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The stress crack resistance (SCR) and standard oxidative induction time (Std-OIT) reduction of high- density polyethylene (HDPE) geomembrane (GMB) fusion seams is examined for a 12-year old 1.5 mm HDPE GMB when welded using nine welding parameter combinations. The GMB experienced a decrease in Std-OIT from 135 to 87 min between manufacturing and welding. Welding did not significantly decrease the Std-OIT in the heat-affected zone (HAZs). In general, individual seam SCR failure times fell between the notched and unnotched sheet, with adhered squeeze-out bead seams exhibiting individual failure times closer to the notched sheet and an average time until nominal failure ( tNF) ∼21%–35% shorter than the unnotched sheet. Differences in tNF are attributed to residual stresses and (or) accelerated craze formation in seams with adhered squeeze-out beads. Non-adhered squeeze-out seams exhibited a slower tNF similar to that of the unnotched sheet, indicating a seam tNF less than that of the sheet may not be intrinsic to all HDPE fusion seams. High sheet temperatures at the time of welding and higher heat-applied welding parameters are thought to increase the likelihood of squeeze-out bead adherence. This paper shows that squeeze-out adherence can have a deleterious effect on seam SCR.

Experiments were conducted to quantify short-term tensile strains induced in a 1.5 mm thick high-density polyethylene geomembrane overlain by tailings. Four subgrades, a poorly graded angular to subangular gravel (GP), a well-graded angular to subangular gravel (GW), a poorly subrounded graded sand (SP), and silty sand (SM), and two nonwoven geotextiles, 450 and 1420 g/m2, were evaluated. All the indentations in the geomembrane (GMB) were due to the subgrade, and with an adequate fraction of sand size particles in the subgrade, the tensile strains could be minimized. At 2000 kPa, the maximum tensile strain was 32% for GP, 16% for GW, 13% for SP, and no discernable indentation was observed for SM prepared at the optimum water content of 11%. For the soft SM subgrade prepared at 20% water content, only one indentation with the tensile strain of 2% was observed. At 2000 kPa, a 450 g/m2 geotextile beneath the GMB reduced the maximum tensile strain from 32% to 23% for GP subgrade, from 16% to 8% for GW subgrade, and from 13% to 6% for SP subgrade; the 1420 g/m2 geotextile reduced the maximum tensile strain to 14% for GP subgrade. Thus, minimal subgrade indentation induced strain and hence the possibility of a very long service life of GMB could be achieved using SM subgrade.

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.

Microstructure showing the involvement of the fine and coarse grains in the soil skeleton is evaluated. Incremental loading tests using a stress-dependent permeameter are conducted on the mixtures of poorly graded sand and nonplastic fines originating from tailings. The results are compared with the published data of various tailings. It is shown that increasing the fines content from 0% to 100%, the involvement of the fine and coarse components of soil skeleton can be classified into four categories: no fines involvement (40% fines). At the same consolidation stress, the void ratio, e, rapidly decreases for fines less than 30%, then almost remains constant between 30% and 50% fines, and gradually increases for fines exceeding 50%. The hydraulic conductivity, k, decreases more than 20-fold as the fines content increases from 12% to 50%, then remains constant. k is proportional to [e3/(1 + e)]A and inversely proportional to S2, where A is a factor describing the effect of particle angularity and S is the specific surface. Finally, the influence of fines content on the seepage-induced internal stability is discussed.

The observation of failure surfaces within column-supported embankments is critical to understanding how the embankment stresses are transferred towards the column heads. In this study, finite element simulations utilising a strain-softening constitutive model, nonlocal regularisation, and the Arbitrary Lagrangian–Eulerian formulation are used to examine these failure surfaces over various embankment geometries. This methodology offers insights into the nature of the failure mechanism, the development of a plane of equal settlement, and the influence of the subsoil settlement profile. Depending on the embankment geometry, the results indicate a punching failure, inverted general bearing failure, or a localised failure develops. The transition between punching and inverted general bearing failure is found to be closely related to the establishment of a plane of equal settlement within the embankment. The height of the plane of equal settlement and the range of failure mechanisms that develop were largely insensitive to the nature of the subsoil settlement profiles simulated. These findings have implications for the practical design of efficient embankments and the effective design of future experimental studies.

This study offers an analytical solution for radial consolidation that captures the bio-geochemical clogging effect in acid sulfate soils. Field sites and personal communication with industry practitioners have provided evidence of piezometers exhibiting retarded pore pressure readings that do not follow conventional soil consolidation and seepage principles when installed in coastal acidic floodplains. This retarded response together with a variation in pH, ion concentrations, and piezometric heads provided evidence of clogging at and around the piezometers. This paper uses the proposed bio-geochemical clogging model, which is an analytically derived system of equations to estimate the excess pore water pressure (EPWP) dissipation of piezometers installed in clogging-prone acid sulfate soils. The inclusion of the total porosity reduction attributed to biological and geochemical clogging improves the predictions of the retarded dissipation of excess pore pressure, especially after about 1 year. This method is validated for two previously identified acidic field sites in coastal Australia, where piezometers measured a very slow rate of dissipation. It is concluded that this model has potential to accurately monitor the performance of critical infrastructure such as dams and embankment foundations built on acidic terrain.

Experiments are conducted to quantify leakage through a circular geomembrane (GMB) hole using two subgrades (underliners) not meeting modern filter criteria. Test results show that a 1.9- and 5.0-fold increase in hole diameter causes a 1.8- and 6.2-fold increase in leakage, respectively, whereas a 2-fold increase in GMB thickness results in an approximate 10% decrease in leakage. For a constant hydrostatic effective stress, a 2- and 3-fold increase in water head above the GMB leads to a 1.8- and 2.5-fold increase in leakage, respectively. Higher leakage arising from the intrusion of subgrade materials through the hole is quantified. A numerical model, considering the localized concentration of head loss around the hole and the consequent heterogeneity in hydraulic conductivity due to the seepage force, is presented. The percent of head loss within the hole and from the hole to any position above the GMB is independent of loading conditions and only influenced by the GMB thickness and hole diameter. Finally, empirical equations for predicting leakage through a circular GMB hole in tailings storage applications are proposed. Compared with published equations that do not consider consolidation induced heterogeneity in hydraulic conductivity, the proposed equation (which does consider these factors) gives a more accurate prediction for the cases examined.

Leakage through a 10 mm diameter circular hole in a geomembrane beneath fine-grained tailings is examined for a range of pore pressures and effective stresses. Leakage was measured in an experiment with coupled physical and hydraulic conditions to simulate the effective stresses and flow conditions near the hole. The leakage rate was as little as 0.16 L/day with 200 kPa pore pressure (10–30 kPa effective stresses) and increased only to 0.46 L/day with 800 kPa pore pressure (200 kPa effective stress). Seepage analysis of the experiment and local measurements of permeability from small samples extracted after the experiment indicate that the tailings hydraulic conductivity controlling flow was 3–6 × 10−9 m/s. Only a subtle decrease in hydraulic conductivity (less than 2 times) near the hole was found. No evidence of seepage-induced migration of fines within the tailings was found. Calculations with the parameters deduced from the experiment show that leakage from a tailings storage facilities containing fine-grained tailings can be limited to 1–2 L/ha/day with a geomembrane liner, even when containing up to five 10 mm diameter holes per hectare, as opposed to an unlined facility with 3 to 4 orders of magnitude more leakage.

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.

This study demonstrates the change in porosity of permeable reactive barrier (PRB) material when it reacts with acidic flow. The laboratory column test data obtained over nine months proves that the porosity of a granular limestone assembly decreases significantly due to bio-geochemical clogging caused by a continuous flow of acidic groundwater. The variations in pH, the pressure measurements, ion concentrations, and the results from X-ray diffraction suggest that clogging at the outlet of the column is much less than at the inlet. About 57% of the total reduction in porosity of the column is attributed to chemical clogging, while the remainder is mainly due to biological clogging. In this paper, a mathematical approach is proposed to estimate the reduction of reactive surface area based on changes in the pore volume. These proposed equations suggest that at the end of experimentation, the dissolution of calcite and bio-geochemical clogging can reduce the total surface area of limestone aggregates by more than 70%. The rigorous approach presented in this paper to determine the dominant clogging component within a granular filter at a given time is vital in estimating the longevity of a PRB and for planning its maintenance.

The degradation in physical and mechanical properties of a 1.5 mm thick HDPE geomembrane immersed in seven different low pH and three high pH simulated mining solutions is examined at temperatures between 95 and 40 °C for 3 years. The solutions with pH between 0.5 and 13.5 encompass solutions found in copper, nickel, uranium, gold, and silver heap leach pads. The geomembrane did not exhibit any chemical degradation during the three years of incubation in all the low-pH solutions. However, in the solutions with pH 9.5, 11.5, and 13.5, some of the geomembrane’s physical and mechanical properties are shown to reach nominal failure at 95 and 85 °C. While the geomembrane examined shows superior performance in the acidic environments than in the basic solutions examined, its performance in such extremely basic environments is still better than in neutral reduced municipal solid waste leachate. Using Arrhenius’ modelling, the predicted times for the antioxidant depletion stage of the geomembrane examined in composite liner configuration range between 31 years in pH 13.5 to 51 years in pH 0.5 for pad liners at 50 °C, exceeding a typical leaching period of the ore of around 20 years in different heap leaching operations.

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.

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.

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 wfdn, GCL mass per unit area, and overburden stress (20–100 kPa). GCL self-healing is better for wfdn = 16% than for wfdn = 10%, which is better than for 5%, when the SSS pore water has negligible cations (ionic strength, I

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 (θGeofilm/GCL), however most of the flow occurs at the coating/GMB interface (θGeofilm/GMB).

The long-term performance of geomembranes with 12 different resin–antioxidant master-batch combinations, including eight high-density polyethylene (HDPE), three linear low-density polyethylene (LLDPE), and one blended polyolefin (BPO) base resins, is investigated. Results are reported for immersion tests in chlorinated water (0.5 ppm) for 35 months at 85 °C. The degradation trends show that the choice of resin type played a key role in the longevity of the geomembranes but also that some hindered amine light stabilizer (HALS) packages contributed to better resistance to degradation in chlorinated water. The results show that the specific antioxidant package is more important than the initial oxidative induction time (OIT) in terms of long-term performance. Finally, it is shown that while increased thickness may be beneficial, a more resistant resin or antioxidant–stabilizer package can be more effective than increasing thickness in improving geomembrane performance in chlorinated water. The conclusion regarding the beneficial role of HALS is specific to chlorinated water and generally is not true in other cases of submerged or buried geomembranes.

Geotextiles and geomembranes have been around for more than 40 years and geosynthetic clay liners (GCLs) for more than 25 years. During that time, they have revolutionized the containment of fluids...

Geosynthetic reinforced column supported embankments predominantly utilise two mechanisms to transfer embankment loads towards column heads, soil arching and membrane actions. When undertaking the design of column supported embankments, it is common practice to perform a two-step design, whereby the arching actions are estimated independently of the subsoil deformation and membrane actions. This approach is unable to capture the deformation dependency exhibited by both arching and membrane actions. This paper presents deformation dependent arching and membrane action models and implements them within an interaction diagram. It is shown that an interaction diagram-based design approach is capable of performing an ultimate and serviceability limit state design of a geosynthetic reinforced column supported embankment. In contrast, most existing analytical design methods only consider the ultimate limit state. The proposed method is applied to a design example where the benefits of such a design approach are demonstrated.

This paper examines the hydration/dehydration behaviour of geosynthetic clay liner (GCL) under polar conditions for four simulated conditions experienced at Australia's Casey Station in Antarctica: (a) hydration during summer, (b) dehydration during a winter-summer cycle, (c) hydration through a fine Antarctic soil, and (d) hydration through a coarse Antarctic soil. Hydration during the summer is found to occur if there is direct contact with the water table. In contrast, the low relative humidity of the environment tends to dehydrate the GCL along a path that depends on the field conditions the GCL is exposed to. Hydration from either fine or coarse Antarctica soil is function of the original gravimetric water content of the subgrade soil and its grain size distribution as well as the low relative humidity prevailing in Antarctica.

The influence of co-disposal of municipal solid waste (MSW) and incinerator ash used as daily cover on the clogging of leachate collection systems (LCSs) from landfills is examined. The “BioClog” model is used to simulate the fate and transport of the nine leachate constituents most responsible for clogging the LCSs as they move through the porous media. It then calculates the thicknesses of five films that attach to the porous media and the effect of this clog mass–volume on the porosity and hydraulic conductivity of the granular material. Then it models the consequent growth in the leachate mound with increasing clog mass over time until the service life of the LCS is reached. The modeling shows that the concentrated source of leachable minerals in the incinerator ash accelerates the clogging rate and reduces the service life of the LCSs compared to inert daily cover. If an LCS is not designed to accommodate these higher concentrations of cations in the influent leachate during the landfill operating period, the ash can significantly reduce the LCS service life. Means of extending LCS service life are discussed. A practical technique is also utilized to estimate the service life of LCSs with conservative and reasonable agreement with BioClog.

The desiccation and subsequent hydraulic conductivity of both a standard (GCL_A) and polymer-enhanced (GCL_B) Na-bentonite GCL hydrated from a well-graded sandy subsoil under 20 kPa, then subjected to a thermal gradient, and finally rehydrated and permeated with distilled water or 0.325 mol/L Na+ synthetic brine are reported. With moderate temperature of 40 °C applied to the top of the liner, GCL_B experienced less cracking than GCL_A, but this advantage disappeared when temperatures increased. Both desiccated specimens of GCL_A and B showed significant self-healing when permeated with distilled water and their hydraulic conductivities quickly reduced to around 10−11 m/s at 20 kPa upon rehydration. However, when GCL_B desiccated specimens were permeated with the synthetic brine, their hydraulic conductivities were found to be one to two orders of magnitude higher than corresponding values obtained with distilled water. On the other hand, GCL_A (with no polymer treatment) maintained its hydraulic conductivities at the same level obtained with distilled water. It is concluded that caution should be exercised in using polymer-bentonite in applications in which GCLs are subjected to significant thermal gradients unless there is data to show they are resistant to thermal effects.

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.

An approach for selecting a high-density polyethylene (HDPE) geomembrane (GMB) for a long design life is described and illustrated for five 2 mm thick textured GMBs when immersed in a simulated municipal solid waste leachate (L3) and two simulated leachates representative of low-level radioactive waste leachates (L7 and L9) for 9–16 months at a range of temperatures. Although made from the same nominal resin, substantial differences are reported in both the initial properties and the rate of antioxidant depletion for the five GMBs. At an expected operating liner temperature of 10 °C and immersed in L3, the projected time to antioxidant depletion for the five GMBs ranges from 125 to over 2000 years. The antioxidant depletion in leachates L7 and L9 were similar or slower than in leachate L3. There was no evidence of traditional thermal-oxidative degradation reported over the 9–16 months of monitoring; however, there was a significant reduction in stress crack resistance due to physical ageing ranging between 30% and 70% of the initial value. Two GMBs are considered highly likely to have service life well in excess of the required design life of 550 years. It is suggested that the proposed approach could be adopted for selecting GMBs for other projects that require a long design life.

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.

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.

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.

The effect of texturing (co-extrusion using blowing agent) on the longevity of a geomembrane (GMB) when immersed in synthetic municipal solid waste leachate is investigated over a ∼3 year period. Based on data at four temperatures (40, 55, 75, and 85 °C), the time to antioxidant depletion of the textured portion of a 1.5 mm core thickness high-density polyethylene (HDPE) GMB is 40% (standard oxidative induction time) and 9% (high-pressure oxidative induction time) faster compared to the 1.5 mm smooth edge of the GMB. However, despite this, stress crack resistance results show that texturing may have no significant effect on the time to nominal failure for this GMB. It is also shown that HDPE GMBs made from nominally the same resin but from different production lots have different rates of stress crack resistance degradation and hence time to nominal failure; this should be considered both in landfill design and landfill construction quality assurance.

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.

Two funnel-and-gate permeable reactive barriers (PRBs) with settling ponds comprised of a composite liner (geomembrane and geosynthetic clay liners (GCL)) were installed at Resolution Island (BAF-5), Nunavut to contain residual polychlorinated biphenyls (PCBs) moving with the sediment and annual snowmelt. The long-term performance of the geosynthetics used in the PRB funnels is studied for physical integrity and diffusive barrier properties after nine years of operation. Exhumed geomembrane specimens are compared with virgin material by index testing: diffusive resistance to benzene, toluene, ethylbenzene, and xylenes (BTEX) compounds, as well as puncture, burst, and tensile strength. Exhumed GCLs are evaluated concerning hydraulic conductivity, the mass of bentonite per unit area, and swell index. The migration of PCBs through the composite liner system by diffusion is modelled and the diffusive and sorptive properties of the geomembrane (Dg = 1.7 × 10−14 m2/s, Sgf = 160,000) and GCL (De = 3.1 × 10−10 m2/s, bentonite plus fibers layer Kd = 15 mL/g, cover geotextile Kd = 12,000 mL/g, and carrier geotextile Kd = 16,000 mL/g) were calculated. Modelling results estimate that the composite liner was successfully containing PCBs. This was confirmed by downgradient monitoring. The challenges of the location and terrain, PRB design, construction, and maintenance are discussed along with recommendations for designing PRBs in other remote and cold region environments.

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.

A high density polyethylene geomembrane (GMB) stabilized with hindered amine (light) stabilizers (HALS or HAS) is immersed in four chlorinated water solutions with a simulated free chlorine concentration range of 0.5–5 ppm at five different temperatures (25, 40, 65, 75, and 85 °C) for 70 months. Standard and high pressure oxidative induction time (OIT) tests are performed to monitor antioxidants depletion while melt flow index, tensile, and stress crack resistance (SCR) tests are conducted to monitor degradation in physical and mechanical properties. Degradation in the GMB properties occurred shortly after immersion in chlorinated water at all temperatures except at 25 °C. Increasing the free chlorine concentration resulted in faster degradation of the tensile properties and SCR. The predicted time to nominal failure based on SCR ranges between 25 years at 40 °C and 5 years at 85 °C in chlorinated water (with 0.5 ppm free chlorine). A comparison between the degradation in SCR of this GMB and a GMB with a different resin and without HALS shows significant difference in their performance in chlorinated water but not in other incubation media.

The transfer of embankment stresses towards pile heads in piled embankments is attributed to the mechanism known as soil arching. Three-dimensional physical models of piled embankments were built to simulate this mechanism. The progressive settlement of subsoil beneath an embankment was modelled and paused at increments of displacements to allow synchrotron X-ray computed tomography to be performed on the models. Image correlation techniques were then applied to the reconstructed volumes to obtain evolving three-dimensional displacement and strain fields. The strain fields show localised (shear bands) and diffuse failure modes occurring above pile heads within the embankment fill. These failure surfaces are seen to progressively develop as the subsoil undergoes settlement. The displacement fields also show the formation of a plane of equal settlement developing at a height above the pile heads, known as the critical height. The critical height is dependent on the height at which the failure surfaces propagate into the embankment fill, and a method is proposed to calculate the maximum height of failure surfaces based on the observed kinematics. The full-field kinematics provide fundamental insight into the soil arching mechanism that develops within piled embankments.

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.

The dependence of the geosynthetic clay liners (GCLs) soil-water characteristic curve (SWCC) on temperature and overburden stress are characterised experimentally. It is shown that changes in void ratio and temperature alter the relationship between suction and moisture content and new forms of existing SWCC equations are developed. To cover a wide suction range, the SWCCs are measured using axis-translation and dew point methods. Based on the available experimental data, both proposed SWCCs are shown to perform well in predicting the effects of void ratio on SWCC along the drying path when compared to the experimental results. It is found that the air-entry value increases as the net vertical stress increases for the experiments under the same temperature. In addition, elevation of temperature reduces retention capacity of the GCL.

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 (θ2-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.

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.

This lecture examines the progress made in the last few decades concerning the use of geosynthetics for aiding in the construction on soft soils. Consideration is given to different soft soils ranging from peat to rate-sensitive soft clay and silt. Both relatively elastic and rate-sensitive reinforcements are examined. Consideration is given to basal reinforcement, prefabricated vertical drains, and embankments with reinforcement and other supports such as piles. Particular emphasis is placed on advances since the senior author’s 2002 Giroud lecture.

Organic contaminant diffusion through a novel multilayer coextruded vapor barrier is examined for benzene, toluene, ethylbenzene, xylenes (BTEX), trichloroethylene (TCE), and tetrachloroeth...

It is becoming increasingly more common to install a layer of highly permeable granular material beneath the primary liner in a landfill. Most commonly, the layer is intended to allow detection and collection of leachate which escapes through the primary liner. The separation between the aquifer and the landfill would be as large as possible for a design involving a secondary leachate collection system. However, in many practical situations the actual thickness may be quite thin. The role of advection for allowing the migration of contaminants from waste disposal facilities is well recognized. More research is required to determine relevant parameters; however, it is evident that diffusive transport through geomembranes and secondary leachate collection systems is an important consideration in the design of these facilities. The processes involved in contaminant migration for hydraulic control systems are better understood than the processes of migration for systems that use the granular layer as a secondary leachate collection system.