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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]

Low-permeability liners are required at the base of municipal solid waste (MSW) landfills to minimize leachate leakage and contaminant migration into groundwater. This paper uses a two-dimensional coupled groundwater flow and contaminant transport model to examine the performance of three types of low-permeability liners specified by the current Chinese landfill standard: (1) a compacted clay liner (CCL), (2) a geomembrane (GMB) overlying a CCL, and (3) a GMB overlying a geosynthetic clay liner (GCL) on a CCL. The model simulates leachate leaking and contaminant migrating over the entire base of the landfill for the CCL and through the holed GMB wrinkles for the GMB composite liners. The performance of each type of low-permeability liners was evaluated and compared in terms of leakage rate and peak impact of chloride on the aquifer. Based on liner cases and conditions examined in this paper, the results show that the three types of low-permeability liners are not equivalent for minimizing the leakage rate and chloride impact on the aquifer. The GMB + GCL + CCL performs the best among the three low-permeability liners, and is effective for limiting the peak chloride impact on the aquifer below the acceptable level in drinking water. [ABSTRACT FROM AUTHOR]

The paper develops an analytical model based on the force equilibrium for the design of steep slope geosynthetic liner systems to overcome downdrag loads from waste settlement. This analytical model calculates the required tensile stiffness for a high strength/stiffness geotextile (HS-GTX) reinforcement over the GMB to limit the maximum HS-GTX tensile strain to 5% and the maximum GMB strain to 4% on the side slope. The numerical model illustrates the need for reducing the GMB tensile strains for a single GMB liner on a steep landfill slope and confirms that the use of a HS-GTX over the GMB is able to limit the maximum HS-GTX and GMB tensile strains to the acceptable strain levels. The analytical model developed in this paper is a practical tool for preliminary design to limit tensile strains of the HS-GTX and GMB in a steep slope geosynthetic liner system. [ABSTRACT FROM AUTHOR]

Two geosynthetic clay liners (GCLs) with sodium bentonite and three GCLs with polymer-amended bentonite were subjected to wet–dry cycles selected to simulate the conditions to which a GCL on an 18° slope might be subjected: for a GCL below an exposed geomembrane wrinkle with a hole. The wetting involved water flowing over the GCL for 8 h each cycle. Three drying cycles (0.67, 7, and 14 days) were examined. After 12–18 months of wet–dry cycles, the samples were X-rayed to identify representative specimens for testing. The changes in the hydraulic conductivity, k, of the GCLs were obtained when permeated with two synthetic municipal solid waste leachates at an applied head of 0.35 m for a range of effective stresses (3–150 kPa). The results showed an up to four orders of magnitude difference in k depending on applied stress and RMD of the leachate permeant. The effects of the number and the duration of the wet–dry cycles, the GCL mass per unit area, presence/absence of polymer modification, the carrier geotextile, the number and the size of the needle-punched bundles, and the bundle thermal treatment are discussed. [ABSTRACT FROM AUTHOR]

• Full-scale real-time tests of a double liner system long-term performance. • Failure of geotextile components of GCL and GCD in 6 years at 85 °C and 250 kPa. • Demonstration that design can finesse problems due to geotextile degradation. • Demonstrates importance of service life all components of double liner systems. The long-term performance of a geosynthetic clay liner (GCL) above a drainage layer and a geocomposite drain (GCD) are investigated. Full-scale tests are used to: (i) assess the integrity of GCL and GCD in a double composite liner below a defect in the primary geomembrane with ageing, and (ii) establish the head at which there was internal erosion in GCL without a carrier geotextile (GTX) such that the bentonite is in direct contact with the underlying gravel drainage. Six years after contact with simulated landfill leachate at 85 °C through an intentional defect on the geomembrane, the GCL resting on the GCD had failed due to degradation of the GTX between the bentonite and the core of the GCD and subsequent erosion of the bentonite into the core structure of the GCD was observed. In addition to complete degradation of its GTX at some locations, the GCD had also experienced extensive stress cracking and rib rollover. The second test demonstrates that if a suitable gravel drainage layer had been used instead of the GCD, the GTX component of the GCL would not have been required for acceptable long-term performance under normal design conditions and indeed could withstand a head of up to 15 m before problems became evident. The findings serve as a warning landfill designers and regulators that more attention must be paid to the service life of all components of double liner systems used in Municipal Solid Waste (MSW) landfills. [ABSTRACT FROM AUTHOR]

A series of laboratory tests were conducted to investigate the interface transmissivity, θ, between a 1.5 mm thick high-density polyethylene geomembrane (GMB) and a multicomponent geosynthetic clay liner (GCL) when permeated with a simulated municipal solid waste landfill leachate under a range of applied stresses (10–150 kPa). The effect of three different prehydration fluids, a 4 mm diameter central coating defect, coating defect position, and the effect of coating orientation of the GCL were investigated. It was found that at an applied stress of less than 70 kPa, the effect of these factors on θ was largely masked by the variability in the initial interface contact condition between the GMB and GCL. At 100–150 kPa, the effects of initial variability were largely eliminated, but there was no notable effect of other factors for multicomponent GCLs on interface transmissivity. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Geotechnical Journal is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Design- and construction-related factors that affect leakage through geomembranes, used either alone or in a composite liner, are examined. The development, detection, and number of holes that can develop during construction and operation of a facility are discussed. The relationship between leakage through holes in a geomembrane, and the hydraulic conductivity of the material above (including tailings) and below a geomembrane, is examined, considering both a relatively permeable subgrade and an underlying clay liner (including the unstressed zone beneath a wrinkle) in a composite liner. The leakage observed in the primary liners of 180 landfill cells is explained by holes in geomembrane wrinkles with a length consistent with those observed in the field and an appropriate choice of hydraulic conductivity for the clay liner below the geomembrane wrinkle with a hole. Leakage through geosynthetic clay liner (GCL) overlaps below wrinkles is examined. The latest research into the physical and chemical aging of high-density polyethylene (HDPE) geomembranes liners is discussed in the context of the compatibility of the antioxidant package and resin with the solution to be retained, liner temperature, nature of exposure, sustained tensile strains, and welds. The estimation of the service life of a geomembrane based on immersion tests and simulated field conditions is examined in terms of antioxidant depletion, stress cracking, and the maximum allowable strain in a geomembrane. It is projected that the service life of a geomembrane may range from just a few years to many centuries. [ABSTRACT FROM AUTHOR]

• Use the maximum gas pressure directly below cover to distinguish different designs. • Address a balance between cover properties and vacuum pressure of horizontal wells. • A design formula defining the required cover properties with others parameters. Horizontal spacing of horizontal extraction gas wells can be designed to achieve a 90% pumping rate of the total generated landfill gas (LFG) from given waste properties (viz: gas permeability, landfill gas generation and non-homogeneity with depth), cover characteristics and vacuum pressure. However, cover characteristics and vacuum pressure are also important design parameters and different combinations of them result in different distributions of gas pressure in the waste, some of which would induce problematic air intrusion while others might pose threat to cover stability. This paper uses the maximum gas pressure directly below cover to distinguish these combinations, and provides the first study of the effects of the above parameters on potential outcomes. The ability of the overlying cover to resist LFG emission from the landfilled waste is suggested not to exceed a critical value, otherwise the maximum gas pressure below it would become at least 1 kPa larger than atmospheric pressure. A design formula for this critical value is proposed with respect to typical values of waste properties, vacuum pressure and the buried depth of horizontal wells in wide ranges. Together with consideration of recovery efficiency, the proposed method can be used to design a horizontal extraction gas collection system and a cover system in better working condition, and to evaluate the maximum gas pressure below cover. These applications are illustrated by a worked example. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Geotechnical Journal is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

• 2D analytical gas flow model for a non-homogeneous landfill with horizontal wells. • A quantitative analysis of over 680,000 scenarios involving typical key variables. • Capture responses of air intrusion and LFG recovery for horizontal collection wells. • A generalized design method for horizontal wells considering waste non-homogeneity. Considering exponential decreases in gas permeability and gas generation of waste with depth, a two-dimensional analytical model is developed to describe the landfill gas (LFG) recovery using horizontal wells. This model is used to simulate more than 680,000 scenarios involving typical values of waste properties, cover characteristics and design parameters for horizontal wells (seven variables in total). The coupled effect of these seven variables on air intrusion and the gas recovery efficiency of horizontal wells are investigated. It is shown that all the variables examined, except for the two variables defining waste non-homogeneity, could be integrated into three dimensionless variables. The horizontal spacing and buried depth of horizontal wells are examined as a function of cover characteristics, waste properties, and vacuum pressure to allow the development of a generalized design method for horizontal wells. An upper limit of horizontal well spacing is defined (for an 85% recovery rate) and a simple formula is provided which can be used to estimate the corresponding level of air intrusion. The upper limit spacing is shown to be affected by the non-homogeneity in gas permeability of waste, cover characteristics, and buried well depth. Using a worked example, the proposed method is shown to be capable of estimating air intrusion into existing horizontal gas collection wells and to optimize the design of horizontal wells considering waste non-homogeneity. [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]

Highlights • Use exponential functions to represent the waste non-homogeneity with depth. • A quantitative analysis of over 500,000 scenarios combining typical key variables. • Capture recovery response of LFG by three dimensionless variables and two constants. • Provide a preliminary estimation of cover properties and vacuum for good recovery. Abstract A two-dimensional axisymmetric and normalized analytical model for landfill gas (LFG) migration around a vertical well is developed. The vertical gas permeability and LFG generation rate of waste are assumed to be subject to exponential decreases with depth. Using a general analytical solution, over 500,000 scenarios involving a combination of typical control variables (viz: cover properties, waste properties, vacuum pressure, well radius and spacing) are modelled. A quantitative analysis of the coupled effects of these control variables on LFG recovery rate indicates that the recovery response could be captured by: (a) three dimensionless variables (denoted as cover resistance, pump capacity, and well spacing parameters), and (b) two constants defining the decreases in gas permeability and LFG generation of waste with depth. For example, if the LFG generation rate of the waste at the top is doubled, a two times increase in the vacuum pressure with other parameters being equal would give a same gas recovery rate, as well as simultaneously doubling the thickness and gas permeability of the cover. The recovery efficiency of a vertical well with a low permeability cover is examined as a function of cover resistance and pump capacity, and design charts are presented that may be used to optimize gas recovery by adjusting cover properties and vacuum pressure. The proposed model makes it possible to consider the waste non-homogeneity in the design process, and the results contribute to a preliminary design of a cover and vertical LFG collection systems. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Geotechnical Journal is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

To safely contain Per-and Polyfluoroalkyl substances (PFAS) in municipal solid waste landfills and contaminated soil monofills, it is necessary to understand how these substances interact with components of engineered systems designed to contain them. This paper examines the interaction between one of the most critical components of the system: a high-density polyethylene (HDPE) geomembrane. The same geomembrane is immersed in PFAS solution and synthetic municipal solid waste leachate containing PFAS for 2.5 years, and the effects of PFAS on antioxidant depletion time is examined. The geomembrane is incubated in ovens at 85-40 °C to obtain data for Arrhenius predictions at typical landfill temperatures. When exposed to PFAS solution alone, the antioxidant depletion times are smaller than when the same geomembrane is immersed in synthetic municipal solid waste leachate alone. The combination of the two has a synergistic effect which leads to an even greater reduction in antioxidant depletion time for this geomembrane, with results showing a 68% decrease in predicted antioxidant depletion time at a typical landfill temperature of 35 °C when PFAS is present in leachate. This study highlights the need to consider the potential impact of PFAS on the service life of geomembranes used to contain them. • First paper to examine the effect of geomembrane exposure to PFAS on antioxidant depletion. • First paper to report the synergistic effect of PFAS and municipal solid waste components on antioxidant depletion times. • First paper indicating the importance of considering PFAS effect on service-life of geomembranes used to contain them. [ABSTRACT FROM AUTHOR]

The time to rupture of a 1.5-mm-thick high-density polyethylene geomembrane aged in a typical landfill composite liner configuration is investigated under a pressure of 250 kPa at 85°C. The geomembrane was underlain with a geosynthetic clay liner and had a 560 g/m2 needle-pinched nonwoven geotextile protection layer separating it from 50 mm of drainage gravel containing leachate. Seventeen (0.6-m-diameter) tests were conducted. In addition to 9 months required to deplete antioxidants (Stage I), the tests indicated a lag period (Stage II) of 5.5 months and a time from the start of degradation to rupture (Stage III) of 20 months, giving a total inferred time to rupture of 34.5 months (2.9 years). There were up to 61 brittle ruptures per sample (i.e., >2 million cracks per hectare). The ruptures were predominately oriented in the machine direction and located (1) directly beneath a gravel contact, (2) at the side of a gravel indentation, or (3) between gravel indentations. The ruptures between gravel indentations were the least frequent but largest. The calculated strains perpendicular to the rupture direction were 24 ± 6%. Rupture occurred, although the average stress-crack resistance for all ruptured geomembrane samples still was approximately 760 ± 200 h, with a minimum of 360 h. These results indicate the importance of minimizing tensile strains in the geomembrane in the design of a liner system. [ABSTRACT FROM AUTHOR]

The effect of geomembrane thickness (1.5, 2.0, and 2.5 mm) on aging when immersed in a synthetic leachate is investigated over a period of approximately 7 years. Based on data at five different temperatures (55, 65, 70, 75, and 85°C), the predicted time required for a reduction in stress crack resistance to 150 h (half the typically specified value) at 35°C is 62% longer for the 2.5 mm than for the 1.5-mm geomembrane tested and 12% longer for the 2.0-mm than for the 1.5-mm geomembrane. Thus, other things being equal, the results suggest a longer time to nominal failure with increasing geomembrane thickness. It is also shown that the data from a proposed stage-parallel testing procedure collected over 2.5 years fit well with data from traditional incubation of virgin samples over almost 7 years and hence provides a viable means of obtaining good data in a reasonable period of time. [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]

A decision support system (Landfill Advisor or LFAdvisor) was developed to integrate current knowledge of barrier systems into a computer application to assist in landfill design. The program was developed in Visual Basic and includes an integrated database to store information. LFAdvisor presents the choices available for each liner component (e.g. leachate collection system, geomembrane liner, clay liners) and provides advice on their suitability for different situations related to municipal solid waste landfills (e.g. final cover, base liner, lagoon liner). Unique to LFAdvisor, the service life of each engineered component is estimated based on results from the latest research. LFAdvisor considers the interactions between liner components, operating conditions, and the existing site environment. LFAdvisor can be used in the initial stage of design to give designers a good idea of what liner components will likely be required, while alerting them to issues that are likely to arise. A systems approach is taken to landfill design with the ultimate goal of maximising long-term performance and service life. [ABSTRACT FROM AUTHOR]

The potential for the desiccation of a geosynthetic clay liner (GCL) forming part of a single composite landfill liner is evaluated. A thermohydromechanical model is used to identify conditions likely to cause desiccation. Simulations for typical landfill conditions show that the potential risk of desiccation exists even at relatively low temperatures (i.e., 35°C). It is found that the water content of the GCL prior to waste placement, the liner temperature, the overburden stress, the grain size and water content of the subsoil, and the depth to aquifer all affect the potential for desiccation. The results suggest that the placement of GCLs directly over coarse-grained soils or drainage layers should be evaluated carefully. This study highlights the need for more research into the potential for GCL desiccation and, in particular, the need to establish water retention curves for different GCLs over a range of stresses and degrees of hydration before heating, so that the effect of stress and hysteresis on the potential for desiccation can be examined. The findings of this study are limited by the approximations and assumptions described in the paper and, even then, apply only to the GCL and conditions examined. Changing the assumptions may change the findings. The findings should not be generalized to other composite liners without independent verification. The results serve to identify some of the situations that may lead to GCL desiccation, and it is hoped that they will prompt more research on this important topic. [ABSTRACT FROM AUTHOR]

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]

A numerical model is used to estimate the service life and clogging of gravel leachate drainage layers with a filter separator layer, such as those used in municipal solid waste landfills. A filter separator layer between the waste and the drainage layer is shown to reduce the leachate strength entering the gravel drainage layer and to extend the time that it takes to clog the drainage layer to the point when the leachate level exceeds the maximum design value (the service life). The filter layer is shown to have a far more significant effect in extending the service life when pea gravel () is used in the drainage layer, and less effect when coarse gravel () is used in the cases examined, although most of the benefit of a filter separator layer was achieved by using a 3-mm-thick needle-punched nonwoven geotextile. This geotextile may be sufficient for many practical purposes that are similar to those examined. The results from modeling leachate collection systems with filter separator layers subjected to both constant and variable leachate strength show that the characteristics of the leachate entering the drainage layer can substantially affect the service life of the drainage layer, and that high-strength leachate entering the systems for a limited time early in the life (acid phase) of a landfill can greatly reduce the service life of the leachate drainage layer. [ABSTRACT FROM AUTHOR]

A two-dimensional numerical model, BioClog, is used to estimate the leachate characteristics and leachate-induced clogging of laboratory mesocosms permeated with real municipal solid waste landfill leachate. The model is used to examine mesocosms with 38-mm (nominal diameter) gravel subjected to different durations of leachate permeation, mesocosms run-in series, and mesocosms with 19-mm gravel. A comparison of the calculated and measured leachate concentrations indicates that the model provided reasonable predictions of the effluent chemical oxygen demand (COD) and calcium concentrations. The calculated porosities within the saturated drainage layers are general in encouraging agreement with the measured values from all experimental mesocosms. The calculated and measured hydraulic conductivities are in reasonable agreement. The modeling results indicated that reducing the mass loading for the drainage layers and increasing the particle size of granular media can be expected to extend the time before clogging of drainage layers, and therefore, to extend the service life of landfill leachate collection systems. [ABSTRACT FROM AUTHOR]

The migration of contaminants through a 10-mm-diameter hole (0.785 cm2) in a geomembrane in direct contact with a geosynthetic clay liner (GCL) and adjacent silty sand is examined. Experiments were conducted in four 0.6-m-diameter cells at a vertical stress of 100 kPa and hydraulic head differences of 0.3 and 1 m. The system was first permeated with distilled water until steady-state flow was attained (the reference case). After 280 days the permeant was changed to a NaCL solution. After 800 days of permeation with 0.14 M NaCl solution there was only a 3% increase in the flow (leakage) compared with the reference case despite up to almost an order of magnitude increase in GCL permeability near the hole. The wetted radius at the end of the experiments was inferred by injection of dye and was found to be about 0.1-0.15 m. This provides the first experimental evidence in support of theoretical predictions that, when the geomembrane is in direct contact with a GCL, leakage through a hole is primarily controlled by the interface transmissivity rather than the GCL hydraulic conductivity when there is interaction between the permeant and the GCL. The observed chloride distribution in the silty sand at the end of the experiments is reported. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Geotechnical Journal is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Canadian Geotechnical Journal is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract: A numerical model ‘BioClog’ is used to examine clogging of granular drainage material within finger drains at the base of landfills. The reduction in porosity of drainage material is evaluated as the accumulation of clog mass reduces the pore spaces of granular media. The leachate mounding within the landfills caused by the reduction in hydraulic conductivity of finger drain is modeled. The effect of overburden pressure on the engineering properties of waste material and consequent leachate mounding is considered. The calculated rates of leachate mound development for two landfills with finger drain leachate collection are shown to be in encouraging agreement with the observed data at the times when data were available. It is demonstrated that a significant leachate mound can be expected to develop over time when the leachate collection system is comprised of finger drains; however the rate of increase and the magnitude at a given time will depend on a number of factors including the grainsize, thickness and spacing of the finger drains, and the thickness of waste as well as the leachate generation rate. [Copyright &y& Elsevier]

Abstract: A forensic modeling study examines the potential causes of gas and water venting observed during the excavation of a landfill in a 40m thick clayey till deposit in southwestern Ontario, Canada. The clayey till is known to be underlain by permeable, natural gas bearing, rock and gas has been diffusing through the clayey deposit over about the last 13,000–15,000years. Attention is focused on 2D modeling of hydrofracturing as well as gassy soil behavior. The model examines the predicted location of venting with and without the presence of gassy sand lenses and a discontinuous layer of basal till between the bedrock and the low permeability till. The modeling shows that hydrofracturing alone can explain the occurrence and location of the observed venting in one of the sub-cells. In the other sub-cell, hydrofracturing explains the occurrence but not the location of the observed venting or the presence of the sand in the venting water at this location. Modeling of appropriately placed gassy sand lenses is shown to explain the occurrence and location of the observed venting as well as the presence of both waters with different geochemistry from the bedrock and the till in the venting waters. [Copyright &y& Elsevier]

Geomembranes (GMBs) are widely used as advective barriers in landfill liner systems. When exposed to the sun, GMBs exhibit a network of wrinkles as a result of thermal expansion. These wrinkles disrupt the intimate contact between the GMB and the underlying layer. If a hole is coincident with a GMB wrinkle then the space under the wrinkle has the potential to act as a preferential pathway for flow of contaminants. Thus, the size and shape of GMB wrinkles have implications for leakage rates through the composite liner system. However, wrinkles are only a concern if they persist after placement of backfill, which is currently a subject of debate. In this paper, wrinkles are induced in a 1.5-mm-thick, black high-density polyethylene strip GMB specimen overlying a geosynthetic clay liner using natural solar and laboratory energy sources. Particle image velocimetry techniques are employed to record cross-sectional wrinkle geometry during growth and subsequent backfilling. This cross-sectional geometry is found to follow a Gaussian shape in which the height increases with the temperature and the width remains relatively constant. The resulting relationships between the height and temperature permit an estimation of wrinkle height for a known coefficient of thermal expansion for the GMB and an estimate of wrinkle spacing. For the GMB material and conditions tested, the results of the backfilling experiments indicate that when covered with 230 mm of cool sand (21°C), wrinkles of initial height less than about 20 mm disappear completely, while larger wrinkles remain with a reduced height. Furthermore, wrinkles of 20 mm in height are observed to form with increases in GMB temperature of less than 5°C. With application to the field, these findings indicate that a GMB must be covered at or below its installation temperature to achieve a wrinkle-free installation. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Geotechnical Journal is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract: Heat generated by the biodegradation of waste and other chemical processes in a landfill can potentially affect the long-term performance of landfill liner system, in particular that of a high-density polyethylene geomembrane. In a double liner system, the difference in leachate exposure and temperature might improve the long-term performance of the secondary geomembrane compared to that of the primary geomembrane. However, in some cases, the temperature is likely to be high enough to substantially reduce the service-life of the secondary geomembrane. This study explores the possible effectiveness of using tire chips as thermal insulation between primary and secondary liners to reduce the temperature of secondary geomembranes as compared to traditional soil materials. Heat and contaminant migration analyses are performed for cases with no insulation and for cases in which a layer of soil or tire chips has been used as thermal insulation between the primary and secondary liners. The effect of insulation on prolonging the service-life of a secondary geomembrane and, consequently, on contaminant transport through a liner system is examined for the case of a volatile organic compound (dichloromethane) found in landfill leachate. The study suggests that the use of tire chips warrants consideration, however there are other practical issues that require consideration in the detailed design and construction of landfill liners. Issues such as finite service-life, low working temperature, excessive settlement, ability to generate internal heat, leaching of tire chips and limitations in performing electrical resistivity leak detection tests are identified. [Copyright &y& Elsevier]

A method to control the increase in landfill liner temperature due to the heat generated by the waste is examined. The design involves installation of an array of cooling pipes beneath the waste. The feasibility of this system for cooling the liner was examined by performing a series of analyses for conditions based on the Tokyo Port Landfill. The results suggest that the introduction of a cooling system can substantially reduce liner temperature and consequently significantly increase the service life of a high-density polyethylene (HDPE) geomembrane liner in an engineered barrier system. The effects of pipe layout, pipe spacing, and coolant flow rate are examined. It is shown that a periodic pipe layout is the most efficient. Liner temperature decreases with increased coolant transfer flow rate [ABSTRACT FROM AUTHOR]

Accelerated aging tests to evaluate the depletion of antioxidants from a high density polyethylene geomembrane are described. The effects of temperature, high pressure, and continuous leachate circulation on the aging of geomembranes in composite liner systems are examined. The antioxidant depletion rates (0.05, 0.19, and 0.41 month-1 at 55, 70, and 85°C, respectively) obtained for the simulated landfill liner at 250 kPa vertical pressure are consistently lower than that obtained from traditional leachate immersion tests on the same geomembrane (0.12, 0.39, and 1.1 month-1 at 55, 70, and 85°C). This difference leads to a substantial increase in antioxidant depletion times at a typical landfill liner temperature (35°C) with 40 years predicted based on the data from the landfill liner simulators tests, compared to 15 years predicted for the same geomembrane based on leachate immersion tests. In these tests, the crystallinity and tensile yield strain of the geomembrane increased in the early stages of aging and then remained relatively constant over the testing period. There was no significant change in other geomembrane properties within the testing period. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The diffusive migration in the aqueous and vapour phases of four volatile organic compounds (VOCs: benzene, toluene, ethylbenzene and xylenes) through samples of 0.76 mm (30 mil) thick flexible poly-vinyl chloride (PVC) and 0.76 mm (30 mil) linear low-density polyethylene (LLDPE) geomembranes are compared based on both sorption and diffusion tests. Diffusion coefficients for the PVC geomembrane were in the range of 5.0--10 x 10-13 m²/s for diffusion from both the aqueous and vapour states. The range for LLDPE was 2.5--5.0 x 10-13 m²/s. Diffusive transport of VOC contaminants through geomembranes in a simulated landfill environment (i.e. in the presence of water vapour) is identical despite the phase they originated from, simplifying the analysis of contaminant transport. The partitioning (sorption) coefficients for PVC were in the range 100--1075 with respect to aqueous phase concentrations. The corresponding values with respect to vapour phase concentrations were 22-290. LLDPE partitioning (sorption) coefficients were 200-475 with respect to aqueous phase concentrations and 44--123 with respect to vapour phase concentrations. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Geotechnical Journal is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Canadian Geotechnical Journal is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Laboratory-accelerated aging experiments conducted to examine the depletion of antioxidant from a geomembrane (GM) underlain by a geosynthetic clay liner (GCL) are described. Three different “protection” layers between the GM and overlying gravel and leachate are examined: (1) A traditional nonwoven geotextile (GT); (2) a GT-GCL; and (3) a GT-sand-GT layer. The GT-GCL protection layer gives an antioxidant depletion rate 0.59 to 0.66 times slower than the GT layer alone. The GT-sand-GT layer gives depletion rates 0.72–0.75 times that of the conventional GT alone. Based on Arrhenius modeling, the time required for depletion of antioxidants at 35°C is estimated to be 65 years for a GM with a GT-GCL protection layer, 50 years for a GT-sand-GT layer, and 40 years for a conventional GT protection layer. These times are all significantly greater than the depletion time for GM immersed in leachate (10 years) for the geomembrane tested. [ABSTRACT FROM AUTHOR]

A numerical model called BioClog is used to backcalculate biological activity rate constants using measured values of water quality and clog chemical characteristics from well-controlled laboratory column experiments that contained a granular-sized material permeated with synthetic and real leachates. BioClog is a multispecies, reactive chemical transport model capable of predicting clogging of a porous media caused by the accumulation of biofilms, chemical precipitates, and entrained particles. Monod kinetic constants for acetate- and butyrate-degrading bacteria were obtained through inverse modelling of granular-sized material permeated with synthetic leachate. The model predicted the changes in concentrations of volatile fatty acids and dissolved calcium and it predicted the changes in clog composition from a juvenile clog containing biofilm to a mature clog containing biofilm with mineral matter. The kinetic constants were then applied to predict spatial and temporal water quality and clog composition for a granular-sized material permeated with real leachate. The kinetic constants deduced through inverse modelling of the synthetic leachate column experiments provided reasonable predictions of the behaviour of the columns permeated with real leachate. On utilise un modèle numérique appelé BioClog pour calculer à rebours les constantes de vitesse de l’activité biologique au moyen de valeurs mesurées de la qualité de l’eau et des caractéristiques du blocage chimique dans des expériences bien contrôlées de colonnes en laboratoire qui contiennent un matériau de dimension granulaire infiltré par des lixiviants réels et synthétiques. Le BioClog est un modèle de transport chimique réactif multiespèce pouvant prédire le colmatage d’un milieu poreux dûà l’accumulation de biofilms, de précipités chimiques, et de particules entraînées. Les constantes cinétiques monodes pour les bactéries dégradables à l’acétate et au butyrate ont été obtenues au moyen d’une modélisation inversée de matériau de dimension granulaire infiltré par un lixiviant synthétique. Le modèle a prédit les changements de concentrations des acides gras volatiles et du calcium dissout, de même que les changements dans la composition du blocage partant d’un blocage juvénile contenant du biofilm, à un blocage mature contenant du biofilm avec de la matière minérale. Les constantes cinétiques ont alors été utilisées pour prédire la qualité de l’eau dans l’espace et dans le temps, et la composition du blocage pour un matériau de grosseur granulaire infiltré par un lixiviant réel. Les constantes cinétiques déduites par la modélisation inversée des expériences de colonnes de lixiviant synthétique ont fourni des prédictions raisonnables du comportement des colonnes infiltrées par un lixiviant réel. [ABSTRACT FROM AUTHOR]

The results of two series of accelerated aging tests are reported. Both series of tests were conducted at temperatures of 85, 70, 55, and 26°C over a period of about 3 years. In the simulated liner series, the top of the geomembrane was covered with a geotextile (protection) layer that was exposed to simulated municipal solid waste (MSW) landfill leachate while the bottom of the geomembrane was in contact with a hydrated geosynthetic clay liner. In the immersion series, the geomembrane was immersed in the simulated MSW leachate, and hence, both sides were exposed to leachate. The results from oxidative induction time tests indicate that the antioxidant depletion is about 2.2–4.8 times faster for the leachate immersed geomembrane than for geomembrane in a composite liner. The higher rates are attributed to the higher extraction of antioxidants from two sides of the geomembrane immersed in leachate. The measured antioxidant depletion rates are extrapolated to a range of temperatures (0–60°C) using Arrhenius modeling. At a liner temperature of 35°C, the calculated time for the depletion of antioxidants is about 40 years for a geomembrane in a composite liner compared to 10 years if it is simply immersed in leachate. These tests suggest that to obtain realistic estimates of geomembrane service life one needs data from tests that simulate the expected field conditions and that prediction based on immersion tests may underestimate the service life. [ABSTRACT FROM AUTHOR]

Ten flow cells, called mesocosms, are used to investigate the effect of different gravel sizes (38 and 19 mm) and operating conditions on clogging of leachate collection systems. These mesocosms simulated in real time and real scale the two-dimensional leachate flow conditions representative of a section of a continuous 300-mm-thick gravel drainage blanket adjacent to a leachate collection pipe in a primary leachate collection system. The tests were terminated after 6–12 years of operation. In some mesocosms the full 300 mm of gravel was saturated. In others, the leachate level was initially set at 100 mm and the upper 200 mm were unsaturated. Although the flow through all mesocosms was similar, the clogging in the fully saturated gravel was substantially more than in the partially saturated gravel. After 6 years of operation, typically, less than 10% of the initial pore space was filled with clog material in the unsaturated gravel. For the saturated zone, 45% of the initial pore space was filled with clog material in the fully saturated design as compared to only 31% in the partially saturated design. The 38 mm gravel performed much better than the 19 mm gravel. For example, it maintained a hydraulic conductivity that was higher than the 19 mm gravel even after operating for twice as long. Up to four mesocosms were placed in series, with the effluent from one mesocosm being the influent for another. The reduction in mass loading within the first mesocosm reduced the amount of clogging within the mesocosm later in series. There was a clear progression of decreasing amounts of initial pore space filled with clog material in the last mesocosm in series, and most of the clogging was due to the vertically percolating leachate. [ABSTRACT FROM AUTHOR]

Geosynthetic clay liners (GCLs) are frequently used as part of composite base liners for landfills. Heat generated by the waste can potentially cause drying of the hydrated GCL. In order to better understand the performance of GCLs under such conditions, an assessment of the water retention characteristics under drying conditions is required. To date, such an investigation has not been undertaken for GCLs, although some data exists for compacted bentonite. This paper presents the results of laboratory testing aimed at obtaining water retention curves (WRCs) for two GCL materials representative of those commonly used in practise. Pressure plate and pressure membrane extractors were used to assess the relationship between the degree of saturation and the applied capillary pressure (suction) for a range of capillary pressures between 10 and 10000kPa. The effect of overburden pressure was investigated, as was the relationship between capillary pressure and bulk GCL void ratio. Volume change behaviour was found to have a significant effect on the WRCs. Representative parameters were obtained by fitting the predictions of three WRC models to the experimental data. Reasonable fits were achieved and the parameters obtained may be used for numerical modelling. [Copyright &y& Elsevier]

Copyright of Canadian Geotechnical Journal is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Canadian Geotechnical Journal is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This lecture describes the latest findings with respect to the long-term performance of modern municipal solid waste (MSW) landfill barrier systems. Field data relating to the clogging of leachate collection systems and the latest techniques for predicting their performance are examined. It is indicated that the primary leachate collection systems may have service lives that range from less than a decade to more than a century, depending on the design details, waste characteristics and mode of operation. Recent data indicate that landfill liner temperatures can be expected to reach at least 30-40°C for normal landfill operations. With recirculation of leachate the liner temperature increases laster than under normal operating conditions, and may be expected to exceed 40°C. Temperatures (up to 40-60°C) may occur at the base of landfills where there is a significant leachate mound. Temperature is shown to have a significant impact on both contaminant migration and the service life of the liner system. Field measurements and theoretical calculations show that composite liners are substantially better than single liners in terms of controlling leakage from landfills. Also, the leakage rates with a composite liner are very small, and diffusion will dominate as a transport mechanism for contaminants that can readily diffuse through a geomembrane (CM). Composite liners involving a CM over a geosynthetic clay liner (CCL) gave rise to substantially less leakage than those involving a compacted clay liner (CCL). The observed leakage through composite liners can be explained by the holes in, or adjacent to, wrinkles/waves in the CM, and this leakage can be calculated using simple equations. High-density polyethylene (HDPE) CMs provide an excellent diffusive barrier to ions. However, some organic compounds readily diffuse through HDPE CMs, and a combination of CM and an adequate thickness of liner and attenuation layer are required to control impact to negligible levels. The long-term performance of HDPE CMs is discussed. Based on the currently available data, the service life for HDPE CM in MSW landfill is estimated to be about 160 years for a primary liner at 35°C and greater than 600 years for a secondary CM provided it is at a temperature of less than 20°C. Clay liners are susceptible to both shrinkage and cracking during construction (due to heating by solar radiation or freezing) and after placement of the waste (due to temperature gradients generated by the waste). The former can be controlled by quickly covering the liner with a suitable protection layer. The latter can be controlled by appropriate design. The use of numerical models for predicting the service lives of engineered systems and long-term contaminant transport is demonstrated. [ABSTRACT FROM AUTHOR]

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]