Performance Of Geosynthetic Clay Liners Research Articles

Modification of bentonite with black cotton soil and carboxyl methyl cellulose for the enhancement of hydraulic performance of geosynthetic clay liners

ABSTRACT Geosynthetic clay liners (GCLs) are mostly used as flow barriers in landfills and waste containments due to their low hydraulic conductivity to prevent the leachate from reaching the environment. The self-healing and swell-shrink properties of soft clays (expansive soils) such as bentonite enable them as promising materials for the GCL core layers. However, it is important to modify their physico-chemical properties in order to overcome the functional limitations of GCL under different hydraulic conditions. In the present study, locally available black cotton soil (BCS) is introduced in the presence of an anionic polymer named carboxymethyl cellulose (CMC) as an alternative to bentonite to enhance the hydraulic properties of GCL under different compositions. The modified GCL is prepared by stitching the liner with an optimum percentage of CMC along with various percentages of BCS mixed with bentonite. Hydraulic conductivity tests were performed on the modified GCL using the flexi-wall permeameter. The results suggest that the lowest hydraulic conductivity of 4.58 × 10−10 m/s is obtained when 25% of BCS is blended with bentonite and an optimum 8% CMC and further addition of BCS results in the reduction of the 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.

Modelling migration of ammonium ion through landfill bottom barriers subjected to elevated temperature and dry-wet cycles, a case study in southern Hanoi, Vietnam

Geosynthetic clay liners and compacted clay liners are common bottom barriers in landfills. One of the important steps in barrier design is to evaluate the effectiveness of the bottom barrier system in practical conditions based on modelling contaminant migration. This study simulates the migration of ammonium ions through proposed bottom barriers for design under consideration of the southern Hanoi area’s temperature and geohydrology conditions, then evaluates potential impacts on normalized ammonium concentration in groundwater. Proposed bottom barriers include geosynthetic clay liner and compacted clay liner constituting from local soils. The results indicated the reclamation of locally natural soils and geosynthetic clay liner (GCL) placement was approximately 5.2 and 13.4 times more effective in slowing ammonium migration than natural soil, respectively. Compacted clay liner of natural soils exhibited few impacts from elevated temperature. However, being sensitive to an elevated temperature at 50°C, the barrier performance of GCL caused the peak normalized ammonium concentration in groundwater to be 4.2 times higher than normal conditions. Notably, normalized ammonium concentration in groundwater increased 29 times when the geosynthetic clay liner was exposed to 2 dry-wet cycles with ammonium solution. These results provided useful references for designers and engineers to consider in selecting barrier materials in the new landfill.

Mining Leachates Effect on the Hydraulic Performance of Geosynthetic Clay Liners under Different Temperatures

Geosynthetic clay liners (GCLs) are often used as anti-seepage systems in landfills and at the bottom of tailing ponds. The anti-seepage performance of GCL will change under different temperatures. In this study, bentonite was mixed with test solutions at different temperatures to measure the basic performance indexes of bentonite components and analyze the permeability. The composition and micro-structure of bentonite at different temperatures were analyzed by X-ray diffraction, X-ray fluorescence spectrum, and SEM, and the change rule of permeability property with the mine leachates at different temperatures was understood by combining the macro-measured parameters with the microscopic analysis results. The research results indicate that the fluid loss of two bentonites increased with the increasing temperature due to the inhibition of ion exchange between bentonite and mixture by the increased temperature. The swelling index of the bentonite increased at high temperatures. The micro-structure analysis showed the increase of the pore size attributed to high temperature, and the uneven distribution of the pore size resulted in the increase of the intrinsic permeability. The study would provide the reference for the application of GCL in mining.

Hydraulic Performance of Geosynthetic Clay Liners Comprising Anionic Polymer–Enhanced Bentonites

The hydraulic performance of geosynthetic clay liners (GCLs) comprising anionic polymer–enhanced bentonites (EBs), or EB-GCLs, based on permeation with 500 mM NaCl and 167 mM CaCl2 at an average effective stress of 27 kPa and hydraulic gradients of ∼300 was evaluated. The EBs included powdered sodium bentonite (NaB) enhanced with one of seven anionic polymers, including linear, noncrosslinked poly(acrylic acid) (PA) with low, medium, and high molecular weights, sodium carboxymethylcellulose (CMC) with either low viscosity or high viscosity, a covalently crosslinked sodium polyacrylate, and an in situ polymerized bentonite polymer composite. The NaB was enhanced with polymer at mass loadings of 5%, 8%, or 10% using dry-sprinkling, dry-mixing, and/or wet-mixing methods. Hydraulic conductivity tests at a low hydraulic gradient (∼35) also were conducted to determine the effect of seepage forces. Overall, the hydraulic performance of the EB-GCLs to the NaCl solution was better than that to the CaCl2 solution. Dry sprinkling (DS) with PA generally provided the best hydraulic performance, with hydraulic conductivity values based on permeation with 500 mM NaCl and 167 mM CaCl2 of 1.3×10−11 and 4.0×10−11 m/s, respectively, for the high-molecular-weight PA added at 5% via DS and 2.9×10−11 and 4.7×10−11 m/s, respectively, for the medium-molecular-weight PA added at 8% via DS. The CMC viscosity grade did not affect the hydraulic conductivity of the EB-GCLs. All EB-GCLs eluted polymer during permeation, regardless of mixing method or polymer type. Polymer elution was correlated with preferential interaggregate flow paths in multiple EB-GCL specimens, indicating the importance of polymer retention in maintaining low hydraulic conductivity of EB-GCLs. Because standard termination criteria for the measurement of the hydraulic conductivity of conventional unenhanced GCLs (e.g., as per current ASTM standards) do not address the potential impact of polymer elution from EB-GCLs, caution should be exercised when adapting these criteria to measure the hydraulic conductivity of EB-GCLs such as those evaluated in this study.

Performance of GCLs after long-term wet–dry cycles under a defect in GMB in a landfill

Two geosynthetic clay liners (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° 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 h 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 four orders 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-punched bundles, and the bundle thermal treatment are discussed.

Permeability and solute sorption of unamended and polymer-enhanced geosynthetic clay liners

Polymer treatment is an increasingly adopted approach to mitigate the impact of aggressive liquids on the hydraulic performance of geosynthetic clay liners (GCLs). Dense prehydrated (DPH) GCLs are particular products that combine polymer treatment, prehydration and densification to improve the overall containment performance. This study investigated the saturated hydraulic conductivity, k, and heavy-metal retention in a conventional GCL and a DPH GCL. Both materials were permeated first with water and then with a synthetic solution containing zinc (Zn), copper (Cu) and lead (Pb), also with the purpose of extending a previous study where a solution of the same solutes at higher concentrations had been used. The k of the conventional GCL increased by one order of magnitude with respect to that of water upon permeation with the synthetic solution. The solutes were partly retained, and the increase in k was associated with the breakthrough of solutes. Upon permeation with the same solution, the k of the DPH GCL decreased by a factor of 1.6 with respect to that of water. No significant breakthrough of metals was observed, despite the considerably longer test duration. Under the adopted testing conditions, owing to polymer amendment and densification, the hydraulic performance and solute retention properties of the DPH GCLs were superior to those of the conventional GCL.

Influence of polymer enhancement on water uptake, retention and barrier performance of geosynthetic clay liners

This paper explores the influence of polymer enhancement on water uptake and retention by geosynthetic clay liners (GCLs) across a wide suction range (up to 106 kPa), including the low suction regime (0.1–10 kPa) typically omitted in past studies. The suction measurement methods used enabled elucidation of water uptake and retention behaviour through the framework of GCL pore structures and their corresponding suction regimes. Polymer enhanced GCLs (PE-GCLs) have high maximum water uptake, and both the water entry and air expulsion values tend to be high. Due to high swelling, the onset of geotextile confinement for PE-GCLs was observed at high suctions. The impact of polymer becomes more apparent when the bentonite achieves a pseudo-two-layer interlayer hydration state at a suction of about 40 MPa (RH = 75%). The hydration mechanism for the polymer fraction in bentonite is unique to the specific polymer type, polymer dosage, and manufacturing process. The water retention behaviour at the low suction range is caused by the in-filling of geotextile pores, bentonite swelling and extrusion, and polymer water adsorption. Insights from this study can form the basis for developing a more suitable bimodal generalised model for fitting the water retention curves of GCLs.

Air and hydraulic flow characteristics of polymer amended bentonite based unsaturated GCLs

Geosynthetic clay liners (GCLs) are used in landfill applications because of their high deformation resistance and excellent self-healing ability. In the present study, air and hydraulic flow characteristics of natural bentonite based GCL (GCL-N) and polymer amended bentonite based GCL (GCL-A) were assessed using air permeability and hydraulic conductivity measurements. The airflow characteristics of GCLs were studied at various apparent degrees of saturation (Sa) and corresponding suction using a custom-designed air permeability test setup. The effect of desiccation on the airflow characteristics of GCL-A was also examined. Further, hydraulic flow characteristics of virgin and desiccated-rehydrated GCLs were assessed using a flexible wall permeameter. The water absorption capacity of GCL-A is relatively higher than GCL-N, irrespective of the stress state. For any chosen Sa, air permeability of GCL-A is lower than GCL-N. Moreover, with the decrease in the suction from 2500 kPa to 1000 kPa, the air permeability of virgin GCL-A has reduced from 9 × 10−12 m2 to 3.57 × 10−16 m2 while the desiccated-rehydrated GCL-A has reduced from 8.86 × 10−11 m2 to 1.9 × 10−13 m2. The hydraulic conductivity of GCL-A is two orders of magnitude less than GCL-N, which shows the better performance of GCL made with amended bentonite.

Effect of overburden confining stress on hydraulic performance of geosynthetic clay liners (GCLs)

Geosynthetic clay liners are a rapidly evolving geosynthetic product used in most hydraulic barrier applications in the geo-environmental industry. Continuous research has led to new insights to overcome the shortcomings faced in deploying GCLs in the field. These include shrinkage due to shear failure on side slopes, the effect of temperature variation, and inadequacy of minimum timely confinement to achieve optimum hydraulic performance. This paper presents previous experimental data and an additional dataset from this research gathered to observe the effect of overburden confining stress on GCL hydraulic conductivity and how the findings can be used to predict the performance of a geosynthetic clay liner for a given field application. An inverse power relationship is identified between these two parameters along with the reduction in the order of the degree of hydraulic conductivity depending on the permeant material passing through. A relationship is determined to estimate the GCL hydraulic conductivity as a function of the overburden confining stress, given that it is pre or post hydrated and the permeant liquid passing through the product. It is proposed that the relationship can be used to predict the GCL hydraulic performance in the field and provide guidance in improving the serviceability of hydraulic barrier designs.

Barrier performance of geosynthetic clay liners to copper (II) chloride solutions

Three needle-punched geosynthetic clay liners (GCLs) which had been manufactured with sodium (Na+) bentonite (Na-GCL) and polymerised sodium bentonites (PLS-GCL-1 and PLS-GCL-2) were permeated with 50 and 100 mM copper (II) chloride (CuCl2) solutions. The hydraulic conductivity of Na-GCL to 50 mM copper (II) chloride gradually increased to 7·7 × 10−9 m/s. In contrast, PLS-GCL-1 and PLS-GCL-2 had hydraulic conductivities of 1·5 × 10−11 and 4·2 × 10−11 m/s to 50 mM copper (II) chloride solution, respectively. Dissolution of polymers and precipitation of copper (II) (Cu2+) ions are two reasons for the low hydraulic conductivity. Metal fittings were replaced with plastic fittings, and hydraulic conductivity tests have been conducted with both solutions. The hydraulic conductivities were still low and were equal to 7·2 × 10−11 and 6·2 × 10−11 m/s with 50 and 100 mM copper (II) chloride solutions, respectively. Chemical analyses showed that cation replacements took place between bentonites and copper (II) ions. The effluents were abundant in monovalent and divalent cations which had been released from bentonites. In contrast, a considerable amount of copper (II) ions was detected in the effluents, indicating the start of the breakthrough tendency of GCLs to copper (II) ions.

Performance of GCLs in high salinity impoundment applications

The interface transmissivity (θ) and hydraulic conductivity (k) are measured for two geosynthetic clay liners (GCLs), one with polymer-enhanced bentonite, when hydrated and permeated with saline (brine) solutions at three different concentrations and Reverse Osmosis (RO) water. Two interface transmissivity values are reported, the 2-week (θ2-week) and the steady-state (θsteady-state) interface transmissivity. For saline solution (brine) permeation, the 2-week interface transmissivity (θ2-week) is one to two orders of magnitude higher than the steady-state values. In addition, the steady-state interface transmissivity (θsteady-state) with respect to brine is almost an order of magnitude higher than that for RO water permeation. Geomembrane (GMB) stiffness has a limited effect on interface transmissivity at 150 kPa, whereas at 10 kPa the interface transmissivity decreases as the GMB stiffness decreases. GMB texture has only a small effect on interface transmissivity at different stress levels. Water prehydration reduces the effect of brine permeation on interface transmissivity and hydraulic conductivity, especially at 150 kPa. Transmissivity tends to increase as the salt concentration increases but the effect was significant at all concentrations considered (440 to 4400 mmol/l). While the effect of bentonite enhancement on interface transmissivity is unclear, the hydraulic conductivity (k) is generally lower for enhanced bentonite.

USE OF GEOSYNTHETIC CLAY LINER AS A WATERPROOFING BARRIER IN SANITARY LANDFILLS

One of the main sources of soil and water (superficial and subterranean) contamination is the inadequate solid waste disposal. In this sense, sanitary landfills have great importance for environmental protection, and in these systems, the geosynthetics materials, such as the Geosynthetic Clay Liner (GCL), are widely employed. However, for landfills with leachate recirculation, the application of GCL is vetoed by many government agencies. In view of this, this study sought to provide an analysis of recent advances in application of GCL in landfill sites. For this reason, the main configurations of landfills and characteristics of its leachate were presented, as well as of the GCL by means of case studies applied to the context. The results indicated that the most important parameter to be evaluated in GCL performance is its hydraulic conductivity, which can be influenced directly by leachate composition (conventional and recirculated) as to the presence of cations and anions. Thus, it is concluded that the evaluation of these characteristics is essential to ensure the proper performance of GCL in landfills.

USE OF GEOSYNTHETIC CLAY LINER AS A WATERPROOFING BARRIER IN SANITARY LANDFILLS

One of the main sources of soil and water (superficial and subterranean) contamination is the inadequate solid waste disposal. In this sense, sanitary landfills have great importance for environmental protection, and in these systems, the geosynthetics materials, such as the Geosynthetic Clay Liner (GCL), are widely employed. However, for landfills with leachate recirculation, the application of GCL is vetoed by many government agencies. In view of this, this study sought to provide an analysis of recent advances in application of GCL in landfill sites. For this reason, the main configurations of landfills and characteristics of its leachate were presented, as well as of the GCL by means of case studies applied to the context. The results indicated that the most important parameter to be evaluated in GCL performance is its hydraulic conductivity, which can be influenced directly by leachate composition (conventional and recirculated) as to the presence of cations and anions. Thus, it is concluded that the evaluation of these characteristics is essential to ensure the proper performance of GCL in landfills.

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.

Performance of Geosynthetic Clay Liner with Polymerized Bentonite in Highly Acidic or Alkaline Solutions

The hydraulic behavior of a polymerized bentonite (PB) and the self-healing capacity of a geosynthetic clay liner (GCL) using the PB as core material (PB-GCL) in corrosive solutions (pH ranging from 1 to 13) were investigated through a series of laboratory tests [i.e., free swelling index, swelling pressure, permeability (k) (consolidation), and leakage rate tests]. The test results indicate that the PB had a higher swelling capacity than that of the corresponding untreated bentonite (UB). Particularly, for pH greater than 12.5 solutions, PB had a higher free swelling index (FSI), higher swelling pressure, and lower k value than that of the PB with deionized water. PB-GCL with a damage hole had lower permittivity for the damage hole (ψhole) and a higher self-healing capacity than that of the GCL using UB as the core (UB-GCL). Based on the test results, it is suggested that PB-GCL can be used as an effective barrier material to contain acidic and alkaline liquids.

Optimising geosynthetic clay liner overlaps: implications on hydraulic performance

Geosynthetic clay liners (GCLs) play a major role in optimising the hydraulic performance of today’s barrier systems. Numerous researches have been carried out to evaluate and enhance the hydraulic performance of GCLs. However, there are still challenges in maintaining consistent performance at roll overlaps. This paper reviews the studies conducted to determine the liquid flow mechanism through the overlap, methodologies developed to assess the hydraulic performance and also how different factors affect the performance at the overlap. The preferential flow at the overlapping seam has been identified as a critical factor affecting the hydraulic performance, and an emphasis on understanding the mechanism of the liquid flow is identified as important. Hence, the necessity of research to quantify the horizontal (transverse) flow in the overlap region is brought into discussion. A new development proposes a potential experimental and numerical method for quantifying the preferential flow and factors affecting it in the overlapping region of GCLs.

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.

GCLs with factory-applied bentonite below geomembrane wrinkle

The hydraulic performance of geosynthetic clay liner (GCL) panels with a geotextile impregnated with factory-applied bentonite that are overlapped by 300 mm to form a seam is reported for when a wrinkle in an overlying geomembrane intersects the underlying GCL seam. Two GCLs with impregnated bentonite are examined. It is shown that the details of how a GCL is needle-punched can have a significant impact on the effectiveness of seams formed using bentonite impregnated in the cover geotextile. In one case, the hydraulic performance was quite good. In the second case, the impregnated bentonite was not sufficient to seal the unstressed portion of the seam. However, 400 g/m of powdered supplemental bentonite piled along the centre of the seam provided an adequate seal and decreased the flow by orders of magnitude. The results show the validity of manufacturers’ recommendations that stress be applied to the overlap or, if used in a composite liner, that the geomembrane be installed with no wrinkles. If these recommendations cannot be followed, then the reported experiments show that about 400 g/m of powdered supplemental bentonite piled along the centre of the seam resolved the issue.

Thermo-elasto-plastic analysis of geosynthetic clay liners exposed to thermal dehydration

Geosynthetic clay liners (GCLs) are designed to minimise contaminant migration into subsoils in landfills and other waste containment systems. The so-called mixture theory has typically been chosen as a basis for numerical models describing the complicated behaviour of GCLs subjected to thermo-hydro-mechanical changes. Despite progress in this area in the past decade, some aspects of the response of GCLs have not been captured by these simulations. For example, a typical assumption of these analyses is that the mechanical response to thermal loads of GCLs can be described by linear or non-linear elastic constitutive equations. However, there is evidence that in many instances plastic deformation cannot be avoided. The primary aim of this study is to analyse the hydration and dehydration behaviour of GCLs within a thermo-elasto-plastic framework. First, a thermo-elasto-plastic constitutive model is introduced and numerically implemented. Next, the results of numerical simulations of the performance of GCLs are presented and discussed and the effects of deviations from elastic behaviour are identified. Finally, a parametric study is conducted to identify the influence of key parameters in the constitutive model on the horizontal stress in the GCL’s bentonite during restrained shrinkage driven by thermal dehydration.