To close a waste containment facility, regulations require installation of a final cover that usually consists from bottom to top of: a graded landfill surface, gas venting layer, geomembrane, drainage layer consisting of soil or a geosynthetic composite, 30 inches of compacted soil, and 6 inches of topsoil for growing vegetation. The main design issues for the geomembrane include differential settlement, wrinkle management, gas migration, low temperature behavior for landfills subjected to sub-freezing temperatures, CQA and CQC procedures, and susceptibility to rats and rodents. This technical bulletin discusses the advantages and disadvantages of PVC geomembranes for each of these design issues. The adjacent photograph shows a PVC geomembrane being used for a large landfill cover on the East Coast.
Differential settlement is a serious potential problem for geomembranes and in some instances is the cause for failure. It is generally considered that low modulus or flexible geomembranes (geomembranes that elongate significantly under a given tension), such as PVC, are more likely to withstand differential settlement without damage than high modulus or less flexible geomembranes (geomembranes that do not elongate significantly under a given tension), such as HDPE and reinforced geomembranes. The co-energy approach developed by Giroud and Soderman (Geosynthetics International, Vol. 2, No. 6, 1995, pp. 953 - 969) can be used to show that the factor of safety with respect to differential settlement is greatest for PVC. In fact, the factors of safety for PVC, CSPE, and HDPE geomembranes are 22, 2.6, and 1.2, respectively, and the ability of these three geomembranes to withstand differential settlement are in order PVC, CSPE, and HDPE. The main reason that the factor of safety is 18 times higher for PVC than HDPE geomembranes is that HDPE exhibits a yield point in its stress/strain behavior that occurs at approximately 12% strain. PVC does not exhibit a yield point and can undergo strains in excess of 300% before breaking at room temperature under uniaxial conditions.
An important geomembrane issue in recent months is the development of wrinkles after installation. Geomembranes usually exhibit wrinkles in the field as soon as they are heated by the sun, (Giroud, Geotechnical Fabrics Report, Vol. 13, No. 3, April, 1995, pp. 14-17), with an increase in interface strength between the geomembrane and the subgrade. As a result, Giroud concludes that flexible geomembranes, such as PVC, exhibit smaller wrinkles than smooth HDPE geomembranes.
The third design issue is the ability of water or gas, such as methane, to migrate through the geomembrane. Usually the water-vapor transmission test is used (ASTM E96) to evaluate the diffusion of water vapor through a geomembrane. The material specification prepared by the PVC Geomembrane
Institute requires an equivalent hydraulic conductivity of less than 5 x 10 -9 cm/sec for a 30 mil geomembrane. A smooth 1.5 mm thick HDPE geomembrane usually exhibits an equivalent hydraulic conductivity of 1 x 10 -11 cm/sec. ASTM Test Method D 1434-V was used to measure similar methane transmission rates through smooth 40 mil PVC and HDPE geomembranes. In summary, PVC and polyethylene geomembranes exhibit a low water-vapor transmission and methane migration rate and thus migration through an intact liner will be limited. In fact, recent case histories suggest geomembranes are effective at containing methane gas because of the presence of whale backs or bubbles underneath the geomembrane and slope instability due to gas buildup below the geomembrane.
The fourth design issue is geomembrane brittleness at low temperatures. This is important because the thickness of the vegetative cover soil and drainage material may not be sufficient to prevent the geomembrane from being subjected to low temperatures in the majority of the states in the U.S. One effect of low temperature on polyethylene geomembranes is to reduce the yield point and thus the allowable strain is 3% instead of about 12%. Since PVC does not exhibit a yield point temperature even at low temperatures, the allowable strain is still greater than 100%. Therefore, PVC geomembranes can withstand the tensile stresses that may be induced by differential settlement at low temperatures.
The fifth design issue is CQA and CQC procedures. It is universally recognized that field prepared seams are potentially the most problematic. PVC geomembrane rolls are factory seamed to produce large panels that greatly reduce the number of field seams. In a given liner area, the length of field seam required in a PVC liner may be 80% less than that required for a similar polyethylene geomembrane. This reduces the potential problems associated with field seaming and the cost and duration of field installation. PVC and polyethylene geomembranes can be seamed using either single or dual-track wedge welding equipment.
This allows long continuous seams to be constructed in a wider range of environmental conditions. In addition, the use of a dual-track wedge welder allows air-channel testing to be conducted to nondestructively test seams of both PVC and polyethylene geomembranes. In summary, the same techniques for conducting CQA and CQC programs can be used for both PVC and polyethylene geomembranes because both liners can be seamed using a dual-track wedge welder.
The last design issue involves the likelihood of rats and rodents eating the geomembrane in the cover system. It has been implied that flexible geomembranes, such as PVC and very flexible polyethylene, are easier to eat or gnaw because they are easier to bend than an HDPE geomembrane. This may be true but it is probably only possible to eat the geomembrane at an edge, fold, or seam on which the animal can grip. Since PVC geomembranes may have 80% less seams than polyethylene geomembranes, the likelihood of having an edge, fold, or seam is significantly less. However, it is important to note the following information about polyethylene and PVC geomembranes: (1) they possess no compounds that have any food or nutritional value to rats and rodents, (2) the materials are not addictive, and (3) the reason for an animal eating through a geomembrane is probably to obtain access to the other side for food, warmth, or some other reason. In summary, neither geomembrane is desired by rats and rodents, and thus there is no danger if the geomembrane is properly fabricated and installed.
In summary, PVC geomembranes have performed well in the past and will perform well in the future in waste containment facilities. Based on the review of the six major design issues addressed in this technical bulletin, PVC geomembranes appear uniquely suited for landfill cover systems.
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