Richard W. I. Brachman

The surface temperature of an exposed composite liner consisting of a geosynthetic clay liner (GCL) covered by a black geomembrane (GMB) can routinely exceed 60°C during summer solar exposure before falling to ambient air temperature at night. Observations at the Queen's University Environmental Liner Test Site (QUELTS) have recently shown that the evaporation and cooling of the air space at the GMB–GCL interface causes condensed water to form underneath the GMB. These water drops flow down-slope until they are released onto the top surface of the GCL at points of contact, seams, or changes in geomembrane curvature. An unforeseen consequence of this solar-driven generation and release of distilled water onto the GCL was discovered to be down-slope erosion of bentonite from the GCL. The objective of the current study was to repeat the QUELTS field exposure experiment to confirm whether the immediate covering of the GMB will supress the mechanism of down-slope bentonite erosion through the elimination of the air gap at the GMB–GCL interface and a reduction in thermal cycling. Further, this study examined whether the use of a white-coloured GMB can reduce the time to onset of erosion if immediate cover is not possible. Results confirm that no erosion occurred on the composite liner test section covered by 0.3 m of gravel after the 15 month duration of the study. In contrast, erosion was observed to occur on the exposed white GMB section, but at a slower rate than that observed on the exposed black GMB section. The timely placement of cover soil on the composite liner as per manufacturers' recommendations is therefore the most effective strategy to avoid the issue of down-slope bentonite erosion.

Field observations of downslope bentonite erosion from a geosynthetic clay liner (GCL) covered by only a black geomembrane are reported for a composite liner left exposed without a protective soil cover for much longer than recommended by the GCL manufacturer. A new nondestructive, light-transmission technique developed to investigate bentonite erosion features in the field is presented. A classification system is developed to describe the type of erosion features observed. Results from five field investigations at the Queen’s University Environmental Liner Test Site (QUELTS II) are reported to quantify the onset, progression, and severity of downslope erosion for one particular geotextile-encased, needle-punched GCL for exposure periods of between 7 weeks and 1.3 years. The first significant erosion feature (type “E”with bentonite loss narrower than 2.5 cm) was observed after 6 months of exposure. Irrecoverable erosion features (type “EE” with bentonite loss wider than 2.5 cm) were observed after 12 months of exposure. These findings highlight the need to follow the manufacturer’s recommendations for timely covering of a composite liner with soil following liner installation.

A black 1.5 mm geomembrane (GMB) and geosynthetic clay liner (GCL) liner were placed on both a 3H: 1V (18.4°) slope and a gently sloping (3%) base (latitude 44°34′15″N) and left exposed for 4.7 years. The observed solar radiation, ambient air temperature, temperature at the interface between the GMB and GCL, and temperature at various depths (to 600 mm) in the underlying silty sand soil are reported. The interface temperature was up to 40°C higher than ambient temperature on a sunny day. The difference between interface temperature on the slope and base was minimal near summer solstice and increased significantly earlier and later in the year (except when covered in snow). There was significant variability in GMB and interface temperature depending on the contact conditions between the GMB and GCL. The interface temperatures at wrinkles could be 15°C higher than other locations where there was intimate contact between GMB and GCL. Snow-cover insulated the liner from solar radiation and extreme temperature. These insulating effects were lost on the south-facing slope before the base, subjecting the GCL on the slope to more daily freeze–thaw cycles than on the base. A correlation between the interface temperature, ambient air temperature, and the solar radiation gave good agreement with the observed temperatures. The cycling of interface temperatures is considered to be central to the mechanism of moisture evaporation from the GCL that may cause GCL panel shrinkage and moisture condensation-driven downslope bentonite erosion. Covering the composite liner with a ballast layer as quickly as possible is recommended.

Manufacturers of geosynthetic liner materials recommend that composite geomembrane/geosynthetic clay liners (GCLs) be covered in a timely fashion to avoid potential issues that may arise under the action of long-term solar exposure. In this paper, field evidence of a new, never before reported solar-exposure driven damage mechanism for GCLs covered only by a black geomembrane and left exposed for more than 3 years is presented. Solar exposure can give rise to a large daily variation in geomembrane temperature, which causes a moisture cycle within the interface between the geomembrane and GCL resulting in the formation and flow of condensed moisture beneath the geomembrane. All four of the GCL products investigated at the Queen's University Environmental Liner Test Site were shown to have experienced significant bentonite erosion after 4.7 years of exposure. Erosion was identified in the field through a tactile survey of GCL panels in which the stiffness response of the GCL to touch was used to identify eroded zones. A change in the colour of the GCL, although useful to identify possibly eroded zones in some GCL products, proved ineffective in others. Erosion features were observed with widths up to and exceeding 200 mm across, making them unlikely to undergo self-healing upon hydration and application of normal stress. As a result, the observed erosion features would have severe adverse consequences for leakage rates through the GCL component of a composite liner barrier system. These observations provide yet another strong motivation for timely covering of composite landfill liner systems.

The potential for downslope bentonite erosion from a geosynthetic clay liner (GCL) subjected to an imposed daily surface flow of deionized water is examined for 10 GCLs with different carrier and cover geotextiles, bentonite (powdered and fine and coarse grained and some with a polymer-enhancement additive), and a polypropylene coating on the carrier geotextile. The erosion was similar but more visible in products with a white geotextile than in those with a black geotextile. Also, the erosion features were similar but more visible with nonwoven geotextiles than with woven geotextiles. Polymer-enhanced sodium bentonite substantially slowed the formation of erosion features. A polypropylene coating over the upper geotextile prevented erosion features from developing by keeping water from penetrating into the bentonite layer when subjected to 60 daily imposed-flow/air-drying cycles.

Local tensile strains in a 1.5-mm-thick high-density polyethylene (HDPE) geomembrane induced from coarse overliner soil materials intended to simulate the physical conditions at the base of a heap leach mineral extraction pad under very deep burial are reported. They were obtained from physical experiments conducted in a 590-mm-diameter pressure vessel at applied vertical pressures up to 3,000 kPa for 100 h and a temperature of 21°C. An applied pressure of 3,000 kPa corresponds to a heap leach depth of around 150 m. Three different coarse-grained overliner materials placed directly above the geomembrane and a silty sand underliner beneath the geomembrane were examined. Both the grain size and grain size distribution of the overliner affected the maximum tensile strain in the geomembrane. At an applied pressure of 3,000 kPa, the largest strain of 27% recorded for the coarsest overliner tested, which had a maximum particle size of 50 mm and 20% sand, well exceeded one proposed maximum allowable strain limit of 6%. Finer overliners with both a smaller maximum particle size (25 mm) and much more sand (35 and 55%) reduced the geomembrane strains, but the maximum values still exceeded 6% by a factor of 2, even for the finest overliner examined. At these high pressures, a 150-mm-thick silty sand protection layer between the geomembrane and even the coarsest overliner examined was found to be very effective at reducing local indentations in the geomembrane from overliner particles and was able to reduce the tensile strain in the geomembrane to 2%.

The susceptibility of a 1.5 mm thick high-density polyethylene geomembrane to brittle rupture from long-term stress cracking in a simulated municipal solid waste landfill liner is examined. The geomembrane was pre-aged in a leachate at 85°C to lower the notched constant tensile load stress crack resistance of the geomembrane to about 75 h. The aged geomembrane was then used as part of a composite liner system in geosynthetic liner longevity simulators (GLLSs) with a geosynthetic clay liner and sand foundation layer below the geomembrane and a 560 g/m² geotextile protection layer and 50 mm drainage gravel above the geomembrane. The GLLSs allow the simulation of field conditions including elevated temperatures, overburden pressure, leachate circulation, and composite liner exposure conditions. The geomembrane experienced brittle rupture on the side slopes of the local gravel indentations for temperatures between 55 and 85°C. The higher the liner temperature, the shorter the time to rupture and the higher the tensile strain at rupture. Arrhenius modelling of the test data gave an activation energy of E_a = 112 kJ/mol.

The effectiveness of layered-geotextile protection layers comprised of combinations of nonwoven needle-punched, woven slit-film, and nonwoven heat-bonded geotextiles to minimize strains in landfill geomembranes is examined. Results from physical experiments are reported where a sustained 700-N force was applied to a 28-mm-diameter machined steel probe on top of the protection layer, which was above a 60-mm-diameter, 1.5-mm-thick high-density polyethylene geomembrane and a 50-mm-thick compressible clay layer. The experiments are intended to simulate the physical conditions in a medium-sized landfill with an average vertical stress of 250 kPa and to capture the mean response with nominal 50-mm coarse gravel above the geomembrane. Screening tests were first conducted for up to 100 h at temperatures up to 55°C to evaluate three different combinations of layered geotextiles. Of those examined, the combination with a low-slack, heat-bonded geotextile above and below a thick, nonwoven, needle-punched geo...