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Geomembranes in Surface Barriers Ronald K. Frobel, R.K. Frobe] & Associates, Lakewood, Colorado Geomembranes are very {ow-permeability synthetic liners used to control fluid or gas migration within soil, rock, earth, or any other geotechnical matenal, as an integral part of a man- made product, structure, or system. As a synthetic component used within the ground, they are technically a geosynthetic, the prefix "geo" indicating usage on or in the earth. The other primary geosynthetics are geotextiles, geonets, geognds, geocomposites, and geocomposite clay liners. Geomembranes have been used in a variety of containment applications since the early 1940's. Within the past 25 years, the growth of flexible membrane liners (FME's) In the form of prefabricated plastic or elastomeric sheet materials has increased rapidly in the construction industry and is now an accepted civil engineering material along with other geosynthetics. In fact, there are over 30 individual applications that have been developed for geomembranes. The original use for geomembranes was for the distribution, storage, and containment of potable/agncultural water supplies. It still remains as an Important element of this market, except now it has been broadened to contain a wide variety of liquids. These liquids span the entire spectrum of liquid wastes (acids to bases), potable water, water for recreation, power, flood control, etc. Closely related to Geomembranes containing liquids in the static state are geomembranes used for canal liners and other hydraulic conveyance systems. The rise of Geomembranes for cover systems is a growing application area. The major concentration is to cover solid waste sites, liquid waste ponds, and potable water reservoirs, but numerous other applications such as odor containment, potential sabotage reduction, methane gas collection, and temporary waste covers are evident. The largest growing segment of the geomembrane industry has to do with cover systems tor solid waste containment. Currently required for hazardous waste containment, there is a growing tendency, fueled by environmental regulations, to use Geomembranes to contain all types of solid waste. These include municipal, industrial, mining, and low-level radioactive wastes. Geomembranes have become the design choice as part of a cover system due to a variety of factors. These factors include such design considerations as imperviousness, chemical resistance, Inertness to surrounding soils, ease and variety of seaming, mechanical strength and elongation, ease of application, and economics. Environmental concerns and subsequent regulations have also compelled engineers to include synthetic geomembrane systems in their waste containment designs, mining applications, retention ponds, and municipal landfi~Is. Although the current U.S. geomembrane market is volatile, all infonnation collected thus far clearly points to a growth rate through 1995 of between 20 and 25 percent, with 20 percent being a conservative estimate. At the Tower-bound 20 percent growth rate, the total estimated installed quantity should approach 1 14 million m2/year by 1996. This growth can be attributed to six factors (Koerner, 19941: 1. exishug companies stepping up their marketing services, 2. new companies with highly visible marketing, 3. new high-tech materials and seaming methods, D-71

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D-72 BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT 4. the emergence of new geocomposites requiring geomembranes, 5. new government regulatory standards in waste disposal, and 6. education through conferences, publications, advertising, etc. Although there are many polymeric types of geomembranes, fully 90 percent of the market has been comprised of four geomembrane systems of varying sheet configurations, i.e., smooth' textured, reinforced, composite, etc. Table 1 lists these geomembranes and their approximate formulations. TABLE 1 Types of Commonly Used Geomembranes and Their Approximate Formulations by Percent - Geomembrane Res~n Plasticizer Filler Carbon Black Additives Type o/O o/O o/O or Pigment % % APES 95-98 0 0 2-3 0.25-1.0 VLDPE 94-96 0 0 2-3 {-4 PVCC 45-60 25-35 0-10 2-5 2-5 CSPEd 40-60 0 30-45 5-40 5- 1 - a HDPE (Medium- to High-Density Polyethylene in the density range of 0.92 to 0.96 gm/cm:) b VLDPE (Very Low-Density Polyethylene). c PVC (Polyvinyl Chlonde - Soft) CSPE (Chlorosofonated Polyethylene or Hypalon) It is evident that the semi-crystalltne and very low-crystall~nity thermoplastics such as HDPE, VLDPE, CSPE, and the newer Flexible Polypropylenes, are taking an increasing part of the waste closure market and will continue to do so. From a low percentage of use in 1981 to over 25 percent in 1988 and over 50 percent in 1995, semi-crystalline/low-crystallinity thermoplastic materials are outperforming competitive products for the following reasons: relatively Tow cost wide extruded sheets wide variety of thicknesses wide variety of surface textures excellent chemical resistance reliable field welding (thermoplastic) good mechanical properties less field seams durability GEOMEMBRANE MANUFACTURE Most geomembranes are made in a manufacturing plant using one of the following highly technical and controlled manufacturing processes: extrusion, spread coating, and calendaring.

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APPENDIX DIAPERS PRESENTED D 73 The extrusion process is a method whereby a polymer of the polyolefin family (such as polyethylene or polypropylene) is extruded by blown vertical or slot die horizontal extrusion methods into a nonre~nforced sheet. Immediately after extrusion, when the sheet is still warm, it can be bonded to a fabric, through light calendaring forming a geocomposite. During extrusion, the sheet matenal can also be textured for high surface friction using a variety of dies or processes. The spread coating process usually consists of coating a fabric (woven, nonwoven, knit) by spreading a polymer or asphalt compound on it. These geomembranes are therefore reinforced. Non-reinforced geomembranes can be made by spreading or spraying a polymer on a sheet of paper that is removed and discarded at the end of the manufacturing process. Calendaring is the most frequently used manufacturing process. Calendared non-reinforced geomembranes usually consist of a single sheet of compound made by passing a heated polymeric component through a series of heated rollers (calendar). Calendared reinforced geomembranes are produced by simultaneously running sheets of compound and seems through heated rollers. A three-ply calendared reinforced geomembrane is compound/ scrim/compound. made of the following layers: Geomembranes manufactured by the above processes are produced In rolls approximately 1.5-10 m In width. Geomembranes that are produced in wide rolls, typically 5-10 m, are commonly transported to the field site where they are seamed together. Geomembranes that are produced in narrow, lighter rolls (PVC and CSPE) are first transported to a fabrication factory where they are seemed into large panels. Panels can be fabncated to any designed shape and are limited only by handling weight and dimension, commonly less than 1860 m2. Panels are packaged and transported to the construction site where they are seamed together. Small sites can often be lined with a single panel, thereby el~m~nahug the need for field seaming. Figure 1 schematically illustrates the geomembrane Industry as it presently exists Mom polymer manufacture through Installation. | Polymer+ Additives ~ / 1. Extrusion hi. Nonreinforced \ Fabric 1 ~ _ _5~ 2. Calendering t. - 3. Spread Coating . Reinforced I_ Fabrication ._~. | Panel l | Installation . FIGURE I Schematic of the Geomembrane Industry (Haxo, 19861.

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D-74 BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT GEOMEMBRANE SEAMING The most critical element of an installed geomembrane system is the seam. The mechanism of seaming polymeric geomembrane sheets together is to melt or otherwise bond the overlapped edges of a geomembrane roll or panel in a controlled manner. The bonding of two polymeric geomembranes results from an input of energy that originated from either thermal or chemical processes. There are generally four categories of seaming methods: extrusion welding, thermal fission (melt bonding), chemical fission, and adhesive bonding. Figure 2 illustrates the four categories. Thermal fusion or melt bonding is the most common seaming method for the thermoplastic geomembranes. In particular, the dual hot-wedge method and hot-air method are predominant. The dual hot-wedge method provides an air channel between weld tracks that can be nondestructively tested by air pressunzation. Extrusion fillet welds are used pnmarily for detail and patch welding. Fillet-Type , ~ . EXTRUSION SEAMS Flat-Type 1 Dual Hot Wedge FUSION SEAMS Single Hot Air (Single Track Is also possible) (Dual Track Is also possible) 1 , ~ _ 1 ~ Chemical CHEMICAL SEAMS Bodied Chemical ,m,,,,1 ";.~.,~ Chemical Adhesive ADHESIVE SEAMS FIGURE 2 Various Geomembrane Seaming Methods (USEPA, ~ 9931. Contact Adhesive

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APPENDIX~PAPERS PRESENTED SUBTITLE C COVER DESIGN INCORPORATING GEOMEMBRANES D-75 The objectives of a surface battier system is to limit the infiltration of water into the contained waste to minimize leachate generation that potentially could escape to groundwater in close proximity to the bottom of the waste cell. Minimizing leachate In a closed cell requires that liquids be kept out and that leachate that does exist be collected and removed. The surface balTier system must be designed at the time the site is selected and the design of the containment system is chosen. The site location, availability of low permeability clay soil, and good topsoil, the use of geosynthetics, and the use of the site after the postclosure care period are typical considerations in design. The ultimate goals of the barrier system are to minimize maintenance and to protect human health and the environment. The Resource Conservation and Recovery Act (RCRA) Subtitle C regulations form the basic requirements for cover systems being designed and constructed tony. In general, after the waste cell is closed, the U.S. Environmental Protection Agency (EPA) recommends as a minimum that the final cover system consist of the following major elements from bottom to top ~JSEPA, 19911: 1. Low Hydraulic Conductivity Geomembrane/Soil Layer. A 60-cm layer of compacted natural or amended soil with a hydraulic conductivity of 1 x 10-7 cm/see in intimate contact with a minimum 0.5-mm thick geomembrane. 2. Drainage Layer. A minimum 30-cm soil layer having a minimum hydraulic conductivity of 1 x 10-2 cm/see, or a layer of geosynthetic material having the same characteristics. 3. Top, Vegetation/Soi! Layer. A top layer with vegetation (or an armored top surface) and a minimum of 60 cm of soil graded at a slope between 3 and 5 percent. The basic generic design incorporates natural soil, geomembranes, drainage, and vegetative layers. It is obvious that the geomembrane/natural soil layer is an integral part of the barrier concept. Figure 3 illustrates the EPA recommended minimum cover system with optional geosynthetics layers as well as banter intrusion layers. _ = cobbles/soil -l top layer biotic bamer (cobbles) drainage layer low hydraulic conductivity geomembrane/soil layer gas vent layer waste o ~ 0 Or ~ ~o D o 60 cm 30 cm geosynthetic filter geosynthetic filter 30 cm ~ geomembrane 60 cm geosynthetic filter FIGURE 3 EPA Recommended Cover System With Optional Layers (USEPA, 19911.

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D-76 BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT Because the design of the final cover must consider the site, climate, waste characteristics, arid other site-specific conditions, the minimum recommendations may be altered providing the alternative design is equivalent to the U.S. EPA-recommended design or will meet the intent of the regulations (USEPA, 1991). In extremely and regions, for example, a gravel top surface will compensate for reduced vegetation. Where burrowing animals might damage the geomembrane/low-hydraulic conductivity soil layer, a biotic barrier layer of large-sized aggregate will be needed above it. Where the type of waste may create gases, soil or geosynthetic gas' vent layers and escape vents would need to be included. The recommended Subtitle C cover design has a relatively short design life of usually no more than 30 years, during which time the barrier can be monitored. Many wastes, however, especially those considered by the Department of Energy (DOE) and the U.S. Nuclear Regulatory Commission (USNRC) must be isolated for much longer periods of from 100 to over 1~000 years. In developing a cap design for the long term' designers of DOE type enclosures generally shy away Mom geosynthetic materials that are assumed to last only decades and intuitively rely on natural materials. GEOMEMBRANE DURABILITY in addition to the known physical/mechanica1 design values used in state of the art geomembrane design, there is always the consideration of product durability and aging over the design life of the containment system. As described earlier, the most frequently utilized geomembrane group in cover applications are the thermoplastics. However, as shown in Table 1, the final compound will include various additives, stabilizers or plasticizer in addition to the predominant resin type. Ideally, each component in the compound formulation must be analyzed when assessing the durability or long-term aging of a particular geomembrane. Those geomembrane polymer systems with very few additives, such as HDPE, are much easier to analyze for durability and aging mechanisms. According to Koerner (1994), there are six separate primary mechanisms that can be attributed to geomembrane degradation over time by causing polymeric chain scission or bond breaking within the polymer structure: ultraviolet degradation radioactive degradation biological degradation chemical degradation thermal degradation oxidation degradation Any combination of the above mechanisms can cause synergistic effects due to interaction. addition, elevated temperatures and stress will accelerate any of the degradation processes. Geomembranes that are exposed to the elements or incorporated in shallow barrier systems (<1 m) with no biotic barrier protection are susceptible to most of the above degradation mechanisms. However, in a buried situation, such as a cap or surface barrier that may be 34 m in layered thickness above the geomembrane, most of the primary degradation mechanisms are nonexistent or of little concern. In addition, elevated temperatures and chemical degradation are not an issue. The only liquid that potentially could come into contact with the geomembrane is water due to potential

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APPENDIX~PAPERS PRESENTED D-77 infiltration or soil moisture. The temperature at 3-4 m depth will remain constant over hme and probably will not be above 14-18 C. Of all of the thermoplastic materials, PVC-soft is the most susceptible to the effects of aging' primarily due to the high content of volatile plasticizer, fillers, and other additives. However, field exhumed samples of 30-year-old PVC geomembrane under shallow burial and saturated soil conditions show an intact geomembrane that is still serving its intended function even with reduced physical/mechanical properties. This same geomembrane will no doubt be in service for an additional 30-100 years. The semi crystalline polyolefins, such as HDPE, which are composed of over 95 percent resin are much less susceptible to many of the degradation processes, especially in surface barriers that are 3-4 m in layered thickness. All geomembranes are subject to the oxidation mechanism. Gases and liquids (water or soil moisture) that come into contact with the geomembrane will cause oxidation and, over time, oxygen will enter the polymer structure. The rate of oxidation reaction is specific to the site and polymer. The reaction is minimized by providing antioxidants in the polymer formulations that neutralize free radicals. Removal of oxygen from the geomembrane's surface (i.e., submerged or covered with waste) will eliminate the potential for oxidation degradation. However, in a deep cover design, the geomembrane will be encapsulated by nonsaturated soil and thus will be susceptible to oxidation. An accelerated test method commonly used in determining reaction time and antioxidant levels is the Oxidation Induction Time (OIT). Arrhenius Modeling at elevated temperatures currently is being used to determine the projected life of semicrystalline geomembranes. The Geosynthetic Research Institute (GRI) is currently exposing HOPE samples to superimposed compressive stress, chemical and oxidative exposure, elevated temperature and long testing time. Samples are removed periodically and evaluated for numerous property changes. Deciding on a maximum property change, such as a 50- percent reduction at each temperature allows for the plotting of the Arrhenius curve, which plots inverse temperature against reaction rate. Thus, Arrhenius Modeling can be used for lifetime prediction using elevated temperature aging (Koerner, 1994~. The pipe and cable industry have used Arrhenius Modeling methodology extensively In the determination of in-service lifetimes for buried plastic pipe and cable, and much of this documented information is being used to model geomembrane aging characteristics. SUMMARY Geomembranes are highly technical engineered systems that form an integral part of today's surface barriers. Obviously, the longevity or durability of a geomembrane is the most important design issue when considering barrier systems that must last 50, 100, 200, or even 1,000 years. However, the primary degradation mechanisms are generally eliminated by burial in long- term cover systems that are in the range of 3-4 m in thickness. In addition, liquid contact, if any, will be limited to potential water infiltration or soil moisture, and thus chemical degradation will not be an issue. Proper stress-limiting desigr and strict manufacture/installation quality assurance coupled with the use of predictive models, such as the Arrhenius Modeling technique, can be used In helping to assure a long-term barrier utilizing geomembranes.

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D-78 BARRIER TECHNOLOGIES FOR ENVIRONMENTAL MANAGEMENT REFERENCES Haxo, H.E., Ir. 1986. Quality Assurance of Geomembranes Used as Linings for Hazardous Waste Containment, Journal of Geotextiles and Geomembranes. 3~4~:225-248. Koerner, R.M. 1994. Designing With Geosynthetics. Englewood Cliffs, N.J.: Prentice Hal! Publishers. U.S. Environmental Protection Agency (USEPA). 1991. Design and Construction of RCRA/CERCLA Final Covers, EPA Technical Guidance Document, EPA/625/4-91/025. U.S. Environmental Protection Agency (USEPA). 1993. Quality Assurance and Quality Control for Waste Containment Facilities, EPA Technical Guidance Document, EPA1600/R-93/182.