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Use of Perforated Base Pedestal to Simulate the Gravel Subbase in Evaluating the Internal Erosion of Geosynthetic Clay Liners
Abstract
When geosynthetic clay liners (GCLs) are placed over coarse-grained gravel subgrades, the permittivity of the GCLs may increase because of internal erosion. To simulate this condition, geosynthetic clay liners typically are placed over gravel and tested in the laboratory under high hydraulic heads. In this study, a perforated base pedestal was used instead of gravel. The base pedestal was designed to have circular voids to represent the voids of a uniform and rounded gravel subgrade. Results obtained from tests where natural gravel and a perforated base pedestal were used were compared. To verify the effectiveness of the new approach, two different geosynthetic clay liners were tested over two different gravel subgrades. Tests also were conducted using rounded, uniform, coarse-grained gravels to compare to the results of the tests with the perforated base pedestals. The void diameter of the perforated base pedestals was chosen to be approximately the same as the maximum void size between the gravel particles. Test results indicated that a perforated base pedestal with uniform voids simulated a rounded, uniform, coarse-grained gravel subgrade in terms of internal erosion. The hydraulic heads that caused internal erosion were similar when a perforated base pedestal or a rounded gravel subgrade was placed beneath a GCL. When the same GCL was used over a base pedestal or over a gravel subgrade with equivalent void size, the difference in hydraulic heads at failure did not alter more than 5 m, except for one comparison. For most of the tests, the performance of the GCL placed over the gravel subgrade was slightly better than that of the perforated base pedestal in terms of internal erosion. These results indicated that the proposed technique of using perforated subbase to simulate gravel remains conservative for the GCLs and gravel subgrades considered as part of this study.
... The collection of surface water into these GCL lined ponds necessitates only a small investment and thereby reduces the demand on groundwater. In the applications where a GCL is used solely, the soil beneath a GCL could range from fine-grained clay to coarse-grained gravel (Ozhan and Guler, 2013). A fresh water reservoir has to be designed by taking very high water levels into consideration. ...
... As the depth of water increases, the hydraulic head and consequently the seepage flow increases and this might cause the bentonite to be migrated out through the carrier geotextile. Because of this interaction, the hydraulic conductivity of the GCL may significantly increase and this increase might even impair the hydraulic barrier capability of the GCL (Ozhan and Guler, 2013). This process is called internal erosion. ...
... The diameter sizes of the voids were chosen as 20, 15, 10 and 5 mm. It was shown by Ozhan and Guler (2013) that the perforated base pedestal with uniform circular voids successfully simulated the rounded coarse-grained gravel in terms of internal erosion. GCLs reinforced with needlepunching and unreinforced GCLs placed over base pedestals with different sized uniform circular voids were tested under high hydraulic heads of up to 50 m. ...
Geosynthetic clay liners (GCLs) are often used as lining materials for freshwater reservoirs. To irrigate agricultural land without depleting groundwater, surface water is stored in these artificial ponds. In this study, hydraulic conductivity tests were performed on GCLs placed in flexible-wall permeameters under hydraulic heads of up to 50 m in order to investigate the risk of internal erosion. In these tests, base pedestals made of Plexiglas with uniform circular voids were placed beneath the GCLs instead of a typical gravel subgrade. The voids in the base pedestal represented the voids between uniform rounded gravel particles. Different types of GCLs were tested. GCL-1 was reinforced using needle-punching technology, whereas GCL-2, GCL-3, and GCL-4 were un-reinforced GCLs that were assembled in the laboratory. We investigated the effects on internal erosion of the void size in the subbase; the geotextile component that was in contact with the subbase; the bentonite component; and the manufacturing process of the GCLs. Test results indicated that internal erosion was directly related to the void diameter of the base pedestal. The resistance of the needlepunched GCL to internal erosion was better than that of the un-reinforced GCLs. The degree of internal erosion was also related to the engineering properties of the geotextile in contact with the base pedestal. Higher tensile strength of the GCL reduced the possible potential for internal erosion within it. The type of bentonite did not have a significant effect on internal erosion.
... The collection of surface water into these GCL lined ponds necessitates only a small investment and thereby reduces the demand on groundwater. In the applications where a GCL is used solely, the soil beneath a GCL could range from fine-grained clay to coarse-grained gravel (Ozhan and Guler, 2013). A fresh water reservoir has to be designed by taking very high water levels into consideration. ...
... As the depth of water increases, the hydraulic head and consequently the seepage flow increases and this might cause the bentonite to be migrated out through the carrier geotextile. Because of this interaction, the hydraulic conductivity of the GCL may significantly increase and this increase might even impair the hydraulic barrier capability of the GCL (Ozhan and Guler, 2013). This process is called internal erosion. ...
... The diameter sizes of the voids were chosen as 20, 15, 10 and 5 mm. It was shown byOzhan and Guler (2013)that the perforated base pedestal with uniform circular voids successfully simulated the rounded coarse-grained gravel in terms of internal erosion. GCLs reinforced with needlepunching and unreinforced GCLs placed over base pedestals with different sized uniform circular voids were tested under high hydraulic heads of up to 50 m., 2007), which was designated as GCL-1, was a reinforced GCL consisting of a layer of granular sodium bentonite between a carrier woven slit-film polypropylene geotextile (108 g/m 2 ) and a cover nonwoven needle-punched polypropylene geotextile (203 g/m 2 ). ...
Geosynthetic clay liners (GCLs) are often used as
lining materials for freshwater reservoirs. To irrigate
agricultural land without depleting groundwater, surface
water is stored in these artificial ponds. In this study,
hydraulic conductivity tests were performed on GCLs
placed in flexible-wall permeameters under hydraulic
heads of up to 50 m in order to investigate the risk of
internal erosion. In these tests, base pedestals made of
Plexiglas with uniform circular voids were placed
beneath the GCLs instead of a typical gravel subgrade.
The voids in the base pedestal represented the voids
between uniform rounded gravel particles. Different
types of GCLs were tested. GCL-1 was reinforced using
needle-punching technology, whereas GCL-2, GCL-3,
and GCL-4 were un-reinforced GCLs that were
assembled in the laboratory. We investigated the effects
on internal erosion of the void size in the subbase; the
geotextile component that was in contact with the
subbase; the bentonite component; and the manufacturing
process of the GCLs. Test results indicated that
internal erosion was directly related to the void diameter
of the base pedestal. The resistance of the needlepunched
GCL to internal erosion was better than that of
the un-reinforced GCLs. The degree of internal erosion
was also related to the engineering properties of the
geotextile in contact with the base pedestal. Higher
tensile strength of the GCL reduced the possible
potential for internal erosion within it. The type of
bentonite did not have a significant effect on internal
erosion.
... The GCL tested consisted of a granular sodium bentonite placed between a woven and a 149 nonwoven geotextile without reinforcement. The difference in permittivity of a needle-150 punched and an unreinforced GCL was found to be only of approximately 0.04 order of 151 magnitude (Ozhan and Guler, 2013). By considering that internal confinement due to needle-152 punching did not cause a decrease in permittivity and it was not easy to add polymer to the 153 bentonite component of a reinforced GCL due to the fibers that held the carrier and the cover 154 geotextiles together, an unreinforced GCL was used in the triaxial permeability tests. ...
Geosynthetic clay liners (GCLs) can be used effectively as barriers in waste containment facilities. However, interaction of the GCL with a saline solution might cause a decrease in the hydraulic performance of the GCL. In order to simulate the field conditions, triaxial permeability tests were performed on a GCL permeated with 0.5 M and 0.1 M MgCl2 solutions at 20 °C, 40 °C and 60 °C. The temperature increase resulted in an increase in the permeability of the GCL and this increase is attributed to a decrease in the viscosity of the permeant fluid. In order to improve the hydraulic capability of the GCL, an anionic and a cationic polymer with a polymer content of 1% and 2% by mass in the bentonite-polymer mixture was added to the GCL. According to the results, polymer treatment caused up to two orders of magnitude decrease in permeability and improved the hydraulic capability of the GCL. Generally, anionic polymer treatment resulted in slightly lower permeabilities than cationic polymer treatment. Furthermore, 2% anionic polymer treatment caused almost no decrease in permittivity when compared with 1% polymer treatment. However, the permeability of 1% cationic polymer-treated GCL was the lowest among all the other anionic and cationic polymer-treated GCLs that were permeated with 0.1 MgCl2 solution at 60 °C.
... Afterwards, the granular Na bentonite was wetted homogenously with deionized water to enhance its bonding to the geotextiles. In the end, the woven geotextile having a diameter of 100 mm was placed over the wetted bentonite without any reinforcement [10]. ...
In this study, first, various geosynthetic clay liner (GCL) application areas were briefly defined and afterwards, the details of GCL usage in mine applications were given. GCLs are barrier materials that are preferred in mining facilities such as heap leach pads, tailing dams or waste landfills. In these applications, GCLs might be in contact with aggressive leachates that could deteriorate the hydraulic properties of the GCLs. The hydraulic capability of the GCLs is decreased by an increase in hydraulic conductivity or a decrease in swell index of the bentonite component of the GCL. Due to the biodegradation of the organic substances in a waste containment area, heat is generated and the temperature of the leachates increases. To investigate the effect of temperature, triaxial permeability and free swell tests were conducted on the GCLs that were permeated with 0.5 M MgCl2 solution and deionized water. The temperature of the permeation fluid was chosen as 20 and 40 °C. Furthermore, a cationic polymer having 1 and 2% amounts of mass was added to the bentonite component of the GCL to enhance the hydraulic capability of the GCL. Test results indicated that temperature increase in 0.5 M MgCl2 solution from 20–40 °C caused both an increase in the swell index and the permittivity of the GCLs. The increase in permittivity could be attributed to the lower viscosity of the fluid at higher temperatures. Finally, adding a cationic polymer up to the amount of 2% by mass to the bentonite resulted in almost two orders of magnitude decrease in the permittivity of the GCLs that were permeated with 0.5 M MgCl2 solution. In conclusion, cationic polymer-treated GCLs might be used effectively in waste containment facilities by both decreasing the permittivity and increasing the swell index of the GCLs.
Geosynthetic clay liner (GCL) is a lining material that is used in waste containment facilities with its low hydraulic conductivity and barrier capability. In this study, %0.5, %1 and %2 cationic polymer by mass was added respectively to the bentonite component of the GCLs and triaxial hydraulic conductivity and free swell tests were performed on the GCLs that were permeated with 0.1 M KCl, 0.5 M KCl and 0.1 M MgCl2 saline solutions respectively in order to evaluate the hydraulic performance of the GCL. As a result, %0.5 cationic polymer in 0.1 M KCl solution and %1 cationic polymer in 0.5 M KCl and 0.1 M MgCl2 solutions improved the hydraulic performance of the GCL by causing almost 0.13, 0.18 and 0.08 times decrease in hydraulic conductivity respectively. However, additional polymer treatment resulted in either no change or increase in hydraulic conductivity. Furthermore, swell index of the GCL increased by adding up to an amount of %2 cationic polymer to the GCL. The effect of adding cationic polymer to the GCL on hydraulic conductivity and swell index was related to the electrostatic forces among polymer-bentonite-saline solution and diffuse double layer respectively. Increasing the concentration and valence of the cation in the saline solutions resulted in both increase in hydraulic conductivity and decrease in swell index. According to the test results, the cationic polymer improved the hydraulic properties of the GCL and resulted in a satisfactory hydraulic performance in saline solutions.
We evaluated the potential use of a geomembrane-laminated geosynthetic clay liner (GCL) along the slopes of a boric acid tailings dam in Emet, Turkey. Even though a compacted clay liner (CCL) had been used at the bottom of the tailings dam, it was not possible to place a CCL along the slopes of the dam due to their steepness. Triaxial permeability tests were conducted on the base GCL without a geomembrane and the results indicated that although the volumetric flow through a cross section of the GCL was measured to be very low initially, it increased after a while due to the interaction of the bentonite in the GCL and the mine leachate. For this reason, using a geomembrane-laminated GCL along the steep slopes was found to be an appropriate solution. The mechanical properties of the barrier material were evaluated by performing a parametric study, including a slope stability analysis and an anchorage design for the geomembrane-laminated GCL. Based on the results, a geomembrane-laminated GCL with appropriate mechanical and hydraulic properties was chosen.
When a geosynthetic clay liner (GCL) containing sodium bentonite is brought into contact with fluids containing other cations, the latter may exchange with the sodium present between clay layers. This modification of clay surface chemistry may change the clay microstructure and hence its hydraulic conductivity. The influence of clay surface chemistry on microstructure and permeability, after prolonged contact between two GCLs (a natural sodium bentonite GCL and a sodium-activated calcium bentonite GCL) and different fluids in oedometer cells, was investigated using exchangeable-cation analysis, small-angle x-ray scattering, and transmission electron microscopy. Results suggest that calcium carbonate in the bentonite, formed during activation of the calcium bentonite, may redissolve during contact with a dilute permeant, releasing calcium ions that exchange with sodium in the clay. This exchange leads to obliteration of a so-called "gel" phase (beneficial in terms of low permeability) and to the development of a more permeable "hydrated-solid" phase. Sodium replacement by calcium during GCL contact with a 0.1 M CaCl2 solution was found to be virtually complete, with or without GCL prehydration with dilute water. No gel phase was observed in these samples. When in contact with real leachate, however, a gel phase appeared, especially when GCL samples were prehydrated. A correlation was observed between the level of hydraulic conductivity and the relative proportions of gel phase and clay interlayer occupation by sodium.
The effect of a gravel subgrade on the hydraulic performance of GCLs is investigated. Laboratory test results show that the GCL specimens exhibit significant variation in thickness when compressed against gravel. The maximum and minimum thicknesses of the specimen were about 20 and 3mm, respectively, after consolidation by an effective stress up to 138kPa. However, the permittivity of GCLs remained very low. The permittivity of both needle-punched and adhesive-bonded geotextile-supported GCLs decreased with increasing confining stress, regardless of the type of subgrade materials. In general, larger particles led to more significant migration of bentonite. Nevertheless, there was no significant difference in the degree of bentonite migration between the two GCLs investigated.
The hydration and subsequent hydraulic performance of geosynthetic clay liners (GCLs) depend on the water-retention curve (WRC) of the GCL. Because of the inherent difficulty in obtaining the WRC for these materials, limited data exists regarding the WRCs of GCLs in the literature. In this study, high-capacity tensiometers and capacitance relative humidity sensors were used to quantify the water-retention behavior of GCLs for four different GCL products that vary both in materials (woven and nonwoven geotextiles) and in fabrication detail (thermal treatment and needle-punching). The water-retention behavior was investigated under wetting and drying paths; we present results in terms of gravimetric and volumetric moisture content and bulk GCL void ratio. The WRCs of the different GCL products showed significant variation among wetting and drying curves, indicating that both needle-punching and thermal treatment have a significant effect on the swelling behavior of the GCL and its WRC. Theoretical equations were fit to the experimental data, establishing the parameters that can be used for numerical modeling of these four GCL products.
Over the past decade, geosynthetic clay liners (GCLs) have gained widespread popularity as a substitute for compacted clay liners in cover systems and composite bottom liners. They are also used as environmental protection barriers in transportation facilities or storage tanks, and as single liners for canals, ponds or surface impoundments. As a result, they are being investigated intensively, especially in regard to their hydraulic and diffusion characteristics, chemical compatibility, mechanical behaviour, durability and gas migration. In this paper, a review of the main findings is presented with the focus on the critical aspects affecting the service life of GCLs. From this work, a general insight is gained on the design implications for systems that incorporate GCLs.
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 400kPa. 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 1kPa), whereas the other half on non-prehydrated samples. The prehydrated samples exposed during shearing to normal stresses of 100kPa 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.
Experimental results from flow tests to assess the hydraulic performance of a geosynthetic clay liner (GCL) when subjected to extrusion beneath coarse (50 mm) gravel are reported. Although the permittivity was slightly higher then when loaded uniformly, it remained very low (6.3 × 10-1 S-1) when the GCL was placed directly beneath coarse gravel and subjected to 250 kPa of overburden pressure. In some cases, hydraulic failure from internal erosion was observed when the final GCL thickness beneath the gravel contact was less than 3.5 mm. The GCL was much more susceptible to failure from internal erosion when a geotextile protection layer was installed above the GCL as a result of the increased transmissivity between the gravel and GCL allowing the large head to act on the thinnest part of the GCL. The head required to result in failure of the GCL by internal erosion decreased as the minimum thickness of the GCL decreased and it was very sensitive to thickness being less than 1 m for a GCL thickness less than 2 mm and at least 100 m for a GCL thickness greater than 3.5 mm. The results suggest that if a protection layer is selected to limit the tensile strains in an overlying geomembrane to less than 3%, the reductions in GCL thickness from bentonite extrusion would be sufficiently small such that the particular GCL tested would not be at risk of hydraulic failure from internal erosion for the conditions tested for most practical cases (i.e. heads much less than 100 m).
Experimental results from physical testing are reported to examine the thickness and hydraulic performance of three geosynthetic clay liners (GCLs) overlying a geonet when subjected to vertical stresses (e.g., as may be found in a secondary leachate collection layer or hydraulic control layer in solid waste landfills). The GCL was found to intrude into the underlying geonet and the effects of GCL type and water content, temperature, applied pressure, and test duration on the final GCL thickness are examined. The results are consistent with GCL deformation from the beneficial consolidation of bentonite as opposed to lateral extrusion of bentonite. Results from fixed ring flow tests suggest that the indentations in the GCL caused by intrusion into the underlying geonet do not appear to negatively impact the hydraulic performance (permittivity or resistance to internal erosion) of the particular GCLs tested for the conditions examined. The flow capacity of the geonet in these tests was found to depend not only on the amount of GCL intrusion but also on the orientation of the geonet relative to the flow direction.
GCLs incorporating different combinations and types of carrier geotextile and resting on three different subgrades are examined using a fixed-ring hydraulic conductivity apparatus. The results demonstrate that for GCLs with a conventional woven or nonwoven carrier geotextile and resting on a gravel or geonet, high hydraulic gradients can cause internal erosion within the GCL that can result in an increase in hydraulic conductivity of at least one order of magnitude. The method of GCL construction is shown to be important and GCLs with a scrim-reinforced nonwoven geotextile carrier layer performed better than those with a single light-weight (woven or nonwoven) geotextile carrier. The tests on GCLs with the scrim-reinforced geotextile carrier layer indicated that hydraulic gradients of up to 7000 (equivalent to about 70m of water head) could be sustained without measurable internal erosion for the cases examined. When the GCLs were resting on a sand subgrade, all GCLs performed adequately (i.e. no evidence of internal erosion) in these tests with applied water heads of up to about 70m.
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.
Hydraulic performance of adhesive-bonded and needle-punched geotextile-supported geosynthetic clay liners (GCLs) was measured for specimens covered with uniformly graded fine, medium, and coarse gravel using flexible-wall permeameters. Local measurements of specimen thickness, water content, and mass per unit area were taken after each test to quantify thickness variability and bentonite migration/consolidation behavior. Control tests were also performed to measure GCL hydraulic performance under conventional testing conditions. Test results showed that the hydraulic performance of control and gravel-covered specimens of both GCL products improved with increasing confining stress. In addition, measured values of hydraulic conductivity for the gravel-covered specimens were less than the manufacturer's specification. The amount of bentonite migration generally increased with increasing cover soil particle size and rate of loading. Thickness variability and bentonite migration were generally less for the needle-punched product, which is attributed to the additional confinement provided by the reinforcement. The placement of an additional non-woven geotextile over the adhesive-bonded GCL was somewhat effective in reducing thickness variability and bentonite migration under rapid loading conditions.
Geosynthetics – Wide-Width Tensile Test
ISO 10319, 2008, " Geosynthetics – Wide-Width Tensile Test, "
International Organization for Standardization, 2nd ed.,
Geneva, Switzerland, p. 16.
Internal Erosion of Geosynthetic Clay Liners Under High Hydraulic Heads
- H O Ozhan
Ozhan, H. O., 2011, "Internal Erosion of Geosynthetic Clay Liners Under High Hydraulic Heads," Ph.D. thesis, Bogaziçi University, Istanbul, Turkey.