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CERCLA landfill closures: Construction considerations
Geotextiles and Geomembranes 10 (1991) 393--402
C E R C L A Landfill Closures: Construction Considerations* David L. Jaros US Army Corps of Engineers, Missouri River Division, PO Box 103, Downtown Station, Omaha, Nebraska 68101, USA
ABSTRACT A properly functioning cover over an uncontrolled landfill which is regulated under the Comprehensive Environmental Response Compensation and Liability Act (CERCLA) depends on both a well-engineered cover design and prudent construction practices. This paper focuses on several geosynthetic construction issues which ultimately impact the performance of the final constructed cover. Preparation and placement issues related to subgrade preparation, geosynthetic placement, cover soil placement and test sections are presented. Storage and seaming of the geosynthetics are discussed. Construction quality control and quality assurance programs are emphasized. In addition, relevant design issues pertaining to settlements, interface friction, and rigid-nonrigid connections are discussed. Although these issues apply to all landfills, this paper is based on experience gained from design, review and construction oversight of CERCLA landfill covers by the US Army Corps of Engineers.
1 INTRODUCTION A cover over an uncontrolled hazardous waste landfill functions as a roof. It p r o m o t e s controlled runoff, minimizes percolation and leachate formation, and thus prevents or minimizes future ground-water contamination. Geosynthetics play a key role in assuring these objectives are met. "The views expressed in this paper are solely those of the author and not the US Army Corps of Engineers. 393 Geotextiles and Geomembranes 0266-1144/91/$03.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain
394
David L. Jaros
The major emphasis of this paper is on construction-related geosynthetic issues pertaining to past and present US Army Corps of Engineers (USACE) landfill projects. Pertinent design issues that have a major impact on construction are also presented.
2 CERCLA
2.1 Regulation The Comprehensive Environmental Response Compensation and Liability Act (CERCLA or SUPERFUND) was originally enacted by Congress in 1980.1 The legislation was amended in 1986 by the Superfund Amendments and Reauthorization Act (SARA).2 The primary purpose of CERCLA and SARA is to provide funding and enforcement for EPA to clean up the thousands of hazardous waste sites created in the United States by unregulated practices and to respond to hazardous substance spills. The CERCLA legislation allowed for the use of existing capabilities within other Federal agencies to meet its objectives. The EPA and USACE signed an interagency agreement (lAG) on 3 February, 1982. Under the agreement, upon EPA request, USACE manages design and construction contracts and provides technical assistance to EPA in support of remedial clean-up of hazardous waste sites. An lAG between EPA and USACE was signed on 3 December, 1984, which extends the partnership indefinitely.
2.2 Cover requirements Currently there are no specific EPA regulations pertaining to design of final covers on CERCLA landfills. 3 Generally, EPA final cover Minimum Technology Guidance for landfills regulated under the Resource Conservation and Recovery Act (RCRA) is applied to CERCLA designs. Consequently, the three-layer RCRA cap system consisting of a vegetative top layer, middle drainage layer and a single or composite bottom liner is common CERCLA design practice.
2.3 Projects To date, the Corps of Engineers' Missouri River Division (CEMRD) has been involved in ten major CERCLA landfill cover projects. Eight are constructed or presently under construction with the remaining two in the
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design phase. High or very low density polyethylene geomembranes have or will be used where a geomembrane barrier layer is required to meet the intent of the EPA's Record of Decision. Geotextiles have been used for separation layers. Geonets have been used for drainage layers and gas transmission layers where necessary. To date, no geogrids or geocomposites have been used.
3 DESIGN CONSIDERATIONS 3.1 Settlement
Landfills settle with time due to a variety of factors including waste composition, landfill density, landfill age, landfill operation and landfill thickness. Unlike controlled R C R A landfills, most of these factors are unknown at uncontrolled C E R C L A landfills. To date, very limited field investigations have been conducted at C E R C L A sites to provide reliable estimates of potential settlement quantities or rates. Difficulties in drilling and sampling heterogeneous landfill masses and health and safety concerns are but a few reasons why such in-depth investigations are not conducted. Differential settlements causing localized stress concentrations are a much greater threat than areal subsidence which generally imposes only low compressive stresses on the cover system. With settlements upwards of 50% of the landfill thickness at C E R C L A landfills as compared to 1.5% at R C R A landfills, the potential for large localized settlements becomes a major design issue and subsequent construction problem. 4 Differential settlements result in cover geosynthetics being stressed biaxially. However, most C E R C L A cover designs to date, have not, in the author's opinion, addressed differential settlements properly. Uniaxial stress-strain relationships provided by manufacturer's index tests have been used in lieu of more realistic biaxial stress-strain relationships provided by design-oriented tests. Although biaxial tests are not yet standardized by ASTM, one method is available from the Geosynthetics Research Institute (GRI) as GRI Standard GM-4. Differential settlements can be detrimental to all cover system components. Depending on the type of geomembrane, the parent material may perform well under large uniaxial strains but fail in tension under much lower biaxial strains. For example, high density polyethylene (HDPE) is more sensitive to biaxial strains than very low density polyethylene (VLDPE) or polyvinyl chloride (PVC) materials. This problem is compounded in seamed areas where stress conditions are more
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complex and variable. Unfortunately, little emphasis has been placed on simulating differential settlements at seamed areas in the laboratory. Yet, field seams are considered the weak link in any geosynthetic system. Differential settlements may distort geonets sufficiently to impede their flow properties resulting in localized ponding. Overlying geotextiles may tear or separate at seams, allowing cover soil migration into the underlying drainage layer resulting in eventual clogging. Clay barriers may crack and become ineffective barriers. Obviously, it is critical that cover systems be designed to handle anticipated differential settlements. Likewise, appropriate designoriented laboratory tests are absolutely necessary when using a design-by-function approach. 5 3.2 Interface friction
Ideally, 100% contact between adjacent cap components is desirable to provide maximum frictional resistance against sliding along interfaces. However, due to subgrade irregularities, differential settlement, relatively low normal stresses, discontinuities at joints, material properties and environmental factors, total interface contact throughout a cover system is impossible. To simplify calculations, static-type interface stability analyses assume 100% interface contact between adjacent materials along uniform slopes. 4 The interface friction angle used in these analyses depends on both the material type and the test method used. To date, there is no consensus on a standard test method for determining this friction angle. Typically, geotechnical-oriented direct shear tests or tilt table tests are used to predict this value. Both tests rely on total interface contact between relatively small specimens. Friction angle results can vary upwards of 10° between test methods. Where low factors of safety (e.g. 1.5 or less) are used for design, this difference becomes critical. Interface friction angles should be determined during design using actual site construction materials. However, Government construction contracts generally prohibit specifying sole source products. Therefore, actual construction materials are unknown prior to the start of construction. Consequently, designers must rely on published friction values and apply appropriate factors of safety. In recent C E M R D contracts, the Contractor is required to run laboratory interface friction angle tests on proposed cover materials using a 30.5 × 30.5 cm minimum size direct shear box to verify design assumptions for slopes greater than or equal to 4 (horizontal) or 1 (vertical). Obviously, without standardized test methods, the accuracy and validity of these results is questionable.
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3.3 Rigid-nonrigid connections C E R C L A landfills can and do settle differentially. Therefore, it is critical that connections between rigid and nonrigid components of the landfill be as flexible as possible. A case in point is the connection between relatively rigid vertical gas vent standpipes or monitoring wells and a flexible geomembrane barrier. Typical pipe boot connections are factory or field fabricated using the same geomembrane materials. The bottom skirt is welded to the geomembrane and the upper sleeve is welded or mechanically fastened to the standpipe. In the case of polyethylene, the skirt and sleeve are typically joined at a 90 ° angle by a relatively rigid extrusion weld. Any differential settlement between the standpipe and the surrounding subgrade induces additional stresses at the sleeve-skirt connection and may cause separation at that point. In addition, any downslope movement of the cover soil may cause rippling of the geomembrane upslope of the standpipe and tension in the geomembrane downslope of the standpipe. Excessive movement caused by differential settlement or improper cover soil placement methods can break the standpipe or tear the boot away from the geomembrane or both. For C E R C L A landfills, where the location, rate and magnitude of differential settlements are unknown, additional emphasis is required in designing more flexible connections in critical areas.
4 P R E P A R A T I O N AND P L A C E M E N T
4.1 Subgrade preparation Poor subgrade preparation can jeopardize the physical and functional properties of overlying geosynthetics. Equipment rutting of relatively soft subgrades can lead to additional stresses on the geosynthetics. As the size and weight of geosynthetic rolls increase, the potential for rutting increases with the improper selection of equipment to deploy such rolls. As a result of rutting, the flowpath through the draining system may be interrupted, causing ponding of water in these ruts and subsequent diffusion through the geomembrane. This problem becomes more critical where geonets are placed over surfaces with 3-5% slopes. The magnitude of differential settlement required to interrupt flow through a 4-mm to 7-5-mm thick geonet is considerably less than that required to interrupt flow through a 305-mm thick granular drainage layer. The problem is compounded by relatively high flow rates and potential turbulent flow through geonets. 5
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David L. Jaros
To prevent puncture of overlying geomembranes, all vegetation, rocks larger than 2.54 cm in diameter, and debris should be removed from the surfaces to be covered with the geomembrane. All subgrades should be proof-rolled prior to geomembrane placement, to provide a relatively uniform, smooth subgrade. A181.4-kg minimum weight roller is currently being specified in CEMRD projects.
4.2 Geosynthetic placement Subgrade preparation is a major construction operation for any landfill cover project. This is particularly true where a clay barrier acts as a subgrade for an overlying geomembrane barrier. Protecting the clay barrier from desiccation, erosion, etc., is a major undertaking. It is crucial that overlying geosynthetics be placed as quickly as possible after completion of the subgrade to maintain the integrity of the subgrade. Side slope benches and swales lined with geomembranes can be problem areas, especially for polyethylene materials with relatively high coefficients of thermal expansion. To prevent 'trampolining' over these areas, geomembranes should be placed during the cooler periods of the day or sufficient slack should be provided for the geomembrane to conform to the bench or swale under anticipated temperature extremes. Otherwise, once cover soil placement begins, it may be impossible to eliminate existing trampoline areas without cutting the geomembrane and splicing in additional material to provide the necessary slack. Where necessary, all splicing should be located away from the bench or swale to reduce the number of seams within these areas and thus minimize the potential for leaks at those seams.
4.3 Cover soil placement The construction method used to place cover soil over a geosynthetic cap system is critical to the survivability of those geosynthetics. Construction equipment may induce larger dynamic stresses in the geosynthetics during placement than the gravity stresses induced by the final cover material. Every site is unique, thus requiring detailed plans and specifications, competent full-time field inspection, and an experienced Contractor. With this in mind the following guidelines are offered: ----Cover soils should be placed when the geosynthetics are fully contracted (i.e. during cooler periods of the day) to prevent excessive thermal stresses in the geosynthetics. This is more critical for
C E R C L A landfill closures: construction considerations
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polyethylene products which have a relatively high coefficient of thermal expansion. ---Cover soils should be advanced from the bottom of a slope to the top to prevent inducing unnecessary dynamic stresses in the geosynthetics resulting from the downhill movement of heavy earth-moving equipment. The geosynthetics must be anchored at the top of the slope during this placement operation to prevent a potential slope stability failure. ---Cover soils should be placed in relatively thin lifts (e.g. 20--30 cm) and should never be stockpiled in large berms over the geosynthetic cap system. Berm heights less than the final cover soil thickness should be specified. - - D e p e n d i n g on the cover soil particle size and shape, the placement drop height may have to be limited. --Strength-oriented compaction criteria for cover soil is unnecessary and could damage the underlying geosynthetics. Cover soil placement specifications should be written to minimize the compactive effort required to produce a stable vegetative layer. --Exposed geosynthetic cap components should be covered as quickly as possible to reduce the potential for damage from ultraviolet (UV) radiation, wind, temperature extremes, ongoing construction activities, etc. 4.4 Test sections
Test sections are field models used to verify the adequacy of the materials, design, equipment and construction procedures proposed for the landfill cover system. Prior to full-scale production, at least one test section should be constructed duplicating the entire cross-section of the landfill above the waste. The actual subgrade soil, geosynthetics, cover soil, and construction equipment proposed for the full-scale cover should be used for constructing all test sections. Test sections should be constructed on the landfill in an area of maximum slope and in an area of maximum anticipated settlement to simulate worst case placement conditions. Test sections should be at least four times wider than the widest piece of construction equipment and long enough to allow compaction equipment to reach operating speed before reaching the area within the test fill that will be used for testing. 6 A portion of the cover soil (e.g. 3 x 3 m) should be removed after completion of the test section to assess the condition of the geosynthetics. Depending on the findings, placement methods may require modifications before being approved for full-scale production.
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5 H A N D L I N G OF GEOSYNTHETICS
5.1 Storage On large C E R C L A projects, geosynthetics may be stored on-site for 12 months or longer. To prevent degradation of materials by environmental forces or damage from improper construction practices, appropriate storage facilities and protective measures are necessary. Geotextiles must be covered to prevent UV degradation as well as contamination by foreign materials (e.g. windblown soil). Some geomembrane types (e.g. PVC) require a UV-resistant cover and all geomembranes should be stored in areas away from extreme heat or cold to prevent reduction of physical properties. Most geonets and geogrids are made of polyethylene and can withstand considerable UV exposure. However, to prevent contamination by foreign material, all rolls of geonet and geogrid material should be covered during storage. In addition, all geosynthetics should be stored on a firm subgrade (preferably not soil) which is free draining and provides adequate airflow around all materials.
5.2 Seaming Seaming methods vary considerably depending on the type of material and its intended function. 7 Geomembranes are generally seamed using thermal or solvent methods. Geotextiles are overlapped, sewn together, or heat seamed depending on the intended function. Geonets are generally overlapped and fastened together with plastic ties. Geogrids are overlapped or joined by mechanical fasteners. Considering the variety of methods and equipment required for connecting geosynthetics, the following guidelines are offered: - - W h e r e a relatively thin geomembrane (i.e. 0.75 mm or less) overlies a geonet, a sacrificial separation medium (e.g. geomembrane strip) may be necessary between the geomembrane and geonet to prevent the fusion of the two materials during extrusion welding. - - M e t a l staples should never be used to tack a geotextile or geonet to the subgrade prior to deployment of an overlying geomembrane. Some metal staples will inevitably end up puncturing the geomembrane. --Seaming of geomembranes should never be conducted in the presence of standing water. Prior to and during seaming, the seam area should be clean and free of moisture, dust, or any other foreign material.
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---Grinding marks resulting from preparation of polyethylene geomembrane extrusion seam areas should be oriented perpendicular to the seam direction. No marks should appear beyond 6.4 mm of the extrudate after placement. The depth of the grinding marks should be no greater than 10% of the sheet thickness. 8 --Temperature gradients across exposed geomembranes may cause condensation to form and collect on the bottom of the geomembrane. Where a highly plastic clay subgrade interface exists, excess moisture can produce a wet sticky clay which bonds to the underside of the geomembrane, resulting in seaming problems with some types of wedge welding equipment. Geomembranes should, therefore, be seamed immediately after being deployed to prevent these seaming problems. Current CEMRD specifications for landfills require wedge welding as the primary seaming method for polyethylene geomembranes. In the author's opinion, this type of seaming method provides a more consistent seam over a wider range of site conditions. Operator sensitivity is removed and non-destructive testing is much easier where the dual wedge weld is used. However, this method does not remove the need for extrusion welds at patch areas, 'T' seams, and appurtenances.
6 QUALITY CONTROL/QUALITY ASSURANCE Many of the potential problems previously addressed can be circumvented by implementation of a good construction quality control (CQC) and construction quality assurance (CQA) program. 6'9 To date, CQA plans written for CERCLA landfill covers being constructed with USACE construction oversight, lack specific requirements for geosynthetics. More reliance has been placed on the Contractor's CQC plan rather than on developing and implementing a good Government CQA plan. On recent CEMRD projects, specifications have been written to include some CQA measures. For instance, a qualified third party inspector, who is independent from the manufacturer, fabricator, and installer, is now being required for overseeing activities related to the quality of the geomembrane from manufacturing through installation. More stringent deployment, seaming and testing requirements are being specified for all geosynthetics. Yet, even with these measures, it is apparent that better CQA plans are necessary and proper geosynthetic training is long overdue for construction inspectors.
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REFERENCES 1. Public Law 96-510, The Comprehensive Environmental Response, Compensation, and Liability Act of 1980. 2. Public Law 99-499, The Superfund Amendments and Reauthorization Act of 1986. 42 USC Sect. 9601 et seq. 3. US Environmental Protection Agency. Minimum Technology Guidance for Final Covers on Hazardous Waste Landfills and Surface Impoundments, EPA/530-SW-89-047. Office of Research and Development, Cincinnati, Ohio, 1989. 4. US Environmental Protection Agency. Requirements for Hazardous Waste Landfill Design, Construction and Closure, EPA/625/4-89/022. Office of Research and Development, Cincinnati, Ohio, 1989. 5. Koerner, R. M., Designing With Geosynthetics, Prentice Hall, Englewood Cliffs, New Jersey, 1990. 6. US Environmental Protection Agency. Construction Quality Assurance for Hazardous Waste Land Disposal Facilities, EPA/530-SW-86-031. Office of Research and Development, Cincinnati, Ohio, 1986. 7. The Seaming of Geosynthetics, Proceedings of the 3rd GRI Seminar, Papers presented at Geosynthetic Research Institute, Drexel University, Philadelphia, Pennsylvania, 14-15 December, 1989. 8. The Fabrication of Polyethylene FML Field Seams, EPA/530/SW-89/069 (1989). EPA, Office of Research and Development, Cincinnati, Ohio, 1989. 9. US Environmental Protection Agency. Covers For Uncontrolled Hazardous Waste Sites, Report No. EPA/540/2-85/002. Office of Research and Development, Cincinnati, Ohio, 1985.
C E R C L A Landfill Closures: Construction Considerations* David L. Jaros US Army Corps of Engineers, Missouri River Division, PO Box 103, Downtown Station, Omaha, Nebraska 68101, USA
ABSTRACT A properly functioning cover over an uncontrolled landfill which is regulated under the Comprehensive Environmental Response Compensation and Liability Act (CERCLA) depends on both a well-engineered cover design and prudent construction practices. This paper focuses on several geosynthetic construction issues which ultimately impact the performance of the final constructed cover. Preparation and placement issues related to subgrade preparation, geosynthetic placement, cover soil placement and test sections are presented. Storage and seaming of the geosynthetics are discussed. Construction quality control and quality assurance programs are emphasized. In addition, relevant design issues pertaining to settlements, interface friction, and rigid-nonrigid connections are discussed. Although these issues apply to all landfills, this paper is based on experience gained from design, review and construction oversight of CERCLA landfill covers by the US Army Corps of Engineers.
1 INTRODUCTION A cover over an uncontrolled hazardous waste landfill functions as a roof. It p r o m o t e s controlled runoff, minimizes percolation and leachate formation, and thus prevents or minimizes future ground-water contamination. Geosynthetics play a key role in assuring these objectives are met. "The views expressed in this paper are solely those of the author and not the US Army Corps of Engineers. 393 Geotextiles and Geomembranes 0266-1144/91/$03.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain
394
David L. Jaros
The major emphasis of this paper is on construction-related geosynthetic issues pertaining to past and present US Army Corps of Engineers (USACE) landfill projects. Pertinent design issues that have a major impact on construction are also presented.
2 CERCLA
2.1 Regulation The Comprehensive Environmental Response Compensation and Liability Act (CERCLA or SUPERFUND) was originally enacted by Congress in 1980.1 The legislation was amended in 1986 by the Superfund Amendments and Reauthorization Act (SARA).2 The primary purpose of CERCLA and SARA is to provide funding and enforcement for EPA to clean up the thousands of hazardous waste sites created in the United States by unregulated practices and to respond to hazardous substance spills. The CERCLA legislation allowed for the use of existing capabilities within other Federal agencies to meet its objectives. The EPA and USACE signed an interagency agreement (lAG) on 3 February, 1982. Under the agreement, upon EPA request, USACE manages design and construction contracts and provides technical assistance to EPA in support of remedial clean-up of hazardous waste sites. An lAG between EPA and USACE was signed on 3 December, 1984, which extends the partnership indefinitely.
2.2 Cover requirements Currently there are no specific EPA regulations pertaining to design of final covers on CERCLA landfills. 3 Generally, EPA final cover Minimum Technology Guidance for landfills regulated under the Resource Conservation and Recovery Act (RCRA) is applied to CERCLA designs. Consequently, the three-layer RCRA cap system consisting of a vegetative top layer, middle drainage layer and a single or composite bottom liner is common CERCLA design practice.
2.3 Projects To date, the Corps of Engineers' Missouri River Division (CEMRD) has been involved in ten major CERCLA landfill cover projects. Eight are constructed or presently under construction with the remaining two in the
CERCLA landfill closures: construction considerations
395
design phase. High or very low density polyethylene geomembranes have or will be used where a geomembrane barrier layer is required to meet the intent of the EPA's Record of Decision. Geotextiles have been used for separation layers. Geonets have been used for drainage layers and gas transmission layers where necessary. To date, no geogrids or geocomposites have been used.
3 DESIGN CONSIDERATIONS 3.1 Settlement
Landfills settle with time due to a variety of factors including waste composition, landfill density, landfill age, landfill operation and landfill thickness. Unlike controlled R C R A landfills, most of these factors are unknown at uncontrolled C E R C L A landfills. To date, very limited field investigations have been conducted at C E R C L A sites to provide reliable estimates of potential settlement quantities or rates. Difficulties in drilling and sampling heterogeneous landfill masses and health and safety concerns are but a few reasons why such in-depth investigations are not conducted. Differential settlements causing localized stress concentrations are a much greater threat than areal subsidence which generally imposes only low compressive stresses on the cover system. With settlements upwards of 50% of the landfill thickness at C E R C L A landfills as compared to 1.5% at R C R A landfills, the potential for large localized settlements becomes a major design issue and subsequent construction problem. 4 Differential settlements result in cover geosynthetics being stressed biaxially. However, most C E R C L A cover designs to date, have not, in the author's opinion, addressed differential settlements properly. Uniaxial stress-strain relationships provided by manufacturer's index tests have been used in lieu of more realistic biaxial stress-strain relationships provided by design-oriented tests. Although biaxial tests are not yet standardized by ASTM, one method is available from the Geosynthetics Research Institute (GRI) as GRI Standard GM-4. Differential settlements can be detrimental to all cover system components. Depending on the type of geomembrane, the parent material may perform well under large uniaxial strains but fail in tension under much lower biaxial strains. For example, high density polyethylene (HDPE) is more sensitive to biaxial strains than very low density polyethylene (VLDPE) or polyvinyl chloride (PVC) materials. This problem is compounded in seamed areas where stress conditions are more
396
David L. Jaros
complex and variable. Unfortunately, little emphasis has been placed on simulating differential settlements at seamed areas in the laboratory. Yet, field seams are considered the weak link in any geosynthetic system. Differential settlements may distort geonets sufficiently to impede their flow properties resulting in localized ponding. Overlying geotextiles may tear or separate at seams, allowing cover soil migration into the underlying drainage layer resulting in eventual clogging. Clay barriers may crack and become ineffective barriers. Obviously, it is critical that cover systems be designed to handle anticipated differential settlements. Likewise, appropriate designoriented laboratory tests are absolutely necessary when using a design-by-function approach. 5 3.2 Interface friction
Ideally, 100% contact between adjacent cap components is desirable to provide maximum frictional resistance against sliding along interfaces. However, due to subgrade irregularities, differential settlement, relatively low normal stresses, discontinuities at joints, material properties and environmental factors, total interface contact throughout a cover system is impossible. To simplify calculations, static-type interface stability analyses assume 100% interface contact between adjacent materials along uniform slopes. 4 The interface friction angle used in these analyses depends on both the material type and the test method used. To date, there is no consensus on a standard test method for determining this friction angle. Typically, geotechnical-oriented direct shear tests or tilt table tests are used to predict this value. Both tests rely on total interface contact between relatively small specimens. Friction angle results can vary upwards of 10° between test methods. Where low factors of safety (e.g. 1.5 or less) are used for design, this difference becomes critical. Interface friction angles should be determined during design using actual site construction materials. However, Government construction contracts generally prohibit specifying sole source products. Therefore, actual construction materials are unknown prior to the start of construction. Consequently, designers must rely on published friction values and apply appropriate factors of safety. In recent C E M R D contracts, the Contractor is required to run laboratory interface friction angle tests on proposed cover materials using a 30.5 × 30.5 cm minimum size direct shear box to verify design assumptions for slopes greater than or equal to 4 (horizontal) or 1 (vertical). Obviously, without standardized test methods, the accuracy and validity of these results is questionable.
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3.3 Rigid-nonrigid connections C E R C L A landfills can and do settle differentially. Therefore, it is critical that connections between rigid and nonrigid components of the landfill be as flexible as possible. A case in point is the connection between relatively rigid vertical gas vent standpipes or monitoring wells and a flexible geomembrane barrier. Typical pipe boot connections are factory or field fabricated using the same geomembrane materials. The bottom skirt is welded to the geomembrane and the upper sleeve is welded or mechanically fastened to the standpipe. In the case of polyethylene, the skirt and sleeve are typically joined at a 90 ° angle by a relatively rigid extrusion weld. Any differential settlement between the standpipe and the surrounding subgrade induces additional stresses at the sleeve-skirt connection and may cause separation at that point. In addition, any downslope movement of the cover soil may cause rippling of the geomembrane upslope of the standpipe and tension in the geomembrane downslope of the standpipe. Excessive movement caused by differential settlement or improper cover soil placement methods can break the standpipe or tear the boot away from the geomembrane or both. For C E R C L A landfills, where the location, rate and magnitude of differential settlements are unknown, additional emphasis is required in designing more flexible connections in critical areas.
4 P R E P A R A T I O N AND P L A C E M E N T
4.1 Subgrade preparation Poor subgrade preparation can jeopardize the physical and functional properties of overlying geosynthetics. Equipment rutting of relatively soft subgrades can lead to additional stresses on the geosynthetics. As the size and weight of geosynthetic rolls increase, the potential for rutting increases with the improper selection of equipment to deploy such rolls. As a result of rutting, the flowpath through the draining system may be interrupted, causing ponding of water in these ruts and subsequent diffusion through the geomembrane. This problem becomes more critical where geonets are placed over surfaces with 3-5% slopes. The magnitude of differential settlement required to interrupt flow through a 4-mm to 7-5-mm thick geonet is considerably less than that required to interrupt flow through a 305-mm thick granular drainage layer. The problem is compounded by relatively high flow rates and potential turbulent flow through geonets. 5
398
David L. Jaros
To prevent puncture of overlying geomembranes, all vegetation, rocks larger than 2.54 cm in diameter, and debris should be removed from the surfaces to be covered with the geomembrane. All subgrades should be proof-rolled prior to geomembrane placement, to provide a relatively uniform, smooth subgrade. A181.4-kg minimum weight roller is currently being specified in CEMRD projects.
4.2 Geosynthetic placement Subgrade preparation is a major construction operation for any landfill cover project. This is particularly true where a clay barrier acts as a subgrade for an overlying geomembrane barrier. Protecting the clay barrier from desiccation, erosion, etc., is a major undertaking. It is crucial that overlying geosynthetics be placed as quickly as possible after completion of the subgrade to maintain the integrity of the subgrade. Side slope benches and swales lined with geomembranes can be problem areas, especially for polyethylene materials with relatively high coefficients of thermal expansion. To prevent 'trampolining' over these areas, geomembranes should be placed during the cooler periods of the day or sufficient slack should be provided for the geomembrane to conform to the bench or swale under anticipated temperature extremes. Otherwise, once cover soil placement begins, it may be impossible to eliminate existing trampoline areas without cutting the geomembrane and splicing in additional material to provide the necessary slack. Where necessary, all splicing should be located away from the bench or swale to reduce the number of seams within these areas and thus minimize the potential for leaks at those seams.
4.3 Cover soil placement The construction method used to place cover soil over a geosynthetic cap system is critical to the survivability of those geosynthetics. Construction equipment may induce larger dynamic stresses in the geosynthetics during placement than the gravity stresses induced by the final cover material. Every site is unique, thus requiring detailed plans and specifications, competent full-time field inspection, and an experienced Contractor. With this in mind the following guidelines are offered: ----Cover soils should be placed when the geosynthetics are fully contracted (i.e. during cooler periods of the day) to prevent excessive thermal stresses in the geosynthetics. This is more critical for
C E R C L A landfill closures: construction considerations
399
polyethylene products which have a relatively high coefficient of thermal expansion. ---Cover soils should be advanced from the bottom of a slope to the top to prevent inducing unnecessary dynamic stresses in the geosynthetics resulting from the downhill movement of heavy earth-moving equipment. The geosynthetics must be anchored at the top of the slope during this placement operation to prevent a potential slope stability failure. ---Cover soils should be placed in relatively thin lifts (e.g. 20--30 cm) and should never be stockpiled in large berms over the geosynthetic cap system. Berm heights less than the final cover soil thickness should be specified. - - D e p e n d i n g on the cover soil particle size and shape, the placement drop height may have to be limited. --Strength-oriented compaction criteria for cover soil is unnecessary and could damage the underlying geosynthetics. Cover soil placement specifications should be written to minimize the compactive effort required to produce a stable vegetative layer. --Exposed geosynthetic cap components should be covered as quickly as possible to reduce the potential for damage from ultraviolet (UV) radiation, wind, temperature extremes, ongoing construction activities, etc. 4.4 Test sections
Test sections are field models used to verify the adequacy of the materials, design, equipment and construction procedures proposed for the landfill cover system. Prior to full-scale production, at least one test section should be constructed duplicating the entire cross-section of the landfill above the waste. The actual subgrade soil, geosynthetics, cover soil, and construction equipment proposed for the full-scale cover should be used for constructing all test sections. Test sections should be constructed on the landfill in an area of maximum slope and in an area of maximum anticipated settlement to simulate worst case placement conditions. Test sections should be at least four times wider than the widest piece of construction equipment and long enough to allow compaction equipment to reach operating speed before reaching the area within the test fill that will be used for testing. 6 A portion of the cover soil (e.g. 3 x 3 m) should be removed after completion of the test section to assess the condition of the geosynthetics. Depending on the findings, placement methods may require modifications before being approved for full-scale production.
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David L. Jaros
5 H A N D L I N G OF GEOSYNTHETICS
5.1 Storage On large C E R C L A projects, geosynthetics may be stored on-site for 12 months or longer. To prevent degradation of materials by environmental forces or damage from improper construction practices, appropriate storage facilities and protective measures are necessary. Geotextiles must be covered to prevent UV degradation as well as contamination by foreign materials (e.g. windblown soil). Some geomembrane types (e.g. PVC) require a UV-resistant cover and all geomembranes should be stored in areas away from extreme heat or cold to prevent reduction of physical properties. Most geonets and geogrids are made of polyethylene and can withstand considerable UV exposure. However, to prevent contamination by foreign material, all rolls of geonet and geogrid material should be covered during storage. In addition, all geosynthetics should be stored on a firm subgrade (preferably not soil) which is free draining and provides adequate airflow around all materials.
5.2 Seaming Seaming methods vary considerably depending on the type of material and its intended function. 7 Geomembranes are generally seamed using thermal or solvent methods. Geotextiles are overlapped, sewn together, or heat seamed depending on the intended function. Geonets are generally overlapped and fastened together with plastic ties. Geogrids are overlapped or joined by mechanical fasteners. Considering the variety of methods and equipment required for connecting geosynthetics, the following guidelines are offered: - - W h e r e a relatively thin geomembrane (i.e. 0.75 mm or less) overlies a geonet, a sacrificial separation medium (e.g. geomembrane strip) may be necessary between the geomembrane and geonet to prevent the fusion of the two materials during extrusion welding. - - M e t a l staples should never be used to tack a geotextile or geonet to the subgrade prior to deployment of an overlying geomembrane. Some metal staples will inevitably end up puncturing the geomembrane. --Seaming of geomembranes should never be conducted in the presence of standing water. Prior to and during seaming, the seam area should be clean and free of moisture, dust, or any other foreign material.
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---Grinding marks resulting from preparation of polyethylene geomembrane extrusion seam areas should be oriented perpendicular to the seam direction. No marks should appear beyond 6.4 mm of the extrudate after placement. The depth of the grinding marks should be no greater than 10% of the sheet thickness. 8 --Temperature gradients across exposed geomembranes may cause condensation to form and collect on the bottom of the geomembrane. Where a highly plastic clay subgrade interface exists, excess moisture can produce a wet sticky clay which bonds to the underside of the geomembrane, resulting in seaming problems with some types of wedge welding equipment. Geomembranes should, therefore, be seamed immediately after being deployed to prevent these seaming problems. Current CEMRD specifications for landfills require wedge welding as the primary seaming method for polyethylene geomembranes. In the author's opinion, this type of seaming method provides a more consistent seam over a wider range of site conditions. Operator sensitivity is removed and non-destructive testing is much easier where the dual wedge weld is used. However, this method does not remove the need for extrusion welds at patch areas, 'T' seams, and appurtenances.
6 QUALITY CONTROL/QUALITY ASSURANCE Many of the potential problems previously addressed can be circumvented by implementation of a good construction quality control (CQC) and construction quality assurance (CQA) program. 6'9 To date, CQA plans written for CERCLA landfill covers being constructed with USACE construction oversight, lack specific requirements for geosynthetics. More reliance has been placed on the Contractor's CQC plan rather than on developing and implementing a good Government CQA plan. On recent CEMRD projects, specifications have been written to include some CQA measures. For instance, a qualified third party inspector, who is independent from the manufacturer, fabricator, and installer, is now being required for overseeing activities related to the quality of the geomembrane from manufacturing through installation. More stringent deployment, seaming and testing requirements are being specified for all geosynthetics. Yet, even with these measures, it is apparent that better CQA plans are necessary and proper geosynthetic training is long overdue for construction inspectors.
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David L. Jaros
REFERENCES 1. Public Law 96-510, The Comprehensive Environmental Response, Compensation, and Liability Act of 1980. 2. Public Law 99-499, The Superfund Amendments and Reauthorization Act of 1986. 42 USC Sect. 9601 et seq. 3. US Environmental Protection Agency. Minimum Technology Guidance for Final Covers on Hazardous Waste Landfills and Surface Impoundments, EPA/530-SW-89-047. Office of Research and Development, Cincinnati, Ohio, 1989. 4. US Environmental Protection Agency. Requirements for Hazardous Waste Landfill Design, Construction and Closure, EPA/625/4-89/022. Office of Research and Development, Cincinnati, Ohio, 1989. 5. Koerner, R. M., Designing With Geosynthetics, Prentice Hall, Englewood Cliffs, New Jersey, 1990. 6. US Environmental Protection Agency. Construction Quality Assurance for Hazardous Waste Land Disposal Facilities, EPA/530-SW-86-031. Office of Research and Development, Cincinnati, Ohio, 1986. 7. The Seaming of Geosynthetics, Proceedings of the 3rd GRI Seminar, Papers presented at Geosynthetic Research Institute, Drexel University, Philadelphia, Pennsylvania, 14-15 December, 1989. 8. The Fabrication of Polyethylene FML Field Seams, EPA/530/SW-89/069 (1989). EPA, Office of Research and Development, Cincinnati, Ohio, 1989. 9. US Environmental Protection Agency. Covers For Uncontrolled Hazardous Waste Sites, Report No. EPA/540/2-85/002. Office of Research and Development, Cincinnati, Ohio, 1985.