Needle-punched Geosynthetic Clay Liners Research Articles

Hydraulic conductivity to Jet-A1 of GCLs after up to 100 freeze–thaw cycles

Needle-punched geosynthetic clay liner (GCL) specimens subjected to up to 100 freeze–thaw cycles in the laboratory, and GCL samples recovered from the field after 3 years, are examined to assess their permeability with respect to both water and Jet A-1 fuel using flexible wall permeameters. The mean GCL hydraulic conductivity with respect to water is shown to be 3·3 × 10–11 m/s before freeze–thaw. The mean hydraulic conductivities after up to 100 freeze–thaw cycles did not change significantly, and were in the range 2·2–5·3 × 10–11 m/s. Freeze–thaw cycles did reduce the entry pressure required for Jet A-1 to begin to permeate through the GCL from about 27 to 55 kPa with no freeze–thaw cycles to 13·8–20·7 kPa for 3 to 50 freeze–thaw cycles, and to 0–13·8 kPa after 100 freeze–thaw cycles. For the GCL specimens subjected to 5, 12 and 50 freeze–thaw cycles in the laboratory, the hydraulic conductivity with respect to Jet A-1 is less than 3 × 10–11 m/s at a pressure just above the entry pressure, compared with less than 3 × 10–12 m/s for GCL samples recovered from the field. The combined effect of many freeze–thaw cycles and permeation with Jet A-1 did result in an increase in hydraulic conductivity; however, the effect was small, and the GCL performed well with a maximum hydraulic conductivity of 1 × 10–10 m/s after 100 freeze–thaw cycles.

Geomembrane strains from coarse gravel and wrinkles in a GM/GCL composite liner

The physical response of a 1.5-mm-thick, high-density polyethylene geomembrane (GM) is reported when placed on top of a needle-punched geosynthetic clay liner (GCL), buried beneath 50-mm coarse gravel and subjected to vertical pressure in laboratory experiments. Local strains in the geomembrane caused by indentations from the overlying gravel and deflections of a wrinkle in the geomembrane are quantified. A peak strain of 20% was calculated when a flat geomembrane was tested without a protection layer at an applied vertical pressure of 250 kPa. Strains were smaller with a nonwoven needle-punched geotextile protection layer between the gravel and geomembrane. Increasing the mass per unit area of the geotextile up to 2200 g/m 2 reduced the geomembrane strain. However, none of the geotextiles tested were sufficient to reduce the geomembrane strain below an allowable limit of 3%, for the particular 50-mm gravel tested and when subjected to a vertical pressure of 250 kPa. Increasing the initial GCL water content and reducing the stiffness of the foundation layer beneath the GCL were found to increase the geomembrane strains. These local strains were greater when a wrinkle was present in the geomembrane. The wrinkle in the geomembrane experienced a decrease in height and width. The wrinkle deformations lead to larger pressures beside the wrinkle and hence producing larger local strains. A 150-mm-thick sand protection layer was effective in limiting the peak strain to less than 0.3% even with a wrinkle in the geomembrane, at a vertical pressure of 250 kPa.

Long-term shear strength of geosynthetic clay liners

Geosynthetic clay liners (GCLs) often have a sandwich-like multilayer structure, e.g. bentonite encased between two geotextile layers connected by fibers or yarns, either by needle-punching or stitch-bonding. Therefore, the internal shear strength of the GCL depends on the strength of reinforcing fiber bundles or yarns and their anchoring strength in the cover and carrier geotextiles. When used on long and steep slopes and covered with thick soil layers, the GCL is permanently exposed to a combined action of compressive and shear stress. Such load conditions are characteristic for landfill covers and the slope stability of the overall cover system in the long run strongly depends on the long-term internal shear strength of the GCL. A new test method was developed to study this long-term shear behavior. The focus was not only on creep, as it is normally done, but on aging effects. The shear test devices allow the measurement of creep curves and times-to-failure at elevated temperatures in different media (tap water and de-ionized water). In this publication, the main findings of the experiments on needle-punched GCLs with and without thermal treatment are summarized. Tap water as a test medium was essential to ensure sodium to calcium ion exchange in the bentonite layer. Under this condition extremely long test durations without failure were achieved. Sliding failure occurred when de-ionized water was used. Two failure modes were observed: brittle failure of the GCLs with thermal treatment and slow disentanglement of fiber bundles for untreated GCLs. Short-term shear strength (e.g. peel strength) is unrelated to the actual long-term shear strength, i.e. to the times-to-failure achieved in long-term shear strength test. Hence, short-term shear strength alone will not provide reliable dimensioning data for product design and choice of resins. Therefore, the often suggested approach, namely, restriction to short-term tests only and application of factors of safety, is challenged by these results.

Oxygen diffusion through partially hydrated geosynthetic clay liners

One of the prime purposes of a cover system in sulphidic mine tailings is to limit oxygen intrusion into the waste deposits so that generation of acid mine drainage is minimised. In areas where humid climates tend to prevail, a resistive barrier is usually used, either as part of a conventional multi-component cover or as a capillary break cover. Geosynthetic clay liners (GCLs) can be used in either case because of their low saturated hydraulic conductivity, provided they can be kept close to saturation to minimise oxygen diffusion. This paper presents the results of a laboratory investigation into oxygen diffusion through partially hydrated needle-punched and stitch-bonded geosynthetic clay liners. It was observed that the diffusion coefficient decreased with an increase in the degree of saturation. The variations in diffusion coefficient followed the same trend for the GCLs investigated, with variation of up to four orders of magnitude for the conditions investigated, indicating that low oxygen diffusivity can be achieved if GCLs have a high degree of saturation. However, it was found that the way that GCLs are held together as a composite material has a significant effect on the variation of gas diffusivity. Finally, unique empirical relationships between the effective diffusion of needle-punched and stitch-bonded GCLs and degree of saturation appear to exist and can be used to obtain an estimate of the diffusion coefficient value for use in calculations of diffusive flux through cover systems containing stitch-bonded or needle-punched GCLs.

Dynamic Shear Behavior of a Needle-Punched Geosynthetic Clay Liner

An experimental investigation of the dynamic internal shear behavior of a hydrated needle-punched geosynthetic clay liner is presented. Monotonic and cyclic displacement-controlled shear tests were conducted at a single normal stress to investigate the effects of displacement rate, displacement amplitude, number of cycles, frequency, and motion waveform on material response. Monotonic shear tests indicate that peak shear strength first increased and then decreased with increasing displacement rate. Cyclic shear tests indicate that cyclic response was primarily controlled by displacement amplitude. Excitation frequency and waveform had little effect on cyclic shear behavior or postcyclic static shear strength. Number of cycles (greater than or equal to 10) also had little effect on postcyclic static shear strength. Shear stress versus shear displacement diagrams displayed hysteresis loops that are broadly similar to those for natural soils with some important differences due to the presence of needle-punched reinforcement. Secant shear stiffness displayed strong reduction with increasing displacement amplitude and degradation with continued cycling. Values of damping ratio were significantly higher than those typical of natural clays at lower shear strain levels. Finally, cyclic tests with increasing displacement amplitude yielded progressively lower postcyclic static peak strengths due to greater levels of reinforcement damage. Postcyclic static residual strengths were unaffected by prior cyclic loading.

Gas permeability of GCLs: effect of poor distribution of needle-punched fibres

Manufacturing defects in a needle-punched geosynthetic clay liner (GCL) may include variation of mass per unit area of the bentonite, uneven thickness of the GCL, poor bonding of the composite components (geotextile/bentonite/geotextile) and, in some cases, a poor distribution of needle-punched fibres. This last case can result in unreinforced patches in the GCL, which can be detrimental to its hydraulic or gas barrier performance and also its internal strength. This paper presents the results of an experimental investigation into the effect of the distribution of needle-punched fibres on the gas permeability of a GCL. It is shown that, at high gravimetric moisture content, a poor distribution of needle-punched fibres can result in gas permeability up to three orders of magnitude higher than in a sample with uniform distribution, whereas the difference was found to be small at low gravimetric moisture content.

The influence of bentonite extrusion on shear strength of GCL/geomembrane interface

A series of shear strength tests of needle-punched geosynthetic clay liner (GCL)/textured geomembrane interface were conducted at normal stresses ranging from 10 to 400 kPa. The geomembrane was in contact with the woven geotextile of GCL. One half of the tests were carried out on prehydrated GCL samples at low normal stress (about 1 kPa), whereas the other half on non-prehydrated samples. The prehydrated samples exposed during shearing to normal stresses of 100 kPa and above demonstrated bentonite extrusion to the area in contact with geomembrane, which was visible to the naked eye. The bentonite extruded was quantified by introducing an extrusion coefficient. The quantity of the bentonite extruded increased with an increase in normal stress, and lubrication of the contact area with bentonite resulted in reduced shear strength. Finally, the testing showed that for tests carried out on prehydrated samples at lower shear rates, lower contact shear strengths were obtained and more extensive bentonite extrusion to the contact area was observed.

Compatibility with Jet A-1 of a GCL Subjected to Freeze–Thaw Cycles

Needle-punched geosynthetic clay liner (GCL) specimens subjected to 0, 5, and 12 freeze—thaw cycles in the laboratory, and GCL specimens recovered from a composite barrier wall in the Canadian Arctic after 1 and 3 years were examined to assess the hydraulic conductivity/permeability with respect to both deionized deaired water and Jet A-l. The GCL specimens recovered from the field after 3 years had a hydraulic conductivity with respect to water that was approximately 30% less than that of the GCL specimens subjected to 12 initial freeze—thaw cycles in the laboratory, suggesting that the laboratory conditions are more severe than field conditions. The combined effects of both the freeze—thaw cycles and Jet A-l permeation increased the permeability. This increase is attributed to the creation of macropores in the GCL due to freezing and to an expansion of free-pore space due to contraction of the double layer caused by permeation of Jet A-l. Although there was an increase in permeability due to the combined effect of freeze—thaw and permeation by Jet A-l, the effect was relatively small and the results suggest that the GCL continued to exhibit good performance as a hydraulic barrier when subject to extreme climatic conditions and hydrocarbons both in the laboratory and in the field.

Discussion of “Analysis of a Large Database of GCL Internal Shear Strength Results” by Jorge G. Zornberg, John S. McCartney, and Robert H. Swan, Jr.

The discusser cites Chiu and Fox (2004) regarding a large published database containing analysis of internal and interface geosynthetic clay liners (GCL) shear strengths, including those of the discusser. One item found in this database conflicts with the authors' results ? the failure envelope. For hydrated needle-punched GCLs, the non-linear regression analysis produced a zero intercept instead of the permitted non-zero intercept. This contradicts the authors' test results. In shear-induced excess pore pressures, the discusser states that the hypothesis of the authors -- that pore water suction causes progressively larger shear strengths with increasing shear displacement rate (SDR) at low normal stresses -- is not borne out in the data from Fox et al. (1998). As for SDR effects at high normal stress conditions, the discusser notes that more research is needed.

Large Dynamic Direct Shear Machine for Geosynthetic Clay Liners

Abstract A large direct shear machina for static and dynamic shear strength testing of geosynthetic clay liners (GCLs) and GCL liner systems is described. The machine tests rectangular GCL specimens measuring 305 × 1067 mm and has a maximum shear displacement of 254 mm, which is sufficiently large to allow for the measurement of residual or near-residual shear strengths in most cases. The basic design concept for the device is to shear a GCL specimen between a bidirectional pullout plate and a stationary reaction plate, each covered with an aggressive gripping surface. The pullout plate is driven by a computer-controlled hydraulic actuator. The maximum normal stress is 2000 kPa, the maximum shear stress is 750 kPa, and the shearing system is capable of imposing general stress-controlled or displacement-controlled dynamic loading to a test specimen. The actuator has a maximum frequency of 4 Hz for sinusoidal loading with a displacement amplitude of 25 mm. The maximum displacement rate for burst loading (i.e., single thrust) at zero force is 1 m/s. The paper describes four main components of the machine: (1) the shearing system; (2) the normal stress and vertical displacement measurement system; (3) the specimen hydration system; and (4) the process control and data acquisition system. The performance of the machine is illustrated using displacement-controlled test data for the static and cyclic internal shear strength of a hydrated needle-punched GCL.

Effect of Wet–Dry Cycles and Cation Exchange on Gas Permeability of Geosynthetic Clay Liners

A series of gas permeability tests were performed on a partially hydrated needle punched geosynthetic clay liner (GCL) after exposure to wet–dry cycles and ion exchange. To be able to evaluate the effect of wetting and drying cycles combined with the effect of cation exchange, three sets of GCL samples were prepared with different types of hydrating liquid. The first set of GCL samples was hydrated with de-ionized water, which formed a baseline test series. The second and third sets were hydrated with 0.0125 and 0.125 M calcium chloride (Ca Cl2 ) solutions, respectively. All three sets of GCL samples were subjected to multiple wetting and drying cycles before undergoing gas permeability tests. Gas permeability of the GCL, hydrated with 0.0125 M calcium chloride solution, was found to be approximately one order of magnitude higher than that of the GCL hydrated with de-ionized water, whereas gas breakthrough flow was observed for all but the first wetting cycle on GCLs hydrated with the stronger 0.125 M cal...

Average bonding peel strength of geosynthetic clay liners after short-term exposure to water and jet fuel A-1

The average peak and bonding peel strengths of needle-punched thermally bonded geosynthetic clay liners (GCLs) exposed to Jet Fuel A-1 are presented. Peel testing was performed on dry and saturated GCLs and the effect of stress during exposure was examined. GCLs exposed to Jet Fuel A-1 under zero stress for up to 14 days showed no statistically significant decrease in average bonding peel strength. GCLs hydrated with water and exposed to Jet Fuel A-1 at 14 kPa showed similar results with no statistically significant decrease in average bonding peel strength for up to 7 days. There was no statistically significant decrease in average bonding peel strength for thermally- and non-thermally-treated needle-punched GCLs hydrated with 14 kPa applied stress for up to 15 days. Variability in peel strength was observed depending on sample location. The average bonding peel strength of samples obtained near the edge of the roll were 45% lower than samples obtained away from the edge.

Effect of straining on gas advective flow of a needle-punched GCL

Although geosynthetic clay liners (GCLs) have gained advantage over compacted clay liners regarding the ability to withstand large differential settlement in cover systems, the ability of strained GCLs to mitigate gas flow is still not known. A series of gas permeability tests on partially hydrated GCLs subjected to straining is presented in this paper. Tests were performed on partially hydrated needle-punched GCLs subjected to straining (up to 30% of area strain). The GCL samples were strained either before hydration or after hydration. It was found that the gas flow rate through the strained GCL depended on GCL moisture content and straining conditions. The tensile strain seemed to have no adverse effect on dry GCL (straining before hydration) as long as the geotextile components were able to maintain their functions, and the mass of bentonite per area was preserved. However, in the case of straining (up to 15% area strain) after hydration, tensile strains possibly induced tension cracks in the bentonite component, providing preferential gas flow paths and higher gas flow rate than that of an unstrained GCL. At higher degree of area strain (15% < ε ≤ 30%), the gas flow rate was found to be similar to the flow through an unstrained GCL, particularly at high moisture content (MC > 90%). Poiseuille's equation for laminar flow of fluid through space between parallel solid planes can be used to describe the flow of gas through cracks in the hydrated bentonite component.

Effect of straining on gas advective flow of a needle-punched GCL

Although geosynthetic clay liners (GCLs) have gained advantage over compacted clay liners regarding the ability to withstand large differential settlement in cover systems, the ability of strained GCLs to mitigate gas flow is still not known. A series of gas permeability tests on partially hydrated GCLs subjected to straining is presented in this paper. Tests were performed on partially hydrated needle-punched GCLs subjected to straining (up to 30% of area strain). The GCL samples were strained either before hydration or after hydration. It was found that the gas flow rate through the strained GCL depended on GCL moisture content and straining conditions. The tensile strain seemed to have no adverse effect on dry GCL (straining before hydration) as long as the geotextile components were able to maintain their functions, and the mass of bentonite per area was preserved. However, in the case of straining (up to 15% area strain) after hydration, tensile strains possibly induced tension cracks in the bentonite component, providing preferential gas flow paths and higher gas flow rate than that of an unstrained GCL. At higher degree of area strain (15% &lt; ε ≤ 30%), the gas flow rate was found to be similar to the flow through an unstrained GCL, particularly at high moisture content (MC &gt; 90%). Poiseuille's equation for laminar flow of fluid through space between parallel solid planes can be used to describe the flow of gas through cracks in the hydrated bentonite component.

Volatile organic compound diffusion and sorption coefficients for a needle-punched GCL

Test apparatus designed to examine volatile organic compound (VOC) diffusion through geosynthetic clay liners (GCLs) are described, together with procedures for evaluating the relevant diffusion and sorption parameters. Test results show that the rate of contaminant migration proceeded through the hydrated GCL in the decreasing order of DCM and DCA &gt; benzene &gt; TCE and toluene. This was attributed to varying degrees of sorption of DCA, benzene, TCE and toluene to the geotextile component of the GCL as well as to the bentonite present in the GCL. Diffusion coefficients deduced from VOC diffusion testing conducted on the GCLs at confining pressures lower than approximately 10 kPa range from 2 × 10–10 m2/s to 3 × 10–10 m2/s. This is generally lower than those reported in the literature for compacted clay liner materials. Assessments of the environmental protection afforded by a landfill liner require that all underlying soil and geosynthetics components be considered in landfill contaminant migration assessments. The results in this paper provide some of the first published data for laboratory GCL diffusion and sorption coefficients, required to perform contaminant migration assessments for five VOC contaminants commonly found in municipal solid waste leachate.

Volatile organic compound diffusion and sorption coefficients for a needle-punched GCL

Test apparatus designed to examine volatile organic compound (VOC) diffusion through geosynthetic clay liners (GCLs) are described, together with procedures for evaluating the relevant diffusion and sorption parameters. Test results show that the rate of contaminant migration proceeded through the hydrated GCL in the decreasing order of DCM and DCA > benzene > TCE and toluene. This was attributed to varying degrees of sorption of DCA, benzene, TCE and toluene to the geotextile component of the GCL as well as to the bentonite present in the GCL. Diffusion coefficients deduced from VOC diffusion testing conducted on the GCLs at confining pressures lower than approximately 10 kPa range from 2 × 10–10 m2/s to 3 × 10–10 m2/s. This is generally lower than those reported in the literature for compacted clay liner materials. Assessments of the environmental protection afforded by a landfill liner require that all underlying soil and geosynthetics components be considered in landfill contaminant migration assessments. The results in this paper provide some of the first published data for laboratory GCL diffusion and sorption coefficients, required to perform contaminant migration assessments for five VOC contaminants commonly found in municipal solid waste leachate.

Internal and interface shear strengths of unreinforced and needle-punched geosynthetic clay liners

This paper presents information from a large database that has been compiled on internal and interface shear strengths of geosynthetic clay liners (GCLs). The data were obtained from the literature, GCL manufacturers, and the authors' own tests, and are representative of the most common materials and interfaces used in the construction of landfills and other facilities that incorporate GCLs as hydraulic barriers. Internal shear strengths are presented for unreinforced and needle-punched GCLs. Interface shear strengths are presented for needle-punched GCLs placed against smooth geomembranes, textured geomembranes, drainage geocomposites, and soil. Peak and large-displacement shear strengths are reported, along with displacements at peak strength and post-peak strength ratios. The database is useful for: (1) gaining insight into the shear behavior of GCLs; (2) illustrating the variability of GCL shear test results; (3) obtaining preliminary strength parameters prior to conducting project-specific shear test...

Internal and interface shear strengths of unreinforced and needle-punched geosynthetic clay liners

This paper presents information from a large database that has been compiled on internal and interface shear strengths of geosynthetic clay liners (GCLs). The data were obtained from the literature, GCL manufacturers, and the authors' own tests, and are representative of the most common materials and interfaces used in the construction of landfills and other facilities that incorporate GCLs as hydraulic barriers. Internal shear strengths are presented for unreinforced and needle-punched GCLs. Interface shear strengths are presented for needle-punched GCLs placed against smooth geomembranes, textured geomembranes, drainage geocomposites, and soil. Peak and large-displacement shear strengths are reported, along with displacements at peak strength and post-peak strength ratios. The database is useful for: (1) gaining insight into the shear behavior of GCLs; (2) illustrating the variability of GCL shear test results; (3) obtaining preliminary strength parameters prior to conducting project-specific shear tests for final design; and (4) comparing with the results of future shear tests on similar or new GCL products.

An apparatus to measure gas permeability of geosynthetic clay liners

This paper describes a test device and test method for measuring the gas permeability of partially saturated geosynthetic clay liners. The tests were carried out on a commercially available needle punched geosynthetic clay liner (GCL). The GCL samples were partially hydrated with de-ionized water for 7–10 days under zero confinement and 20 kPa surcharge, respectively, prior to testing. Measurements of the differential pressure across the GCL samples at varying flow rates were used to calculate gas permeability at different gravimetric moisture contents. The differential pressure used in the present investigation ranged from 1 to 40 kPa. The highest pressures (>10 kPa) were used to verify the validity of Darcy's equation. The results highlighted the influence of moisture content on gas flow kinetics and also showed that the apparatus can provide a reliable and simple method of measuring gas permeability. For the GCL and conditions examined, it was found that the intrinsic permeability decreased as the volumetric moisture content increased. More importantly it was found that the pre-hydration curing process could affect the intrinsic permeability. Higher intrinsic permeabilities (when volumetric water content >40%) were obtained when the moisture was not distributed uniformly. (i.e. for samples pre-hydrated under no confinement). At lower volumetric water content (<40%) the pre-hydrating conditions did not seem to affect the intrinsic permeability values simply because there was less water available for further hydration.

Creep testing and data extrapolation of reinforced GCLs

Presented herein are the results of creep tests on three internally reinforced geosynthetic clay liners (GCLs) conducted for 1000 h at each increment of loading. Two of the GCLs were reinforced by needle punched and one was by stitch bonding. All three GCLs sustained loads up to 60% of their short-term strengths without any evidence of fiber pullout or breakage. The recorded deformations were nil at 20% loadings and were small, but measurable, at the higher loadings. Having the experimental data, the Kelvin–Chain model was used for extrapolation out to 1.0×10 6 h (114 years) from which isochronous time curves were plotted. The 100-year predicted deformation data at a 30% stress level is approximately 5 and 10 mm for the two needle punched GCLs, and approximately 10 mm for the stitch bonded GCL. It is concluded that such deformations are reasonable and can be accommodated by the reinforced GCLs currently being produced. There is a need, however, for additional investigation and to formalize a GCL specification to ensure that the proper polymer formulation is used in these materials.