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The effect of applied stress (20, 60, and 150 kPa) on the diffusion of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) through a geosynthetic clay liner (GCL) is examined. The diffusion coefficients deduced from GCL diffusion tests for PFOA and PFOS decrease linearly with decreasing final bulk GCL void ratio (increasing applied stresses). The different components of the same GCL are also tested for PFOA and PFOS sorption. No statistically significant sorption of PFOA is observed for any of the components of the GCL. However, some sorption of PFOS onto the cover and carrier geotextiles of the GCL is observed with in an average distribution coefficient, K d ∼2.22 ml/g for the GCL. Permeants containing different PFAS compounds are tested to assess their impact on the Geomembrane (GMB) - GCL interface transmissivity in composite liners. Results show PFAS concentrations up to 20 ppm had negligible impact on the GMB-GCL interface transmissivity. Lastly, the GCL specimens extracted from the diffusion tests are tested for hydraulic conductivity. No impact of PFAS is seen on the hydraulic conductivity of GCLs subjected to high applied loads, but a small increase is seen on the GCLs subjected to relatively low applied stresses. • First diffusion test and parameters on PFAS migrating through a GCL and evaluation of a diffusion coefficient for PFOS and PFOA. • First interface transmissivity test with a PFAS-contaminated solution and assessment of the effect of PFAS on interface transmissivity and evaluation of the effect of PFAS on the hydraulic conductivity of GCL used in transmissivity tests. • Use of a diffusion test to independently verify batch test results for partitioning of PFOS and PFOA to a GCL. [ABSTRACT FROM AUTHOR]

The correlation between the single-point notched constant tensile load-stress crack resistance (SP-NCTL SCR) Test (ASTM D5397; Appendix) of smooth high density polyethylene (HDPE) geomembranes and their strain hardening modulus is investigated for both unaged and aged specimens. The strain hardening modulus was calculated based on the (force-elongation) raw data from the tensile strength test conducted at room temperature using Type IV and/or Type V specimens (as described in ASTM D638) at a test speed of 7 mm/min. Three different approaches are used to define the strain hardening modulus and to compare the representative strain hardening modulus with the SP-NCTL SCR. It is shown that the high test speed of 7 mm/min performed at room temperature provides a good correlation with the SP-NCTL SCR of different smooth black HDPE geomembranes. Additionally, the proposed method using Type V specimens predicts the SCR values during oxidative degradation close to those observed using the SP-NCTL SCR test. For the resins and conditions examined, the proposed method provides a quick assessment of the SP-NCTL SCR of unaged geomembranes when the SP-NCTL SCR takes long testing times (e.g., >1000 h) or in jurisdictions in which the use of surfactants becomes prohibited to allow conducting the SP-NCTL SCR tests. • Presents a correlation between the NCTL-SCR of HDPE GMBs and strain hardening slope at room temperature at 7 mm/min. • Presents a correlation with a transformed post-yield load-elongation curve for unaged specimens using Type V and IV specimens. • Presents a correlation with a transformed post-yield load-elongation curve for aged and unaged specimens using Type V specimens. • Shows that the strain hardening test conducted at room temperature at 7 mm/min can predict the SCR with the same level of accuracy as the NCTL tests. [ABSTRACT FROM AUTHOR]

Experiments are conducted to investigate the effect of a drainage layer in/below saturated tailings on leakage through a hole of various shapes and sizes in a 2-mm-thick HDPE geomembrane. Three cases: no drainage (Case 1), drainage layer between two layers of tailings (Case 2), and drainage layer below tailings and above the GMB (Case 3) are examined. Analytical solutions predicting leakage through a circular GMB hole overlain by tailings with an internal drainage in (Case 2) and below (Case 3) tailings are developed, and match the experimental and numerical results well. Results show that a drainage layer separated from the GMB by a thin layer of tailings (Case 2) gives leakage slightly greater than if no drainage layer is present, but 3-5 orders of magnitude lower than when the drainage is placed directly over the GMB (Case 3). In Case 3, leakage is dependent on the hydraulic conductivity k of both the drainage and subgrade, and is not affected by the tailings. Unlike Cases 1 and 2 where the subgrade has negligible effect on leakage, a low permeability subgrade with k less than 10% of k for the drainage layer is recommended in Case 3 to minimize leakage through geomembrane defects. • first investigation of the effect of an internal drainage within tailings on leakage through a GMB defect. • first evaluation of where to place the internal drainage in order to minimize leakage through a GMB defect. • first guidance on how to predict leakage through a GMB defect when a drainage is placed at different positions. [ABSTRACT FROM AUTHOR]

The stress crack resistance (SCR) of high-density polyethylene (HDPE) geomembrane extrusion welds is examined for a 1.5 mm HDPE geomembrane and three different welding parameter combinations (denoted as "Cool", "Good", and "Overheated"). Results are reported for unnotched welds, unnotched sheet, and notched sheet. The average SCR for a Good extrusion weld is 23% of that of the unnotched sheet SCR. Little variation is found between the three welding parameter combinations for low geometry irregularity SCR weld specimens. There is no statistically significant difference between a good-quality fusion and extrusion weld. However, operator-dependent weld induced geometric irregularity (WIGI) greatly affects the SCR of extrusion welds. Extrusion welds with high WIGI have an average unnotched SCR of only 9% of the unnotched sheet. Extrusion welds with an overground surface can have an unnotched SCR as little as 1% of the best extrusion weld. Deleterious weld bead geometries are identified to provide a framework with which engineers can identify "high-risk" extrusion welds with respect to stress cracking. • Paper highlights: • First examination of the effects of weld-induced geometric irregularity (WIGI) in extrusion welds on unnotched stress crack resistance. • Highlights the impact of poor quality control for extrusion welds (e.g., overgrinding, high WIGI, and scratches) on the likelihood of stress cracking in the field. • Demonstrates that a good extrusion weld (i.e., low WIGI, good grinding, and appropriate welding temperature) has a comparable unnotched stress crack resistance to that of a good fusion weld. • Demonstrates that the operator-dependent nature of extrusion welds substantially increases the variability in the stress crack resistance of extrusion welds compared to fusion welds. • Highlights the importance of doubling the standard oxidative induction time (Std-OIT) of welding rod to reach a post-welded bead's Std-OIT similar to the sheet's Std-OIT (i.e., it shows a 50% decrease in Std-OIT of the extruded bead compared to the initial rod entering the extruder). [ABSTRACT FROM AUTHOR]

The short-term and long-term performance of bituminous geomembrane (BGM) seams are examined using both small-scale and large-scale tests. Different BGM products, different sustained tensile loads, different weld qualities, and different overburden stresses are examined. The BGM seams are shown to be very susceptible to creep rupture under sustained tensile loads. Time to rupture and strain at rupture for acceptable welds are both exponentially correlated with the sustained load, for the different BGM products examined. With the increasing tensile load from 5%, 10%, 20%, 30%, 40% of sheet maximum tensile strength, the time to rupture decreased from 30–50 days (5%), 5 days (10%), 0.8 day (20%), 0.2 day (30%), to 0.03 day (40%) and, the strain at rupture increased from 5%, 7%, 13%, 17%, to 20–30%. In large-scale tests simulating field conditions, the BGM seam creep ruptured within 24 days when the overburden stress was 20 kPa, and within ≤0.2 day when the overburden stress reached 50 kPa. The consequences of liquids or gases readily permeating through failed seam should be evaluated before using BGMs in an environment where tensile stresses can develop (e.g. due to differential settlement, subgrade irregularity, or downdrag). • first investigation of the creep rupture of different BGM seams at sustained tensile loading as small as 1 kN/m. • first evaluation of the creep behavior of different BGM seams under simulated field conditions. • first correlations of time to rupture and strain at rupture both with a wide range of sustained tensile loads. [ABSTRACT FROM AUTHOR]

The performance of four 1.5-mm HDPE geomembranes from the same manufacturer each with a different resin and additive package is examined in a synthetic municipal solid waste leachate at five temperatures (40, 65, 75, 85, 95 °C) for 7.5 years. Two geomembranes used polyethylene with raised temperature resistance (PE-RT) resins and two used more conventional polyethylene resins. All four geomembranes were inferred to contain hindered amines stabilizers (HAS) and had very high off-the-roll stress crack resistance (SCR o) values that decreased to stable, more representative SCR m values shortly after immersion and it was found that the 3-stage GMB degradation model applied to both Std-OIT and HP-OIT. The time to nominal failure is predicted for a range of temperatures using Arrhenius modelling and the representative SCR m values. Despite the fact that all four GMBs had similar times to nominal failure at constant elevated temperatures (65–95 °C), it is shown that the HAS used in one of the PE-RT GMBs served a useful function in extending its life in situations such as elevated temperature landfills where there is a time-temperature history to be accommodated by the GMB. The value of considering both Std- and HP-OIT is also demonstrated. • Paper highlights: • First long-term study on PE-RT HDPE geomembranes to report the change in physical-mechanical properties with time all the way to nominal failure. • First In-depth analysis on the three stages of geomembrane degradation for two PE-RT GMBs. • Shows the beneficial role that hindered amine stabilizers (HAS) played in protecting a leachate immersed HDPE GMB not subjected to UV exposure. • In-depth analysis and discussion on polymer annealing, Arrhenius modelling, and the representative stress crack resistance of HDPE geomembranes. [ABSTRACT FROM AUTHOR]

The durability of five 1.5-mm thick geomembranes (GMBs) is investigated in pH 0.5 and 13.5 synthetic mining solutions using immersion tests. Two high density polyethylene (HDPE), two linear low density polyethylene (LLDPE), and one blended polyethylene (BPO) GMBs are investigated at 85 °C for incubation durations of 4.5–6.5 years. It is shown that the degradation of all five GMBs in the high pH solution is faster than in the low pH solution. In the pH 0.5 solution, one of the HDPEs and the BPO GMBs exhibited polymer degradation before or at the time of the depletion of their antioxidants. In pH 13.5, four out of the five GMBs exhibited degradation and followed the conceptual three-stage degradation model until nominal failure. However, there is no correlation between the long-term performance of these GMBs and their resin type or their initial properties since one of the examined LLDPEs outperformed all the higher density/crystallinity GMBs with higher initial properties while the other LLDPE did not perform well. Thus, when selecting a GMB for a desired application, the relative performance of different candidate GMBs can be only assessed using immersion tests using the solutions expected in the field. • First comparison of the degradation behaviour of HDPE, LLDPE, and BPO geomembranes incubated for up to 6.5 years at 85 °C. • Estimates the time to nominal failure in pH 0.5 and pH 13.5 synthetic mining solutions. • Examines the role of the stabilizers and the resin type on the longevity of five different geomembranes. • Compares the morphology of the failure surfaces after stress cracking from HDPE, LLDPE, and BPO geomembranes. [ABSTRACT FROM AUTHOR]

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

Experiments are conducted to quantify leakage through saturated tailings (with 30% fines) underlain by a geomembrane with either a 100-mm-length knife cut slit defect or a 110-mm-length defective extrusion seam. Various subgrades, a poorly graded gravel (GP) with or without nonwoven geotextile (450 or 1420 g/m2) above, a well-graded gravel (GW), and a poorly graded sand (SP) are evaluated. Test results show that leakage through the slit defect and defective seam increase with the subgrade coarseness and subgrade unevenness. The inferred upper and lower bounds opening width of a slit based on the measured leakage increase with the overburden pressure and are categorized for each subgrade. Indentations in the overlapped defective extrusion seam area arise from both the entrapped materials inside the overlap and the materials underlying/overlying the geomembrane, incrementally inducing a greater interface transmissivity with the increasing stress. Overlain by tailings at total overburden pressure of 510 kPa and water head of 35m, leakage through the slit defect is 1.1L/day for SP, 2.2L/day for GW, 2.6–3.3L/day for both GP and uneven GW; leakage drops to 0.8L/day with a geotextile layer directly above the GP; leakage through the defective extrusion seam is 0.4L/day for SP and 0.8L/day for GW. • First indication of the effect of water pressure on leakage through a slit defect and inferred opening size. • First evaluation of how overburden stress and subgrade affect leakage and opening size of a narrow slit or seam defect. • First guidance regarding a reasonable range for the opening width of a slit defect for different subgrades. • First comparison of leakage between a slit defect and defective extrusion weld. [ABSTRACT FROM AUTHOR]

The difference in seam squeeze-out antioxidant loss (in terms of standard oxidative induction time, Std-OIT loss) and thickness reduction are evaluated for three different 1.5 mm-thick high density polyethylene (HDPE) geomembranes (GMBs) seamed using a variety of welding parameters and two different wedge welders. Partial squeeze-out antioxidant loss was detected in seams created from each of the three materials examined, with the majority off loss occurring when seam thickness reductions fell between 0.4 mm and 0.8 mm. Seams with thickness reduction exceeding 0.8 mm were found to exhibit greater squeeze-out Std-OIT loss, with near full Std-OIT depletion for one material. Wedge welder size was found to influence this relationship, some seams created with the large wedge welder exhibiting a near full Std-OIT depletion from squeeze-out at approximately 0.6 mm thickness reduction. Variation in seaming pressure and high load melt index (HLMI) were found to shift the degree of thickness reduction a seam may experience for a given welding speed and temperature, with higher seaming pressure and HLMI values generally resulting in greater thickness reduction. Although, for a given welding speed, wedge temperature, and sheet temperature combination, changes in seaming pressure had a limited effect on squeeze-out Std-OIT loss. This paper provides a rational basis into defining a practical 1.5 mm fusion seam thickness reduction criteria based on limiting antioxidant loss within a seam's squeeze-out and also provides a framework for identifying potentially higher risk fusion seams for future research. • First detailed evaluation of relationship between seam squeeze-out antioxidant loss and thickness reduction. • First detailed evaluation of effect of choice of welding parameters and wedge welder. • Provides a rational basis into defining a 1.5 mm fusion seam thickness reduction criteria based on limiting antioxidant loss. • Provides a framework for identifying potentially high risk fusion seams. [ABSTRACT FROM AUTHOR]

Experimental data is presented for leakage through slits, square, and rectangular geomembrane defects overlain by silty sand tailings and underlain by a well-graded gravel. The rectangular GMB defects have a range of widths, B , lengths, L , and aspect ratios (L / B), and ratios of defect length to thickness of tailings above the geomembrane (L / T). A blade cut slit (or simulated stress crack) widens to an extent dependant on the subgrade and stress level. For L= 100 mm, increasing B (0.15 < B / L ≤ 1) from 1.6 (slit) to 100 mm (square), decreases the leakage from about twice to essentially the same as that for a circular hole of equal area. The ratio (h 1 / H) of head loss within the hole, h 1 , relative to the total head loss, H , is independent of loading conditions and constant for any particular hole shape (B / L), area (B ∙ L), and relative depth (L / T). A semi-empirical general solution is developed for a rectangle within the range of 0 ≤ B / L ≤ 1 with the solution converging to that for a strip for B / L → 0 and to a circle for B / L → 1. The solution gives calculated leakage in encouraging agreement with the experimental data. • Investigates the factors affecting leakage through a rectangular hole in a geomembrane overlain by tailings. • Studies how hole geometry affects leakage. • Provides a semi-empirical general solution for predicting leakage through quasi circular, rectangular, and strip (L » B) holes. • Investigates the physical and hydraulic response of a blade cut slit with flat and uneven ground conditions. [ABSTRACT FROM AUTHOR]

Experiments are conducted to study leakage and piping through circular geomembrane holes from the overlying silty sand tailings. Three subgrades, a poorly graded gravel, a well-graded gravel, and a poorly graded sand are studied. Test results show that leakage through the geomembrane hole is not proportional to the surface area of the hole. A 5-fold increase in hole diameter from 10 to 50 mm results in an approximately 6-fold increase in leakage. With the filter incompatible subgrade, piping is prone to occur at small consolidation stress and/or with a larger hole. Open voids within soil skeleton induced by piping result in a 2- to 4.5-fold higher leakage than in the no-piping cases. The extent of silty sand piping and internal erosion depends on the filter compatibility and surface regularity of the subgrade, which could be significantly improved by increasing its fine particle component or placing a layer of nonwoven needle-punched geotextile beneath the geomembrane. 1. Investigates the factors affecting piping through geomembrane defects overlain by tailings 2. Evaluates the increase in leakage and potential for failure due to piping 3. Explores the tailings erosion process with piping 4. Provides a solution to prevent piping and mitigate tailings erosion [ABSTRACT FROM AUTHOR]

Experiments are conducted to quantify leakage through holes in a geomembrane (GMB) overlain by tailings with 30% fines and underlain by a well-graded gravel. Analytical and empirical equations for predicting leakage through a circular hole are used to analyze the data. Test results show that changing the hole shape from circular to noncircular increases the leakage. Leakage through 2000 mm2 triangular, diamond, and square holes are all 15% higher than the 2000 mm2 circular hole. An increase by up to 33% is observed for a rectangular hole. The analytical equations for predicting leakage and head loss through a hole given by the modified Rowe-Booker equation are experimentally verified. The spatial distribution of fines content within about five diameters of the hole is shown to have a notable impact on leakage. Up to a 100% increase in leakage is detected due to 3% reduction in the fines content around the hole. For homogeneous tailings, the ratio of head loss within the hole and above the hole to total head loss are independent of consolidation stress and water head and primarily depend on the hole geometry (i.e., shape and size). Finally, the combined effect of tailings thickness and water head on leakage is discussed. • First experimental research on the effect of noncircular hole shape on leakage through geomembrane in tailings storage application. •First experimental research on the impact of tailings heterogeneity on leakage. •Verify the analytical and empirical equations to predict leakage through a circular geomembrane hole overlain by saturated tailings. •Explore the hole shape factors for the noncircular hole in terms of leakage prediction. [ABSTRACT FROM AUTHOR]

A laboratory investigation was conducted on two different conventional GCLs (one with fine granular and another one with powdered bentonite) to explore the effect of prehydration and permeant fluid; GCL desiccation on the interface transmissivity, θ , between the interfaces of a 1.5 mm-thick high-density polyethylene (HDPE) geomembrane (GMB) and a GCL. The study also aimed to assess the self-healing capacity of desiccated GCLs for three different permeant solutions under a range of applied stresses (10–150 kPa). It was found that at stresses less than 70 kPa, θ was dominated by variability in the initial contact condition between the GMB-GCL interfaces. The effect of other factors was largely masked by the contact variability. At 100–150 kPa, the effects of initial variability were largely eliminated, but there was no notable effect of other factors on θ in the absence of desiccation. GCL desiccation increased θ by up to three orders of magnitude than an intact specimen at 10–100 kPa. Even at 150 kPa, desiccated specimens had a θ ≤ 8.0 × 10−9 m2/s for all specimens tested. The chemical composition of the permeant solutions, crack width, and nature of bentonite could play an important role in healing the cracks of desiccated GCLs. • First study of the effects the GCL desiccation on interface transmissivity between a geomembrane and GCL. • First study of the effects of different permeants on the interface transmissivity between a geomembrane and desiccated GCL. • First assessment of the effect of contact conditions on interface transmissivity between a geomembrane and desiccated GCL. [ABSTRACT FROM AUTHOR]

The performance of five different GCLs (two GCLs with standard sodium bentonite and three GCLs with polymer enhanced bentonite) subjected to three different climatic modes of wet-dry cycles simulating conditions to which a GCL might expose in cover systems over a prolonged time is reported. The wetting cycles lasted for 8 h, while the drying cycles varied between 16 h, seven days, and 14 days. It is shown that after around a year of accelerated aging, the hydraulic conductivity of the aged GCLs increased notably when permeated with tap water at an applied effective stress of 15 kPa for a range of heads (0.07, 0.14, 0.21, 0.49, and 1.2 m). The combined effects of the number and the duration of the wet-dry cycles, the GCL's mass per unit area, the carrier geotextile, the size and the number of the needle punch bundles, and the thermal treatment to bond the needle-punch bundles to the carrier geotextile are discussed. The poor hydraulic performance of the polymer-amended/modified bentonite GCLs is discussed. • First study of the effects water flow over the GCL on inferred washout of polymers. • First study of the effects of different lengths of drying cycles. • First assessment of the relative performance of standard and polymer amended GCLs when subjected to wet-dry cycles. [ABSTRACT FROM AUTHOR]

The self-healing of a GCL with a circular hole is examined in experiments where the GCL, overlain by geomembrane, is hydrated from a silica sand subgrade (SSS) having three different pore water chemistries. Factors considered included: hole size, subgrade initial moisture content w fdn , GCL mass per unit area, and overburden stress (20–100 kPa). GCL self-healing is better for w fdn = 16% than for w fdn = 10%, which is better than for 5%, when the SSS pore water has negligible cations (ionic strength, I < 0.1 mM). However, only the 14.3 mm-diameter hole fully self-healed and only when w fdn = 16%. In contrast, when the GCL is hydrated from SSS with pore water having an ionic strength, I , of 20 and 30 mM, the self-healing for w fdn = 5% is better than for w fdn = 10%, which is better than for w fdn = 16%, although none of the holes self-healed. When a ~0.5 m hydraulic head was applied above the GCL under σ v = 20–100 kPa, a 38.1 mm-diameter hole self-healed with water having I < 0.1 mM, a 25.4 mm-diameter hole self-healed with pore water with I = 20 mM and 30 mM, but none self-healed with simulated synthetic landfill leachate (SSL). Post-hydration hydraulic conductivity (k) tests with SSL suggest that a hole up to 14.3 mm-diameter would not pose a significant adverse impact on the k compared to an intact GCL; however, this is not the case for the larger holes tested. • Hole D , GCL M A , w fdn , σ v , chemistry of subgrade pore water, ponded fluid and permeant all affect self-healing and k of GCL. • There is better self-healing with increasing w fdn for the GCL hydrated from SSS with DI pore water. • The self-healing was less with increasing w fdn when the GCL is hydrated from SSS with PW1 or PW2 pore water. • Other things being equal, self-healing improves with increasing M A of the GCL. • A hole as large as 14.3 mm self-heal at w fdn = 16% but not at w fdn = 5 and 10% upon hydration from SSS with DI pore water. • No hole examined (14.3 - 25.4 mm) completely self-heal upon hydration from SSS with PW1 or PW2 pore water. • The cations in pore water PW1 and PW2 inhibit the self-healing of holes in the GCL and result in a higher k. • All holes examined experience sufficient self-healing when permeated with TW or SSL to give k less than 2.4×10-9 m/s. [ABSTRACT FROM AUTHOR]

The interface transmissivity (θ) of two multicomponent geosynthetic clay liners (GCLs) is investigated upon hydration and permeation with a highly saline solution (TDS ≈ 260,000 mg/l; Na+ ~ 95,000 mg/l; K+~12,000 mg/l) at two stress levels (10 kPa and 150 kPa). One GCL had a smooth 0.2 mm-thick coating whereas the second GCL had a textured 1 mm-thick coating. For both GCLs, the interface transmissivity after 2-weeks is shown to be higher than at steady-state. The lower the geomembrane's (GMB) stiffness, the lower interface transmissivity. However, the effect is generally diminished at steady state and higher stress. The effect of GMB stiffness at 10 kPa is shown to be 1.6-times that at 150 kPa. Similarly, the 2-week and steady state interface transmissivity for the textured GMB was higher at 10 kPa than at 150 kPa. Coating texture and coating orientation are shown to have a significant effect on GMB/multicomponent GCL interface transmissivity. A hole in the coating aligned with GMB hole creates an additional flow path at the coating/GCL interface (θ Geofilm/GCL), however most of the flow occurs at the coating/GMB interface (θ Geofilm/GMB). [ABSTRACT FROM AUTHOR]

Aqueous diffusion of trichloroethylene (TCE) and tetrachloroethylene (PCE) is examined for high density polyethylene (HDPE), linear low density polyethylene (LLDPE), polyurethane/urea, and two polyethylene (PE) geomembranes coextruded with ethylene vinyl alcohol (EVOH). Additionally, the diffusion of benzene, toluene, ethylbenzene, and xylenes through polyurethane/urea geomembrane is examined. Permeation coefficients for HDPE, LLDPE, and polyurethane/urea range from 0.4-1.2 × 10−10 m2/s for TCE and 1.0-2.5 × 10−10 m2/s and for PCE. Experiments using the coextruded geomembranes have not reached equilibrium at 500 days, however parameters for the EVOH layer are deduced using data from these experiments. Using the parameters of the individual layers, single layer parameters were calculated. These single layer parameters range from 0.37-2.2 × 10−12 m2/s for TCE to 0.28-0.93 × 10−12 m2/s for PCE. Two hypothetical vapour intrusion cases are modelled using the parameters developed for the five geomembranes, and the calculated airspace concentrations decrease depending on the choice of vapour barrier in the following order: no barrier >0.75 mm LLDPE >1.5 mm polyurethane/urea >1.5 mm HDPE >0.75 mm LLDPE/EVOH/LLDPE >1.5 mm HDPE/EVOH/HDPE. • First BTEX diffusion and TCE and PCE for polyurethane/urea geomembrane. • Full layer parameters for diffusion through co-extruded geomembranes with EVOH core. • First evaluation of the relative performance as a vapour barrier of five GMBs for TCE and PCE. [ABSTRACT FROM AUTHOR]

The paper examines the effects of settlement-induced downdrag on geosynthetic liner systems for a waste containment facility with steep side slopes for different design scenarios, and conducts the stability analysis of the waste mass during waste filling operations. Without the presence of a reinforcing layer above a geomembrane (GMB) liner, the liner experiences unacceptable tensile strains under both short- and long-term downdrag waste settlements. It is shown that an anchored high strength/stiffness geotextile (HS-GTX) reinforcement over the GMB can reduce the GMB tensile strains to less than 3%, but the HS-GTX itself may be overloaded. A geosynthetic slip layer over the full or partial HS-GTX reinforcement overlying the GMB can reduce the tensile strains of the GMB to less than 3% and of the HS-GTX to less than 5% by providing a preferential slip plane between the geosynthetic slip layer and the HS-GTX. A rupture of the geosynthetic slip layer is likely to occur resulting in the exposure of the HS-GTX to the waste, but the protection of the GMB by the HS-GTX is still expected. The results from the stability analysis show that, during waste filling operations under a given factor of safety, there is a critical relationship between the width of the top of the waste pile and the total waste thickness. • Examines the strains developed in a geomembrane due to downdrag together with stability considerations. • Examines role of a preferential slip plane and high stiffness reinforcement. • First evaluation of the effect of geonet composite as a drainage layer on downdrag strains developed in the geomembrane. [ABSTRACT FROM AUTHOR]

The behaviour of geosynthetic clay liners (GCLs) as part of a physical-environmental system is examined. Consideration is given to: (a) both the physical and hydraulic interactions with the materials, and the chemical interactions with the fluids, above and below the liner, (b) time-dependent changes in the materials, (c) heat generated from the material to be contained, as well as (d) the climatic conditions both during construction and during service. This paper explores some common perceptions about GCL behaviour and then examines the misconceptions that can arise and their implications. It demonstrates how what may first appear obvious is not always as one expects and that more is not always better. It discusses: (i) the pore structure of a GCL, (ii) the dependency of the water retention curve of the GCL on its structure, bentonite particle sizes and applied stress, (iii) the effect of the subgrade pore water chemistry, (iv) the mineralogy of the subgrade, and (v) thermal effects. The desirability of a GCL being reasonably well-hydrated before being permeated is examined. The critical size of needle-punch bundles at which preferential flow can increase hydraulic conductivity by orders of magnitude is illustrated. The dependency of self-healing of holes on the interaction between GCL and subgrade is discussed. Finally, the transmissivity of the geomembrane/GCL interface is shown to be a function of GCL and geomembrane characteristics and to be poorly correlated with GCL hydraulic conductivity. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract The implications of the tensile stress/strain developed in high density polyethylene (HDPE) geomembranes (GMB) is explored in the context of a reduction in stress crack resistance due to ageing in contact with leachate in a municipal solid waste (MSW) landfill. The experimental evidence of GMB cracking and ultimately failure when subject to excessive tensile strains is discussed to highlight the need to limit the maximum tensile strain sustained by an HDPE GMB to an acceptable level if good long-term performance is to be ensured. The effect of both local GMB indentations induced by gravel in an overlying drainage layer or an underlying clay liner on tensile strain is reviewed. In addition, the tensile strains caused by down-drag in the GMB on side slopes with settlement of the waste is examined. The key research related to tensile strains developed in GMBs from these sources is reviewed and new data presented. It is shown that an appropriate protection layer over the GMB can limit local GMB tensile strains to less than 3% and that the selection of a suitable slope inclination and stiffness of a geotextile reinforcement layer can limit the GMB strains due to down-drag to less than 2% and geotextile strains to less than 4% after long-term waste settlement. [ABSTRACT FROM AUTHOR]

Abstract A laboratory investigation of the interface transmissivity is reported for five different geosynthetic clay liners (GCLs) and a range of different geomembranes (GMBs) for a range of stresses from 10 to 150 kPa. The GCLs were prehydrated under normal stress before permeation. The GCLs examined comprised three multicomponent (a smooth coated, a smooth laminated, and textured coated) and two conventional (one with granular and one with powdered sodium bentonite) GCLs. The effect of a 4 mm circular defect in the coating of a multicomponent GCL directly below the 10 mm diameter hole in the GMB is investigated. The effect of GMB stiffness and texture is examined. Additionally, the effect of hydration and permeation of smooth coated GCL with highly saline solution and synthetic landfill leachate (SL3) is presented. It is shown that the 2-week interface transmissivity (θ 2-week) can be one to two orders of magnitude higher than steady-state interface transmissivity (θ steady-state) at low stresses (10 kPa–50 kPa), whereas at high stresses (150 kPa) the variation is substantially less. For a smooth coated GCL hydrated and permeated with reverse osmosis (RO) water, GMB stiffness and texture has a limited effect on interface transmissivity when the coating is placed in contact with GMB at normal stresses of 10 kPa–150 kPa, whereas coating indentations result in much high interface transmissivity when placed in contact with GMB. GCL prehydration and permeation with highly saline solutions leads to higher interface transmissivity compared to RO water. With a 4.0 mm defect in the coating, the interface transmissivity between the coating and woven geotextile is higher than that between the coating and GMB for the stress levels and GCL examined. [ABSTRACT FROM AUTHOR]

Ethylene vinyl alcohol (EVOH) copolymers can provide a superior barrier to hydrocarbons and are increasingly being used in co-extruded geomembranes for geoenvironmental applications. These thin-films behave differently under different humidity conditions. This study investigated the permeation properties of toluene through two EVOH thin-films (32 mol% ethylene and 44 mol% ethylene) for both non-aqueous and aqueous solutions. The results of this study are used to gain a better understanding of the behaviour of the EVOH layer used in co-extruded geomembranes. The thin-film results are compared with published values for co-extruded linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) geomembranes with an EVOH core. Permeation coefficients are presented over a range of contaminant concentrations from 25 ppm to 99% toluene based on almost five years of continuous testing and the effect of moisture is discussed. A number of EVOH thin-films were affected by humidity (i.e., where moisture diffused into the film) prior to diffusion testing under non-aqueous conditions. This observation led to an investigation of the effect of moisture uptake on the permeation of toluene under non-aqueous testing. In these cases, the 44 mol% thin-film had lower toluene permeation coefficient values than the 32 mol% thin-film. These values were similar to toluene permeation coefficient values from tests with aqueous solutions. When relative humidity was less than 60%, the 32 mol% had slightly lower permeability values than 44 mol% thin-film. However, even when affected by humidity, the permeability of both thin-films were considerably (two to three orders of magnitude) lower than previously observed in a water-saturated solution. Permeation of toluene from a 1/1 toluene/hexane solution was also examined for the 32 mol% EVOH thin-film at temperatures of 23–50 °C and results fit well with a conventional Arrhenius relationship of increasing P g values with increasing temperature. [ABSTRACT FROM AUTHOR]

The development of tensile strains in geomembrane liners due to loading and waste settlement in waste containment facilities is examined using a numerical model. Two different constitutive models are used to simulate the waste: (a) a modified Cam-Clay model and (b) a Mohr-Coulomb model. The numerical analyses indicate the role of the slope inclination on the maximum geomembrane liner strains for both short-term loading (immediately post closure) and long-term waste settlement. A geosynthetic reinforcement layer over the geomembrane liner is shown to reduce the maximum geomembrane liner strains, but the strain level of the geosynthetic reinforcement itself may become an engineering concern on steeper slopes (i.e., greater than 3H:1V) for cases and conditions examined in this paper. The paper considers some factors (e.g., slope inclination, use of a high stiffness geosynthetic over the geomembrane liner) and notes others (e.g., the designer selection of interface characteristics below and above the geomembrane, use of a slip layer above the geomembrane) that warrant consideration and further investigation to ensure good long-term performance of geomembrane liners in waste containment facilities. [ABSTRACT FROM AUTHOR]

The time-dependent performance of deep-mixing-method column supported embankments reinforced by viscous reinforcement is investigated for different long-term (at 99% degree of consolidation) reinforcement strains using fully coupled three-dimensional finite element method. The influence of long-term reinforcement strains on long-term net embankment height, maximum crest settlement, maximum differential settlement at the crest and horizontal toe movement of embankments numerically constructed over two soft foundations is explored with the consideration of viscosity of two reinforcement products. Based on a series of numerical simulations, an approach to controlling the deformations of column supported embankments to modest levels while maximizing their long-term service heights is proposed. Also, a correlation between long-term reinforcement strain and end-of-consolidation reinforcement strain is suggested for the studied cases. [ABSTRACT FROM AUTHOR]

GCL manufacturers recommend that composite liners (i.e., a geomembrane (GMB) over geosynthetic clay liner (GCL)) be covered in a timely fashion. This paper highlights the importance of following this recommendation by reporting on significant down-slope bentonite migration first noted at the Queen's University Environmental Liner Test Site (QUELTS) constructed in 2006 (QUELTS I). The down-slope erosion is attributed to thermal cycles that caused evaporation of moisture from the GCL on sunny days (when the black geomembrane heated to 60–70 °C) followed by condensation of moisture on the underside of the geomembrane at night when the geomembrane cooled. The condensed moisture would drip onto the GCL and run down-the slope. Repetition of this process over an extended period of time caused the erosion of bentonite at some locations in all four GCLs examined in the 3.7 years the liner was exposed before the full inspection of the GCL which detected the mechanism. A series of laboratory experiments confirmed that dripping of evaporative water could cause down-slope erosion in relatively few cycles. These tests also identified several GCL products with a high resistance to down-slope erosion prompting the desire to construct a second field study to examine the issue. Thus, in 2012, the liner system was removed and QUELTS II was constructed with a new series of 7 composite liners. This paper highlights the key findings from these studies with particular emphasis on issues of importance to designers, regulators and installers. [ABSTRACT FROM AUTHOR]

Partitioning of bisphenol A (BPA) to geotextile (GTX), and geosynthetic clay liner (GCL) is investigated. BPA distribution coefficients range from 3.3 to 5.0 mL/g for the geotextile component of the GCL and 10–16 mL/g for the entire GCL. The diffusion of BPA through the GCL is also investigated and the diffusion coefficient is estimated to be 1.5 × 10 −10 m 2 /s at a bulk void ratio of 4.4. The estimated parameters are used to study the potential mass transfer of BPA through a composite liner of a municipal solid waste landfill for a number of scenarios. The results show that HDPE geomembrane is an excellent diffusive barrier for BPA but the advective transfer of BPA through the composite liner needs to be limited by minimizing the number of holes and length of the wrinkles in the geomembrane. [ABSTRACT FROM AUTHOR]

The influence of reinforcement viscosity on the post-construction performance of embankments with floating columns and fully penetrating columns is explored. It is shown that the viscous behaviour of geosynthetic reinforcement can increase the long-term shear deformations of foundation soil and increase the horizontal toe movement. The effects of reinforcement strain at the end of construction of embankments over two soft clay deposits are investigated. The deep-mixing-method column supported embankments with viscous and with inviscous reinforcement are numerically constructed to identify the effect and magnitude of reinforcement creep and stress relaxation. [ABSTRACT FROM AUTHOR]

The short-term and long-term performance of reinforced, deep mixing method (DMM) floating-column supported embankments with and without prefabricated vertical drains (PVDs) are investigated using a fully 3D coupled model. The concurrent use of PVDs with the other two forms of soft ground improvement significantly shortens time for subsoil consolidation and also substantially reduces post-consolidation lateral displacements and post-construction crest settlements. The combined use of PVDs and surcharge preloading to meet a requirement of 100 mm post-construction crest settlement for a bridge approach embankment is examined. Compared to a comparable case with surcharge preloading but without PVDs, the synergistic effects of PVDs and 0.5 m fill surcharge preloading diminish the calculated post-construction crest settlement from 0.228 m to 0.036 m and reduce the post-consolidation horizontal movement below the toe at the ground surface level from 0.115 m to 0.067 m even though embankment is constructed 5 times faster in the case with PVDs. [ABSTRACT FROM AUTHOR]

The long-term performance of three multilayered textured white conductive-backed geomembranes (GMBs) is compared to the comparable textured nonconductive GMBs and their smooth edge/equivalent to investigate the effect of the conductive layer on their longevity. Oven immersion in synthetic municipal solid waste leachate is used at a range of temperatures to accelerate the ageing during an incubation period of 50 months. It is shown that the conductive layer can antagonistically or synergistically affect the antioxidant depletion of conductive-backed GMBs relative to nonconductive GMBs produced by the same GMB manufacturer and formulated using the same nominal resin and antioxidant package. However, their relative degradation at 85 °C does not necessarily follow their relative antioxidant depletion times implying that the manufacturing process and the interaction between the additive packages of these GMBs can affect their relative degradation beyond the antioxidant depletion stage. Arrhenius modelling predicts the antioxidant depletion stage at field temperatures ranging between 180 and 1400 years at 20 °C for two different conductive-backed GMBs produced by two different manufacturers. With such variation in the long-term performance of conductive-backed GMBs currently available in the market, their durability should be investigated before their use in barrier systems to ensure they can meet the required design life of the desired geoenvironmental application. • First investigation into the degradation behaviour of conductive-backed geomembranes. • Examines the effect of texturing on the durability of conductive-backed geomembranes. • Estimates antioxidant depletion time of multilayered geomembranes at field temperatures. [ABSTRACT FROM AUTHOR]

The effect of thickness on the antioxidants depletion time for five high density polyethylene geomembranes (with nominal thicknesses of 0.5, 1.0, 1.5, 2.0, and 2.4 mm) immersed in synthetic leachate at six different temperatures (25, 40, 55, 70, 85 and 95 °C) is investigated. The geomembranes were manufactured from the same geomembrane resin (i.e., polymer resin, antioxidant/stabilizer package, and carbon-black batch). Four (1.0, 1.5, 2.0, and 2.4 mm) of the five geomembranes were manufactured during the same production run by changing the pulling speed of the geomembrane from the extrusion die. The depletion of antioxidants/stabilizers inferred from both standard (Std-) and high pressure (HP-) oxidative induction times (OIT)s show that increasing geomembrane thickness resulted in longer depletion times for temperatures above 40 °C but little to negligible difference in projected depletion times below 40 °C for this antioxidant package. The increase in depletion times was not proportional to the square of the geomembrane thickness as theoretically predicted if depletion was diffusion controlled and the diffusion coefficient was the same for each GMB. Thus, the depletion of antioxidants is not fully governed by diffusion and/or the GMB's diffusion coefficients are different. Regardless of the GMB thickness, the time for Std-OIT depletion was shorter than the time to residual HP-OIT above 70 °C but was longer at or below 55 °C. For example, for 2.4 mm GMB, the inferred time for Std-OIT depletion was 0.2–0.3 (at 85 °C), 2–3.5 (at 55 °C) and 9.5 (at 40 °C) times that for the HP-OIT to deplete to residual. The projected Std-OIT depletion times for 1.0 and 2.4 mm GMBs were: 2.7 and 4 years, respectively, at 60 °C; 23 and 26 years, respectively, at 40 °C; and about 330 years at 20 °C. [ABSTRACT FROM AUTHOR]

Leaving a composite liner exposed for an extended period can sometimes lead to down-slope bentonite erosion from geosynthetic clay liners (GCLs). This laboratory study examines a number of factors that can affect the erosion of bentonite particles with an imposed flow of water for one particular geotextile-encased, needle-punched GCL. The factors examined include the effect of an initial wet/dry cycle, water chemistry, flow rate, slope, prior cation exchange, and the effect of no-drying phase in the test cycle. No erosion was observed unless the GCL had been hydrated and dried to create a wet/dry cycle. The most critical factor was found to be the water chemistry. No erosion was observed with tap water (39 ppm calcium) with up to 360 cycles and a flow of 3 L/hour. Tests simulating the evaporation and condensation of water below an exposed composite liner by imposing deionized water on the GCL surface developed erosion holes within 5–6 cycles. [ABSTRACT FROM AUTHOR]

Aging of the same geomembrane immersed in the same four synthetic municipal solid waste leachates tested by Rowe et al. (2008) for 2.65 years is continued for an additional 6.5 years (i.e., providing data over more than 9 years of aging). The additional data is used to (a) update the initial estimates for antioxidant depletion stage, and (b) investigate the effect of leachate constituents on the timing and magnitude of the changes in the physical properties of the geomembrane. While all the examined leachates had surfactant and trace metals in common, reduced Leachate 1 (full leachate) had salts and volatile fatty acids (VFAs), Leachate 2 was not reduced and had neither salts nor VFAs, reduced Leachate 3 had salts, reduced Leachate 4 had VFAs, and Leachate 5 was similar to Leachate 2 except that it was reduced. Incubation in all leachates gave very similar predictions of the antioxidant depletion stage, however, the salts in Leachates 1 and 3 are shown to have the largest effect on the geomembrane mechanical properties, especially stress-crack resistance, resulting in a shorter time to nominal failure than for leachates without salts. Furthermore, reduced Leachates 1 and 3 showed faster degradation in stress-crack resistance than Leachate 2 which was not reduced and without salts. Arrhenius modeling is used to extrapolate the time to nominal failure (i.e., a reduction in stress-crack resistance to 150 h) at a range of temperatures. [ABSTRACT FROM AUTHOR]

The effect of elevated temperatures, typically 95-115 °C, on antioxidant depletion from a high-density polyethylene (HDPE) geomembrane (GMB) incubated in air, water and synthetic leachate is examined. It is shown that the antioxidant depletion in synthetic leachate at 95-115 °C is consistent with what would be expected from Arrhenius modeling based on data from lower temperatures (25-85 °C). A similar finding is reached for incubation in air. However, when incubated in water the antioxidant depletion is more complicated. At temperatures above 100 °C a four-parameter exponential model was needed to fit oxidative induction time data that exhibited quite different early-time and later-time depletion rates. The early-time depletion rate decreases with an increase of the temperature while the later-time depletion rates follow the more typical pattern of increasing with increasing temperature. Three additional HDPE GMBs with different antioxidant packages are examined at elevated temperatures in air. The GMB with the lowest initial standard (Std) oxidative induction time (OIT) and without hindered amine light stabilizer (HALS) has the longest antioxidant depletion stage based on Std-OIT at these elevated temperatures. GMBs stabilized with HALS showed only a slight change in their high pressure OIT during the current study. It is shown also that degradation in physical properties can start at Std-OIT values above the residual OIT values. [ABSTRACT FROM AUTHOR]

Abstract: The degradation of three high density polyethylene geomembranes (GMBs) (denoted xA, xB and xC) when immersed in simulated landfill leachate at 85 °C is examined. All three GMBs met the requirements of the generic industry specification GRI-GM13 with respect to their performance properties. The large high-pressure oxidative induction time (HP-OIT) (i.e., 790 min for xB and 960 min for xC) combined with the relatively high level of trace nitrogen detected, suggest the presence of hindered amine light stabilizers (HALS) as part of the antioxidant package in these GMBs, whereas, the relatively low initial HP-OITs (i.e., 260 min) and the low level of trace nitrogen for xA, suggest the absence of HALS in xA. Although xA had the lowest initial standard Std-OIT (Std-OIT was 115, 158 and 175 min for xA, xB and xC, respectively) it exhibited the longest time to antioxidant depletion based on Std-OIT. For the three GMBs, the HP-OIT depleted following exponential decay model until reaching a residual value. xB had the slowest HP-OIT depletion (0.016 month−1) and was still depleting without reaching a residual value at the end of this study (after 46 months). xA experienced the fastest HP-OIT depletion (∼62 times faster than of xB) and reached a residual value of 78 min. For xC, HP-OIT depleted 40 times faster than for xB reaching a residual value (∼610 min) that was still higher than the HP-OIT of 400 min specified by GRI-GM13 for a new GMB. xB had the longest time to nominal failure despite having the lowest initial SCR value (330 h for xB compared to 910 and 800 h for xA and xC, respectively) and not having the highest OITs values. Although xC had a residual HP-OIT of 610 min, the SCR, melt index (MI) and tensile properties for xC had decreased to the point that xC was at nominal failure, indicating that the degradation can take place without the total depletion of antioxidants/stabilizers captured by the HP-OIT. [Copyright &y& Elsevier]

Abstract: A review of 92 heap leach projects from 15 countries provides a starting point for a series of experiments, at 22 °C and a vertical pressure of 2000 kPa, to examine short-term puncturing and the development of geomembrane strains that could affect longer-term performance. Underliners of gravel with some sand or those of gravel and sand caused significant puncturing and excessive strains in the geomembrane for the conditions examined. The shape of the underliner grading curve had a much greater effect on the potential for puncturing and the magnitude of the strains in the geomembrane than just the maximum particle size. Of the six granular underliners examined, the best performance was for the well graded gravelly sand with some silt which offered sufficient support to minimize the strains in the geomembrane due to the overliner while not inducing significant strains directly from the underliner. Nevertheless even in this case the maximum strain of 11% is almost double the maximum recommended in the literature for ensuring good long-term performance of the geomembrane. Consideration of composite liners with GCLs and compacted clay liners shows that the more deformable the foundation, the larger are the indentations and strains induced in the geomembrane by a given overliner. For the specific conditions examined, it is shown that there was no apparent improvement in performance for an LLDPE geomembrane versus the HDPE geomembrane tested. A 540 g/m2 geotextile protection layer above the geomembrane was also found to be insufficient to prevent significant strains in the geomembrane due to the overliner examined. [Copyright &y& Elsevier]

A physical model was employed to evaluate the potential shrinkage of a needle-punched nonwoven/nonwoven geosynthetic clay liner (GCL) subjected to simulated daily heating/cooling cycles (23 °C for 16 h; 60 °C for 8 h; 23 °C for 16 h etc. at the top) modelling solar exposure conditions of an overlying black geomembrane and simulated field moisture conditions where the GCL must rehydrate with water from a silty sand foundation soil with an initial moisture content of 16–17% in a closed system. It is shown that when the GCL was allowed to hydrate to 110% gravimetric moisture content before being subjected to daily heating/cooling cycles there is a relatively rapid increase in maximum shrinkage strain to 1.7% (corresponding to shrinkages of ∼75 mm for a 4.42 m wide roll) over the first 5 cycles and 3.8% (∼170 mm) over the first 14 cycles. The shrinkage rate then decreases giving a maximum shrinkage strain of 4.5% (∼200 mm) after 50 heating/cooling cycles. Experiments are conducted for the GCL with different initial gravimetric moisture contents and it is shown that the initial moisture content at the time the heating/cooling cycle commenced has a substantial effect on shrinkage. Consistent daily cycles of the magnitude examined are shown to suppress GCL shrinkage, but a cool period during which the GCL can hydrate from the subsoil, followed by significant daily thermal cycles, increases the shrinkage. The implications with respect to field shrinkage are discussed. [ABSTRACT FROM AUTHOR]

The performance of three geosynthetic clay liners (GCLs) above arsenic-bearing gold mine tailings is examined based on a series of laboratory column experiments conducted over a two year period. The GCLs examined had either untreated or polymer-enhanced sodium bentonite and had different carrier geotextiles (a woven geotextile, a scrim-reinforced nonwoven geotextile, and a woven geotextile laminated with a polypropylene film). After 24 months, the hydraulic conductivity (k) of GCL with untreated sodium bentonite increased by almost an order of magnitude from 3×10−11 m/s to 2×10−10 m/s with the swell index decreasing from 26mL/2g to 7–11mL/2g and the exchangeable sodium percentage (ESP) decreasing from 65% to 12–21%. Samples of this GCL exhumed after 9 months of moisture uptake and cation exchange and then subjected to 15 extreme wet–dry cycles experienced an increase in k up to 6×10−10 m/s. The GCL with polymer-enhanced bentonite maintained k less than 5×10−11 m/s after 24 months (virgin k =1×10−11 m/s) and experienced less cation exchange (ESP decreased from 78% to 39–42%) than the GCL with untreated bentonite. For this GCL, 15 extreme wet–dry cycles gave k <9×10−11 m/s provided that the soil stress was 15kPa or higher (≥1.0m of cover soil above the GCL). The GCL with polymer-enhanced bentonite and carrier geotextile laminated with a polypropylene geofilm demonstrated the best performance with k <7×10−12 m/s even with exposure to the extreme wet–dry cycling. In all cases, the GCL prevented arsenic migration into the overlying cover soil over the two year period examined. [ABSTRACT FROM AUTHOR]

Abstract: The potential for desiccation of GCLs in double composite liner systems under thermal gradients is experimentally investigated. The effects of key initial and boundary conditions such as the GCL mass per unit area, initial GCL and subsoil water content, time lag between waste placement and temperature increase, the applied temperature gradient and the foundation layer thickness are investigated and discussed. The results suggest that surface temperatures of 39–45 °C, corresponding to thermal gradients of 59–67 °C/m, can induce sufficient thermally driven moisture redistribution to cause desiccation of GCLs. For surface temperatures of 29–37 °C and thermal gradients of 20–29 °C/m there was occasional slight cracking observed in about a quarter of the cases examined. Results of laboratory permeability tests on the virgin and exhumed samples are used to assess the self-healing capacity of GCLs. [Copyright &y& Elsevier]

Abstract: The behavior of heat-tacked GCL seams under cyclic wetting and drying is examined. It is shown that the transverse shrinkage behavior of heat-tacked seams was similar to that observed in previous shrinkage tests performed on unseamed GCL. It is shown that the heat-tacked seam has strength comparable to the GCL adjacent to the seam and hence it is likely the strength of the GCL itself that will govern failure of the GCL in applications where there can be significant shrinkage. The strength of the heat-tacked seam subjected to 40 wet-dry cycles was at least as high as that of virgin heat-tacked samples, suggesting that 40 wet-dry cycles did not weaken the heat-tacked seam. After 40 cycles, the samples remained heat-tacked, suggesting the technique has promise as one method of preventing panel shrinkage for GCLs highly susceptible to shrinkage. It is noted that these tests are small-scale laboratory tests, under idealized conditions, and that the behavior in the field may differ due to more extreme conditions that may occur in the field and due to the greater amount of material between seams available to shrink and hence induce forces in the heat-tacked GCL seam. [Copyright &y& Elsevier]

Abstract: A series of fluorinated high density polyethylene (f-HDPE) geomembrane (GM) samples of different thickness (1, 1.5 and 2.5mm) was exhumed from the backfill immediately upstream of a barrier system constructed to contain a hydrocarbon spill in the Canadian Arctic. The samples were tested for oxidation induction time (OIT), crystallinity, melt index (MI) and tensile properties. The results of these tests are reported and it is shown that the durability of the GM was maintained well beyond the initial 3-year design life of the barrier system. Based on 7 years of field data, the std-OIT depletion time for the 1.5mm thick GM used in the barrier system was inferred to be over 140 years while the antioxidant depletion time based on the HP-OIT is estimated to be about 200 years. No significant temporal changes in the crystallinity, MI or tensile properties of the exhumed GM samples were detected. [Copyright &y& Elsevier]

Abstract: A numerical study of the behavior of geosynthetic-reinforced embankments constructed on soft rate-sensitive soil with and without prefabricated vertical drains (PVDs) is described. The time-dependent stress–strain-strength characteristic of rate-sensitive soil is taken into account using an elasto-viscoplastic constitutive model. The effects of reinforcement stiffness, construction rate, soil viscosity as well as PVD spacing are examined both during and following construction. A sensitivity analysis shows the effect of construction rate and PVD spacing on the short-term and long-term stability of reinforced embankments and the mobilized reinforcement strain. For rate-sensitive soils, the critical period with respect to the stability of the embankment occurs after the end of the construction due to a delayed, creep-induced, build-up of excess pore pressure in the viscous foundation soil. PVDs substantially reduce the effect of creep-induced excess pore pressure, and hence not only allow a faster rate of consolidation but also improve the long-term stability of the reinforced embankment. Furthermore, PVDs work together with geosynthetic reinforcement to minimize the differential settlement and lateral deformation of the foundation. The combined use of the geosynthetic reinforcement and PVDs enhances embankment performance substantially more than the use of either method of soil improvement alone. [Copyright &y& Elsevier]

Abstract: Composite basal liners for municipal solid waste landfills may be comprised of geosynthetic materials such as geomembranes and geosynthetic clay liners. Exothermic degradation of organic matter or hydration of incinerator ash within the landfill generates heat within the waste mass. This creates thermal gradients through the composite liner, which have the potential to induce a net movement of moisture away from the warmer liner. The result is a potential for desiccation that may impair the long-term performance of the GCL. This paper presents the results of an investigation into the potential for desiccation of GCLs under thermal gradients using the numerical model SUMMIT. The results from a series of large-scale laboratory experiments are compared with predictions made using the numerical model in terms of volumetric water content and temperature distributions. The influence of key thermal and hydraulic parameters is discussed, as well as the ability of the model to predict situations where cracking is likely. Key limitations of the model, especially the assumption of a rigid media, are identified and discussed within the context of GCL desiccation behaviour. It is concluded that although the SUMMIT model may be suitable for general investigations of thermally induced moisture movement, a more comprehensive model is required to extend the investigation to the case of GCLs as part of a composite lining system. [Copyright &y& Elsevier]

Abstract: The potential improvement in the sorption of volatile organic compounds (VOCs) to bentonite used in Geosynthetic Clay Liners (GCLs) is examined for mixtures of bentonite and several organoclays or activated carbon. Results of batch tests performed with dichloromethane (DCM), 1,2-dichloroethane (1,2-DCA), trichloroethylene (TCE), benzene and toluene are reported. It is shown that all organoclays could potentially increase VOC sorption to GCLs by several orders of magnitude but activated carbon generally appears to provide the most improvement for the samples tested. The engineering relevance of this potential improvement is assessed with respect to design of municipal solid waste landfill liner systems by performing contaminant transport modeling. It is shown that only slight changes in contaminant migration will occur, despite the large potential increases in GCL sorption. This suggests that increased costs associated with modifying GCLs may outweigh the benefit of such additives to GCLs for municipal solid waste landfill applications for the conditions examined herein. [Copyright &y& Elsevier]

Geotextiles have been widely used in soil structures for separation, filtration, reinforcing, and drainage. They are often used to provide reinforcement and drainage for retaining walls and embankments. It has been reported, however, that geotextiles may not drain water as effectively as was initially expected. In this study, published data on the hydraulic properties of unsaturated geotextiles are compiled and analyzed in order to highlight the hydraulic characteristics of unsaturated geotextiles.The application of the van Genuchten equations originally developed for the water characteristic curve and the hydraulic conductivity curve of unsaturated soil to unsaturated geotextiles is then examined and discussed. Finally, the drainage from a one-dimensional sand column having a horizontal geotextile layer was analyzed using the finite element method and the van Genuchten equations to assess the utility of this procedure for further study of unsaturated/saturated water flow within the soil–geotextile system. [Copyright &y& Elsevier]

Geosynthetic reinforced embankments can fail before the ultimate tensile strength of the reinforcement is mobilized. For the purpose of embankment design, engineers must often rely on experience when selecting a reinforcement strain for analysis using limit equilibrium methods or resort to complicated numerical methods such as those based on the finite element method. This paper presents a simple procedure for estimating the undrained stability of geosynthetic reinforced embankments founded on soft clayey soils where the shear strength increases with depth. Finite element results are summarized in a design chart for establishing geosynthetic reinforcement strains suitable for design. The procedure is illustrated using worked examples and tested against a well documented case history. [Copyright &y& Elsevier]

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 70 m 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 70 m. [Copyright &y& Elsevier]

This paper summarizes the basic concepts and mechanisms related to the durability of HDPE geomembranes and discusses the factors influencing the service life of geomembrane liners. Geomembrane durability is addressed in terms of field performance and laboratory test results under various conditions. It discusses their projected service lives that may range from many centuries to less than a decade depending on the type of material and exposure conditions. [Copyright &y& Elsevier]