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Geosynthetics in landfill closures design considerations
Geotextiles and Geomembranes 10 (1991) 403--410
Geosynthetics in Landfill Closures Design Considerations M i c h a e l T. F e e n e y Golder Associates Inc., 3730 Chamblee Tucker Road, Atlanta, Georgia 30341, USA
ABSTRACT Geomembranes can provide an effective low permeability cover at many sites. Maintenance of overall stability is essential to design performance. The beneficial effects of textured geomembrane, drainage terraces and tensile geogrids are discussed. The use of both vegetative and nonvegetative covers to resist erosion is presented. The effects of construction equipment traffic, freezing, root growth and vermin on the geomembrane are also discussed. Finally, geomembrane penetrations and a closure concept which does not physically join the closure geomembrane to the bottom geomembrane are discussed.
INTRODUCTION The use of geosynthetics in landfill closures can be an effective means of providing low permeability closure covers, particularly at sites with limited or no supply of natural clay. Geosynthetics have the additional advantage of relatively low cost when compared to covers of natural materials at many sites. This paper discusses three categories of design considerations related to the use of geosynthetics in closure covers: general stability, reduction of infiltration, and design details.
GENERAL STABILITY In order to fulfill any design function, the constructed closure cover must remain stable under gravitational and erosive forces. Providing resistance to these forces generally governs the geometric configuration of the 403 Geotextiles and Geomembranes 0266-1144/91/$03.50© 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain
404
Michael T. Feeney
closure. Additionally, for solid waste landfills, the closure cover must remain stable, during and after construction, and under the positive pressures resulting from gas generation with the landfill.
Stability under gravitational forces Stability under gravitational forces is typically the most carefully considered stability analysis performed during a closure cover design. Since many existing landfills are reaching their permitted capacity and a significant amount of time and difficulty is involved in permitting new landfills, there is a trend toward the design and construction of the steepest sideslopes possible. The use of geosynthetics is often a significant factor in requiring that cover slopes be flatter, rather than steeper. Therefore, careful stability analysis and selection of geosynthetic products is needed to construct the steepest possible geosynthetic slope, even though this slope may be flatter than a cover consisting only of soils. Design and construction of the steepest possible geosynthetic cover slopes involve careful evaluation of both driving and resisting forces. This paper discusses the following three components of resisting force: friction between the cover soil and the geosynthetic, buttress support at the toe of slope, and tensile reinforcement at the crest of the slope. The friction developed between the cover soils and the geosynthetics is dependent on the force normal to the geosynthetics and the friction angle between the cover material, the uppermost geosynthetic and among the cover synthetics. While a complete discussion of friction angles is beyond the scope of this paper, the controlling friction angle in a cover design involving geosynthetics is often between the drainage layer and the geomembrane. When this interface consists of geonet overlying smooth geomembrane, the author is aware of test results suggesting the friction angle may be as low as 8 ° or 9 °. The friction angle on this geonet/geomembrane interface may be increased through the use of textured geomembrane, at least when polyethylene membranes have been selected. The use of textured geomembrane can significantly increase the minimum friction angle of a closure cover geosynthetic/soil system. In some cases, the friction angle of a geonet/textured geomembrane remains low because of low normal loads. For these cases, a geocomposite involving a geotextile in contact with the liner may be necessary. In the author's experience, several factors are important when considering the use of textured geomembrane. First, it is recommended that the friction angle be measured in a laboratory using samples of the geonet and textured geomembrane products under consideration, along with soil from the project site. The use of textured geomembrane is relatively new
Geosynthetics in landfill closures design considerations
405
and the author believes the current body of available literature on friction angles to be rather limited. Second, laboratory tests should measure the friction angle under normal loads representative of those expected in the field condition. As closure covers are typically only a few feet thick, normal loads may be relatively small and the geonet might slide above, or 'skip over', the texturing on the geomembrane. Third, it is suggested that the elongation properties of the textured geomembrane be carefully evaluated by laboratory testing. The author is aware of test results that indicate some textured geomembranes develop holes and rupture at elongations of about 100%. The apparent lack of elongation in some textured geomembranes is, in the author's opinion, a needless forfeiture of a desirable physical property. Fourth, the author's experience suggests that the variation in membrane thickness and overall quality is greater with textured geomembranes than with smooth geomembranes. This added variation introduces a higher degree of uncertainty as to the quality of the cover geomembrane. Fifth, and finally, installation and quality control testing is more difficult with textured geomembrane than with smooth liners. In summary, the use of textured geomembranes can significantly increase the minimum friction angle of closure cover systems, although several factors are recommended for careful consideration. The author has incorporated textured polyethylene geomembranes into the steep portions of closure covers of several projects. Another component of resisting force which contributes to the stability of closure covers is the shear strength of the soil growth media above the cover geosynthetics. Closure cover systems are often analyzed as infinitely long slopes, a procedure which ignores the resistance provided by soil shear strength at the toe of the slope. In the author's experience, inclusion of this soil shear strength in the stability analysis can have a measurable effect. This is especially true if the cover soil is a cohesive material. Additionally, the calculated stability of long steep slopes can often be significantly increased by incorporating benches or terraces into the design to decrease the unbroken slope length. When the soil shear strength just above the bench/terrace is included in the stability analysis, the angle of the slope above the bench may be able to be designed steeper than would otherwise be allowable. Since benches/terraces are often needed in long slopes for erosion control purposes, including their effect in stability analyses often benefits the client without requiring additional cover elements to be designed and constructed. Of course, a bigger soil bench/terrace can always be designed if the steeper allowable slope angle which results is sufficiently beneficial to justify the bigger bench/terrace. A third means of increasing the angle at which closure slopes can be designed and constructed is to incorporate tension reinforcing elements,
406
Michael T. Feeney
such as geogrids, into the cover design. The embedment of a tension element into the cover soil above the closure geomembrane allows the development of tension forces which resist the gravitational forces acting on cover soils. The development of tension forces on each side of the cover crest is balanced. Essentially, the soil on one side of the cover crest acts as a counterweight for the soil on the other side of the crest. Strain compatibility is also important in evaluating the use of a geogrid. The tensile force required to maintain stability of the cover soils will result in certain strain in the geogrid. If the magnitude of this strain is large enough to have unacceptable physical consequences, such as distortion or slippage of the cover soils, then a stiffer geogrid must be chosen or the slope angle reduced.
Stability under erosive forces Resistance to erosive forces is an essential requirement in maintaining the long-term function of a closure cover system. In many areas of the country, erosion resistance is provided by vegetation established on the cover soils. In some areas of the country, rainfall is insufficient to reliably establish and maintain vegetation. In such regions, gravel, soil-cement, or other non-vegetative erosion-resistant materials could be used to 'armor' the slope. On long, steep slopes drainage benches or terraces are often included in an attempt to reduce erosion. Because surface water runoff from a slope often becomes concentrated by irregularities in the slope, the terrace collects all sheet flow and shallow concentrated flow from the slope. This collected flow then travels along a known high flow area, the terrace. Some designers have made the frequent use of terraces a routine part of their cover systems. The author prefers to reduce the use of terraces and to expect intensive efforts to establish vegetative cover. Experience indicates that excellent vegetative covers can be established in very harsh soil environments. However, it is recognized that some owners are either unfamiliar with the aggressive effort needed to establish a vigorous vegetative cover or lack the site personnel to maintain the required effort. A frank discussion with the owner at the initiation of the project can establish a proper design philosophy regarding terraces which matches the owner's desires with the need to resist erosion.
Stability under pressure of gas generation For solid waste landfills, the liner must remain stable under the pressure of gas generated by the landfill being closed. Provision is usually made for
Geosynthetics in landfill closures design considerations
407
active extraction of landfill gas or passive venting of gas through a completed closure. Passive systems often rely upon granular trench or blanket drains over the waste to transmit gas to the vent. The thickness of cover soil could be chosen such that the weight of the soil exceeds the anticipated gas pressure. It should be recognized that passive systems relying on granular trench or blanket drains may not function until the geomembrane is completely covered with soil. This would be particularly true for landfills with sandy soils covering the waste. Prior to replacement of the geomembrane, landfill gas exits the landfill through the path of least resistance. This least resistance path may be directly upward through the cover soils instead of through the granular drains. Placement of the geomembrane alone does not change this flow path. Instead, the gas continues to seep out behind the geomembrane and, in the absence of cover soil, simply lifts the geomembrane. Left unattended, the gas bubble beneath the geomembrane simply continues to build until the pressure is sufficient to rupture the geomembrane. Obviously, the potential for this situation to exist is greatest when sandy soils have been used in the landfills. One means of avoiding this problem is to leave temporary vents in the geomembrane so that the landfill gas is dissipated rather than allowed to collect beneath the liner. These vents would be progressively sealed immediately prior to the placement of soil cover over the vent. It would also be helpful to place the cover soil over the geomembrane in stages, as portions of the geomembrane are completed. The author is aware of one project where a geomembrane liner failed catastrophically after being completed, but not promptly covered with soil.
R E D U C T I O N OF I N F I L T R A T I O N A primary function of the closure cover is to reduce the potential for infiltration into the covered waste. This reduction is greatly advanced by promoting surface water runoff from the cover. Runoff promotion is primarily a function of cover slope, not geosynthetics, and is therefore beyond the scope of this paper. Geosynthetics can play a role in lateral drainage above the geomembrane, however. Additionally, geomembrane survivability under various construction and environmental conditions is vital in maintaining the integrity of the closure cover system. Provision of lateral drainage above a geomembrane can dissipate any excess head on the geomembrane that may result from infiltration through the vegetative cover soil. Either geosynthetics or natural materials may be
408
Michael T. Feeney
used for the lateral drain. Both materials will likely require a filter between the cover soil and the drain. The required capacity of the drain can be estimated using water balance and hydraulic calculations. A proper estimate must include the water removed by evapotranspiration through the closure cover vegetation. Allowances for evapotranspiration will avoid unnecessary oversizing of the drain. The author is familiar with one project where inclusion of evapotranspiration indicated that no drain was necessary. This conclusion, confirmed by field observations one year after construction, could allow steeper closure cover slope angles by eliminating potentially low friction geonet/geomembrane interfaces. Geomembrane survivability under various construction and environmental conditions is necessary to reduce infiltration into the underlying waste materials. Construction equipment used to place cover soils above the geomembrane exerts stresses on the geomembrane. To evaluate the potential effects of this traffic, the maximum particle size of the soil particle placed on top of the geomembrane must be known. The stress of overburden and equipment traffic on this particle can be calculated using conventional geotechnical engineering techniques. The stress exerted by the equipment should be reduced from actual wheel loads because of the 'spreading' effect of the stress through the thickness of cover soil. The puncture strength required of the geomembrane can then be calculated. In the author's experience, only low ground pressure tracked equipment is suitable for operating on top of 0.3 m (one foot) of sand or gravel above a geomembrane. Wheeled scrapers and trucks typically require about one meter (two to three feet) of cover material above the geomembrane, depending on the specific equipment and geomembranes involved. The author recommends that all equipment operating above geosynthetics travel at reduced speeds and avoid sharp turns. Environmental conditions such as cold weather, root penetration, and vermin have the potential for affecting the integrity of the liner. Freezing conditions, which can damage clay liners, should have no effect on the polymeric geomembranes. However, the author is aware of some anecdotal evidence which suggests that rapid cooling may have some effect on exposed geomembranes. The mechanism by which the cooling affects the exposed geomembrane is not known to the author at this time. The author knows of no detrimental effects of freezing on buried geomembranes. Root penetration through geomembranes has not been encountered by the author. The polymeric geomembrane does not provide any nutrients to plants and, therefore, has no attraction to roots. Also, most of the geomembranes used in covers today are highly resistant to puncture, so the author considers them highly resistant to root penetration also. The
Geosynthetics in landfill closures design considerations
409
author would welcome any information on the penetration of thinner geomembranes by root networks. The author is also not aware of any instance of penetration of geomembranes by vermin. Again, as there are no nutrients or food odors in the geomembrane, there should be no attraction to vermin. Also, since most geomembranes form an effective barrier to vapors, odors from buried waste should be minimized, thereby hopefully eliminating attractive signals to vermin. The author would welcome any information regarding vermin penetration of geomembranes, especially thinner geomembranes.
DESIGN DETAILS Two design details for closure covers are often discussed, anchor trenches and cover penetrations. For anchor trenches, the author strongly advocates the use of the 'umbrella' concept. In this concept, the cover geomembrane is not physically joined to the geomembranes forming the landfill's bottom liner. Instead, the cover geomembrane is terminated in an anchor trench outside of, and at an elevation approximately equal to, the bottom liner anchor trench. In this manner, the cover geomembrane forms an 'umbrella' over the closed landfill. It is extremely improbable, even under the influence of large capillary forces, that a significant quantity of water could be drawn upward between the two anchor trenches and into the closed landfill. The umbrella concept also simplifies construction, primarily by avoiding the practical difficulty of exhuming, then cleaning, the bottom liner sufficiently to make a high-quality weld with the closure liner. Penetrations through the completed closure cover are also inevitable. Leachate risers, landfill gas extraction wells or vents, and groundwater monitoring wells are the most frequent penetrations required. In early designs completed by the author, the penetration was made by pouring a concrete collar around the penetration, then using anchor bolts, a steel batten plate, and a compressible gasket material to secure the geomembrane to the collar. In the last six years, however, the author has developed confidence in the use of geomembrane boots to make required penetrations as watertight as possible. The lower portion of the boot is joined to the geomembrane, while the upper portion is secured around the penetration with compressible gasket material, such as neoprene, and a stainless steel clamp. Field experience indicates that these boots are at least as effective as the previous concrete collar details and are much easier and cheaper to install.
410
Michael T. Feeney
SUMMARY G e o m e m b r a n e s can be an effective means of providing low permeability closure covers at, for many sites, a cost less than that of natural materials. Stability under gravitational and erosive forces is essential if the cover is to carry out its design function. The use of geosynthetics in closure covers generally results in flatter slope angles than covers using only soil materials. The use of textured geomembrane on steep portions of closure covers is one technique that can be used to construct the steepest possible geosynthetic slope. The author recommends that laboratory tests of friction angle and elongation be conducted before using textured geomembrane. Including the effect of cover soil shear strength can have a measurable effect on the calculated stability of closure cover soils. Since many designs incorporate drainage terraces regardless of stability, accounting for the soil strength just above the terrace when conducting the stability analysis results in a no-cost benefit to the final design. Also, geogrids can be used as tensile elements in cover soils above the closure geomembranes. Geogrid use allows the cover soil on one side of the closure to counterbalance the weight of soil on the other side. The integrity of the geomembrane must be maintained if the closure cover is to achieve its design purpose of reducing infiltration into the covered waste materials. An analysis of the puncture stresses on the g e o m e m b r a n e due to the weight of construction equipment and the overlying soil should be made. In the author's experience, one foot of cover soil is often sufficient for low ground pressure tracked equipment, while two to three feet of cover soil is often required for wheeled equipment and trucks. The author is not aware of any detrimental effects of freezing, root penetration or vermin on buried geomembranes. The author strongly advocates use of the 'umbrella' concept for closure cover geomembranes. The closure geomembrane is not physically attached to the geomembrane underlying the waste. Instead, the closure geomembrane is anchored adjacent to, and at about the same elevation as, the b o t t o m geomembrane. This concept greatly simplifies construction. Penetrations through the closure cover can be effectively sealed with g e o m e m b r a n e boots and stainless steel clamps.
Geosynthetics in Landfill Closures Design Considerations M i c h a e l T. F e e n e y Golder Associates Inc., 3730 Chamblee Tucker Road, Atlanta, Georgia 30341, USA
ABSTRACT Geomembranes can provide an effective low permeability cover at many sites. Maintenance of overall stability is essential to design performance. The beneficial effects of textured geomembrane, drainage terraces and tensile geogrids are discussed. The use of both vegetative and nonvegetative covers to resist erosion is presented. The effects of construction equipment traffic, freezing, root growth and vermin on the geomembrane are also discussed. Finally, geomembrane penetrations and a closure concept which does not physically join the closure geomembrane to the bottom geomembrane are discussed.
INTRODUCTION The use of geosynthetics in landfill closures can be an effective means of providing low permeability closure covers, particularly at sites with limited or no supply of natural clay. Geosynthetics have the additional advantage of relatively low cost when compared to covers of natural materials at many sites. This paper discusses three categories of design considerations related to the use of geosynthetics in closure covers: general stability, reduction of infiltration, and design details.
GENERAL STABILITY In order to fulfill any design function, the constructed closure cover must remain stable under gravitational and erosive forces. Providing resistance to these forces generally governs the geometric configuration of the 403 Geotextiles and Geomembranes 0266-1144/91/$03.50© 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain
404
Michael T. Feeney
closure. Additionally, for solid waste landfills, the closure cover must remain stable, during and after construction, and under the positive pressures resulting from gas generation with the landfill.
Stability under gravitational forces Stability under gravitational forces is typically the most carefully considered stability analysis performed during a closure cover design. Since many existing landfills are reaching their permitted capacity and a significant amount of time and difficulty is involved in permitting new landfills, there is a trend toward the design and construction of the steepest sideslopes possible. The use of geosynthetics is often a significant factor in requiring that cover slopes be flatter, rather than steeper. Therefore, careful stability analysis and selection of geosynthetic products is needed to construct the steepest possible geosynthetic slope, even though this slope may be flatter than a cover consisting only of soils. Design and construction of the steepest possible geosynthetic cover slopes involve careful evaluation of both driving and resisting forces. This paper discusses the following three components of resisting force: friction between the cover soil and the geosynthetic, buttress support at the toe of slope, and tensile reinforcement at the crest of the slope. The friction developed between the cover soils and the geosynthetics is dependent on the force normal to the geosynthetics and the friction angle between the cover material, the uppermost geosynthetic and among the cover synthetics. While a complete discussion of friction angles is beyond the scope of this paper, the controlling friction angle in a cover design involving geosynthetics is often between the drainage layer and the geomembrane. When this interface consists of geonet overlying smooth geomembrane, the author is aware of test results suggesting the friction angle may be as low as 8 ° or 9 °. The friction angle on this geonet/geomembrane interface may be increased through the use of textured geomembrane, at least when polyethylene membranes have been selected. The use of textured geomembrane can significantly increase the minimum friction angle of a closure cover geosynthetic/soil system. In some cases, the friction angle of a geonet/textured geomembrane remains low because of low normal loads. For these cases, a geocomposite involving a geotextile in contact with the liner may be necessary. In the author's experience, several factors are important when considering the use of textured geomembrane. First, it is recommended that the friction angle be measured in a laboratory using samples of the geonet and textured geomembrane products under consideration, along with soil from the project site. The use of textured geomembrane is relatively new
Geosynthetics in landfill closures design considerations
405
and the author believes the current body of available literature on friction angles to be rather limited. Second, laboratory tests should measure the friction angle under normal loads representative of those expected in the field condition. As closure covers are typically only a few feet thick, normal loads may be relatively small and the geonet might slide above, or 'skip over', the texturing on the geomembrane. Third, it is suggested that the elongation properties of the textured geomembrane be carefully evaluated by laboratory testing. The author is aware of test results that indicate some textured geomembranes develop holes and rupture at elongations of about 100%. The apparent lack of elongation in some textured geomembranes is, in the author's opinion, a needless forfeiture of a desirable physical property. Fourth, the author's experience suggests that the variation in membrane thickness and overall quality is greater with textured geomembranes than with smooth geomembranes. This added variation introduces a higher degree of uncertainty as to the quality of the cover geomembrane. Fifth, and finally, installation and quality control testing is more difficult with textured geomembrane than with smooth liners. In summary, the use of textured geomembranes can significantly increase the minimum friction angle of closure cover systems, although several factors are recommended for careful consideration. The author has incorporated textured polyethylene geomembranes into the steep portions of closure covers of several projects. Another component of resisting force which contributes to the stability of closure covers is the shear strength of the soil growth media above the cover geosynthetics. Closure cover systems are often analyzed as infinitely long slopes, a procedure which ignores the resistance provided by soil shear strength at the toe of the slope. In the author's experience, inclusion of this soil shear strength in the stability analysis can have a measurable effect. This is especially true if the cover soil is a cohesive material. Additionally, the calculated stability of long steep slopes can often be significantly increased by incorporating benches or terraces into the design to decrease the unbroken slope length. When the soil shear strength just above the bench/terrace is included in the stability analysis, the angle of the slope above the bench may be able to be designed steeper than would otherwise be allowable. Since benches/terraces are often needed in long slopes for erosion control purposes, including their effect in stability analyses often benefits the client without requiring additional cover elements to be designed and constructed. Of course, a bigger soil bench/terrace can always be designed if the steeper allowable slope angle which results is sufficiently beneficial to justify the bigger bench/terrace. A third means of increasing the angle at which closure slopes can be designed and constructed is to incorporate tension reinforcing elements,
406
Michael T. Feeney
such as geogrids, into the cover design. The embedment of a tension element into the cover soil above the closure geomembrane allows the development of tension forces which resist the gravitational forces acting on cover soils. The development of tension forces on each side of the cover crest is balanced. Essentially, the soil on one side of the cover crest acts as a counterweight for the soil on the other side of the crest. Strain compatibility is also important in evaluating the use of a geogrid. The tensile force required to maintain stability of the cover soils will result in certain strain in the geogrid. If the magnitude of this strain is large enough to have unacceptable physical consequences, such as distortion or slippage of the cover soils, then a stiffer geogrid must be chosen or the slope angle reduced.
Stability under erosive forces Resistance to erosive forces is an essential requirement in maintaining the long-term function of a closure cover system. In many areas of the country, erosion resistance is provided by vegetation established on the cover soils. In some areas of the country, rainfall is insufficient to reliably establish and maintain vegetation. In such regions, gravel, soil-cement, or other non-vegetative erosion-resistant materials could be used to 'armor' the slope. On long, steep slopes drainage benches or terraces are often included in an attempt to reduce erosion. Because surface water runoff from a slope often becomes concentrated by irregularities in the slope, the terrace collects all sheet flow and shallow concentrated flow from the slope. This collected flow then travels along a known high flow area, the terrace. Some designers have made the frequent use of terraces a routine part of their cover systems. The author prefers to reduce the use of terraces and to expect intensive efforts to establish vegetative cover. Experience indicates that excellent vegetative covers can be established in very harsh soil environments. However, it is recognized that some owners are either unfamiliar with the aggressive effort needed to establish a vigorous vegetative cover or lack the site personnel to maintain the required effort. A frank discussion with the owner at the initiation of the project can establish a proper design philosophy regarding terraces which matches the owner's desires with the need to resist erosion.
Stability under pressure of gas generation For solid waste landfills, the liner must remain stable under the pressure of gas generated by the landfill being closed. Provision is usually made for
Geosynthetics in landfill closures design considerations
407
active extraction of landfill gas or passive venting of gas through a completed closure. Passive systems often rely upon granular trench or blanket drains over the waste to transmit gas to the vent. The thickness of cover soil could be chosen such that the weight of the soil exceeds the anticipated gas pressure. It should be recognized that passive systems relying on granular trench or blanket drains may not function until the geomembrane is completely covered with soil. This would be particularly true for landfills with sandy soils covering the waste. Prior to replacement of the geomembrane, landfill gas exits the landfill through the path of least resistance. This least resistance path may be directly upward through the cover soils instead of through the granular drains. Placement of the geomembrane alone does not change this flow path. Instead, the gas continues to seep out behind the geomembrane and, in the absence of cover soil, simply lifts the geomembrane. Left unattended, the gas bubble beneath the geomembrane simply continues to build until the pressure is sufficient to rupture the geomembrane. Obviously, the potential for this situation to exist is greatest when sandy soils have been used in the landfills. One means of avoiding this problem is to leave temporary vents in the geomembrane so that the landfill gas is dissipated rather than allowed to collect beneath the liner. These vents would be progressively sealed immediately prior to the placement of soil cover over the vent. It would also be helpful to place the cover soil over the geomembrane in stages, as portions of the geomembrane are completed. The author is aware of one project where a geomembrane liner failed catastrophically after being completed, but not promptly covered with soil.
R E D U C T I O N OF I N F I L T R A T I O N A primary function of the closure cover is to reduce the potential for infiltration into the covered waste. This reduction is greatly advanced by promoting surface water runoff from the cover. Runoff promotion is primarily a function of cover slope, not geosynthetics, and is therefore beyond the scope of this paper. Geosynthetics can play a role in lateral drainage above the geomembrane, however. Additionally, geomembrane survivability under various construction and environmental conditions is vital in maintaining the integrity of the closure cover system. Provision of lateral drainage above a geomembrane can dissipate any excess head on the geomembrane that may result from infiltration through the vegetative cover soil. Either geosynthetics or natural materials may be
408
Michael T. Feeney
used for the lateral drain. Both materials will likely require a filter between the cover soil and the drain. The required capacity of the drain can be estimated using water balance and hydraulic calculations. A proper estimate must include the water removed by evapotranspiration through the closure cover vegetation. Allowances for evapotranspiration will avoid unnecessary oversizing of the drain. The author is familiar with one project where inclusion of evapotranspiration indicated that no drain was necessary. This conclusion, confirmed by field observations one year after construction, could allow steeper closure cover slope angles by eliminating potentially low friction geonet/geomembrane interfaces. Geomembrane survivability under various construction and environmental conditions is necessary to reduce infiltration into the underlying waste materials. Construction equipment used to place cover soils above the geomembrane exerts stresses on the geomembrane. To evaluate the potential effects of this traffic, the maximum particle size of the soil particle placed on top of the geomembrane must be known. The stress of overburden and equipment traffic on this particle can be calculated using conventional geotechnical engineering techniques. The stress exerted by the equipment should be reduced from actual wheel loads because of the 'spreading' effect of the stress through the thickness of cover soil. The puncture strength required of the geomembrane can then be calculated. In the author's experience, only low ground pressure tracked equipment is suitable for operating on top of 0.3 m (one foot) of sand or gravel above a geomembrane. Wheeled scrapers and trucks typically require about one meter (two to three feet) of cover material above the geomembrane, depending on the specific equipment and geomembranes involved. The author recommends that all equipment operating above geosynthetics travel at reduced speeds and avoid sharp turns. Environmental conditions such as cold weather, root penetration, and vermin have the potential for affecting the integrity of the liner. Freezing conditions, which can damage clay liners, should have no effect on the polymeric geomembranes. However, the author is aware of some anecdotal evidence which suggests that rapid cooling may have some effect on exposed geomembranes. The mechanism by which the cooling affects the exposed geomembrane is not known to the author at this time. The author knows of no detrimental effects of freezing on buried geomembranes. Root penetration through geomembranes has not been encountered by the author. The polymeric geomembrane does not provide any nutrients to plants and, therefore, has no attraction to roots. Also, most of the geomembranes used in covers today are highly resistant to puncture, so the author considers them highly resistant to root penetration also. The
Geosynthetics in landfill closures design considerations
409
author would welcome any information on the penetration of thinner geomembranes by root networks. The author is also not aware of any instance of penetration of geomembranes by vermin. Again, as there are no nutrients or food odors in the geomembrane, there should be no attraction to vermin. Also, since most geomembranes form an effective barrier to vapors, odors from buried waste should be minimized, thereby hopefully eliminating attractive signals to vermin. The author would welcome any information regarding vermin penetration of geomembranes, especially thinner geomembranes.
DESIGN DETAILS Two design details for closure covers are often discussed, anchor trenches and cover penetrations. For anchor trenches, the author strongly advocates the use of the 'umbrella' concept. In this concept, the cover geomembrane is not physically joined to the geomembranes forming the landfill's bottom liner. Instead, the cover geomembrane is terminated in an anchor trench outside of, and at an elevation approximately equal to, the bottom liner anchor trench. In this manner, the cover geomembrane forms an 'umbrella' over the closed landfill. It is extremely improbable, even under the influence of large capillary forces, that a significant quantity of water could be drawn upward between the two anchor trenches and into the closed landfill. The umbrella concept also simplifies construction, primarily by avoiding the practical difficulty of exhuming, then cleaning, the bottom liner sufficiently to make a high-quality weld with the closure liner. Penetrations through the completed closure cover are also inevitable. Leachate risers, landfill gas extraction wells or vents, and groundwater monitoring wells are the most frequent penetrations required. In early designs completed by the author, the penetration was made by pouring a concrete collar around the penetration, then using anchor bolts, a steel batten plate, and a compressible gasket material to secure the geomembrane to the collar. In the last six years, however, the author has developed confidence in the use of geomembrane boots to make required penetrations as watertight as possible. The lower portion of the boot is joined to the geomembrane, while the upper portion is secured around the penetration with compressible gasket material, such as neoprene, and a stainless steel clamp. Field experience indicates that these boots are at least as effective as the previous concrete collar details and are much easier and cheaper to install.
410
Michael T. Feeney
SUMMARY G e o m e m b r a n e s can be an effective means of providing low permeability closure covers at, for many sites, a cost less than that of natural materials. Stability under gravitational and erosive forces is essential if the cover is to carry out its design function. The use of geosynthetics in closure covers generally results in flatter slope angles than covers using only soil materials. The use of textured geomembrane on steep portions of closure covers is one technique that can be used to construct the steepest possible geosynthetic slope. The author recommends that laboratory tests of friction angle and elongation be conducted before using textured geomembrane. Including the effect of cover soil shear strength can have a measurable effect on the calculated stability of closure cover soils. Since many designs incorporate drainage terraces regardless of stability, accounting for the soil strength just above the terrace when conducting the stability analysis results in a no-cost benefit to the final design. Also, geogrids can be used as tensile elements in cover soils above the closure geomembranes. Geogrid use allows the cover soil on one side of the closure to counterbalance the weight of soil on the other side. The integrity of the geomembrane must be maintained if the closure cover is to achieve its design purpose of reducing infiltration into the covered waste materials. An analysis of the puncture stresses on the g e o m e m b r a n e due to the weight of construction equipment and the overlying soil should be made. In the author's experience, one foot of cover soil is often sufficient for low ground pressure tracked equipment, while two to three feet of cover soil is often required for wheeled equipment and trucks. The author is not aware of any detrimental effects of freezing, root penetration or vermin on buried geomembranes. The author strongly advocates use of the 'umbrella' concept for closure cover geomembranes. The closure geomembrane is not physically attached to the geomembrane underlying the waste. Instead, the closure geomembrane is anchored adjacent to, and at about the same elevation as, the b o t t o m geomembrane. This concept greatly simplifies construction. Penetrations through the closure cover can be effectively sealed with g e o m e m b r a n e boots and stainless steel clamps.