17 Enormous amounts of solid waste are generated every year in the United States and other industrialized countries. These waste materials, in general, can be classified into four major categories: (1) municipal waste, (2) industrial waste, (3) hazardous waste, and (4) low- level radioactive waste. Table 17.1 lists the waste material generated in 1984 in the United States in these four categories (Koerner, 1994). The waste materials generally are placed in landfills. The landfill materials interact with moisture received from rainfall and snow to form a liquid called leachate. The chem- ical composition of leachates varies widely, depending on the waste material involved. Leachates are a main source of groundwater pollution; therefore, they must be contained properly in all landfills, surface impoundments, and waste piles within some type of liner system. In the following sections of this chapter, various types of liner systems and the materials used in them are discussed. 17.1 Landfill Liners— Overview Until about 1982, the predominant liner material used in landfills was clay. Proper clay liners have a hydraulic conductivity of about 10 7 cm/sec or less. In 1984, the U.S. Environmental Protection Agency’s minimum technological requirements for hazardous-waste landfill design and construction were introduced by the U.S. Congress in Hazardous and Solid Waste amend- ments. In these amendments, Congress stipulated that all new landfills should have double liners and systems for leachate collection and removal. 611 Landfill Liners and Geosynthetics Table 17.1 Waste Material Generation in the United States Approximate quantity in 1984 Waste type (millions of metric tons) Municipal 300 Industrial (building debris, degradable waste, nondegradable waste, and near hazardous) 600 Hazardous 150 Low-level radioactive 15 612Chapter 17: Landfill Liners and Geosynthetics To understand the construction and functioning of the double-liner system, we must review the general properties of the component materials involved in the system—that is, clay soil and geosynthetics (such as geotextiles, geomembranes, and geonets). Section 17.2 gives the details for the compaction of clay soils in the field for liner construction. A brief review of the essential properties of geosynthetics is given in Sections 17.3 through 17.6. 17.2 Compaction of Clay Soil for Clay Liner Construction It was shown in Chapter 6 (Section 6.5) that, when a clay is compacted at a lower mois- ture content, it possesses a flocculent structure. Approximately at the optimum moisture content of compaction, the clay particles have a lower degree of flocculation. A further increase in the moisture content at compaction provides a greater degree of particle orien- tation; however, the dry unit weight decreases, because the added water dilutes the con- centration of soil solids per unit volume. Figure 17.1 shows the results of laboratory compaction tests on a clay soil as well as the variation of hydraulic conductivity on the compacted clay specimens. From the labo- ratory test results shown, the following observations can be made: 1. For a given compaction effort, the hydraulic conductivity, k, decreases with the increase in molding moisture content, reaching a minimum value at about the optimum moisture content (that is, approximately where the soil has a higher unit weight with the clay particles having a lower degree of flocculation). Beyond the optimum moisture content, the hydraulic conductivity increases slightly. 2. For similar compaction effort and dry unit weight, a soil will have a lower hydraulic conductivity when it is compacted on the wet side of the optimum moisture content. Benson and Daniel (1990) conducted laboratory compaction tests by varying the size of clods of moist clayey soil. These tests show that, for similar compaction effort and molding moisture content, the magnitude of k decreases with the decrease in clod size. In some compaction work in clayey soils, the compaction must be done in a manner so that a certain specified upper level of hydraulic conductivity of the soil is achieved. Examples of such works are compaction of the core of an earth dam and installation of clay liners in solid-waste disposal sites. To prevent groundwater pollution from leachates generated from solid-waste dis- posal sites, the U.S. Environmental Protection Agency (EPA) requires that clay liners have a hydraulic conductivity of 10 7 cm/sec or less. To achieve this value, the contractor must ensure that the soil meets the following criteria (Environmental Protection Agency, 1989): 1. The soil should have at least 20% fines (fine silt and clay-sized particles). 2. The plasticity index (PI) should be greater than 10. Soils that have a PI greater than about 30 are difficult to work with in the field. 3. The soil should not include more than 10% gravel-sized particles. 4. The soil should not contain any particles or chunks of rock that are larger than 25 to 50 mm (1 to 2 in.). In many instances, the soil found at the construction site may be somewhat non- plastic. Such soil may be blended with imported clay minerals (like sodium bentonite) to achieve the desired range of hydraulic conductivity. In addition, during field compaction, a heavy sheepsfoot roller can introduce larger shear strains during compaction that create a more dispersed structure in the soil. This type of compacted soil will have an even lower hydraulic conductivity. Small lifts should be used during compaction so that the feet of the compactor can penetrate the full depth of the lift. There are a few published studies on the variation of hydraulic conductivity of mix- tures of nonplastic soils and Bentonite. Sivapullaiah, et al. (2000) evaluated the hydraulic conductivity of Bentonite-sand and Bentonite-silt mixtures in the laboratory. Based on this study the following correlation was developed (17.1) where k  hydraulic conductivity of Bentonite-nonplastic soil mixture (m/sec) e  void ratio of compacted mixture LL  liquid limit of mixture (%) Equation (17.1) is valid for mixtures having a liquid limit greater than 50%. log k  e 0.05351LL2 5.2860.00631LL2  0.2516 17.2 Compaction of Clay Soil for Clay Liner Construction 613 8 10 12 14 16 18 20 8 10 12 14 16 18 20 1500 1600 1700 1800 1900 2000 10 9 10 8 10 7 10 6 Hydraulic conductivity (cm/s) Dry density (kg/m 3 ) (a) (b) Moisture content (%) Moisture content (%) Figure 17.1 Tests on a clay soil: (a) modified Proctor compaction curve; (b) variation of k with molding moisture content 614Chapter 17: Landfill Liners and Geosynthetics The size of the clay clods has a strong influence on the hydraulic conductivity of a compacted clay. Hence, during compaction, the clods must be broken down mechanically to as small as possible. A very heavy roller used for compaction helps to break them down. Bonding between successive lifts is also an important factor; otherwise, permeant can move through a vertical crack in the compacted clay and then travel along the inter- face between two lifts until it finds another crack, as is shown schematically in Figure 17.2. Poor bonding can increase substantially the overall hydraulic conductivity of a com- pacted clay. An example of poor bonding was seen in a trial pad construction in Houston in 1986. The trial pad was 0.91 m (3 ft) thick and built in six, 15.2 mm (6 in.) lifts. The results of the hydraulic conductivity tests for the compact soil from the trial pad are given in Table 17.2. Note that, although the laboratory-determined values of k for various lifts are on the order of 10 7 to 10 9 cm/sec, the actual overall value of k increased to the order of 10 4 . For this reason, scarification and control of the moisture content after compac- tion of each lift are extremely important in achieving the desired hydraulic conductivity. In the construction of clay liners for solid-waste disposal sites where it is required that k
10 7 cm/sec, it is important to establish the moisture content–unit weight criteria in the laboratory for the soil to be used in field construction. This helps in the development of proper specifications. Lift 4 Lift 3 Lift 2 Lift 1 Figure 17.2 Pattern of flow through a compacted clay with improper bonding between lifts (After U.S. Environmental Protection Agency, 1989) Table 17.2 Hydraulic Conductivity from Houston Liner Tests * Location Sample Laboratoryk (cm /sec) Lower lift 76 mm (
3 in.) tube 4  10 9 Upper lift 76 mm (
3 in.) tube 1  10 9 Lift interface 76 mm (
3 in.) tube 1  10 7 Lower lift Block 8  10 5 Upper lift Block 1  10 8 Actual overall k  1  10 4 cm/sec * After U.S. Environmental Protection Agency, 1989 17.2 Compaction of Clay Soil for Clay Liner Construction 615 Dry unit weight (a) Moisture content (c) Moisture content Hydraulic conductivity, k (b) Moisture content Dry unit weight Maximum allowable k, kall Acceptable zone Modified Proctor Standard Proctor Figure 17.3 (a) Proctor curves; (b) variation of hydraulic conductivity of compacted specimens; (c) deter- mination of acceptable zone Daniel and Benson (1990) developed a procedure to establish the moisture con- tent–unit weight criteria for clayey soils to meet the hydraulic conductivity requirement. The following is a step-by-step procedure to develop the criteria. Step 1: Conduct Proctor tests to establish the dry unit weight versus molding mois- ture content relationships (Figure 17.3a). 616Chapter 17: Landfill Liners and Geosynthetics Step 2: Conduct permeability tests on the compacted soil specimens (from Step 1), and plot the results, as shown in Figure 17.3b. In this figure, also plot the maximum allowable value of k (that is, kall ). Step 3: Replot the dry unit weight–moisture content points (Figure 17.3c) with different symbols to represent the compacted specimens with k kall and k
k all . Step 4: Plot the acceptable zone for which k is less than or equal to k all (Figure 17.3c). 17.3 Geosynthetics In general, geosynthetics are fabric-like material made from polymers such as polyester, polyethylene, polypropylene, polyvinyl chloride (PVC), nylon, chlorinated polyethylene, and others. The term geosynthetics includes the following: • Geotextiles • Geomembranes • Geogrids • Geonets • Geocomposites Each type of geosynthetic performs one or more of the following five major functions: 1. Separation 2. Reinforcement 3. Filtration 4. Drainage 5. Moisture barrier Geosynthetics have been used in civil engineering construction since the late 1970s, and their use currently is growing rapidly. In this chapter, it is not possible to provide detailed descriptions of manufacturing procedures, properties, and uses of all types of geosynthetics. However, an overview of geotextiles, geomembranes, and geonets is given. For further information, refer to a geosynthetics text, such as that by Koerner (1994). 17.4 Geotextiles Geotextiles are textiles in the traditional sense; however, the fabrics usually are made from petroleum products such as polyester, polyethylene, and polypropylene. They also may be made from fiberglass. Geotextiles are not prepared from natural fabrics, which decay too quickly. They may be woven, knitted, or nonwoven. Woven geotextiles are made of two sets of parallel filaments or strands of yarn sys- tematically interlaced to form a planar structure. Knitted geotextiles are formed by inter- locking a series of loops of one or more filaments or strands of yarn to form a planar structure. Nonwoven geotextiles are formed from filaments or short fibers arranged in an oriented or a random pattern in a planar structure. These filaments or short fibers first are arranged into a loose web. They then are bonded by using one or a combination of the fol- lowing processes: • Chemical bonding—by glue, rubber, latex, cellulose derivative, and so forth • Thermal bonding—by heat for partial melting of filaments • Mechanical bonding—by needle punching The needle-punched nonwoven geotextiles are thick and have high in-plane hydraulic con- ductivity. Geotextiles have four major uses: 1. Drainage: The fabrics can channel water rapidly from soil to various outlets. 2. Filtration: When placed between two soil layers, one coarse grained and the other fine grained, the fabric allows free seepage of water from one layer to the other. At the same time, it protects the fine-grained soil from being washed into the coarse- grained soil. 3. Separation: Geotextiles help keep various soil layers separate after construction. For example, in the construction of highways, a clayey subgrade can be kept separate from a granular base course. 4. Reinforcement: The tensile strength of geotextiles increases the load-bearing capac- ity of the soil. Geotextiles currently available commercially have thicknesses that vary from about 0.25 to 7.6 mm (0.01 to 0.3 in.). The mass per unit area of these geotextiles ranges from about 150 to 700 g/cm2. One of the major functions of geotextiles is filtration. For this purpose, water must be able to flow freely through the fabric of the geotextile (Figure 17.4). Hence, the cross-plane hydraulic conductivity is an important parameter for design purposes. It should be realized that geotextile fabrics are compressible, however, and their thickness may change depending on the effective normal stress to which they are being subjected. The change in thickness under normal stress also changes the cross-plane hydraulic 17.4 Geotextiles 617 Direction of flow Geotextile Figure 17.4 Cross-plane flow through geotextile 618Chapter 17: Landfill Liners and Geosynthetics Direction of flow Geotextile Figure 17.5 In-plane flow in geotextile conductivity of a geotextile. Thus, the cross-plane capability is generally expressed in terms of a quantity called permittivity, or (17.2) In a similar manner, to perform the function of drainage satisfactorily, geotextiles must possess excellent in-plane permeability. For reasons stated previously, the in-plane hydraulic conductivity also depends on the compressibility, and, hence, the thickness of the geotextile. The in-plane drainage capability can thus be expressed in terms of a quan- tity called transmissivity, or (17.3) The units of k n and kp are cm/sec or ft/min; however, the unit of permittivity, P, is sec 1 or min 1. In a similar manner, the unit of transmissivity, T, is or . Depending on the type of geotextile, kn and P, and kp and T can vary widely. The following are some typical values for . • • • • Transmissivity T: Woven: 1.5  10 8 to 2  10 8 m3/sec # m Nonwoven: 2  10 6 to 2  10 9 m3/sec # m Woven: 2  10 3 to 4  10 3 cm/sec Nonwoven: 1  10 3 to 5  10 2 cm/sec Hydraulic conductivity, k p : Permittivity, P: 2  10 2 to 2.0 sec 1 Hydraulic conductivity, k n: 1  10 3 to 2.5  10 1 cm/sec k n, P, k p, and T ft 3/min # ftm3/sec # m k p  hydraulic conductivity for in-plane flow 1Figure 17.52 where T  transmissivity T  k pt t  thickness of the geotextile k n  hydraulic conductivity for cross-plane flow where P  permittivity P  kn t When a geotextile is being considered for use in the design and construction of landfill liners, certain properties must be measured by tests on the geotextile to determine its applicability. A partial list of these tests follows. 1. Mass per unit area 2. Percentage of open area 3. Equivalent opening size 4. Thickness 5. Ultraviolet resistivity 6. Permittivity 7. Transmissivity 8. Puncture resistance 9. Resistance to abrasion 10. Compressibility 11. Tensile strength and elongation properties 12. Chemical resistance 17.5 Geomembranes Geomembranes are impermeable liquid or vapor barriers made primarily from continuous polymeric sheets that are flexible. The type of polymeric material used for geomembranes may be thermoplastic or thermoset. The thermoplastic polymers include PVC, polyethyl- ene, chlorinated polyethylene, and polyamide. The thermoset polymers include ethylene vinyl acetate, polychloroprene, and isoprene-isobutylene. Although geomembranes are thought to be impermeable, they are not. Water vapor transmission tests show that the hydraulic conductivity of geomembranes is in the range of 10 10 to 10 13 cm/sec; hence, they are only “essentially impermeable.” Many scrim-reinforced geomembranes manufactured in single piles have thick- nesses that range from 0.25 to about 0.4 mm (0.01 to 0.016 in.). These single piles of geomembranes can be laminated together to make thicker geomembranes. Some geomembranes made from PVC and polyethylene may be as thick as 4.5 to 5 mm (0.18 to 0.2 in.). The following is a partial list of tests that should be conducted on geomembranes when they are to be used as landfill liners. 1. Density 2. Mass per unit area 3. Water vapor transmission capacity 4. Tensile behavior 5. Tear resistance 6. Resistance to impact 7. Puncture resistance 8. Stress cracking 9. Chemical resistance 10. Ultraviolet light resistance 11. Thermal properties 12. Behavior of seams 17.5 Geomembranes 619 620Chapter 17: Landfill Liners and Geosynthetics (a) (b) (c) Factory vulcanized (d) (e) (f) Adhesive Gum tape The most important aspect of construction with geomembranes is the preparation of seams. Otherwise, the basic reason for using geomembranes as a liquid or vapor barrier will be defeated. Geomembrane sheets generally are seamed together in the factory to pre- pare larger sheets. These larger sheets are field seamed into their final position. There are several types of seams, some of which are described briefly. Lap Seam with Adhesive • A solvent adhesive is used for this type of seam (Figure 17.6a). After application of the solvent, the two sheets of geomembrane are overlapped, then roller pressure is applied. Lap Seam with Gum Tape • This type of seam (Figure 17.6b) is used mostly in dense thermoset material, such as isoprene-isobutylene. Tongue-and-Groove Splice • A schematic diagram of the tongue-and-groove splice is shown in Figure 17.6c. The tapes used for the splice are double sided. Figure 17.6 Configurations of field geomembrane seams: (a) lap seam; (b) lap seam with gum tape; (c) tongue- and-groove splice; (d) extrusion weld lap seam; (e) fillet weld lap seam; (f) double hot air or wedge seam (After U.S. Environmental Protection Agency, 1989) Extrusion Weld Lap Seam • Extrusion or fusion welding is done on geomembranes made from polyethylene. A ribbon of molten polymer is extruded between the two surfaces to be joined (Figure 17.6d). Fillet Weld Lap Seam • This seam is similar to an extrusion weld lap seam; however, for fillet welding, the extrudate is placed over the edge of the seam (Figure 17.6e). Double Hot Air or Wedge Seam • In the hot air seam, hot air is blown to melt the two opposing surfaces. For melting, the temperatures should rise to about 500F or more. After the opposite surfaces are melted, pressure is applied to form the seam (Figure 17.6f). For hot wedge seams, an electrically heated element like a blade is passed between the two opposing surfaces of the geomembrane. The heated element helps to melt the geomembrane, after which pressure is applied by a roller to form the seam. 17.6 Geonets Geonets are formed by the continuous extrusion of polymeric ribs at acute angles to each other. They have large openings in a netlike configuration. The primary function of geonets is drainage. Figure 17.7 is a photograph of a typical piece of geonet. Most geonets currently available are made of medium-density and high-density polyethylene. They are available in rolls with widths of 1.8 to 2.1 m (
6 to 7 ft) and lengths of 30 to 90 m (
100 to 300 ft). The approximate aperture sizes vary from 30 mm  30 mm (
1.2 in.  1.2 in.) to about 6 mm  6 mm (
0.25 in.  2.5 in.). The thickness of geonets available commercially can vary from 3.8 to 7.6 mm (
0.15 to 0.3 in.). Seaming of geonets is somewhat more difficult. For this purpose, staples, threaded loops, and wire sometimes are used. Figure 17.7 Geonet (Courtesy of Braja M. Das, Henderson, Nevada) 17.6 Geonets 621 622Chapter 17: Landfill Liners and Geosynthetics 17.7 Single Clay Liner and Single Geomembrane Liner Systems Until about 1982—that is, before the guidelines for the minimum technological require- ments for hazardous-waste landfill design and construction were mandated by the U.S. Environmental Protection Agency—most landfill liners were single clay liners. Figure 17.8 shows the cross section of a single clay liner system for a landfill. It consists primarily of a compacted clay liner over the native foundation soil. The thickness of the compacted clay liner varies between 0.9 and 1.8 m (3 and 6 ft). The maximum required hydraulic conduc- tivity, k, is 10 7 cm/sec. Over the clay liner is a layer of gravel with perforated pipes for leachate collection and removal. Over the gravel layer is a layer of filter soil. The filter is used to protect the holes in the perforated pipes against the movement of fine soil particles. In most cases, the filter is medium coarse to fine sandy soil. It is important to note that this system does not have any leak-detection capability. Around 1982, single layers of geomembranes also were used as a liner material for landfill sites. As shown in Figure 17.9, the geomembrane is laid over native foundation soil. Over the geomembrane is a layer of gravel with perforated pipes for leachate collec- tion and removal. A layer of filter soil is placed between the solid waste material and the gravel. As in the single clay liner system, no provision is made for leak detection. Waste Filter soil Gravel Clay liner Native foundation soil Perforated pipe Figure 17.8 Cross section of single clay liner system for a landfill Waste Filter soil Gravel Geomembrane Native foundation soil Perforated pipe Figure 17.9 Cross section of single geomembrane liner system for a landfill 17.8 Recent Advances in the Liner Systems for Landfills Since 1984, most landfills developed for solid and hazardous wastes have double liners. The two liners are an upper primary liner and a lower secondary liner. Above the top liner is a primary leachate collection and removal system. In general, the primary leachate col- lection system must be able to maintain a leachate head of 0.3 m (
12 in.) or less. Between the primary and secondary liners is a system for leak detection, collection, and removal (LDCR) of leachates. The general guidelines for the primary leachate collection system and the LDCR system are as follows: 1. It can be a granular drainage layer or a geosynthetic drainage material, such as a geonet. 2. If a granular drainage layer is used, it should have a minimum thickness of 0.3 m (
12 in.) 3. The granular drainage layer (or the geosynthetic) should have a hydraulic conductiv- ity, k, greater than 10 2 cm/sec. 4. If a granular drainage layer is used, it should have a granular filter or a layer of geotextile over it to prevent clogging. A layer of geotextile also is required over the geonet when it is used as the drainage layer. 5. The granular drainage layer, when used, must be chemically resistant to the waste material and the leachate that are produced. It also should have a network of perforated pipes to collect the leachate effectively and efficiently. In the design of the liner systems, the compacted clay layers should be at least 1 m (
3 ft) thick, with k
10 7 cm/sec. Figures 17.10 and 17.11 show schematic diagrams of two double-liner systems. In Figure 17.10, the primary leachate collection system is made of a granular material with perforated pipes and a filter system above it. The pri- mary liner is a geomembrane. The LDCR system is made of a geonet. The secondary liner 17.8 Recent Advances in the Liner Systems for Landfills 623 Waste Filter soil Gravel Geomembrane (primary liner) Geonet (leak detection and leachate collection) Native foundation soil Geomembrane (secondary composite liner) Clay liner Perforated pipe Figure 17.10 Cross section of double-liner system (note the secondary composite liner) 624Chapter 17: Landfill Liners and Geosynthetics Waste Filter soil Gravel Primary composite liner (geomembrane) Primary composite liner (clay liner) Native foundation soil Geotextile Geonet Secondary composite liner (geomembrane) Secondary composite liner (clay liner) Perforated pipe Figure 17.11 Cross section of double-liner system (note the primary and secondary composite liners) is a composite liner made of a geomembrane with a compacted clay layer below it. In Figure 17.11, the primary leachate collection system is similar to that shown in Figure 17.10; however, the primary and secondary liners are both composite liners (geomem- brane-clay). The LDCR system is a geonet with a layer of geotextile over it. The layer of geotextile acts as a filter and separator. The geomembranes used for landfill lining must have a minimum thickness of 0.76 mm (0.03 in.); however, all geomembranes that have a thickness of 0.76 mm (0.03 in.) may not be suitable in all situations. In practice, most geomembranes used as liners have thick- nesses ranging from 1.8 to 2.54 mm (0.7 to 0.1 in.). 17.9 Leachate Removal Systems The bottom of a landfill must be graded properly so that the leachate collected from the primary collection system and the LDCR system will flow to a low point by gravity. Usually, a grade of 2% or more is provided for large landfill sites. The low point of the leachate collection system ends at a sump. For a primary leachate collection, a manhole is located at the sump, which rises through the waste material. Figure 17.12 shows a schematic diagram of the leachate removal system with a low-volume sump. A typical leachate removal system for high-volume sumps (for primary collection) is shown in Figure 17.13. Leachate can be removed from the LDCR system by means of pumping, as shown in Figure 17.14, or by gravity monitoring, as shown in Figure 17.15. When leachate is removed by pumping, the plastic pipe used for removal must penetrate the primary liner. On the other hand, if gravity monitoring is used, the pipe will penetrate the sec- ondary liner. 625 Standpipe Air space RCP pipe Operational cover 2% minimum Sand Compacted clayGeomembrane Concrete base Reinforced concrete pipe (RCP) 1–1.2 m (36–48 in.) Solid waste Sand Geomembrane Steel plateGravel Primary leachate Figure 17.12 Primary leachate removal system with a low-volume sump (After U.S. Environmental Protection Agency, 1989). Figure 17.13 Primary leachate removal system with a high-volume sump (After U.S. Environmental Protection Agency, 1989) 626 Leachate removal Submersible pump Leak detection andcollection system Native foundation soil Waste Clay linerSecondary geomembranePrimary geomembrane Plastic pipe Figure 17.14 Secondary leak detection, collection, and removal (LDCR) system—by means of pumping. (Note: The plastic pipe penetrates the primary geomembrane) Leak detection system Native foundation soil Sump Waste Clay liner Secondary geomembranePrimary geomembrane Pipe Figure 17.15 Secondary LDCR system, by means of gravity monitoring. (Note: The plastic pipe penetrates the secondary geomembrane) 17.10 Closure of Landfills 627 600 mm (24 in.) k
10 7 cm/sec Geomembrane [minimum thickness 0.5 mm (0.02 in.)] k 10 2 cm/sec WasteCover topsoil Compacted clay capDrainage layer 600 mm (24 in.) Figure 17.17 Schematic diagram of the layering system for landfill cap 17.10 Closure of Landfills When the landfill is complete and no more waste can be placed into it, a cap must be put on it (Figure 17.16). This cap will reduce and ultimately eliminate leachate gener- ation. A schematic diagram of the layering system recommended by the U.S. Environmental Protection Agency (1979, 1986) and Koerner (1994) for hazardous-waste landfills is shown in Figure 17.17. Essentially, it consists of a compacted clay cap over the solid waste, a geomembrane liner, a drainage layer, and a cover of topsoil. The manhole used for leachate collection penetrates the landfill cover. Leachate removal continues until its generation is stopped. For hazardous-waste landfill sites, the EPA (1989) recommends this period to be about 30 years. Cap (prevents infiltration) Liner (prevents migration of leachates) Waste Figure 17.16 Landfill with liner and cap 628Chapter 17: Landfill Liners and Geosynthetics 17.11 Summary and General Comments This chapter provided a brief overview of the problems associated with solid- and haz- ardous-waste landfills. The general concepts for the construction of landfill liners using compacted clayey soil and geosynthetics (that is, geotextiles, geomembranes, and geonets) were discussed. Several areas were not addressed, however, because they are beyond the scope of the text. Areas not discussed include the following: 1. Selection of material. The chemicals contained in leachates generated from hazardous and nonhazardous waste may interact with the liner materials. For this reason, it is essential that representative leachates are used to test the chemical compatibility so that the liner material remains intact during the periods of landfill operation and closure, and possibly longer. Selection of the proper leacheates becomes difficult because of the extreme variations encountered in the field. The mechanical properties of geomembranes also are important. Properties such as workability, creep, stress cracking, and the thermal coefficient of expansion should be investigated thoroughly. 2. Stability of side-slope liner. The stability and slippage checks of the side-slope liners of a landfill site are important and complicated because of the variation of the frictional characters of the composite materials involved in liner construction. For a detailed treat- ment of this topic, refer to any book on geosynthetics (e.g., Koerner, 1994). 3. Leak-response action plan. It is extremely important that any leaks or clogging of the drainage layer(s) in a given waste-disposal site be detected as quickly as possible. Leaks or cloggings are likelihoods at a site even with good construction quality control. Each waste-disposal facility should have a leak-response action plan. References BENSON, C. H. and DANIEL, D. E. (1990). “Influence of Clods on Hydraulic Conductivity of Compacted Clay,” Journal of Geotechnical Engineering, ASCE, Vol. 116, No. 8, 1231–1248. DANIEL, D. E. and BENSON, C. H. (1990). “Water Content-Density Criteria for Compacted Soil Liners,” Journal of Geotechnical Engineering, ASCE, Vol. 116, No. 12, 1811–1830. KOERNER , R. M. (1994). Designing with Geosynthetics, 3rd ed., Prentice-Hall, Englewood Cliffs, N.J. SIVAPULLAIAH, P. V., S RIDHARAN, A., and STALIN , V. K. (2000). “Hydraulic Conductivity of Bentonite-Sand Mixtures,” Canadian Geotechnical Journal, Vol. 37, No. 2, 406–413. U.S. E NVIRONMENTAL PROTECTION AGENCY (1979). Design and Construction for Solid Waste Land- fills, Publication No. EPA-600/2-79-165, Cincinnati, Ohio. U.S. ENVIRONMENTAL PROTECTION AGENCY (1986). Cover for Uncontrolled Hazardous Waste Sites, Publication No. EPA-540/2-85-002, Cincinnati, Ohio. U.S. ENVIRONMENTAL PROTECTION AGENCY (1989). Requirements for Hazardous Waste Landfill Design, Construction, and Closure, Publication No. EPA-625/4-89-022, Cincinnati, Ohio.