Geomembranes, predominantly high-density polyethylene (HDPE), have been used as bottom liners in landfills for almost 30 years. They have also been used as caps and, more recently, for floating covers on leachate ponds. Generally they have performed extremely well but, not unexpectedly, there have been a few exceptions that have guided us in developing even better systems.Defining the terms
Before embarking on a detailed review of what is happening in this field, let us begin by defining the terms:
Liner the barrier layer on the floor and sides of a containment facility
Cap the barrier layer on top of a solid waste containment facility
Floating cover a geomembrane floating on contained liquid that is sealed to the bottom liner around the edges of the pond and that rises and falls with the liquid level.
Therefore, a geomembrane liner contains valuable product and/or protects the groundwater, a geomembrane cap contains landfill gas and prevents precipitation becoming leachate, and a floating cover prevents odour emissions and prevents evaporation/contamination of a valuable resource such as potable water.The evolution of HDPE use
In the early 1980s HDPE essentially displaced PVC as the geomembrane of choice because of its broad chemical resistance, its high strength, its relative inherent flexibility achieved without additives, its weathering resistance that allows it to be left uncovered, and its ability to be integrally fusion-welded by thermal methods rather than by using solvents and adhesives. At that time PVC seams could be peeled apart, HDPE seams could not.
HDPE geomembrane lining system being installed in quarry MSW landfill Click here to enlarge image
It was soon found, particularly in cold environments, that when HDPE contracted at low temperatures the tensile stresses induced in the liner could generate brittle stress cracking (SC) a phenomenon that had previously been experienced and studied in HDPE natural gas distribution pipes. Stress cracking is a brittle cracking that occurs at a constant stress lower than the tensile yield or break stress of the material. It was found that SC is a function of the formulation of the resin and therefore can be quite different in HDPEs from different geomembrane manufacturers. However, formulations have been improved over the years to the point where SC now rarely occurs in materials from the established international HDPE resin and geomembrane manufacturers.
Because of this susceptibility to SC it is a standard design objective that geomembranes function as a barrier only and not as a load bearing member of the lining system hence the requirement that a geomembrane liner be fully supported. While this can be achieved easily on a drawing it is very difficult to achieve in practice due to the high coefficient of thermal expansion of HDPE. Wrinkles occur that are very difficult to eliminate in practice. Therefore HDPE geomembranes are unavoidably stressed when covered. Partly because of this, double lining systems were developed using the secondary (lower) liner as a safety back-up. This worked very well provided the leachate detection system (LDS) between the two geomembranes was not allowed to fill with leaked liquid. Avoiding the hydraulic head on the secondary liner would result in the lining system not leaking. Both primary and secondary geomembranes have a similar number of unavoidable defects, but provided the liquid leaking through the primary (upper) liner is not allowed to accumulate over the few flaws in the secondary liner the double system does not leak into the subgrade.Locating leaks
Collecting the leaking liquid allowed the leak flow rates through the primary liner to be determined. At the same time, in the late 1980s electrical techniques were being introduced to locate leaks in geomembranes covered by liquids and by soils. Approximately 25% of leaks are introduced during installation of the geomembrane itself and 75% are introduced as the geomembrane is covered. The number of leaks per unit area decreases with liner area and as the relative amount of detail work in the installation decreases. Leak frequency drops from about 12/ha to about 2/ha above 2 ha.
The electrical surveys showed that most liner installations leaked to different degrees mostly as a function of the effectiveness of the construction quality assurance (CQA) that was performed. In an attempt to minimize leakage, geomembranes were combined with compacted clay liners, and later geosynthetic clay liners (GCLs), to make composite liners. With GCLs, typically two layers of geotextiles with bentonite granules between them and needlepunched together, the principle is that a leak in the geomembrane will hydrate the bentonite causing it to swell into and seal the leak. ‘Intimate’ contact between GCL and geomembrane precludes the lateral transmission of liquid along this interface, thereby significantly reducing the individual primary liner leakage. However, the wrinkles in the geomembrane still remain a potential problem, generating areas where the geomembrane is not in contact with the GCL, and areas where the liner is stressed and has a higher potential for failure. It is also quite common for bulldozers spreading cover soil to scrape the tops off wrinkles. Hence the development of double composite lining systems.
However, while desirable in theory, the actual practice is that wrinkled secondary liners have wrinkled HDPE geonet/geotextile composite LDSs placed on them, which are in turn covered by wrinkled primary geomembranes. Stresses are not avoided; intimate contact is not achieved, and wrinkles in the secondary liner result in ponding water on the secondary. Nevertheless, most lining systems appear to be working satisfactorily.Is zero leakage attainable?
‘Satisfactorily’ does not mean with zero leakage through the primary liner. Electrical surveys have shown statistically that zero leakage is an unreasonable expectation. Non-leaking primary liners do exist, but they certainly cannot be guaranteed. There have been many double lining systems with only drips coming out of the LDS but with which owners have not been satisfied. Demands to repair the liner have only resulted in increasing leakage flow rates. In the United States an acceptable maximum allowable leakage rate through a primary geomembrane with 300 mm of hydrostatic head (the regulated maximum), above which the leaks shall be found and repaired, is typically 200 litres/ha per day. In municipal wastewater treatment plants, under a 2 metre head, it is 5000 litres/ha day. This is too high, and could more reasonably be about 2000 litres/ha day. Therefore, sensible liner designs should assume a maximum allowable leakage flow rate and incorporate an underdrain system that will handle this flow rate without further damage to the subgrade and the liner.The importance of good drainage
Nowhere is this more important at present than in wastewater treatment plants where bacterial activity in leaked water can continue and where additional reaction with subgrade vegetative matter generates large volumes of methane and other gases. With standing water under the liner there is no pathway for these gases to escape to the gas vents placed at the tops of the slopes. The gases accumulate and the liner rises until it breaks the water surface and becomes a ‘whale’. Whales usually contain gas above the water level and the water level under the liner is the same as that above the liner. There must be a good drainage system under the liner from which the water can be removed and it must have sufficient capacity that any generated gases can vent upgradient to the slope vents. A typical non-woven geotextile cushion is not adequate.
With the importance of identifying leaks before major problems occur it is becoming a regulated requirement for geoelectrical liner integrity surveys to be performed on all primary geomembrane liners as the final stage of CQA after the liner has been covered to depths between 600 mm and 1 metre. At this stage, further activity on the soil cover surface is not likely to cause further damage to the geomembrane, yet it can still easily be exposed to make repairs. In fact, New York State Department of Environmental Conservation, having seen the practical benefits of performing surveys on the primary geomembrane, is adding a requirement to perform surveys on the floor area of secondary geomembranes.Landfill capping
To minimize the amount of primary leachate that has to be treated and disposed of, geomembranes have also been introduced to cap landfills, both to prevent the ingress of precipitation and to capture and remove landfill gas (LFG). While HDPE has also been used in caps, the preferred material is often linear low density polyethylene (LLDPE) or another more flexible material such as PVC or flexible polypropylene (fPP) that will better accommodate the strains and resultant stresses that are associated with differential settlements within the waste. Thus, the geomembrane must be able to conform to the profile of the differential settlement without inducing stresses that could, for instance, initiate stress cracking. The less crystalline materials such as LLDPE and fPP are not susceptible to SC in the as-manufactured condition. This requirement also clearly demonstrates that the ductility of a geomembrane is far more important than its strength in relation to long-term performance. Strength is only of importance in being able to handle the rigours of installation.
BGM cap on industrial waste landfill with exposed top and vegetated slopes Click here to enlarge image
In order to stop liners and caps from sliding down slopes they are often textured or structured to enhance friction with the underlying soil or non-woven geotextile on GCLs and geocomposites. Texturing is a random ‘rough sea’ profile, while structured profiles are engineered profiles designed to interact optimally with different mating surfaces. To keep cover soils from slipping on top of the geomembrane the upper surface may also be textured or structured. However, if the interface shear strength of the upper interface is higher than that of the lower interface a tensile stress is induced in the geomembrane that may be damaging in the long run, particularly for HDPE. Hence another reason for using a cap material that is not susceptible to SC.
There have been many side-slope failures on landfill caps when cover soils have slipped, mostly due to design not taking account of saturated soil conditions. In some cases the cover soil slips on the geomembrane, leaving the geomembrane undamaged, and in other cases the cover soil takes the geomembrane with it and tears the geomembrane. This is a function of double or single surface friction enhancement. As a consequence there is now a trend to the use of exposed geomembrane caps (EGC).What can exposed geomembrane caps offer?
Exposed caps eliminate problems with soil covers, the cost and construction time of a cap are reduced, and air space is increased. Conversely, design must accommodate the possibility of wind uplift, LFG pressure uplift, exposure to UV radiation and higher temperatures, and high volumes and speeds of precipitation run-off. Approximately eight exposed geomembrane caps (EGCs) have been constructed in the USA using reinforced fPP, HDPE, and ethylene interpolymer alloy (EIA). One of the HDPE EGCs in Florida successfully survived two hurricanes. The fPP-R cap has performed very well over 10 years and is about to be permitted for another 10 years. PP performs well because of its low coefficient of thermal expansion and lack of wrinkles. Wrinkles in HDPE tend to migrate down slopes but will not move back up slopes at lower temperatures. This requires a carefully considered pattern of anchor trenches. LLDPE is not used for EGCs due to its lower weathering resistance.
A new candidate for EGCs, particularly in cold regions, is prefabricated bituminous geomembrane (BGM). BGMs are custom engineered geomembranes consisting of a fibreglass fleece and a non-woven geotextile saturated with blown oxidized or elastomerically modified bitumen. They can be deployed and welded at well below 0°C, they have very low expansion coefficients, are heavy to resist wind and LFG uplift, and welds can be 100% ultrasonically tested. They can tolerate rough subgrades and cover soils.Floating covers
Reinforced PP, HDPE, and chlorosulfonated polyethylene (CSPE Hypalon) have been used for floating covers on leachate ponds to reduce odours and to prevent incident precipitation from becoming leachate that needs treatment. In some other applications, particularly in potable water reservoirs, PP floating covers and liners have suffered stress-cracking failures along the tops of wrinkles and at other stressed locations. This appears to be synergism between stress and liquid environment, stress and thermal oxidation, and stress and UV radiation accelerating the loss of additives. However, the same material that has cracked in floating cover applications has performed very well in an exposed cap. There is more to understand about PP, but this is no different from the SC problems in the early years of HDPE.
Reinforced PP floating cover on potable water reservoir Click here to enlarge image
What might we see in the future? Single-unit composite liners, five- and seven-layer geomembranes with self healing capabilities, infrared thermography for the non-destructive measurement of seam bond strength over 100% of the width and length of a weld and feedback for welding machine control, increased use of electrical (and other) surveys for leak location, spray applied seamless liners on GCLs and geocomposites, and exposed geomembranes with nanophotovoltaic surface layers for the generation of electricity.
Lining technology is only just beginning to develop.
Dr Ian D. Peggs is President of I-CORP INTERNATIONAL, Inc., a geosynthetic materials performance consulting company with clients worldwide, located in Ocean Ridge, Florida, USA