Conductivity Of Geosynthetic Clay Liners Research Articles

Numerical Investigation of Confining Pressure Effects on Microscopic Structure and Hydraulic Conductivity of Geosynthetic Clay Liners

A series of COMSOL numerical models were developed to explore how confining pressure impacts the microscopic structure and hydraulic conductivity of Geosynthetic Clay Liners (GCLs), taking into account the bentonite swelling ratio, mobile porosity, pore size, and tortuosity of the main flow path. The study reveals that the mobile porosity and pore size are critical factors affecting GCL hydraulic conductivity. As confining pressure increases, the transition of mobile water to immobile water occurs, resulting in a reduction in mobile water volume, the narrowing of pore channels, decreased flow velocity, and diminished hydraulic conductivity within the GCL. Mobile porosity undergoes a slight decrease from 0.273 to 0.104, while the ratio of mobile porosity to total porosity in the swelling process decreases significantly from 0.672 to 0.256 across the confining pressure range from 50 kPa to 500 kPa, which indicates a transition of mobile water toward immobile water. The tortuosity of the main flow path shows a slight increase, fluctuating within the range of 1.30 to 1.36, and maintains a value of around 1.34 as the confining pressure rises from 50 kPa to 500 kPa. At 50 kPa confining pressure, the minimum pore width measures 5.2 × 10−5 mm, with a corresponding hydraulic conductivity of 6.2 × 10−11 m/s. With an increase in confining pressure to 300 kPa, this compression leads to a narrower minimum pore width of 1.81 × 10−5 mm and a decrease in hydraulic conductivity to 5.11 × 10−12 m/s. The six-fold increase in confining pressure reduces hydraulic conductivity by one order of magnitude. A theoretical equation was derived to compute the hydraulic conductivity of GCLs under diverse confining pressure conditions, indicating a linear correlation between the logarithm of hydraulic conductivity and confining pressure, and exhibiting favorable agreement with experimental findings.

Evaluating the impact of bentonite granule size distribution and swelling on the hydraulic conductivity of geosynthetic clay liners

The impact of granular size, distribution, total intergranular porosity, mobile intergranular porosity, and the tortuosity of the flow paths on the hydraulic conductivity of geosynthetic clay liners (GCLs) was assessed using a COMSOL hydrodynamic model. Results showed that as the intergranular pore spaces become smaller as the bentonite granules swell, the hydraulic conductivity of the GCL decreases. This effect is more significant when the density of the bentonite is lower. Outcomes from the model also illustrate that flow in GCLs with low hydraulic conductivity occurs in fine pore spaces with a width on the order of 1 um. The mobile intergranular porosity through which flow occurs is approximately 0.05 after the bentonite swells, and is small relative to the total intergranular porosity. This indicates that most water within bentonite having low hydraulic conductivity is occluded within isolated pores. As the hydraulic conductivity decreases, the tortuosity of intergranular flow paths increases, varying from 1.38 to 1.03. The findings provide insight into the complex flow behavior in GCLs and factors that affect achieving low hydraulic conductivity.

Hydraulic conductivity of geosynthetic clay liners to trona ash leachate: Effects of mass per unit area, bundles of fiber existence, and preydration conditions

The barrier performance of geosynthetic clay liners (GCLs) to coal combustion products (CCPs) is of primary importance. One of the CCPs leachates that has a damaging effect on hydraulic conductivity is trona ash leachate (TAL). In this study, the hydraulic conductivity of sodium GCL (Na-GCL) to TAL was investigated in terms of mass per unit area (MPUA). The hydraulic conductivity of GCLs to TAL was 2.6 × 10-6 and 7.6 × 10-7 m/s when the MPUA was 3.0 kg/m2 (Mb3) and 4.0 kg/m2 (Mb4), respectively. Dye tests conducted on these GCLs showed that flow preferentially occurred through bundles of fibers existing in the GCLs. In contrast, increasing the MPUA to 5.0 kg/m2 (Mb5) led to a decrease in the hydraulic conductivity (i.e. 4.1 × 10-11 m/s). Additional tests were performed on fiber-free GCLs to determine the role of fiber bundles. Regardless of MPUA, the fiber-free GCLs had low hydraulic conductivity (6.7 × 10-11 m/s). Prehydrating Mb3 and Mb4 with deionized water (DIW) before permeation with TAL also decreased the hydraulic conductivity. The hydraulic conductivities of prehydrated Mb3 and Mb4 were 1.6 × 10-10 and 4.8 × 10-11, respectively. Chemical analyses showed that the cation exchange reaction had a negligible influence on the hydraulic conductivity. Because TAL was a potential source of Na+ throughout the tests.

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.

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.

Index tests for geosynthetic clay liners containing bentonite–polymer composites

The hydraulic conductivity of geosynthetic clay liners (GCLs) containing bentonite–polymer composites (BPCs) is controlled by a pore clogging mechanism related to the swelling of sodium bentonite (NaB) granules and formation of polymer gel in the pore space. Three index testing procedures that are influenced by bentonite swelling and rheology of the polymer gel were evaluated in this study for screening BPC GCLs for hydraulic conductivity: viscosity, liquid limit by fall-cone penetration and active water content. Experiments were conducted using polyacrylamide (PAM) polymer, dry-blended mixtures of PAM and NaB and a BPC from a commercially available GCL. These materials were contacted with inorganic solutions representing leachates from a range of waste containment scenarios. For the BPC that was evaluated, hydraulic conductivities less than 10−10 m/s were obtained when the viscosity exceeded 220 mPa s and the active water content exceeded 300%. Hydraulic conductivities less than 10−11 m/s were achieved for viscosity exceeding 350 mPa s, fall-cone liquid limit exceeding 190 and active water content exceeding 300%. Additional testing is needed to assess the generality of these index tests and the identified thresholds when applied to other BPC GCLs and other permeant liquids.

Study on the Effect of Desiccation Cracks on the Hydraulic Conductivity of Geosynthetic Clay Liner

Study on the Effect of Desiccation Cracks on the Hydraulic Conductivity of Geosynthetic Clay Liner

A Numerical Study of Chemical Compatibility of GCLs

A series of COMSOL numerical models were established to study the chemical compatibility of GCLs (geosynthetic clay liner). The effect of chemistry on the mesoscopic structure and the hydraulic conductivity of GCLs was investigated. The factors, including the initial mobile porosity, the swelling ratio, the pore size, and the ionic strength, were discussed as well. The mesoscopic mechanism of the physical and chemical processes of GCLs was explored by the COMSOL models. The hypothesis that the final mobile porosity and the final pore size are the key factors of the hydraulic conductivity of GCLs was proven by the simulation. Meanwhile, when the ionic strength increased from low to medium, the changes in pore size, mobile porosity, and hydraulic conductivity were obvious. However, when the ionic strength increased from medium to high, the changes of these parameters tended to be gentle, and the changes in hydraulic conductivity were not obvious. Moreover, a theoretical model considering the effect of the initial particle size, the initial mobile porosity, and the ionic strength was developed to predict the hydraulic conductivity of GCLs in a chemical solution. This theoretical model was verified by experimental data. A good agreement was obtained.

Evaluation of the Hydraulic Conductivity of Geosynthetic Clay Liners

Abstract The hydraulic conductivity of geosynthetic clay liners (GCLs) is not fully understood and certain gaps in knowledge are still present, such as the effect of coupled mechanical and chemical processes. The current study aimed to develop a simplified mathematical model to predict the hydraulic conductivity of GCLs, particularly regarding the coupled effects of mechanical and chemical processes. Based on Darcy's Law and Poiseuille’s Law, the method combines diffuse double layer (DDL) theory and fractal theory. External factors such as confining pressure and the concentration of the permeating solution, and inherent properties such as exchangeable cations, ionic radius, montmorillonite surface fractal dimension, the distance between two montmorillonite layers (m) after swelling at the exchangeable cation i (i denotes the primary exchangeable cations, such as Na+, Ca2+, K+, and Mg2+ in bentonite), density, and coefficient of viscosity of interlayer water between two montmorillonite layers, were considered. The proposed theoretical model gave relatively accurate predictions. A practical estimate of GCL hydraulic conductivity was also derived. The predictions were compared with experimental results and good qualitative agreement was found. From the experimental results, the proposed prediction model has a maximum deviation of ~1:10–10:1, and the empirical model has a mean deviation of ~1:15–15:1.

Combined effects of ammonium permeation and dry-wet cycles on the hydraulic conductivity and internal properties of geosynthetic clay liners

The hydraulic conductivity of geosynthetic clay liners (GCLs) permeated with deionized water (S0) and NH4+ solutions, with concentrations of 100 mg/L (S100) and 1000 mg/L (S1000), was examined under six dry-wet cycles. The internal properties of virgin, desiccated, and healed GCLs were analyzed and quantified using X-ray computed tomography images. The hydraulic conductivity of the GCLs permeated with S0 and S100 underwent a negligible change during the six dry-wet cycles, whereas that of S1000 increased by almost three orders of magnitude after two desiccations. Each desiccation, after permeating with S0 and S100, generated a completely different macro-crack pattern; however, generation of macro-cracks at the same locations from dry cycles 2 to 6 and an abundance of micro-cracks were typical for S1000. This implies the severe deterioration of bentonite due to multi-desiccations and chemical compatibility with S1000. Moreover, the swell index of bentonite exposed to S1000 was reduced by approximately half, after six dry-wet cycles. Despite the lower volume percentage of macro-cracks for S1000 compared to S0 and S100, the swelling capacity of this bentonite was insufficient to fully heal these cracks. Hence, the swelling properties of bentonite dominate crack volume with regard to determining the hydraulic conductivity of GCLs.

Hydraulic Conductivity of Bentonite–Polymer Composite Geosynthetic Clay Liners Permeated with Coal Combustion Product Leachates

AbstractExperiments were conducted to evaluate the hydraulic conductivity of geosynthetic clay liners (GCLs) containing bentonite–polymer composites (BPCs) permeated with coal combustion product (C...

Characterization of Leachates from Landfills Containing MSW-I Residues

The objective of this study was to characterize the leachate collected from landfills containing municipal solid waste incineration (MSW-I) ashes in the range of 20%–100% by weight. Results suggested that the conductivity and concentrations of major cations (Na+,K+,Ca2+) and anions (Cl− and Br−) increased with ash content, while alkalinity decreased with ash content. MSW-I ash contents had little effect on pH, TOC, NH4+, trace metals, and SO42−. Comparing the metal composition in the MSW-I ashes and associated ash monofill leachate, the leaching of metals, including Mg, Fe, Cu, Ni, and Al, were not as significant as expected. As, Cd, Pb, and Zn in ash monofill leachate were even below the quantification limit. The effect of MSW-I ash on landfill systems is further discussed. Leachate containing the MSW-I ash might increase the risk of leachate collection system (LCS) clogging. The risk of mineral precipitation in the LCS increased with the ash content mainly due to the increasing concentration of Ca. High ionic strength as well as the high anion ratio of the leachates might increase the hydraulic conductivity of geosynthetic clay liners (GCLs).

Hydraulic Conductivity of Geosynthetic Clay Liners Permeated with Acid Mine Drainage

We investigated the potential use of two geosynthetic clay liners (GCLs) as basal liners for acid mine drainage (AMD) from a coal gangue impoundment. One contained natural sodium bentonite (GCL-N), while the other contained sodium-activated bentonite (GCL-S). Chemical compatibility and the effects of effective stress and type of bentonite were evaluated. The test results showed that permeation with a synthesized AMD caused the hydraulic conductivity of the GCL-N and GCL-S to increase by 33 and 104 times, respectively, at an effective stress of 10 kPa. Under an effective stress of 50 kPa, the hydraulic conductivity of the GCL-N and GCL-S permeated with AMD was 1.1 and 6.6 times higher, respectively, than the values based on permeation with distilled water. However, when the effective stress was increased to 200 kPa, the AMD had no negative effect on hydraulic performance. The difference in hydraulic conductivity because of the type of bentonite was only observed at effective stresses of 10 and 50 kPa, with the GCL-N consistently having less hydraulic conductivity than the GCL-S. Thus, the detrimental effects of AMD permeation and low quality bentonite can be countered by applying high effective stress to the GCLs. Both GCLs types may be suitable as effective basal liners for coal gangue impoundments where relatively high stress can be applied.

Insights into Desiccation and Self-Healing of Bentonite in Geosynthetic Clay Liners under Thermal Loads

Geosynthetic Clay Liners (GCLs) are widely used for protecting groundwater from pollution sources at the surface, including applications in which they are subject to significant thermal gradients. Hence, sodium bentonite in the GCL may undergo significant dehydration and cracking, and the GCL might fail as a result. The paper presents outcomes of a set of recent experimental and numerical investigations exploring the propensity of bentonite to desiccate and self-heal, as well as means of mitigating the effect of thermal gradients on the hydraulic conductivity of GCLs. An elasto-plastic thermo-hydro-mechanical model was found to yield reasonable predictions of experimental behaviour, except for the transient phase of pre-heating hydration. Introducing an airgap between the GCL and the heat source can reduce the extent of desiccation and its effects on hydraulic conductivity. However, the effectiveness of the solution will depend on other factors including subgrade, magnitude of thermal and mechanical loads and type of GCL.

Hydraulic conductivity of geosynthetic clay liners to inorganic waste leachate

Hydraulic conductivity (k) tests were conducted to evaluate how the permeation of solidified/stabilized inorganic hazardous waste (SIHW) leachate affected the k values of two geosynthetic clay liners (GCLs): a natural sodium bentonite GCL (GCL-N) and a sodium-activated calcium bentonite GCL (GCL-S). The two GCLs were permeated with distilled water (DW) and a SIHW leachate. The effects of the type of bentonite, effective stress and prehydration were assessed. At effective stress of 30 kPa, the k to DW was 1.5 × 10–11 m/s and 5.1 × 10–11 m/s for GCL-N and GCL-S, respectively. In contrast, at the same effective stress, permeation with SIHW leachate caused the k to increase by factors ranging from 1.2 to 3733 for GCL-N and from 86 to 5294 for GCL-S, respectively. Compared with GCL-N, GCL-S had poorer hydraulic performance. Increasing the effective stress and prehydration with DW reduced the detrimental effect induced by permeation with SIHW leachate. Some GCL specimens maintained low k values to SIHW leachate even though the corresponding bentonites had small or modest swell indexes in SIHW leachate. The results of this study suggest that further evaluation is needed before using swell index test to assess the effect of SIHW leachate on the hydraulic performance of GCLs.

Hydraulic Conductivity of Geosynthetic Clay Liners with Sodium Bentonite to Coal Combustion Product Leachates

AbstractExperiments were conducted to evaluate the hydraulic conductivity of geosynthetic clay liners (GCLs) containing granular sodium bentonite that were permeated with coal combustion product (C...

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.

Thermal conductivity of geosynthetic clay liners

The thermal conductivities of powdered and granular bentonite based needle punched geosynthetic clay liners (GCLs) were investigated at different gravimetric water contents under 25, 50, 75, and 100 kPa vertical stresses. Both types of GCLs exhibited an increase in thermal conductivity with increasing vertical stress at all water contents. The effect of vertical stresses was more pronounced for the specimens hydrated at lower gravimetric water contents and this was attributed to their high initial volumetric air content. The variability of water distribution in partially hydrated GCLs has been identified as a factor that may affect their thermal conductivity. The forms of bentonites (i.e., powder or granular) affected their thermal conductivities; however, this effect was less apparent at higher gravimetric water contents due to the reduced air content and gel formation in the bentonites. Finally, the GCL thermal conductivity calculated from the measured thermal conductivities of its various constituents (i.e geotextile and bentonite) components differed from the measured values. This was attributed to the nonuniform water distribution across the GCL specimen and change in material properties when components of GCL were disassembled.

Hydraulic Conductivity of Geosynthetic Clay Liners to Low-Level Radioactive Waste Leachate

AbstractHydraulic conductivity was evaluated for eight commercially available geosynthetic clay liners (GCLs) permeated with leachate characteristic of low-level radioactive waste (LLW) disposal facilities operated by the U.S. Department of Energy (DOE). Two of the GCLs (CS and GS) contained conventional sodium bentonite (Na-B). The others contained a bentonite–polymer mixture (CPL, CPH, GPL1, GPL2, and GPH) or bentonite–polymer composite (BPC). All GCLs (except GPL2 and GPH) were permeated directly with two synthetic LLW leachates that are essentially identical, except one has no radionuclides (nonradioactive synthetic leachate, or NSL) and the other has radionuclides (radioactive synthetic leachate, or RSL). Hydraulic conductivities to RSL and NSL were identical. For the CS and GS GCLs, the hydraulic conductivity gradually increased by a factor of 5–25 because divalent cations in the leachate replaced native sodium cations bound to the bentonite. The CPL, GPL1, and GPL2 GCLs with low polymer loading (1....

Hydraulic Conductivity of Geosynthetic Clay Liners to Recirculated Municipal Solid Waste Leachates

AbstractTests 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 (∼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 (1.0–2.0×1011 m/s) was slightly less than the long-term hydraulic conductivity to the conventional leachate (2.0×10−11 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...