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Performance of GCLs after long term wet-dry cycles under a defect in GMB in a landfill
Abstract
Two GCLs with sodium bentonite and three GCLs with polymer amended bentonite were subjected to wet-dry cycles selected to simulate the conditions to which a GCL on an 18 o slope might be subjected: for a GCL below an exposed geomembrane wrinkle with a hole. The wetting involved water flowing over the GCL for 8 hours each cycle. Three drying cycles (0.67, 7, and 14 days) were examined. After 12-18 months of wet-dry cycles, the samples were x-rayed to identify representative specimens for testing. The changes in the hydraulic conductivity, k, of the GCLs were obtained when permeated with two synthetic municipal solid waste leachates at an applied head of 0.35 m for a range of effective stresses (3 - 150 kPa). The results showed an up to a four order of magnitude difference in k depending on applied stress and RMD of the leachate permeant. The effects of the number and the duration of the wet-dry cycles, the GCL mass per unit area, presence/absence of polymer modification, the carrier geotextile, the number and the size of the needle punch bundles, and the bundle thermal treatment are discussed.
... GCLs typically comprise a layer of bentonite sandwiched between two geotextiles, which are joined together through needling, bonding, or sewing techniques. This construction effectively restricts the outward movement of contaminants, mitigating the potential pollution of subsurface areas [1][2][3]. The engineering performance needs to be provided by the bentonite after hydration and mainly controlled by its water saturation level [1,4,5]. ...
... Understanding the water retention performance of GCLs under various salt concentrations is crucial to evaluating their longterm serviceability. The water retention properties of GCLs directly influence their function to prevent the migration of contaminants and pollutants from landfills into the surrounding environment [1,2]. Therefore, comprehensive research is necessary to investigate the effect of different salt concentrations on the water retention behavior of GCLs. ...
... Equation (2) presents an approximate linear relationship between parameter 'a', which is related to the air entry value, and the osmotic suction for GCL in the drying path. Similar findings were reported by Karagunduz et al. [34]. ...
The water retention curve (WRC) of a geosynthetic clay liner (GCL) is influenced by the presence of exchangeable cations in the leachate during changes in water content in a landfill construction. This research aims to investigate the impact of salinity on the WRC of GCL. To measure the WRC of GCL under different sodium chloride (NaCl) concentrations on the drying path, a chilled-mirror dew-point device capable of controlling the GCL’s volume was employed. Additionally, the dry state microstructure of the GCL was examined using electron microscopy. The test outcomes indicate that GCL hydrated with higher salinity has greater suction at the same water content during drying. This influence can be attributed to changes in salinity and the precipitation of NaCl crystals within the bentonite when water evaporates, which in turn affects the bentonite’s microstructure and leads to increased matric suction. By introducing the Fredlund and Xing model and parameter relationship, it is possible to predict the WRC of GCL under salinity effects after measuring the WRC under different salinity conditions on the drying path.
... The combination of these two liners in composite systems capitalizes on the strengths of both materials, providing enhanced containment and durability in landfill applications. However, despite the effectiveness of these composite liners under ideal conditions, defects in the GM liner can occur due to various factors such as construction issues, material degradation, or external stresses such as temperature fluctuations and chemical exposure [17][18][19][20]. These defects play a critical role in altering pollutant migration dynamics by creating preferential pathways for contaminants to bypass the intended barriers. ...
This study presents an analytical model for two-dimensional pollutant transport within a three-layer composite liner system, which comprises a geomembrane (GM), a geosynthetic clay liner (GCL), and a soil liner (SL), with particular attention to defects in the geomembrane. The model integrates key processes such as convection, diffusion, adsorption, and degradation, offering a more accurate prediction of pollutant behavior. Through Laplace and Fourier transforms, pollutant concentration distributions are derived, providing a comprehensive view of pollutant migration in landfill settings. Verification against COMSOL 6.0 simulations underscores the model’s robustness. Results show that there is an optimal thickness for the SL that balances the effectiveness of pollutant containment and material usage, while higher diffusion coefficients and advection velocity accelerate migration. The degradation of organic pollutants reduces concentrations over time, especially with shorter half-lives. These findings not only improve the design of landfill liners but also support more sustainable waste management practices by reducing the risk of environmental contamination. This research contributes to the development of more effective, long-lasting landfill containment systems, enhancing sustainability in waste management infrastructure.
... By replacing conventional materials, GCLs help conserve resources, mitigating waste and associated risks. Their efficient installation, capac-ity for pollutant containment, and adherence to regulations all contribute to ecological conservation and the reinforcement of climate resilience [7][8][9][10][11][12][13][14]. However, when bentonite swelling is highly suppressed under aggressive liquid, the hydraulic conductivity can approach hydraulic conductivities like silty sand (~10 −6 m/s) [10,[14][15][16][17][18][19]. ...
Flow in an idealized bentonite polymer composite geosynthetic clay liner (BPC-GCL) containing bentonite comprising two idealized circular granules was simulated using a COMSOL hydrodynamic model. The effect of the polymer rheology properties, including viscosity, surface tension, and contact angle, on the hydraulic conductivity of BPC-GCLs was investigated. The results showed that the hydraulic conductivity of BPC-GCLs significantly decreased by 2–4 orders of magnitude with polymer loadings of 3.3%, 6.5%, and 9.8% compared to conventional geosynthetic clay liners (GCLs). The polymer rheology properties are critical to the residence time and the hydraulic conductivity of BPC-GCLs. The residence time increases with the viscosity, surface tension, and contact angle of polymer hydrogel. In the overall study, the hydraulic conductivities increased significantly from 2.80 × 10⁻⁹ m/s to 1.40 × 10⁻⁷ m/s when the residence time was insufficient. When the viscosity of the polymer hydrogel is 5000 Pa∙s, 1 × 10⁴ Pa∙s, and 1 × 10⁵ Pa∙s, the residence time of the polymer hydrogel in the domain of BPC-GCLs is 14 min, 23 min, and 169 min, respectively. When the surface tension of the polymer hydrogel is 0 N/m, 0.01 N/m, and 0.02 N/m, the residence time of the polymer hydrogel in the domain of BPC-GCLs is 9 min, 17 min, and 23 min, respectively. When the contact angle between the polymer hydrogel and the NaB granules is 30° to 60°, the residence time of the polymer hydrogel in the domain of BPC-GCLs is 9 min and 33 min. These few minutes can approximate the actual passage of several days in physical time. When the viscosity, the surface tension, and the contact angle are higher than 1 × 10⁶ Pa∙s, 0.03 N/m, and 60°, the residence time of the polymer hydrogel in the domain of BPC-GCLs tends to be very long, which means that a very low hydraulic conductivity of BPC-GCLs can be maintained in the very long term. This research unveils a nuanced and profound correlation between the rheological properties of the polymer hydrogel and the resulting hydraulic conductivity. This discovery enhances the understanding of the potential to tailor hydrogel characteristics for BPC-GCLs. The advanced model developed in this study also lays the groundwork for constructing a more realistic model that considers irregular geometries, interconnected pores, and diverse polymer distributions within the pore spaces.
In order to efficiently estimate the properties of landfill composite liners, equivalent analytical models based on time moment are proposed. The service performance of geomembrane layer (GMBL)/geosynthetic clay layer (GCL)/attenuation layer (AL) and GMBL/compacted clay layer (CCL)/AL are equivalent analyzed. The equivalent analysis and quantitative correlation of contaminants at the bottom of GMBL/GCL/AL and GMBL/CCL/AL are considered according to relative concentration (C N ), instantaneous flux (J I ) and accumulative flux (J A ). We showed that C N is the most conservative equivalent index. The thickness equivalent relation of AL for different composite liners is consequently obtained using the pure diffusion equivalent model and the advection-diffusion equivalent model, respectively. It is found that the pure diffusion thickness equivalent relationship was a simplified linear correlation and time independent. The quadratic polynomial equivalent equations of AL thickness for different service time are obtained by group method of data handling approach (GMDH). The results show that GCL composite liner has better environmental protection performance than CCL composite liner. When the thickness of GMBL/CCL/AL composite liner is fixed, the corresponding equivalent thickness of AL in GMBL/GCL/AL composite liner decreased with the rise of leachate head and wrinkle length for the same value of C N .
The state-of-the-art for barriers systems control of pollution migration and hydraulic structures including liner systems for landfills, mine waste, and dams/lagoons are discussed along with some comments on current significant environmental issues.
Polymer gels are porous fluid-saturated materials which can swell or shrink triggered by various stimuli. The swelling/shrinking-induced deformation can generate large stresses which may lead to the failure of the material. In the present research, a nonlinear stress-diffusion model is employed to investigate the stress and the deformation state arising in hydrated constrained polymer gels when subject to a varying chemical potential. Two different constraint configurations are taken into account: (i) elastic constraint along the thickness direction and (ii) plane elastic constraint. The first step entirely defines a compressed/tensed configuration. From there, an incremental chemo-mechanical analysis is presented. The derived model extends the classical linear poroelastic theory with respect to a prestressed configuration. Finally, the comparison between the analytical results obtained by the proposed model and a particular problem already discussed in literature for a stress-free gel membrane (one-dimensional test case) will highlight the relevance of the derived model.
The sensitivity of sodium bentonite (Na-B) to adverse chemical interactions has spurred development of enhanced bentonites (EBs) for geosynthetic clay liners (GCLs) that provide superior properties for containment systems. EB-GCLs are engineered to control contaminant transport by maintaining low hydraulic conductivity (k) when exposed to solutions with high ionic strength, a preponderance of divalent cations, and/or extreme pH (<2 and >12). An overview of current EB-GCL technologies is provided. Engineering properties, including k, the effective diffusion coefficient (D∗), and the membrane or chemico-osmotic efficiency coefficient (ω), are summarized for EBs and compared to properties of conventional Na-B. Applicability of indicator parameters currently used to assess GCLs containing Na-B (swell index, fluid loss, and liquid limit) is evaluated for EBs. Mechanisms proven or postulated to influence the behavior of EBs are presented and discussed. EBs generally have superior transport properties (lower k, lower D∗, higher ω) in elevated concentration solutions, although some bentonites amended with proprietary additives (broadly termed contaminant resistant clays, or CRCs) have been found to be similar or inferior to Na-B. Compatibility tests conducted with containment liquids are necessary to assess the transport properties of EB-GCLs for site-specific applications.
Experiments were conducted to evaluate the hydraulic conductivity of geosynthetic clay liners (GCLs) containing granular sodium bentonite that were permeated with coal combustion product (CCP) leachates. Chemical properties of the CCP leachates were selected from a nationwide survey of CCP disposal facilities. Five synthetic leachates were selected from this database to represent a range of CCP disposal facilities: typical CCP leachate (geometric mean of CCP chemistry), strongly divalent cation leachate (aka low-RMD leachate), flue gas desulfurization (FGD) residual leachate, high ionic strength ash leachate, and trona ash leachate. Typical GCLs from two U.S. manufacturers were used. Hydraulic conductivity tests were conducted on non-prehydrated and subgrade hydrated (by compacted soil for 60 days) GCL specimens at effective stresses ranging from 20 to 450 kPa. At 20 kPa, GCLs permeated directly had high hydraulic conductivity (>10−6 m/s) to trona leachate and moderate to high hydraulic conductivity (10−10 to 10−7 m/s) to the other CCP leachates. Hydraulic conductivity was strongly related to the ionic strength of the leachate and inversely related to the swell index of the bentonite when hydrated in leachate, as demonstrated in past studies on leachates from other waste streams. Increasing the effective stress from 20 to 450 kPa caused the hydraulic conductivity to decrease up to three orders of magnitude. Hydration on a subgrade prior to permeation has only modest impact on the hydraulic conductivity to CCP leachate. Hydration by permeation with deionized (DI) water prior to permeation with trona leachate resulted in hydraulic conductivity up to three orders of magnitude lower than obtained by direct permeation, suggesting that deliberate prehydration strategies may provide chemical resistance to CCP leachates.
Four geosynthetic clay liners (GCLs) serving as single liners were exhumed from below 0.7 m of silty sand on a 3:1 (horizontal:vertical) north-facing slope at the QUELTS site in Godfrey, Ontario, after 5 and 7 years. The 300 mm GCL overlaps with 0.4 kg/m supplemental bentonite were all physically intact. The exchangeable bound sodium was completely replaced with divalent cations. The GCL with the smallest needle-punched bundle size (average of 0.7 mm) and percentage area covered by bundles (4%) maintained low hydraulic conductivity (k) when tested under 0.07–1.2 m head with 10 mmol/L CaCl2 solution as the permeant. For GCLs with larger bundles (1.1–1.6 mm) and higher percentage area covered by bundles (9%–14%), k was low when the head was low (0.07 m). Once the applied head increased, k increased by 1–4 orders of magnitude depending on the (i) hydraulic gradient, (ii) size and number of the needle-punched bundles, and (iii) structure and mass of the bentonite per unit area. The results suggest that the GCLs can perform effectively as a single hydraulic barrier in covers providing that the head above the GCL is kept low (e.g., by a suitable drainage layer above the GCL).
The hydration of a needle-punched, thermally treated and powdered bentonite-based geosynthetic clay liner (GCL) from four different subsoils was studied at optimum moisture content +2% under isothermal conditions. Contrary to the belief that GCL hydration was strongly dependent on the percentage of clay-sized particles present in the subsoils, it was shown in this investigation that this dependency cannot be generalised to all subsoil types as the presence of smectite in the subsoils can substantially impact GCL water uptake. Smectite content of the subsoil has been found to enhance its water retention capacity, and therefore the relative amount of water available in the subsoil for hydration of the GCL was strongly dependent on the amount of smectite available in the subsoil (i.e., GCL water absorption from smectite-rich soils is impeded). The hydration process from subsoils dominated by smectite mineralogy was governed by the vapour phase, whereaswhen the smectite content was very low or nil the hydration process involved both vapour and liquid phases if the subgrade was at a water content close to optimum.
Tests were conducted to assess how permeation with actual recirculated leachate from municipal solid waste (MSW) landfills affects the hydraulic conductivity and exchange complex of geosynthetic clay liners (GCLs). Long-term hydraulic conductivity tests were conducted (similar to 1 to 5 years) on GCL specimens with conventional sodium bentonite using seven recirculated MSW leachates and one conventional (nonrecirculated) MSW leachate as permeant liquids. Effects of stress and temperature were also assessed. Hydraulic termination criteria were achieved for all tests, and chemical termination criteria were achieved for nearly all. The long-term hydraulic conductivity of the GCL specimens to recirculated leachates (1.0-2.0 x 10(11) m./s) was slightly less than the long-term hydraulic conductivity to the conventional leachate (2.0 x 10(-11) m/s). Furthermore, the ratio of the hydraulic conductivity to recirculated MSW leachate to the hydraulic conductivity to DI water for GCL specimens (1.4-2.9) fell within the range reported in other studies where GCLs were permeated with conventional MSW leachate (0.05-2.6). The long-term hydraulic conductivities to recirculated and conventional MSW leachate at 70 kPa were low (<2.0 x 10(-11) m/s) because of the relatively low ionic strength of the leachates (<249 mM) and the preponderance of monovalent cations in the leachate. Geosynthetic clay liner specimens tested at higher stress (70 versus 20 kPa) had lower hydraulic conductivity (2.3-3.0 times lower), which slowed mass delivery of exchangeable cations and thereby delayed exchange processes and corresponding changes in hydraulic conductivity. Gas production from biological activity can he minimized during leachate compatibility testing by reducing the temperature to 4 degrees C without compromising cation exchange processes and related effects on hydraulic conductivity. Leakage data from two landfills showed that leakage rates from composite liners with a GCL can be very low (<0.34 L/m(2)-year) when the landfill is operated with leachate recirculation. (C) 2015 American Society of Civil Engineers.
The performance of geosynthetic clay liners (GCLs) above arsenic-rich gold mine tailings was investigated by the construction of a test cover comprised of three GCL products placed above the tailings at an abandoned gold mine in Nova Scotia, Canada. In parallel research, the performance of the three commercially available GCLs was assessed based on a series of laboratory column experiments. Of the three needle-punched GCLs tested, one has untreated Wyoming bentonite and a woven carrier geotextile; the second has a polymer-enhanced Wyoming bentonite and a scrim-reinforced nonwoven carrier geotextile, and the third has a polymer-enhanced Wyoming bentonite and woven carrier geotextile coated by a thin polypropylene geofilm. After 4 years of exposure in the field and laboratory, the bentonite structure of all exhumed GCLs appeared to be well hydrated with no visible cracks provided that the cover soil thickness was > 0.7 m. The GCL with untreated bentonite exhibited the highest reduction in the exchangeable sodium percentage (ESP; decreased to 1-3% in field and 4-29% in laboratory) and swell index (decreased to 7-12 ml/ 2 g in both field and laboratory tests). Despite this reduction in the swelling capacity, the maximum measured hydraulic conductivity (k)for this GCL was 1 3 1010 m/s, which is still considered low. The GCLs with polymer-enhanced bentonite experienced less cation exchange (ESP 10-27% in the field and 17-53% in the laboratory) than the GCL with untreated bentonite. The k values of the GCL with polymer-enhanced bentonite and non-coated carrier geotextile (3.6 x 10(-11) to 7.9 x 10(-11) m/s in the field and 1.6 3 10 11 to 6.7 3 10 11 m/s in the laboratory) were lower than those measured for the GCL with untreated bentonite. The lowest measured k values (3.6 3 10(-12) to 9.4 3 1012 m/s) were for the GCL with polymer-enhanced bentonite and geofilm coating the carrier geotextile. All three GCLs prevented the migration of arsenic to the overlying cover soil, even when the GCL was resting directly above the tailings without a foundation layer.
Bentonite was modified to prevent alterations in hydraulic conductivity when permeated with aggressive inorganic solutions. Acrylic acid within bentonite slurry was polymerized to create a bentonite-polymer composite (BPC). Tests indicate that BPC generally swells more and retains low hydraulic conductivity compared with natural sodium bentonite (Na-bentonite) when contacted with aggressive inorganic solutions. BPC in deionized water swelled greater than 3.8 times the swell of the Na-bentonite used to create BPC (73 versus 19 mL/2 g). In 500 mM CaCl2, however, swell of BPC was similar to swell of calcium bentonite (<10 mL/2 g). Thin layers of BPC simulating geosynthetic clay liners were permeated directly with 5-500 mM calcium chloride (CaCl2) solutions and extreme pH solutions (1 M NaOH with pH 13.1, 1 M HNO3 with pH 0.3). BPC maintained low hydraulic conductivity (<8x10-11 m/s) for all solutions for the duration of testing (>2 years). In contrast, Na-bentonite and superabsorbent polymer (similar to the polymer in BPC) permeated with the same solutions had hydraulic conductivities at least three orders of magnitude higher (except for 5 mM CaCl2). Hydraulic conductivity of BPC does not follow the classical hydraulic conductivity-swell relationship typical of Na-bentonite. BPC eluted polymer during permeation but maintained low hydraulic conductivity. Polymer elution was lower in more concentrated CaCl2 solutions.
The influence of single-species salt solutions of various concentration, cation valence, and pH on swelling and hydraulic conductivity of nonprehydrated GCLs was examined. At similar concentration, swell was largest with NaCl, KCl, and LiCl solutions (monovalent cations Na+, K+, and Li+) and smallest with LaCl3 solutions (trivalent cation La3+). Intermediate swell volumes were obtained with divalent solutions (CaCl2, MgCl2, ZnCl2, and CuCl2). Analogous results were obtained from hydraulic conductivity tests. GCL specimens permeated with solutions containing divalent or trivalent cations had higher hydraulic conductivity than GCLs permeated with monovalent solutions or deionized water, unless the divalent or trivalent solutions were very dilute (less than or equal to0.01 M). Hydraulic conductivity increased as the concentration increased, and at high concentration (1 M) only small differences existed between hydraulic conductivities measured with all solutions. Swelling and hydraulic conductivity were related to size of the hydrated cation for monovalent cations, but no relationship was observed for different species of divalent and trivalent cations provided that the valence was the same. However, pH only influenced swelling and hydraulic conductivity when the pH was very low (<3) or very high (> 12).
Experiments were conducted to evaluate cation exchange during hydration of geosynthetic clay liners (GCLs) used in composite hydraulic barriers and the effect on their hydraulic conductivity. GCLs arranged in a composite barrier configuration were hydrated by contact with moist compacted subgrades (two clays, one silt, and one sand) under a confining stress of 10 kPa for 30 days to 1 year. No measurable exchange occurred in GCLs hydrated for 30 days. For hydration periods longer than 30 days, the exchange increased as the duration of hydration increased. The exchange during subgrade hydration had no measurable effect on the hydraulic conductivity to deionized (DI) water. However, if the GCL was desiccated after hydration, the hydraulic conductivity increased more than 1,000-fold. Dissolution of calcite within the bentonite during permeation with DI water also induced the replacement of sodium by calcium; however, this additional exchange had no measurable effect on the hydraulic conductivity to DI water. Data from two case histories indicate that calcium and/or magnesium in the subgrade, or in calcite within the GCL, eventually will replace nearly all sodium in GCLs used in composite barriers. The data also indicate that cover soil should be deployed expediently on composite barriers with GCLs to prevent wet-dry cycling and corresponding impacts on hydraulic conductivity. DOI:10.1061/(ASCE)GT.1943-5606.0000793. (C) 2013 American Society of Civil Engineers.
Tests were conducted to assess how permeation with synthetic and real municipal solid waste (MSW) leachate affects the hydraulic conductivity and exchange complex of geosynthetic clay liners (GCLs). GCLs were arranged in a composite barrier configuration for hydration on moist compacted subgrades for 30 or 90 days. After hydration, the geomembrane and subgrade soil were removed and the GCLs were permeated with typical and strong synthetic and real MSW leachates at different confining stresses for 342 to 1,281 days (4.9 to 148.9 pore volumes of flow). For most tests, hydraulic and chemical termination criteria were met (referred to as equilibrium), and in many cases the tests were conducted long after these criteria were achieved. Hydraulic conductivity of the GCLs increased no more than 5.6 times the hydraulic conductivity to deionized water (DIW), even though more than 80% of the sodium (Na) in the bentonite was replaced by other cations [predominantly calcium (Ca) and magnesium (Mg)]. Very long testing times can be required to reach equilibrium, especially at higher stresses. However, the combined effects of stress and cation exchange can be evaluated conservatively and more rapidly by first permeating the GCL to equilibrium at low stress, and then increasing the stress to simulate field conditions. Hydraulic conductivities obtained using this approach are no more than three times hydraulic conductivities obtained at equilibrium under elevated stress. Adding a requirement for equality of effluent and influent cation concentrations results in hydraulic conductivities to MSW leachate representative of long-term conditions.
Breakthrough time based design methods for determining the design thickness of landfill composite liners are proposed. The composite liners consist of either 1) a geomembrane (GMB) and a compacted clay liner (CCL), 2) a geomembrane (GMB), a geosynthetic clay liner (GCL) and a soil liner (SL), or 3) a GCL and a SL. The design methods are based on analytical solutions developed for solute advection and dispersion of leachate contaminants in composite liners. Both solute concentration and solute flux are considered in the analyses. Two dimensionless parameters involving contaminant concentration and contaminant flux are introduced and the design curves are proposed for breakthrough time based design of composite liners. The results obtained by the proposed analytical solutions are in good agreement with the experimental data published in the literature. Illustrative examples are presented to demonstrate the application of the design methods for composite liners consisting of a GMB and a compacted soil liner and those consisting of a GMB, a GCL, and a SL. The effect of leachate head on the design thickness of composite liners was investigated. It is shown that the design methods presented in this paper is easy to be used to design the landfill composite liners in different leachate head cases.
The differences in hydraulic conductivity for two geosynthetic clay liners (GCLs) containing different qualities of bentonite are evaluated based on permeation with water and chemical solutions containing 5, 10, 20, 50, 100, and 500 mM CaCl 2 . The GCL with the higher quality bentonite (GCL-HQB) is characterized by a greater content of sodium montmorillonite (86 versus 77%), a higher plasticity index (548 versus 393%), and a higher cation exchange capacity (93 versus 64 meq/ 100 g) relative to the GCL with the lower quality bentonite (GCL-LQB). The tests using CaCl 2 solutions as permeant liquids were continued until chemical equilibrium between the influent and effluent was established, resulting in test durations that ranged from less than 1 to more than 900 days. The hydraulic conductivity for GCL-HQB, k HQB , is 3 lower than the hydraulic conductivity for GCL-LQB, k LQB , when specimens of both GCLs are permeated with water. However, the hydraulic conductivity for GCL-HQB is always higher than that for GCL-LQB when the specimens are permeated with the CaCl 2 solutions. For example, k HQB / k LQB ranges from 2.0 to 2.6 for the tests performed using 5, 10, and 20 mM CaCl 2 solutions as the permeant liquids, whereas the k HQB / k LQB values are 230, 100, and 40 for the tests performed using 50, 100, and 500 mM CaCl 2 solutions as the permeant liquids, respectively. Thus, the GCL with the higher quality bentonite is more susceptible to chemical attack than the GCL with the lower quality bentonite.
Three index properties liquid limit, sedimentation volume, and swell index of two sodium bentonites from geosynthetic clay liners GCLs are correlated with the hydraulic conductivity k of the same GCLs to evaluate the suitability of index properties for evaluating chemical compatibility. Deionized water DIW and calcium chloride CaCl 2 solutions were used for hydration index tests and permeation hydraulic conductivity tests. In general, increasing the CaCl 2 concentration caused each index property to decrease and the hydraulic conductivity to increase relative to values obtained with DIW, with the strongest correlations obtained with the liquid limit. The correspondence between index properties and hydraulic conductivity differed by index property, the quality of the bentonite, and the effective stress applied during the hydraulic conductivity test. Thus, correlations used for compatibility assessments are specific to the bentonite in the GCL and the stress conditions being applied. Results of the study also show that appreciable changes in hydraulic conductivity can occur with little or no change in index properties and that the greatest changes in index properties may correspond to conditions causing low or modest changes in hydraulic conductivity. However, in this study, a critical threshold existed for each index property, beyond which further decreases in an index property correlated with substantial increases 10 in hydraulic conductivity.
The factors that may affect short-term leakage through composite liners are examined. It is shown that the leakage through composite liners is only a very small fraction of that expected for either a geomembrane (GM) or clay liner (CL) alone. However, the calculated leakage through holes in a GM in direct contact with a clay liner is typically substantially smaller than that actually observed in the field. It is shown that calculated leakage taking account of typical connected wrinkle lengths observed in the field explains the observed field leakage through composite liners. Provided that care is taken to avoid excessive connected wrinkle lengths, the leakage through composite liners is very small compared to a typical GM or CL alone. It is shown that the leakage through composite liners with a geosynthetic clay liner (GCL) is typically much less than for composite liners with a compacted clay liner (CCL). Finally, factors that will affect long-term leakage through composite liners are discussed. It is concluded that composite liners have performed extremely well in field applications for a couple of decades and that recent research both helps understand why they have worked so well and provides new insight into issues that need to be considered to ensure excellent long-term liner performance of composite liners — especially for applications where the liner temperature can exceed about 35 °C.
A general framework for calculating the rate of liquid flow through a composite liner (geomembrane plus soil liner) with holes is presented. Solutions given for a circular hole and a damaged wrinkle can be used for interpreting data from laboratory tests, modelling expected field conditions, and interpreting field leakage data. A number of existing solutions arise from the general solution as special cases. Finally, the paper is extended to consider the potential interaction between damaged wrinkles, and it is theoretically shown that while this may be an important issue for composite liners incorporating a compacted clay liner, it is far less likely to be significant for those incorporating a geosynthetic clay liner.
The performance of five different GCLs (two GCLs with standard sodium bentonite and three GCLs with polymer enhanced bentonite) subjected to three different climatic modes of wet-dry cycles simulating conditions to which a GCL might expose in cover systems over a prolonged time is reported. The wetting cycles lasted for 8 h, while the drying cycles varied between 16 h, seven days, and 14 days. It is shown that after around a year of accelerated aging, the hydraulic conductivity of the aged GCLs increased notably when permeated with tap water at an applied effective stress of 15 kPa for a range of heads (0.07, 0.14, 0.21, 0.49, and 1.2 m). The combined effects of the number and the duration of the wet-dry cycles, the GCL's mass per unit area, the carrier geotextile, the size and the number of the needle punch bundles, and the thermal treatment to bond the needle-punch bundles to the carrier geotextile are discussed. The poor hydraulic performance of the polymer-amended/modified bentonite GCLs is discussed.
The behaviour of geosynthetic clay liners (GCLs) as part of a physical-environmental system is examined. Consideration is given to: (a) both the physical and hydraulic interactions with the materials, and the chemical interactions with the fluids, above and below the liner, (b) time-dependent changes in the materials, (c) heat generated from the material to be contained, as well as (d) the climatic conditions both during construction and during service. This paper explores some common perceptions about GCL behaviour and then examines the misconceptions that can arise and their implications. It demonstrates how what may first appear obvious is not always as one expects and that more is not always better. It discusses: (i) the pore structure of a GCL, (ii) the dependency of the water retention curve of the GCL on its structure, bentonite particle sizes and applied stress, (iii) the effect of the subgrade pore water chemistry, (iv) the mineralogy of the subgrade, and (v) thermal effects. The desirability of a GCL being reasonably well-hydrated before being permeated is examined. The critical size of needle-punch bundles at which preferential flow can increase hydraulic conductivity by orders of magnitude is illustrated. The dependency of self-healing of holes on the interaction between GCL and subgrade is discussed. Finally, the transmissivity of the geomembrane/GCL interface is shown to be a function of GCL and geomembrane characteristics and to be poorly correlated with GCL hydraulic conductivity.
The desiccation and subsequent hydraulic conductivity of both a standard (GCL_A) and polymer-enhanced (GCL_B) Na-bentonite GCL hydrated from a well-graded sandy subsoil under 20 kPa, then subjected to a thermal gradient, and finally rehydrated and permeated with distilled water or 0.325 mol/L Na⁺ synthetic brine are reported.
With moderate temperature of 40 °C applied to the top of the liner, GCL_B experienced less cracking than GCL_A, but this advantage disappeared when temperatures increased. Both desiccated specimens of GCL_A and B showed significant self-healing when permeated with distilled water and their hydraulic conductivities quickly reduced to around 10⁻¹¹ m/s at 20 kPa upon rehydration. However, when GCL_B desiccated specimens were permeated with the synthetic brine, their hydraulic conductivities were found to be one to two orders of magnitude higher than corresponding values obtained with distilled water. On the other hand, GCL_A (with no polymer treatment) maintained its hydraulic conductivities at the same level obtained with distilled water. It is concluded that caution should be exercised in using polymer-bentonite in applications in which GCLs are subjected to significant thermal gradients unless there is data to show they are resistant to thermal effects.
The self-healing of fully penetrating artificial defects (circular holes and rectangular slits) in geosynthetic clay liners (GCLs) on full hydration in deionized water and 10 mM calcium chloride (CaCl 2 ) solution under 2 kPa overburden stress are compared. Circular holes with diameters up to 41 mm self-healed in deionized water but with an indentation of about up to 2 mm deep remaining at the center of the larger self-healed zones for holes of 30 mm diameter and larger. Holes of up to 35 mm diameter completely closed-up in 10 mM CaCl 2 solution, but with an indentation of about up to 6 mm deep remaining at the center of the larger self-healed zones for holes of 30 mm diameter and larger. A fully penetrating 15 mm wide × 120 mm long single slit in the center of a GCL specimen completely closed up in deionized water but not in 10 mM CaCl 2 solution. Even in deionized water, the slit does not fully close when 25 mm (or more) wide. Double parallel silts 15 mm wide × 240 mm long closed up in deionized water, but not in 10 mM CaCl 2 solution when there was a 20 mm-wide strip of undamaged GCL between the slits, but did not fully close up when the undamaged GCL strip between the slits was reduced to 10 mm or 5 mm. The difference in self-healing based on the hydrating fluid chemistry is discussed.
Installed geomembranes typically contain holes, which can be located for repair in most cases using electrical leak location (ELL) technologies. It is of special interest to quantify the likelihood that a geomembrane has holes, the impact of such holes with respect to a facility's expected performance, and subsequent remedial actions. In addition to providing a summary of research to date on these topics, the aim of this paper is to answer these questions with modern case studies, contextual hole frequency statistics and a recapitulation and reexamination of leakage data from double-lined landfills. Finally, the physics of electrical leak location technology is explained in order to illustrate the capabilities and limitations of the methods as well as to provide guidelines for maximizing the effectiveness of the technologies.
The effects of the silt aggregation, compaction density, and water content of the subgrade on the hydration of five different geosynthetic clay liner (GCL) products is reported based on a series of laboratory column experiments conducted over a six-year period. GCLs meeting typical specifications in terms of minimum hydraulic conductivity and swell index are hydrated to equilibrium from the same subgrade soil with sufficient cations to cause cation exchange during hydration. It is then shown that the GCL bentonite granularity and GCL structure can have a significant (~four orders of magnitude) effect on hydraulic conductivity under the same test conditions (from 8 × 10⁻¹² m/s for one GCL to 6 × 10⁻⁸ m/s for another GCL product). The effect of subgrade water content on the hydraulic performance of GCLs are not self-evident and quite dependent on the bentonite granularity, GCL structure, and permeant. Varying the subgrade water content from 5 to 16% and allowing the GCL to hydrate to equilibrium before permeation led to up to 5-fold difference in hydraulic conductivity when permeated with tap water and up to 60-fold difference when the same product is permeated with synthetic municipal solid waste leachate. When permeated with synthetic leachate, increasing stress from 70 kPa to 150 kPa led to a slight (average 37%; maximum 2.7-fold) decrease in hydraulic conductivity due to a decrease in bulk void ratio. It is shown that hydraulic conductivity is lower for GCLs with a scrim-reinforced geotextile, and/or with finer bentonite. It is shown that selecting a GCL based on the initial hydraulic conductivity and swell index in a manufacturers product sheet provides no assurance of good performance in field applications and it is recommended that designers pay more attention to selection of a GCL and preparation of the subgrade for important projects.
This paper investigates whether the introduction of an airgap above a composite liner made of a geomembrane (GMB) and a Geosynthetic Clay Liner (GCL) can decrease thermal loads on the GCL, reduce the risk of bentonite desiccation and/or help maintain its low hydraulic conductivity. A composite liner, subject to 20 kPa overburden load, over a well graded sand was subjected to a thermal gradient. In addition, to the reference base case in which no airgap was present, two designs included air gaps through the placement of a 10 mm and 20 mm-thick geocomposites (GC) on top of the GCL-GMB, respectively. Temperatures on top of the GCLs were found to be significantly reduced by the presence of air gaps, relative to the reference base case. All three designs resulted in GCL desiccation cracks at the end of the tests, due to the relatively high temperature gradients and low water retention of the subsoil, even in the presence of air gaps. However, X-Ray imaging revealed that crack patterns in bentonite samples from designs with air gaps were finer and narrower. Subsequent rehydration (and permeation tests) with distilled water indicated that significant self-healing of bentonite was in evidence in all three cases. However, while in the absence of an air gap the saturated hydraulic conductivity was found to be 2.8 times its pre-heating value, no significant increase was recorded for other two cases. X-Ray imaging of rehydrated samples confirmed that more effective healing had occurred in samples with an air gap.
Exposed composite GMB-GCL liners are at risk of downslope bentonite erosion caused by the release of low ionic strength condensed water onto the top surface of the GCL following daily solar heating. This paper investigates the use of X-ray computed tomography (X-ray CT) to quantify the thinning of the bentonite layer and the application of X-ray diffraction techniques (XRD) to investigate the changes in clay chemistry (if any) of the bentonite from the virgin GCL to the eroded bentonite. The effect of specimen size and scanning orientation was investigated resulting in a revised testing procedure in which the CT scanning orientation was changed from horizontal to vertical to permit a longer test specimen which was also sealed at the bottom edge to minimise the edge boundary condition. The X-ray CT results provide highly visual evidence that a) bentonite thinning immediately under the upper cover geotextile is the initial location of erosion, and b) the bentonite core erodes at a significantly higher rate when not covered by a geotextile than when covered by a geotextile. These observations indicate that the upper geotextile of the GCL plays a significant role in controlling the rate of bentonite erosion. Finally, a comparison of the virgin and runoff bentonite properties was conducted to investigate potential changes in swell index, X-ray diffraction results, and concentration of Na and Ca cations. The runoff bentonite was observed to had a significantly higher swell index (40 ml/2 g) than the virgin bentonite (28 ml/2 g) and lower Na and Ca concentrations. This finding is consistent with the observation from XRD analyses of the runoff bentonite which illustrate that the clay fraction of the bentonite is preferentially eroded by the application of DI water.
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 study reports supportive evidence, from scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (TEM) of bentonite cakes, of the textural effects caused by reaction of concentrated sulfuric acid on the bentonite component of geosynthetic clay liners (GCLs) and how these effects may influence observed bentonite hydraulic performance. SEM images revealed that changes to the natural micro-textures of bentonite were due to interaction with acidic leachate, the effects being greater with increased concentration. It was observed from high resolution TEM images that most of the effects detrimental to hydraulic performance were related to rearrangement of smectite quasi-crystals, as expected for textural responses to increased ionic strength. Significant bentonite dissolution was also observed in both SEM and TEM images after reaction at room temperature for as much as 48 h with sulfuric acid. The observed changes in the microtexture of the bentonite component by both SEM and TEM provide a physical explanation to the bulk experimental results as well as a benchmark for the application of GCLs under acidic conditions.
High-resolution X-ray tomography was used to observe a partially hydrated geosynthetic clay liner (GCL) specimen to gain a better understanding of the interaction of its compnents (i.e., geotextiles, fibres and bentonite) on partial hydration when deployed as part of a composite liner system. Detailed in-situ studies of hydration processes in GCLs has proven difficult despite more than two decades of effort. X-ray tomographs were collected at spatial resolutions of 12 and 7 μm to identify the different components within a GCL, as well as to examine in finer detail their interaction within the GCL after initial partial hydration. Tomograph projections provided an excellent aspect of the interaction of these components and some concepts, such as the presence of shearing features within the bentonite component, may require re-consideration based on evidence from X-ray tomography.
Water retention and hydration tests are reported for three needle punched geosynthetic clay liners (GCLs). GCLs hydration and their maximum hydration capacity were assessed against subgrade soils prepared at different initial gravimetric water contents. The subgrade soil mineralogy and particle size distribution, as well as the carrier geotextiles used in GCLs, are shown to have a significant impact on the GCLs hydration behaviour. This work highlights the need to consider the unsaturated properties of both the GCLs and the subgrade soil when assessing the hydration of the GCLs. At gravimetric water contents above the GCL water entry value (≈30%), some forms of GCL configuration may be better than others with respect to ability to hydrate from a given soil. However, the partial hydration of GCL is mostly controlled by the bentonite microstructure for gravimetric water contents below the water entry value of the GCLs.
The use of geosynthetic clay liners (GCLs) in waste containment applications can induce long-term normal and shear stresses as well as expose GCLs to elevated temperatures and non-standard hydration solutions. Considering the importance of GCL internal shear strength to the design and integrity of waste containment barrier systems, innovative laboratory testing methods are needed to assess shear behavior of GCLs. There were two main objectives of this study: (i) develop a stress-controlled direct shear apparatus capable of testing GCLs exposed to elevated temperatures and hydrated in non-standard solutions; and (ii) assess internal shear behavior of GCLs under varying experimental conditions (e.g., stress, temperature, solution). These two objectives were partitioned into a two-paper set, whereby Part I (this paper) focuses on the shear box design and Part II focuses on an assessment of shear behavior. The direct shear apparatus includes a reaction frame to mitigate specimen rotation that develops from an internal moment within needle-punched reinforced GCLs. Rapid-loading shear tests were conducted to assess functionality of the apparatus and document baseline shear behavior for a heat-treated and a non-heat treated needle-punched GCL with comparable peel strength. These two GCLs failed at comparable applied shear stress; however, the heat-treated GCL yielded lower shear deformation and failure occurred via rupture of reinforcement fiber anchors, whereas the non-heat treated GCL yielded larger shear deformation and failure via pullout of reinforcement fibers.
This study investigates and discusses the hydration and hydraulic conductivity of low performance (LP), medium performance (MP) and high performance (HP) GCLs. The performance description is made in terms of liquid limit rather than cation exchange capacity or smectite content. The liquid limits of LP, MP and HP GCLs were 108, 320, and 1163%, respectively. GCLs were initially hydrated over compacted silty sand subsoils for 7–90 days. After hydration, water contents of GCLs were determined. Regardless of GCL type, the water contents remarkably increased in the first 7 days of hydration and reached equilibrium after 30 days of hydration for LP and HP GCLs. The water content of MP-GCL continued increasing even at the end of 90 days of hydration. The final water contents were 69, 84, and 120% for LP, MP and HP GCLs, respectively. In other words, increase in the liquid limit of bentonite corresponds to increasing the final water contents of GCLs. The findings of this study are in agreement with literature findings. However, there was no such kind of a trend when smectite content or cation exchange capacity was the dependent variable. The hydraulic conductivity behaviors were totally dependent on GCL performance. Hydrated LP and MP GCLs were not able to reduce their hydraulic conductivity at the beginning of the test. The pore volumes of flow (PVF) required to reducing the hydraulic conductivity to around 3.0 × 10⁻¹¹ m/s were 270 for LP-GCL and 77–109 for MP-GCL. The hydraulic conductivity of some specimens of LP and MP GCLs were more than >1.0 × 10⁻⁷ m/s even at the end of test duration. Observations showed that particle erosion took place during permeation. In contrast, the hydraulic conductivity of HP-GCLs decreased below 3.0 × 10⁻¹¹ m/s within a few PVF. This is due to polymer-treated bentonite used in HP-GCL. Post-test measurements on GCLs showed that the water contents kept increasing during hydraulic conductivity. Although water contents increased, the height of LP-GCL did not increase even after hydration and hydraulic conductivity testing, indicating lateral swelling only. MP and HP GCLs, however, had swollen laterally and vertically, resulting in greater heights for HP-GCL than that for MP-GCL.
Downslope bentonite erosion in four geosynthetic clay liners (GCLs) with granular bentonite (denoted GCL1–GCL4) left covered only by a black high-density polyethylene (HDPE) geomembrane for 3.6 and 4.7 years was recently published. Reported herein are the results of a field study of the potential for erosion for seven new test sections over a 28-month period. These include four previously unstudied GCLs: two with powdered bentonite (GCL5 and GCL6), one with polymer-enhanced granular bentonite (GCL7), and one multicomponent product installed coating-up (GCL8). One GCL (GCL2) known to experience erosion below an exposed black geomembrane was installed beneath both a black and a white exposed geomembrane and also in a section where the composite liner was promptly covered by 0.3 m of gravel. After 28 months, there was significant erosion of GCL2 and GCL7, but no significant erosion of GCL5, GCL6, GCL8, or GCL2 covered by 0.3 m gravel.
Alternative laboratory approaches for cyclic permeation-desiccation experiments on geosynthetic clay liner (GCL) specimens are compared. Permeation cycles for hydraulic conductivity (k) are conducted in flexible-wall permeameters. Subsequent desiccation cycles are conducted using one of three approaches: (1) unconfined desiccation involving drying under controlled relative humidity (RH) with no external stress; (2) axially constrained desiccation involving drying under controlled RH and axial stress simulating one meter of cover soil; and (3) a novel in-permeameter method, where specimens are desiccated without physical disturbance by flushing the permeameter endcaps with controlled-RH gas. Comparison metrics include image analysis of bentonite crack patterns, cycle duration, uniformity of water content, and evolution of k for up to seven wet-dry cycles. All three methods result in similar water content, crack intensity, and hydraulic conductivity response, despite the fact that stress conditions during drying and disturbance to specimens are very different. Observed similarity indicates that internal flaws in the bentonite (e.g., reinforcing fibers), rather than external stress conditions, control desiccation crack initiation and propagation.
GCL manufacturers recommend that composite liners (i.e., a geomembrane (GMB) over geosynthetic clay liner (GCL)) be covered in a timely fashion. This paper highlights the importance of following this recommendation by reporting on significant down-slope bentonite migration first noted at the Queen's University Environmental Liner Test Site (QUELTS) constructed in 2006 (QUELTS I). The down-slope erosion is attributed to thermal cycles that caused evaporation of moisture from the GCL on sunny days (when the black geomembrane heated to 60–70 °C) followed by condensation of moisture on the underside of the geomembrane at night when the geomembrane cooled. The condensed moisture would drip onto the GCL and run down-the slope. Repetition of this process over an extended period of time caused the erosion of bentonite at some locations in all four GCLs examined in the 3.7 years the liner was exposed before the full inspection of the GCL which detected the mechanism. A series of laboratory experiments confirmed that dripping of evaporative water could cause down-slope erosion in relatively few cycles. These tests also identified several GCL products with a high resistance to down-slope erosion prompting the desire to construct a second field study to examine the issue. Thus, in 2012, the liner system was removed and QUELTS II was constructed with a new series of 7 composite liners. This paper highlights the key findings from these studies with particular emphasis on issues of importance to designers, regulators and installers.
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.
Under some circumstances, leaving a composite geomembrane/geosynthetic clay liner (GCL) exposed to solar radiation in the field has been shown to cause shrinkage of the underlying GCL. Recent field studies have shown that leaving a composite liner exposed can also lead to erosion of bentonite from the GCL due to downslope moisture migration. This paper reports an experimental technique that reproduced similar erosion in the laboratory on a typical landfill side slope of 3H:1V. The test method simulates the features that occur with the erosion of bentonite caused by downslope migration of evaporative water in the field. The laboratory tests demonstrate that erosion features can be present but may not be visible unless the appropriate back lighting is used. Erosion features measuring over 25 mm in width were produced. The test method simulated the features that were observed with bentonite erosion in the field and has the potential for use in examining other factors that may affect this type of erosion. The test method developed can be used for examining the response of GCLs when part of a composite liner that will be left exposed. The findings from this laboratory study provide additional motivation for timely covering of composite liner systems as recommended by GCL manufacturers.
To investigate systematically the effects of electrolytic solutions on the barrier performance of geosynthetic clay liners (GCLs), a long-term hydraulic conductivity test for 3 years at longest was conducted on a nonprehydrated GCL permeated with inorganic chemical solutions. The hydraulic conductivity test for waste leachates was also conducted. The results of the test show that the hydraulic conductivity of GCLs significantly correlates with the swelling capacity of bentonite contained in GCLs. GCLs have excellent barrier performance of k < 1.0 X 10(-8) cm/s when the free swell is larger than 15 mL/2 g-solid regardless of the type and concentration of the permeant solution. In addition, when the results of the hydraulic conductivity test with chemical inorganic solutions were compared to those with waste leachates, the hydraulic conductivity of GCL permeated with chemical solution was almost the same within the electric conductivity of 0-25 S/m as that permeated with waste leachate having similar electric conductivity. The hydraulic conductivity of GCLs to be used in landfill bottom liners can be estimated by the hydraulic conductivity values obtained from the experiment using chemical solutions having the similar electric conductivity values, if the chemical solution had the electric conductivity within =25 S/m.
A black 1.5 mm geomembrane (GMB) and geosynthetic clay liner (GCL) liner were placed on both a 3H: 1V (18.4°) slope and a gently sloping (3%) base (latitude 44°34 ¢15²N) and left exposed for 4.7 years. The observed solar radiation, ambient air temperature, temperature at the interface between the GMB and GCL, and temperature at various depths (to 600 mm) in the underlying silty sand soil are reported. The interface temperature was up to 40°C higher than ambient temperature on a sunny day. The difference between interface temperature on the slope and base was minimal near summer solstice and increased significantly earlier and later in the year (except when covered in snow). There was significant variability in GMB and interface temperature depending on the contact conditions between the GMB and GCL. The interface temperatures at wrinkles could be 15°C higher than other locations where there was intimate contact between GMB and GCL. Snow-cover insulated the liner from solar radiation and extreme temperature. These insulating effects were lost on the south-facing slope before the base, subjecting the GCL on the slope to more daily freeze–thaw cycles than on the base. A correlation between the interface temperature, ambient air temperature, and the solar radiation gave good agreement with the observed temperatures. The cycling of interface temperatures is considered to be central to the mechanism of moisture evaporation from the GCL that may cause GCL panel shrinkage and moisture condensation-driven downslope bentonite erosion. Covering the composite liner with a ballast layer as quickly as possible is recommended.
Manufacturers of geosynthetic liner materials recommend that composite geomembrane/ geosynthetic clay liners (GCLs) be covered in a timely fashion to avoid potential issues that may arise under the action of long-term solar exposure. In this paper, field evidence of a new, never before reported solar-exposure driven damage mechanism for GCLs covered only by a black geomembrane and left exposed for more than 3 years is presented. Solar exposure can give rise to a large daily variation in geomembrane temperature, which causes a moisture cycle within the interface between the geomembrane and GCL resulting in the formation and flow of condensed moisture beneath the geomembrane. All four of the GCL products investigated at the Queen’s University Environmental Liner Test Site were shown to have experienced significant bentonite erosion after 4.7 years of exposure. Erosion was identified in the field through a tactile survey of GCL panels in which the stiffness response of the GCL to touch was used to identify eroded zones. A change in the colour of the GCL, although useful to identify possibly eroded zones in some GCL products, proved ineffective in others. Erosion features were observed with widths up to and exceeding 200 mm across, making them unlikely to undergo self-healing upon hydration and application of normal stress. As a result, the observed erosion features would have severe adverse consequences for leakage rates through the GCL component of a composite liner barrier system. These observations provide yet another strong motivation for timely covering of composite landfill liner systems.
The performance of a geosynthetic clay liner (GCL) installed as part of a geocomposite barrier (geomembrane/GCL) to contain a hydrocarbon spill adjacent to the Arctic Ocean was evaluated by examining sacrificial samples exhumed after 1, 4, 6, 7, and 10 years in service. The hydraulic and chemical characteristics of the GCL were most affected by the location within the soil profile relative to the water table (typically about 1.3 m below ground level). The bentonite in the GCL samples exhumed from a depth of 0.0-0.8 m was well hydrated with a dispersed structure. Despite the significant cation exchange that took place between these GCL samples and the surrounding soil (the percentage of the exchangeable sodium decreased from 68% to 10-15%), there was no change in the hydraulic conductivity (k) of GCL for tap water or jet fuel. A similar bentonite structure was observed for GCL samples exhumed from 0.8 to 1.3 m below ground level; however, an observed network of horizontal and vertical microcracks (200e400mmwide) in the bentonite layer was attributed to the formation of ice lenses. As a result, the k of theseGCLsamples increased by one to two orders of magnitude when permeated by both tap water and jet fuel. The bentonite in GCL samples exhumed from below the water table after 6 and 10 years was flocculated with relatively high free pore space. The k values of these samples increased by one to four orders of magnitude for tap water and jet fuel. Despite this increase in k at some depths, there was no evidence of migration of hydrocarbons through the barrier over the last 10 years, indirectly suggesting that the subsurface geocomposite barrier system is still performing well.
The available evidence suggests that both geosynthetic clay liners (GCLs) and composite liners with a geomembrane (GMB) over a clay liner have performed extremely well at controlling leakage in field applications for a couple of decades. However, there have also been some problems reported and recent research has allowed us to have a much better understanding of the key design and construction factors affecting good and poor performance. This paper examines some of these issues including factors affecting GCL performance such as the water retention curve of GCLs, subgrade grain size and initial water content, GCL water content and normal stress on the GCL, the effect of daily thermal cycles on hydration, GCL panel shrinkage and cation exchange. Factors affecting composite liner performance examined include the potential for desiccation of the clay liner under a sustained thermal gradient, GMB/GCL interface transmissivity, wrinkles in the GMB when the ballast layer is placed over the composite liner and the potential interaction between wrinkles and GCL panel overlaps. Recent insights regarding leakage through composite liners are discussed. Although a number of potential issues with liner performance are discussed, it is concluded that all can be addressed by appropriate design, material selection, construction and operations.
A GCL may desiccate as a result of one or more wet–dry cycle. This may occur because the GCL is in an exposed composite liner (i.e., the mechanisms giving rise to shrinkage discussed earlier), the GCL is in a cover liner without adequate cover soil to protect it from significant wet–dry cycles due to climatic cycles or because it is in a composite bottom liner that initially hydrates and is then dried by the thermal gradient generated by hot waste (e.g., municipal solid waste where there is leachate recirculation or disposal of combustion ash). When it desiccates, the GCL k value will be high but, provided that there is not too much cation exchange, it can quickly reduce again to low values (Southen and Rowe, 2005) because of the ability of the sodium bentonite to swell and self-heal on re-wetting (i.e., when it comes into contact with the fluid that is to be contained). However, as indicated by some of the cases cited in the previous section, when desiccation is combined with cation exchange the self-healing capacity is reduced or lost, with the magnitude of the effect depending on (a) the amount of cation exchange, (b) the extent of the cracking and the size of the desiccation cracks and (c) the stress on the GCL (higher stress increases the ability of the GCL to self-heal, other things being equal). The ability to rehydrate to a low k may also be reduced by the chemical composition of the permeant (Petrov and Rowe, 1997) even if there was little initial cation exchange.
Wrinkles are buckles or waves that develop from restrained thermal expansion when the geomembrane is left exposed to solar heating. Wrinkles can substantially reduce the effectiveness of the geomembrane as a hydraulic barrier if a hole is at or near a wrinkle, depending on the number, length, and width of wrinkles. Low altitude aerial photography and digital image analysis are used to quantify the nature and extent of wrinkles that developed over one hot and sunny day in a smooth, black, 1.5-mm-thick high-density polyethylene (HDPE) geomembrane over a 55 m by 140 m area. Wrinkles were found to significantly vary over the course of the day, increasing from the fewest wrinkles in the morning to a maximum just after noon before decreasing toward the late afternoon. For the specific conditions examined, wrinkles were found to occupy 3%, 21%, and 7% of the entire area surveyed at 8:45, 12:25, and 17:15, respectively. Connections between adjacent wrinkles were observed to create significant interconnected wrinkle features greater than 2,000 m long. The shortest maximum interconnected wrinkle feature of 80 m/ha was measured at 8:45 while the longest such feature was 6; 600 m/ha at 13:45. The observations and results provide data to support the approach that limiting the time of day when cover is placed on geomembrane can be effective at reducing the extent of wrinkling. DOI:10.1061/(ASCE)GT.1943-5606.0000643. (C) 2012 American Society of Civil Engineers.
Geomembranes (gmbs) are widely used as advective barriers in landfill liner systems.when exposed to the sun,gmbs exhibit a network of wrinkles as a result of thermal expansion. thesewrinkles disrupt the intimate contact between thegmband the underlying layer. if a hole is coincident with agmbwrinkle then the space under thewrinkle has the potential to act as a preferential pathway for flowof contaminants. thus, the size and shape of gmb wrinkles have implications for leakage rates through the composite liner system. however, wrinkles are only a concern if they persist after placement of backfill, which is currently a subject of debate. in this paper, wrinkles are induced in a 1.5-mm-thick, black highdensity polyethylene strip gmb specimen overlying a geosynthetic clay liner using natural solar and laboratory energy sources. particle image velocimetry techniques are employed to record cross-sectional wrinkle geometry during growth and subsequent backfilling. this crosssectional geometry is found to followa gaussian shape inwhich the height increases with the temperature and the width remains relatively constant. the resulting relationships between the height and temperature permit an estimation ofwrinkle height for a known coefficient of thermal expansion for thegmband an estimate of wrinkle spacing. for thegmbmaterial and conditions tested, the results of the backfilling experiments indicate that whencoveredwith230mmof cool sand(21°c),wrinkles of initial height less than about 20mmdisappear completely,while largerwrinkles remain with a reduced height. furthermore, wrinkles of 20 mm in height are observed to form with increases in gmb temperature of less than 5°c. with application to the field, these findings indicate that a gmb must be covered at or below its installation temperature to achieve a wrinkle-free installation.
The effect of geomembrane thickness (1.5, 2.0, and 2.5 mm) on aging when immersed in a synthetic leachate is investigated over a period of approximately 7 years. Based on data at five different temperatures (55, 65, 70, 75, and 85°C), the predicted time required for a reduction in stress crack resistance to 150 h (half the typically specified value) at 35°C is 62% longer for the 2.5 mm than for the 1.5-mm geomembrane tested and 12% longer for the 2.0-mm than for the 1.5-mm geomembrane. Thus, other things being equal, the results suggest a longer time to nominal failure with increasing geomembrane thickness. It is also shown that the data from a proposed stage-parallel testing procedure collected over 2.5 years fit well with data from traditional incubation of virgin samples over almost 7 years and hence provides a viable means of obtaining good data in a reasonable period of time.
The potential shrinkage of eight different geosynthetic clay liners (GCLs) subjected to wetting and drying cycles is examined. It is shown that the initial (e.g, off-the-roll) moisture content may affect the initial shrinkage but did not notably affect the final equilibrium shrinkage. For GCLs with granular bentonite and wetted to a moisture content of about 60% (or greater) in the hydration phase, the actual moisture content did not appear to affect the magnitude of the final equilibrium shrinkage. However, it did affect the rate of shrinkage. Specimens brought to about 100% moisture content in each cycle reached a constant shrinkage value much faster than those brought to about 60% in each wetting cycle. GCLs containing powdered bentonite generally shrank more than those containing granular bentonite. All of the powdered bentonite specimens continued a slow accumulation of strain with increasing cycles, even up to 75 cycles. The shrinkage of a needle-punched GCL with a thermally treated scrim-reinforced nonwoven carrier geotextile and granular bentonite was less than that for a needle-punched GCL with a simple nonwoven carrier and granular bentonite. For some products, there was considerable variability in GCL shrinkage for specimens from the same roll and tested under nominally identical conditions, whereas for other products, the variability was relatively small. The shrinkage strain required to cause the loss of a 150–300 mm panel overlap is shown to be able to be mobilized in about five wet-dry cycles in the experiments reported.
Leaving a composite liner exposed for an extended period can sometimes lead to down-slope bentonite erosion from geosynthetic clay liners (GCLs). This laboratory study examines a number of factors that can affect the erosion of bentonite particles with an imposed flow of water for one particular geotextile-encased, needle-punched GCL. The factors examined include the effect of an initial wet/dry cycle, water chemistry, flow rate, slope, prior cation exchange, and the effect of no-drying phase in the test cycle. No erosion was observed unless the GCL had been hydrated and dried to create a wet/dry cycle. The most critical factor was found to be the water chemistry. No erosion was observed with tap water (39 ppm calcium) with up to 360 cycles and a flow of 3 L/hour. Tests simulating the evaporation and condensation of water below an exposed composite liner by imposing deionized water on the GCL surface developed erosion holes within 5–6 cycles.
Over the past decade, geosynthetic clay liners (GCLs) have gained widespread popularity as a substitute for compacted clay liners in cover systems and composite bottom liners. They are also used as environmental protection barriers in transportation facilities or storage tanks, and as single liners for canals, ponds or surface impoundments. As a result, they are being investigated intensively, especially in regard to their hydraulic and diffusion characteristics, chemical compatibility, mechanical behaviour, durability and gas migration. In this paper, a review of the main findings is presented with the focus on the critical aspects affecting the service life of GCLs. From this work, a general insight is gained on the design implications for systems that incorporate GCLs.
The results of a comprehensive testing program conducted to evaluate the hydraulic conductivity (k) of two geosynthetic clay liners (GCLs) considered as a liner component for a tailings impoundment at a proposed zinc and copper mine are reported. The two GCLs were permeated with a relatively low ionic-strength ground water (GW) from the mine site and two electrolyte solutions, a process water (PW) and a simulated leachate (SL), with chemical compositions consistent with those expected during operation of the impoundment. A total of 22 flexible-wall tests were performed to determine the effects of prehydration with the GW, type of GCL, type of permeant liquid, and duration of the back-pressure stage of the test. The k values for both GCLs permeated with the GW were 1.7 × 10−9 cm/s, which is within the range 1–3 × 10−9 cm/s typically reported for GCLs permeated with low ionic-strength liquids, such as deionized water. However, the mean values of k based on permeation of duplicate specimens of both types of GCL with either PW or SL relative to the values of k based on permeation with GW, or k/kw, ranged from a factor of 200 (2.3 orders of magnitude) to a factor of 7600 (3.9 orders of magnitude). Thus, both tailings impoundment solutions had significant adverse impacts on the hydraulic performance of both GCLs. Given the overall range of k/kw values, factors such as prehydration, type of GCL, type of permeant liquid, and duration of back pressure, were relatively insignificant. The results of this study serve to emphasize the need to perform hydraulic conductivity testing using site specific materials.