONTAMINANT TRANSPORT
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The equivalence of composite liners involving a geomembrane (GMB) and a geosynthetic clay liner (GCL) to regulatory composite liners with a GMB and a compacted clay liner (CCL) can offer greater environmental protection to the underlying aquifer. It is suggested that GCLs and GMBs can play a very beneficial role in providing environmental protection even though GCLs are altered by their environment due to cation exchange and wet-dry cycles or there are defects in the GMB. The performance of GMB-GCL composite liners is accessed in terms of diffusion of contaminants and in terms of advective transfer due to the presence of defects in GMBs. Experimental, numerical and empirical quantification of advective transfers are examined through single GMBs and GCLs and are compared to GMB-CCL composite liners included in the case of aged GCLs.
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The aim of this paper is to present empirical equations developed to calculate advective flow rates through composite liners for hydraulic and environmental applications. The situation studied is that of geomembranes (GMB) involving circular holes combined with aged geosynthetic clay liners (GCLs). Indeed, the alteration by the environment consisting in cation exchange and extreme climatic conditions (i.e wet-dry cycling) is investigated in this paper. The development of these empirical equations is based on a GMB-GCL contact condition obtained from laboratory experiments, in the case of composite liners involving altered GCLs, in which, the hydraulic conductivity is larger than 10−10 m/s. Empirical equations were developed for circular holes with diameters in two different ranges of 2 to 20 mm and 100 to 600 mm. Results show that empirical equations developed in this study are different from previous equations using virgin GCL-GM contact condition for the different kinds of defects studied in the range of parameters studied. To analyse these differences, a comparative study was undertaken in order to review the accuracy between new empirical equa-tions and conventional analytical solutions. Results show that the new equations are much closer to analytical solutions than conventional equations which shows an improvement of the estimation of the flow rate by tak-ing into account a realistic set of conditions
A laboratory investigation of the interface transmissivity is reported for five different geosynthetic clay liners (GCLs) and a range of different geomembranes (GMBs) for a range of stresses from 10 to 150 kPa. The GCLs were prehydrated under normal stress before permeation. The GCLs examined comprised three multicomponent (a smooth coated, a smooth laminated, and textured coated) and two conventional (one with granular and one with powdered sodium bentonite) GCLs. The effect of a 4 mm circular defect in the coating of a multicomponent GCL directly below the 10 mm diameter hole in the GMB is investigated. The effect of GMB stiffness and texture is examined. Additionally, the effect of hydration and permeation of smooth coated GCL with highly saline solution and synthetic landfill leachate (SL3) is presented. It is shown that the 2-week interface transmissivity (θ 2-week) can be one to two orders of magnitude higher than steady-state interface transmissivity (θ steady-state) at low stresses (10 kPa-50 kPa), whereas at high stresses (150 kPa) the variation is substantially less. For a smooth coated GCL hydrated and permeated with reverse osmosis (RO) water, GMB stiffness and texture has a limited effect on interface transmissivity when the coating is placed in contact with GMB at normal stresses of 10 kPa-150 kPa, whereas coating indentations result in much high interface transmissivity when placed in contact with GMB. GCL prehydration and permeation with highly saline solutions leads to higher interface transmissivity compared to RO water. With a 4.0 mm defect in the coating, the interface transmissivity between the coating and woven geotextile is higher than that between the coating and GMB for the stress levels and GCL examined.
This paper, which is based on an Invited Lecture for the 7th International Conference on Environmental Geotechnics, gives an updated overview of the properties of transfer of geosynthetic liner materials used in environmental applications. To begin, the water-retention curves of geosynthetic clay liners (GCLs) are discussed, with the focus being on the high temperatures that can be encountered and the concomitant risk of desiccation. Next, an overview is given of quantifying advective transfer through intact geomembranes (virgin or after exposure on site) and through multicomponent GCLs. Experimental quantification of advective transfer through composite liners is also addressed, whereby geomembranes or the film or coating of a multicomponent GCL is damaged. Finally, based on a literature review including the most recent data, the discussion turns to the diffusion of organic and inorganic species through virgin and aged geomembranes and GCLs. The synopsis of the most recent data presented here in terms of elementary transfer mechanisms, either advective or diffusive, should contribute to improving the quantification of transfer through barrier systems. These four topics were selected as they correspond to the fields of expertise of the co-authors in which they have been publishing in the past 20 years.