Chemical interaction and hydraulic performance of geosynthetic clay liners isothermally hydrated from silty sand subgrade

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.     

Laboratory investigation of GCL hydration from Lateritic subsoils

A laboratory investigation on the hydration behavior of GCLs from lateritic soils was conducted under isothermal and thermal conditions (tropical climate), varying subsoil moisture contents, GCLs bentonite particle size and mineralogy. GCL hydration levels from lateritic subsoils under isothermal conditions (55%) were similar to literature findings. A slight decrease in water content of some GCLs after long periods of contact with the lateritic soils indicates that equilibrium can demand long time in these soils. GCL with granular bentonites were less efficient to hydrate from lateritic subsoils. GCLs with activated-calcium bentonites maintained hydration levels in long-term. Nonwoven geotextile facing down favored capillary effects. Thermal cycles significantly influenced GCLs hydration from subsoils. Capillary connections developed during hydration under isothermal conditions due to suction gradient reductions. Post-hydration tests under isothermal conditions showed more alterations in GCLs swelling and cation exchange properties than thermal cycles test. An increase in the saturated hydraulic conductivity of GCLs was observed in both lateritic soils, mainly for isothermal condition, although continued attending hydraulic conductivity requirements for barrier applications.     

Hydraulic Conductivity of Nonprehydrated Geosynthetic Clay Liners Permeated with Inorganic Solutions and Waste Leachates

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<1.0×10^-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.     

Irrigated composite liner designs for fast hydration and prevention of thermal desiccation of geosynthetics clay liners

In composite liners made of geomembrane (GMB)-geosynthetics clay liners (GCLs), maintaining bentonite in the GCL in a suitably hydrated state is critical for their performance. Hydration of GCL from subsoil, following industry best practice, is time consuming and conditional on suitable water chemistry in subsoil. In addition, under thermal gradients, dehydration occurs, with moisture migrating downwards to the subsoil, leading to the development of cracks in the bentonite and hence loss of performance.Two novel ideas are proposed in this paper, namely hydration of GCLs by artificial irrigation and hydraulic separation of the liner system from the underlying subsoil. Three new composite liner designs allowing for actively irrigating a geosynthetic clay liner (GCL) through a geocomposite layer were investigated. In two of the three designs, the hydraulic connection between the GCL and the subsoil was broken by placing an additional GMB between them. The new designs were tested in column experiments under 20 kPa overburden pressure and temperatures of up to 78 °C applied to the top of the liner. The performances of the new designs were compared to that of a standard GCL-GMB design where GCL was allowed to hydrate from a well-graded sandy subsoil. Three scenarios for the staging of hydration and thermal load application were investigated.Under active hydration of the composite liners, it took less than 14 days for the GCLs to reach a gravimetric water content ω of 110–130%, compared to 49 days taken to reach ω~95% under hydration from the subsoil. GCLs in the new designs in which the hydraulic connection with the subsoil was broken, remained well-hydrated (ω>100%) after 14 days of heating and no cracks appeared in the bentonite. On the other hand, the GCL in the conventional design experienced severe desiccation under the same conditions. The new designs hence offer a viable solution to the problem of slow hydration and/or thermal desiccation of GCLs.     

Investigating factors influencing polymer elution and the mechanism controlling the chemical compatibility of GCLs containing linear polymers

A study was conducted to investigate (1) physicochemical factors that influence polymer elution from GCLs containing a blend of bentonite and linear (water-soluble) polymer (LPB GCLs) and (2) the mechanism that controls the chemical compatibility of LPB GCLs when polymer elutes. A series of hydraulic conductivity (k), free swell and viscosity tests were performed on a commercial LPB GCL using DI water, varying concentrations of NaCl and CaCl?. Comparable tests were also performed on a conventional bentonite (CB) GCL containing the same untreated bentonite and the same physical properties as the LPB GCL. The LPB GCL showed improved swelling and hydraulic performance compared to the CB GCL when permeated with salt solutions. Total organic carbon analysis of the effluents showed that polymer eluted from the LPB GCL regardless of the permeant solution. However, the rate at which polymer eluted increased as the concentration and valence of the dominant cation increased. The rate at which polymer eluted also increased with hydraulic gradient. The mass of polymer retained inside the GCL matrix did not correlate with the k of the LPB GCL. Free swell tests coupled with chemical analysis suggest that, the improved chemical compatibility of the LPB GCL was due to the ability of the polymer to scavenge cations from the solution which allows the bentonite to undergo adequate swelling during the initial hydration period. Analogous to water-prehydrated CB GCLs, the dispersed structure of the bentonite fabric and increased adsorbed water molecules attained during initial swelling controls the k of the LPB GCL when polymer elutes.     

Effects of Water Content Distribution on Hydraulic Conductivity of Prehydrated GCLS against Calcium Chloride Solutions

When geosynthetic clay liners (GCLs) are applied as bottom liners at waste containment facilities, they are naturally prehydrated by absorbing moisture in the underlying base layers. In order to evaluate the effects of cations contained in waste leachates, this study investigated the effects of the water content distribution of the GCLs prehydrated with actual soils on their hydraulic conductivities against CaCl₂ solutions. The “prehydration tests”, which were conducted prior to the hydraulic conductivity tests, showed that the water content distribution of the prehydrated GCLs depends on the properties of the GCLs and the base layers. In particular, drastic differences between GCLs with powdered bentonite and GCLs with granular bentonite were observed in the prehydration water content and its distribution. Prehydrated GCLs with powdered bentonite had a higher water content and a more homogenous distribution than those with granular bentonite. The hydraulic conductivity tests showed that most of the prehydrated GCLs exhibit a low hydraulic conductivity of k?1.0×10^-8 cm/s against CaCl₂ solutions with 0.1-0.5 M. However, GCLs with granular bentonite may be difficult to homogeneously prehydrate and exhibit an unstable hydraulic conductivity, which varies from k=2.9×10^-9 cm/s to k=1.5×10^-6 cm/s. The homogeneity of the water content distribution has been considered an important factor to obtain a required barrier performance under prehydration conditions, which are naturally generated in actual sites.     

Insufficient initial hydration of GCLs from some subgrades: Factors and causes

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.     

Hydraulic conductivity of bentonite-polymer composite geosynthetic clay liners permeated with bauxite liquor

The high ionic strength of the porewater in red mud (bauxite liquor from digestion) can suppress swelling of montmorillonite, resulting in geosynthetic clay liners (GCLs) that are too permeable to be effective as liners in red mud disposal facilities. Bentonite-polymer composite GCLs (BPC GCLs) have been developed as more resilient lining materials, and some BPC GCLs have been shown to have very low hydraulic conductivity to bauxite liquors that have extreme ionic strength and pH. In this study, a nationwide investigation was conducted in China to evaluate the characteristics of bauxite liquor in Chinese impoundments, and to evaluate the suitability of GCLs containing granular sodium bentonite or BPCs for containment. Hydraulic conductivity tests were conducted on six BPC GCLs with two characteristic Chinese bauxite liquors that are hyperalkaline (pH > 12) and had ionic strengths of 76.9 mM and 620.3 mM. The BPC GCLs had hydraulic conductivity ranging from 10^?8-10^?12 m/s, which is higher than the hydraulic conductivity of BPC GCLs to deionized water (10^?12-10^?13 m/s), but lower than the hydraulic conductivity of conventional GCLs with granular sodium bentonite GCLs to the same liquors (10^?7-10^?8 m/s). The hydraulic conductivity of the BPC GCLs depends on the chemical properties of the leachate, the polymer loading, and the type of polymer. Microstructural analysis by scanning electron microscopy (SEM) suggests that the hydraulic conductivity of BPC GCLs is controlled by pore-blocking by polymer hydrogel, which is affected by the bauxite liquor.     

Hydrodynamic assessment of bentonite granule size and granule swelling on hydraulic conductivity of geosynthetic clay liners

Flow in an idealized geosynthetic clay liner (GCL) containing bentonite comprised of equisized and equispaced square granules was simulated using a hydrodynamic model to quantitatively evaluate the premise that the hydraulic conductivity of GCLs diminishes as the bentonite granules hydrate and swell into adjacent intergranular pores, creating smaller and tortuous intergranular flow paths. Predictions with the model indicate that hydraulic conductivity decreases as granules swell and intergranular pores become smaller, and that greater granule swelling during hydration is required to achieve low hydraulic conductivity when the bentonite is comprised of larger granules, or the bentonite density is lower (lower bentonite mass per unit area). Predictions made with the model indicate that intergranular pores become extremely small (<1 μm) as the hydraulic conductivity approaches 10⁻¹¹ m/s. These outcomes are consistent with experimental data showing that GCLs are more permeable when hydrated and permeated with solutions that suppress swelling of the bentonite granules, and that the hydraulic conductivity of GCLs with bentonite having smaller intergranular pores (e.g., GCLs with smaller bentonite granules, more broadly graded particles, or higher bentonite density) is less sensitive to solutions that suppress swelling.     

Water exchange across a subgrade-GCL interface as impacted by polymers and environmental conditions

This paper examines the hydration behaviour of polymer enhanced geosynthetic clay liners (GCLs) under both isothermal conditions and diurnal cyclical temperatures as recorded at a Melbourne landfill site. The diurnal temperature cycles introduced thermal gradients, which induced water movement downwards into the subgrade. It was found that the bulk of the thermal and hydraulic effects occurred within the upper 50 mm of the subgrade layer. Compared to isothermal conditions, diurnal temperature cycling's main effect was to suppress GCL hydration. Thermal gradients induced capillary breaks, and GCL hydration was thus limited to vapour phase hydration from the subgrade, which restricted the gravimetric water content of the GCLs to <30%. The hydration process at the interface was observed to gradually return to isothermal condition hydration levels once the temperature cycle conditions were removed. While the polymer's presence ensures the adsorption of more water, it also renders GCLs more sensitive to the effects of temperature. This study provides insights on the hydration behaviour of uncovered GCLs in field applications and the impact that diurnal temperature cycles can have on GCL hydration in a Mediterranean type climate.     

Hydraulic conductivity of geosynthetic clay liners to tailings impoundment solutions

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/k_w, 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/k_w 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.     

Effect of wet-dry cycles on standard & polymer-amended GCLs in covers subjected to flow over the GCL

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.     

Impact of a synthetic leachate on permittivity of GCLs measured by filter press and oedopermeameter tests

Geosynthetic clay liners (GCLs) are used in landfill liner applications due primarily to their low hydraulic conductivity to water. The low hydraulic conductivity of GCLs comes from the structure of the clay in the bentonite. However, the interaction between clay and aggressive liquids may alter the structure of the clay and, thus, result in an increase in the hydraulic conductivity of the GCL. This paper presents the results of a project aimed at evaluating the impact of a synthetic leachate on the structure of four different bentonites used in the manufacturing of GCLs. The preparation of bentonite dispersions increased the interaction between the bentonites and the various liquids. The hydraulic properties of the dispersions also were tested using filter press tests to obtain flow curves. Results of these tests were correlated with the cationic concentration, electrical conductivity and pH of the dispersions, swell indexes of the bentonite extracted from the GCLs, and permittivities of the intact GCLs determined in oedopermeameter tests. The results showed that one bentonite was more sensitive to the synthetic leachate than the other bentonites. For example, the permittivities of the more sensitive bentonite based on the oedopermeameter tests and filter press tests were respectively 2.11 × 10⁻⁸ s⁻¹ and 5.6 × 10⁻⁸ s⁻¹, whereas the permittivities for other bentonites, including a natural sodium bentonite and two sodium-activated calcium bentonites, were respectively 5.7 to 6.5 × 10⁻⁹ s⁻¹ and 3.2 to 3.5 × 10⁻⁸ s⁻¹. The filter press test served as a quick and easy-to-use test to compare the performance of the various bentonites in containing a given liquid. However, the oedopermeameter test or direct permeation test is preferable to filter press tests or fluid loss tests for evaluating the long-term impact of a liquid on a bentonite.     

Performance-based indicators for controlling geosynthetic clay liners in landfill applications

The results of a project aimed at identifying performance-based indicators that can be used by landfill operators to check the suitability of GCLs for bottom barrier applications are presented. The general methodology consisted of performing detailed characterization of the prevalent GCLs used in France for landfill barrier applications, before and after prolonged contact with several fluids during oedo-permeameter tests. Results of mineralogical analysis illustrate the variety of composition of the tested bentonites, which in addition to smectite clay contain a large number of accessory minerals. For one of the GCLs tested, the proportion of smectite was lower than 30 wt%, which highlights the limitations of the generic designation “bentonite” when referring to GCLs destined to landfill applications. Results also underline the correlation between cation exchange capacity (CEC) and smectite content, the correlation between free swell volume and proportion of exchangeable sodium and the influence of the bentonite's calcium carbonate fraction on hydraulic conductivity. Transmission electron microscopy (TEM) photographs illustrate the effect of cation exchange on clay microstructure, with the formation of clay particles which lead to increased hydraulic conductivity. The exchange is also documented by exchangeable cation analyses. Results of isotopic analyses indicate that information provided by suppliers with respect to the “natural” versus “activated” nature of the bentonite, may sometimes be arbitrary and related to factors that are very difficult to check in practice, even by the suppliers themselves. This further underlines the need for performance-based indicators, rather than generic designations, to provide objective information regarding GCL suitability for landfill applications. Several performance-based indicators are selected in order to provide practical tools for checking the suitability of sodium-bentonite GCLs in bottom barrier applications and limit values are proposed.     

Hydraulic conductivity of bentonite-polymer geosynthetic clay liners to coal combustion product leachates

Hydraulic conductivity of seven geosynthetic clay liners (GCLs) to synthetic coal combustion product (CCP) leachates were evaluated in this study. The leachates are chemically representative of typical and worst scenarios observed in CCP landfills. The ionic strength (I) of the synthetic CCP leachates ranged from 50 mM to 4676 mM (TCCP-50, LRMD-96, TFGDS-473, LR-2577, HI-3179 and HR-4676). One of the GCLs contained conventional sodium bentonite (Na–B) and the other six contained bentonite-polymer (B–P) mixture with polymer loadings ranging from 0.5% to 12.7%. Hydraulic conductivity tests were conducted at an effective confining stress of 20 kPa. The hydraulic conductivity of the Na–B GCLs were >1 × 10⁻¹⁰ m/s when permeated with all six CCP leachates, whereas the B–P GCLs with sufficient polymer loading maintained low hydraulic conductivity to synthetic CCP leachates. All the B–P GCLs showed low hydraulic conductivity (<1 × 10⁻¹⁰ m/s) to low ionic strength leachates (TCCP-50, I = 50 mM and LRMD-96, I = 96 mM). B–P GCLs with P > 5% showed low hydraulic conductivity (<1 × 10⁻¹⁰ m/s) up to HI-3179 leachates. These results suggest that B–P GCLs with sufficient polymer loading can be used to manage aggressive CCP leachates.     

Effect of geosynthetic component characteristics on the potential for GCL internal erosion

Experiments quantifying GCL permittivity and the ultimate water head the GCLs can sustain before the initiation of internal erosion when underlain by a 50 mm angular to subangular gravel subgrade are conducted. The influence of different geotextiles over the subgrade, water heads, hydration periods before testing, masses per unit area of bentonite within the GCL, and ionic strengths of the solution (cation exchange) are considered. Test results show that GCL with the scrim-reinforced nonwoven geotextile over the subgrade has the best hydraulic performance against internal erosion, followed by the woven geotextile coated with a 110 g/m² polypropylene film. A woven or nonwoven is the least useful for preventing internal erosion, with the corresponding threshold water head initiating internal erosion >39 m for scrim-reinforced nonwoven, 21 m for lightly coated woven, 4–5 m for woven and nonwoven alone, respectively. Cation exchange, length of hydration, and mass per unit area of bentonite do not notably affect the threshold water head for the subgrade examined. Once internal erosion occurs, there is a 3-order of magnitude increase in permittivity. The practical implications are discussed.     

Effect of ammonium on the hydraulic conductivity of geosynthetic clay liners

Hydraulic conductivity and swell index tests were conducted on a conventional geosynthetic clay liner (GCL) containing sodium-bentonite (Na-B) using 5, 50, 100, 500, and 1000 mM ammonium acetate (NH₄OAc) solutions to investigate how NH₄⁺ accumulation in leachates in bioreactor and recirculation landfills may affect GCLs. Control tests were conducted with deionized (DI) water. Swell index of the Na-B was 27.7 mL/2 g in 5 mM NH₄⁺ solution and decreased to 5.0 mL/2 g in 1000 mM NH₄⁺ solution, whereas the swell index of Na-B in DI water was 28.0 mL/2 g. Hydraulic conductivity of the Na-B GCL to 5, 50, and 100 mM NH₄⁺ was low, ranging from 1.6–5.9 × 10^?11 m/s, which is comparable to the hydraulic conductivity to DI water (2.1 × 10^?11 m/s). Hydraulic conductivities of the Na-B GCL permeated with 500 and 1000 mM NH₄⁺ solutions were much higher (e.g., 1.6–5.2 × 10^?6 m/s) due to suppression of osmotic swelling. NH₄⁺ replaced native Na⁺, K⁺, Ca²⁺, and Mg²⁺ in the exchange complex of the Na-B during permeation with all NH₄⁺ solutions, with the NH₄⁺ fraction in the exchange complex increasing from 0.24 to 0.83 as the NH₄⁺ concentration increased from 5 to 1000 mM. A Na-B GCL specimen permeated with 1000 mM NH₄⁺ solution to chemical equilibrium was subsequently permeated with DI water. Permeation with the NH₄⁺ converted the Na-B to “NH₄-bentonite” with more than 80% of the exchange complex occupied by NH₄⁺. Hydraulic conductivity of this GCL specimen decreased from 5.9 × 10^?6 m/s to 2.9 × 10^?11 m/s during permeation with DI water, indicating that “NH₄-bentonite” can swell and have low hydraulic conductivity, and that the impact of more concentrated NH₄⁺ solutions on swelling and hydraulic conductivity is reversible.     

Interface transmissivity of conventional and multicomponent GCLs for three permeants

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.     

Investigation of initial hydration and rehydration of geosynthetic clay liners from sandy subgrades via X-ray computed tomography images

The hydraulic conductivity of geosynthetic clay liners (GCLs) widely used as barrier systems considerably depends on their hydration status after the initial hydration of virgin GCLs and the rehydration of desiccated GCLs. Free hydration tests were performed on virgin and desiccated GCLs over sandy subgrades to compare their hydration level. In addition, high-resolution micro-X-ray computed tomography (CT) images of both GCLs and sandy subgrades with different gravimetric water content (i.e. 15%, 20%, and 25%) after the initial hydration were analyzed for better insights. The results show significant influences of subgrade water content on moisture content and thickness of virgin GCLs. Water loss of sandy subgrades and the time interval necessary for reaching a steady state of desiccated GCLs during rehydration was greater and longer than virgin GCLs during initial hydration. X-ray CT images verified a dense distribution of bentonite particles, macropores, and minor desiccation cracks that existed in poorly-hydrated GCLs over unsaturated sand. On the other hand, the completely saturated sandy subgrade facilitated the hydration of GCLs, leaving a lot of macropores in the sand. The relationship between water distribution and the frequency of macropore generation observed in the upper contact zone of sandy subgrades was also indicated via these X-ray CT images.     

Factors affecting GCL hydration under isothermal conditions

The hydration of different GCLs from the pore water of the underlying foundation soil is investigated for isothermal conditions at room temperature. Results are reported for three different reinforced (needle punched) GCL products. Both a silty sand (SM) and sand (SP) foundation soil are examined. GCL hydration is shown to be highly dependant on the initial moisture content of the foundation soil. GCLs on a foundation soil with a moisture content close to field capacity hydrated to a moisture content essentially the same as if immersed in water while those on soil at an initial moisture content close to residual only hydrated to a gravimetric moisture content of 30-35%. The method of GCL manufacture is shown to have an effect on the rate of hydration and the final moisture content. The presence or absence of a small (2 kPa) seating pressure is shown to affect the rate of hydration but not the final moisture content. The GCL hydration did not change significantly irrespective of whether a nonwoven cover or woven carrier GCL rested on the foundation soil.