PVC GEOMEMBRANES IN MUNICIPAL WASTE LANDFILL LINERS AND COVERS: THE FACTS
I. D. Peggs, Ph.D., P.Eng.
I-CORP INTERNATIONAL
INTRODUCTION
"The liner system shall consist of the following:
i. a leachate collection and removal system designed
ii. a 3-foot recompacted clay liner
iii. a leak detection system designed
iv. a geomembrane liner at least 60 mil thick."
This is a rule in the draft of one state's Solid Waste Regulations. The impression is left that the leachate and leak systems are designed but that as long as a 60 mil geomembrane (of any type) is used, it will be adequate. Unfortunately, as experience has shown, geomembranes cannot be so simply regulated.
The selection and design of the geomembrane components of landfill lining and cover systems are not simple matters if optimum durability is required. A designer cannot blindly take any material of a predetermined regulated (implying satisfactory) minimum thickness "off the shelf" and apply it for all systems, on all slopes, on all subgrades, with all leachates, in all environments, and for all landfill operating procedures. If such a practice is followed, and often it is, the liner is not 'designed' and will probably fail before its intended service life. And there have been a- significant number of geomembrane failures in landfills and liquid impoundments, mostly the latter.
There is not one universally acceptable geomembrane material because all materials have their Achilles Heel - it is simply a matter of recognizing the negative feature of each material and designing around it, to take advantage of the positive aspects of each material. This is why geomembrane lining systems need to be competently designed in the first place. Subsequently, they must then be reviewed and approved by informed regulatory design engineers' Geomembrane thickness is only one of the integral parameters in the design process and should not be arbitrarily legislated at one value to cover all potential materials. The flexibility to properly design a given system is essential if optimum performance is to be achieved, and if advances in materials are to be taken advantage of.
My objective in this document is to show that polyvinyl chloride (PVC), in its variant forms, has a large number of very desirable properties and, therefore, is an excellent candidate material for consideration in all lining systems. However, that does not mean that it is a suitable material to use in all applications.
Similarly, HDPE cannot arbitrarily be used in all lining systems. I am in the midst of a failure analysis of an HDPE linen, system that failed after 18 months use despite there being a written guarantee from the manufacturer that the material was chemically compatible with the impoundment contents. I have a second failure that occurred after 5 years in a plant that produced the HDPE resin from which the geomembrane was manufactured to line its own waste facilities. In this latter case, the resin manufacturers were not aware that their resin was inadequate for geomembrane applications.
There is a general feeling at the grass roots level of the regulatory and designing arenas that HDPE is the geomembrane of choice and will meet all waste containment performance requirements. It will not do that, and those who have had failures certainly would not agree with that feeling I believe that the extreme coafidence that the geomembraae users place in HDPE is as unjustified as the lack of confidence they place in other materials such as PVC. This is somewhat strange since PVC has, apparently, been so successfully used for waste containment for over 36 years. I would like to dispel the myths about PVC so that it will come out of the starting gate being considered by competent designers on its technical merits (and demerits) only - not with preconceived emotional feelings.
The regulatory agencies must play a large part in this procedure by positively requiring comprehensive design processes, and not by imposing restrictions that unintentionally discourage necessary design efforts. Certainly it may be necessary to define the 'entry level', or to make the 'initial cut', but this must not be done in such a way that the intended minimum criteria be perceived as the only criteria that need to be met. And the minimum criteria should not be written in such a way that, unintentionally, adequate candidate materials, both old and new, are eliminated.
There are two groups of people and institutions that always have, and always will lose their battles: those who stick will the old, ignoring new developments, and those who blindly jump to the new, not fully recognizing the problems of the new (see Figure 1). Those who remain flexible and take advantage of both will be the winners. The PVC geomembrane manufacturers are attempting to recover from the former. Those who have had HDPE failures are suffering from the latter.
As one considers PVC and HDPE (and other) geomembranes, one must remember that there are many types and grades of each of them. Saying 'HDPE' and 'PVC' is like saying 'steel': there are many different types of steels (ferritic, austenitic, martensitic, and others) formulated to meet different service conditions. There are many PVC geomembranes formulated to meet specific environmental applications. HDPEs also are different but, at the present stage of geomembrane development, they are different not so much by design but by default, due to the different resin types. 'Me factors responsible for the widely variable fundamental stress cracking resistance (mechanical durability) of HDPE are not yet fully understood, and, therefore, cannot be intentionally engineered. It, therefore, follows that just because a failure has occurred in a PVC or HDPE geomembrane, it does not automatically mean that all other PVC and HDPE products will fall under the same circumstances.
Except for a relatively small number of failures that have occurred due to the selection of inappropriate materials,, the majority of geomembrane failures have occurred due to inadequate design, poor installation workmanship, or inattentive Construction Quality Assurance (CQA). There is no question that, to prevent failures, the emphasis must be placed on proper design and CQA. And this, most certainly, includes proper design after the correct material has been selected.
PVC PERFORMANCE
To dismiss some of the perceived concerns of PVC, the following, generally-expressed myths about PVC are addressed:
- It contains pinholes and therefore a lining cannot be made leak-free.
- It does not have adequate chemical resistance.
- It does not have adequate weathering resistance to ultraviolet (UV) and thermal radiation.
- It becomes brittle at low temperatures.
- Plasticizers leach out, or volatilize, causing, unacceptable material degradation.
- The CQA associated with PVC is inadequate.
In addressing these misconceptions, one should keep in mind that the major factor is whether the material continues to function as intended. The fact that some changes occur is irrelevant unless they affect the functional performance of the geomembrane. In other words, different materials may change at different rates as they age, but both could still provide in excess of the required performaace (Figure 2). The fact that one changes more than another is irrelevant
Pinholes
Since a geomembrane is intended to be in impermeable barrier, pinholes are not desirable features. However, no geomembrane is absolutely impermeable, a fact that is difficult for -any of us to accept, but one that is the basis for our acceptance of the need to install double lining systems.
In all CQA plans, project specifications, and regulatory aspects, it is necessary to consider that ideals may not be practically achievable and that real world (practical) situations inevitably require some compromise.
PVC sheeting has been made in thin gages (as low as 4 mil) for vapor barriers, swimming pool liners, and critical medical applications for many years without pinholes. With continuous inplant backlighting QC techniques for continuous examination of manufactured geomembranes and more detailed examination under stress of QC samples, geomembranes with pinholes should not appear on site. There are ASTM standards(e.g.D4451)and Federal Specifications(e.g. L-P-375C) that demand that PVC membranes in thicknesses greater than 10 mil contain absolutely no pinholes. Alberta Environment(1) has used many millions of square feet of 20 mil PVC geomembrane for lining irrigation canals and, several years ago, had a specification of 1 pinhole(maximum) per 100 ft2 of geomembrane. After thorough examination of a large amount of geomembrane, they concluded that pinholes were statistically nonexistent. Improved methods of calendering have effectively eliminated pinholes in PVC sheet and geomembranes: the geomembrane passes through 3 or 4 sets of caleadering rolls (Figure 3) in which semi-molten material is squeezed to fill any existing, small voids.
The US Bureau of Reclamation has also used immense areas of PVC geomembrane to line irrigation canals and reservoirs and has had a similar experience with (the lack of) pinholes in PVC geomembrane. Many years ago (1960's), they experienced a few pinholes in 10 mil material, but after changing to 20 mil geomembrane, they found it unnecessary to have a pinhole specification. They have done a large amount of laboratory testing with 20 mil PVC, such as hydrostatic burst and puncture testing, and have never had a premature test failure due to pinholes. They have also used 20 mil PVC, in 5 x 10 ft panels, to provide critical water proofing in other laboratory tests, without premature failures.
It is inappropriate, and incorrect to propagate the myth that PVC contains pinholes and HDPE does not. In a recent CQA project, I-CORP found a 60 ft. length of installed 60 @l textured HDPE geomembrane that contained about 10 holes up to 80 mil in diameter. This, however, was a remote occurrence, just as it would be to see pinholes in PVC geomembranes.
PVC, when used for medical collection bags (solids and liquids), and internal feeding and plasma bags must be, and is, totally free of pin holes.
Chemical Resistance
For landfill applications, the chemical resistance of the proposed geomembrane to the leachate is assessed by EPA Method 9090 "Compatibility Test for Waste and Membrane Liners"(3). In this test, the geomembrane is exposed to leachate at 23 and 50° C for 120 days. Changes in properties are measured every 30 days. If there is no continuing degradation trend, or if changes reach an equilibrium condition within certain limits, the geomembrane is considered to be compatible with the leachate.
As previously stated, for practical purposes, it is immaterial whether one of two types of the same material, or one of two different materials, ages more than the other if both still, and will continue to, adequately meet the project specifications.
There is no question that for an given chemical containment requirement, there is more chance y .
that HDPE (but not all HDPE's), rather than PVC, will provide the better absolute chemical resistance. However, for most municipal waste leachate applications, it is probable that PVC may provide more than adequate resistance for the required service. The historical performance of PVC geomembrane has proven this. Certainly, there have been a few well-publicized failures, as with HDPE, but there have been many more successes and a large number of successful laboratory test programs. It would be an unreasonable trade-off to use a fundamentally more chemically resistant material if other performance characteristics and durability are sacrificed. In preparing for this report, a survey of lining system designers and installers in North America and Europe was performed, asking the questions shown in Appendix 1. (Responses are also included in Appendix 1.) Most of the installers surveyed felt that PVC was much easier to install correctly than HDPE. A European respondent stated, appropriately and realistically: 'PVC is not suitable for ALL chemical contacts, but PVC is one of the best materials for the ratio price/properties, mechanical properties, weldability, and permanent elongation.' In other words, the selection of the most appropriate geomembrane becomes a project-specific decision.
Three laboratories that perform EPA Method 9090 testing were surveyed for this project: GeoSyntec Consultants, Precision Laboratories, and Texas Research Institute. All have performed, or are presently performing, tests on PVC in hazardous and municipal waste leachates. None of the PVC geomembranes has failed the test. One of the tests in hazardous leachate his been performed at 85'C and, even though there has been some loss of plasticizer, the PVC geomembrane successfully passed the test. This confirms that it is extremely important to recognize that, even though some changes in properties occur, the material will still provide satisfactory service. It should be remembered that PVC can be formulated to provide protection against specific environments, e. oil-resistant PVC.
It is clear that to obtain the optimum combination of material and performance, it is essential to assess the site-specific leachate resistance of each material and its seams. Semi-crystalline materials, such as HDPE, should, in addition, be tested under stress in the leachate to assess their stress cracking resistances.
Chemical resistance work, presently being performed in France by Artieres(4), includes the exposure of one HDPE and two PVC geomembranes to two different municipal solid waste (MSW) leachates. While this study provides an opportunity to directly compare the performances of PVC and HDPE in MSW leachates, it again must be remembered that only two PVCs and one HDPE of the many variants that exist have been assessed. The surface reactions of the exposed specimens were carefully examined by the sophisticated Fourier Transform Infrared (FTIR) technique. After 16 months at 20° C and 3.5 months at 50° C, the surface of the PVC had been affected only to a depth of 80 m m. In the same time period, the HDPE had been affected to a slightly larger depth of 100 m m. There is, therefore, very little difference in the chemical resistance of these PVCs and this HDPE to the two MSW leachates - both materials could probably adequately contain the leachates.
In assessing chemical resistance, the last thing that should be used is a standard chemical compatibility table. Such tables are notoriously inconsistent.
Artieres(4) states:
"But the environmental durability must not be the only choice criterion for a geomembrane. Its long-term mechanical behavior, its aptitude for laying and seaming, its adequation (sic) with the other elements of the tightness system, are also of great importance and must be taken into account."
I have investigated three HDPE liner failures in which the standard chemical resistance tables indicate that the HDPE is compatible with the contents of the ponds (nitric acid and black liquor). In one case, the liner manufacturer had provided a written guarantee that the HDPE would be resistant to the black liquor for 10 years. The black liquor pond liner failed after 12 to 18 months by environmental stress cracking. The nitric acid pond liner failed after 9 months, also by environmental stress cracking. In the latter case, there was even sufficient residual stress in the extruded fillet seam bead to initiate stress cracking in the bead, independently of the service stress on the geomembrane.
Two of the laboratories performing EPA Method 9090 tests commented that MSW leachates are getting weaker, as the wastes placed in landfills become more controlled. The third agreed in principal, but identified two municipal leachates they have tested as containing 'bad actors'. In the recently promulgated Part 258 of 40 CFR 'Criteria for Classification of Solid Waste Disposal Facilities and Practice' (colloquially known as Subtitle D), there are indications (pp 24 and 25) that there is little difference in the toxic constituents of leachates generated in true municipal waste landfills (built since 1980) and those operated prior to 1980 that contained industrial wastes in addition to municipal wastes. It is possible, therefore, that municipal landfills that accept industrial wastes may not have significantly more obnoxious leachates than those that accept municipal waste only. Such co-disposal of municipal and industrial wastes can be used to advantage since there is evidence(5) that it can be done in such a way as to promote the degradation of the municipal waste.
There is a large amount of evidence, not only from EPA Method 9090 testing,, but also from field experience, that PVC is apparently (many installations have not been directly measuring leakage rates) satisfactorily containing municipal waste leachates. For instance, samples of PVC geomembrane removed from the sump of Lycoming County, PA, landfill after exposure to leachate for 11 years are still very flexible and show no visible signs of degradation(6). The few problems that have occurred are, as in most HDPE liner failures, related to inadequate design, poor installation, and/or poor CQA.
In summary, there is no justification for taking the following two approaches to geomembrane chemical resistance: 1) to dismiss PVC out of hand saying it does not have adequate leachate resistance; and 2) to assume that HDPE will perform adequately without appropriately testing it.
Weathering and Thermal Resistance
This is the most confusing area of concern with PVC. In North America, we are adamant that PVC should not be left exposed to the elements; yet in Europe, PVC is quite regularly left exposed on critical installations, such as the upstream faces of hydroelectric dams. In one installation(7), PVC has provided excellent service for more than 12 years at an elevation of over 6000 ft. In Sicily, an exposed PVC roofing membrane has provided a watertight seal at a chemical plant for over 18 years(8).
The US Bureau of Reclamation(9) has investigated 10 mil thick PVC geomembrane that has been installed on irrigation canal side slopes for up to 27 years, and while it has lost some plasticizer, there is still sufficient plasticizer remaining for the geomembrane to have adequate ductility and flexibility for continued service. Once again, the important factor is that material still performs its intended function despite the fact that it has aged. Plasticizers are added in sufficient quantities for the intended service, recognizing that some will be lost in service. Similarly, antioxidant packages are added to HDPE to prevent damaging thermal oxidation during extrusion, seaming, repairing, and service. Just as PVC can lose some (not all) plasticizer during aging, HDPE can age by consumption of antioxidant as it experiences temperatures as high as 80°C or more in summer sunshine. In other words, it is expected that a portion of these additives will be lost during different phases of service, but that fact alone is immaterial provided sufficient additive remains.
Figure 4, developed from the Bureau of Reclamation data(9) shows the rate of plasticizer loss in PVC. Initially, plasticizer is lost rapidly from the surface layers of the geomembrane: 30% of the plasticizer may be lost in the first 4 years of service. As plasticizer begins to diffuse from the interior of the geomembrane, the rate of loss decreases resulting in less than 50% being lost after 19 years. The rate of loss will continue to decrease. These figures were generated on 10 mil PVC geomembrane. The thicker the geomembrane and the lower the surface area to volume ratio, the lower the rate of loss of plasticizer will be. An appropriately formulated 30 mil PVC geomembrane should, therefore, lose no significant amounts of plasticizer in service.
Figure 5 shows that even if 75% of the plasticizer is removed from PVC, causing the geomembrane strength and puncture and tear resistances to increase, and the ductility to decrease, the elongation at break may still exceed 100%. last value is still more than 10 times greater thin the useful strain (the yield strain) allowable in HDPE. In HDPE, once the yield strain has been exceeded the material will continue to elongate at a stress lower than the yield stress.
It has been implied that rats and rodents eat PVC geomembrane because they become addicted to plasticizers. Yet the documents that report this work clearly state(10) that "the undigested debris could be detected in their (the rats) excrement." This does not suggest, as often implied, that the rats eat the PVC for its nutritional (food) value, but rather that they find it easy to gnaw on, as rats must continually do. Rats do gnaw on the edges of HDPE geomembrane and on folded corners with an angle of less than 90° (11), i.e., in regions they can get their teeth into. Elsewhere, the HDPE is too hard and smooth for them to get a bite. Reportedly(10), PVC is not attacked when it does not contain plasticizer; however, without plasticizer, PVC is very hard and may not be possible to bite into. The report itself states: "At the moment, it has not been determined whether the 'addiction' (quotations by Peggs) is of a chemical or tactile nature."
In a related document(12), it is stated: "The polyethylene and polyvinyl chloride membranes used to date in The Netherlands have shown good resistance to such attack. Damage due to rats, wood borers, algae, barnacles, or mussels was not observed . . . " However, consistent with the Dutch findings, a survey of PVC geomembrane users identified only one or two positive cases where holes have been gnawed in liners. Where rodents have penetrated PVC in the field, they have only done it to gain access to the warmth behind the liner, not because they find the plasticizer good to eat. & the other band, HDPE is normally claimed to be inedible by rodents. However, at a power station on one of the Great Lakes, there has been ample evidence, in the form of 1 in. diameter holes, that vole-type rodents do burrow through HDPE. These cases may be the exceptions that prove the rule. They also identify the danger of generic statements, and the resultant possibility of avoiding perfectly adequate materials based on misstatements.
PVC geomembranes are now being successfully heat seamed in the U.S. They have been successfully heat seamed for many years in Europe. If the geomembrane can tolerate seaming temperatures without degrading in service at the seams, it can tolerate the thermal effects of exposure to sunlight for extended service periods, as proven on dams and roofs.
For landfill liners and covers, most weathering/thermal problems are eliminated by the regulatory requirements for covering the liner with various types of soil and other geosynthetic layers. Such layers protect the geomembrane from ultraviolet and severe thermal effects.
Low Temperature Brittleness and Mechanical PropertiesPVC canal liners have been successfully installed during Canadian winters with a maximum brittleness temperature specification of -20° C(ASTM D1790). When deployed, a covering layer of stones is dumped onto the geomembrane from a conveyor belt. This is a severe cold impact test which the geomembrane withstands. National Sanitation Foundation (NSF) International Standard 54 'Flexible Membrane Liners'(13) specifies a maximum brittleness temperature of -29° C for PVC geomembrane. This is even more stringent than the Canadian specifications which have proven satisfactory in harsh service environments. Since most landfill CQA plans require that geomembrane shall not be seamed (thereby meaning 'installed') below 5'C, PVC should be able to withstand most installation environments. Once installed, the soil cover will protect the geomembrane from extremely low temperatures. Even if the temperature does reach the -20° C range under the soil, a PVC geomembrane should continue to provide adequate protection. If the geomembrane is only subject to static loading, it will be protective to much lower temperatures than -30'C, since the brittleness temperature is determined by an impact procedure (ASTM D1790). The measured brittleness temperature of HDPE (ASTM D746) appears to be considerably lower than that for PVC, but once again, is this really of practical significance?
It will depend on the particular installation. It should be noted that PVC is tested by impacting a beat strip, while HDPE is tested by striking a single thickness cantilevered specimen. The PVC test is far more severe and will, therefore, show a higher brittleness temperature. If PVC has other favorable properties, but for some project specific reason must be installed when it is cold, it may be only necessary to take a little extra care during its installation. This is no different to the extra care that is required when installing HDPE to minimize its potential for stress cracking in service.
The majority of designers and installers surveyed for this project agree (Appendix 1) that PVC has better mechanical properties and is easier to install than HDPE. HDPE is a problem because of the yield point in its stress/strain curve (Figure 6) that occurs at approximately 12% strain. This point of instability is of major concern to the designer. When allowing for biaxial stress conditions, as occur in geomembranes in the field, and low temperatures, it is necessary to design for maximum strains in the order of 2 or 3 %. Since PVC does not have a yield point, the designer can make use of strains to the breaking point - in excess of 300% at room temperature under uniaxial conditions. The level of comfort gained by the designer when using a material that has a steadily changing stress/strain curve is significant; the designer is able to give a facility owner an installation with a larger factor of safety and therefore, a wider range of operating conditions. However, once again, mechanical properties are not the only factors that should be considered in liner/cover design. If the subgrade is not subject to settlement, tensile properties may be of less significance. If slopes are steep, and friction angles are high on one side of the geomembrane and low on the other, mechanical properties could be extremely important.
Quality Control and Construction Quality Assurance
There is a feeling among regulators and users that the qualities of HDPE geomembranes and geomembrane seams are higher than those of PVC since the HDPE industry has been required to provide more visible QC documentation and CQA documentation. Comprehensive CQA is absolutely essential for HDPE, particularly in locations where it can be very cold, since its service performance is critically dependent upon proper installation in many respects - it is not as forgiving a material as PVC. HDPE's window of seaming parameters is narrow, allowance for expansion and contraction must be provided, and the conditions that produce stress cracking must be avoided. It is not surprising that more CQA attention is paid to HDPE.
Many owners of facilities have demanded detailed CQA plans for HDPE, but only recently have requested that similar CQA plans be prepared for PVC installations. The manufacturers and installers of PVC geomembrane, at their own discretion, have also recently generated comprehensive QC documents and CQA plans. Such documents can become part of CQA plans, and can be useful to regulators in those instances when adequate CQA Plans are not available.
The past absence of CQA plans for PVC has unquestionably been a function of most designers’ experiences with PVC, and the fact that PVC has generally performed satisfactorily without major installation controls. The survey (Appendix 1) of nationally recognized designers and geomembrane installers (those who regularly install both PVC and HDPE) in the USA and Canada, elicited the fact that most felt most comfortable working firstly with PVC, secondly with VLDPE, and thirdly with HDPE.
A number of significant comments returned with the survey are as follows:
- "Some HDPE resins are a (expletive) to weld and some are easier, the whole gamut of seamability".
- "PVC is hard to generalize, there are so many types available".
- "Although the mechanical and temperature characteristics of HDPE are less than ideal, its chemical inertness and weldability make it the material of choice".
- "PVC has the best overall mechanical properties".
- "I feel most comfortable with all the types of geomembranes and all design aspects, except repairability".
- "PVC is low tech seaming, high performance product. HDPE is high tech seaming".
- "Eliminate chemical resistance and long term stability and HDPE doesn't have much going for it".
It is clear that neither PVC nor HDPE is the better geomembrane in all applications. It is also clear that a selection must be made on a project specific basis, with due allowance being made for the less desirable features of each material. Experienced designers will take and work with each material on its own technical merits, not on pre-selected generalities.
The CQA plan will have to be prepared for the specific material being used in the Project in order to complement the advantageous materials properties being utilized. For instance, it has been pointed out that the uniaxial stress-strain curve of HDPE (Figure 6) contains a yield point at approximately 12% strain while the curve for PVC shows no point of instability up to the break strain of over 300 %: there will be instances when it will be necessary to work with the uniformity and predictability of the PVC curve, and them will be other instances when the point of instability (yield point) of the HDPE is immaterial.
Another significant difference between PVC and HDPE occurs in their puncture performances as shown schematically in Figure 7. The maximum force required to puncture an HDPE geomembrane is higher thin that required to puncture a PVC geomembrane, which might indicate that HDPE is mom puncture resistant than PVC. However, the PVC geomembrane has a much higher puncture strain than the HDPE, which may be of more importance, for instance, in the case of subgrade settlement and in the case of conformance to a rough subgrade. In other words, the PVC will retain effective barrier properties under a much higher strain than HDPE. For PVC, the puncture performance is complete at this stage, but for HDPE there are additional implications: even though the puncture strength and s@ may not be instantaneously exceeded, a constantly applied puncture stress (even 40 % of the yield stress) may ultimately cause a stress cracking failure. Here again is evidence that the interaction and synergism of several individual properties of any material must be considered in order to achieve the optimum design.' The CQA plan for a PVC geomembrane liner need not, apparently, be as extensive as one for HDPE.
An increasing number of state regulations are requiring comprehensive CQA programs on lining system installation, thereby putting HDPE, PVC, and other materials, on an equal footing. However, it will still be necessary to ensure that the correct information is requested in the CQA plan, and that each plan is customized to account for the problematic parameters of each material, whether that material be PVC or HDPE.
Seams
All geomembranes require some field seaming. It is universally recognized that field prepared seams are potentially the most problematic features of lining systems.
The width of HDPE geomembrane rolls is steadily increasing in order to minimize the number of field seams required. PVC, on the other hand, is seamed under controlled conditions in the fabrication plant to produce larger panels, thereby reducing the number of field seams required. In a given liner area, the length of field seam required in a PVC liner may be 20% of that required in an HDPE liner. For any project, it is better to have fewer field seams to minimize the potential problems associated with them. Whether to have a few field seams or a larger number of field seams is a decision only the designer can make. Such a point was concluded by the Bureau of Reclamation(14) in its study on the chemical exposure and weathering of FML field seams: 'Generic-type material specifications are not sufficient to ensure satisfactory performance of FML seams when used for hazardous waste containment applications'.
Until recently, most field seams in PVC have been made with a chemical (adhesive or solvent). Now hot wedge equipment, the same as that preferred for long-run HDPE seaming, is used for PVC geomembrane seaming. It can be used in a wider range of environmental conditions than chemicals. Thermal fusion methods of joining PVC have been used for many years in the roofing industry and in the European geomembrane industry. Double hot wedge (and hot air) fusion offers all the control features and nondestructive testing capabilities in PVC as it does for HDPE.
Thickness
For many years EPA has recognized, as now have the more enlightened states, that a single minimum thickness is not an adequate criterion for geomembranes used in landfill liner and cover applications. All materials are not equal, and cannot so simply be reduced to a single common denominator. Due to its semi-crystalline nature, HDPE is a different breed of geomembrane and within its ranks are many sub-species. Even if a single thickness requirement is based on HDPE, there are many types of HDPE geomembrane that would not perform satisfactorily even at twice a minimum thickness of 60 mil. At this moment, I am investigating, a stress cracking failure that occurred, after five years service, in an HDPE liner 100 mil thick that was not exposed to low temperatures. In such instances, thickness has absolutely no influence on the performance of the geomembrane.
Other parameters must be considered for every type of traditional and novel geomembrane lining installation. A single thickness value, such as 60 mil, is selected based on HDPE, but where realistic technical considerations have been given to other materials, such as PVC and Hypalon, thicknesses in the range of 30 rail are considered adequate. In Subtitle D (p32), a composite bottom liner is required to have a primary geomembrane with a minimum thickness of 30 mil, but if the geomembrane is HDPE, it must be at least 60 mil thick. A higher thickness for HDPE is understood to be required to make allowance for its problematic features; it is difficult to seam at thicknesses less than 40 mil, and the grinding required on preparation for fillet extrusion seaming produces reduced thickness in the notch sensitive area at the edge of the seam. la fact, with a tolerance of ± 10%(13) on HDPE geomembrane thickness and the supposed maximum grinding depth(15) of 10 % of geomembrane thickness, 60 mil HDPE geomembrane could be almost 40 mil thick adjacent to extruded seams, patches, and penetrations. EPA has stated(16): "the design engineer should recognize that some geomembrane materials may require greater thicknesses to prevent failure or to accommodate unique seaming requirements.". The norm is the 30 mil figure. HDPE is the exception that requires additional thickness. PVC and the other geomembranes should not be penalized because of HDPE's perceived deficiencies.
There is also some feeling that thicker HDPE is necessary to provide improved stress cracking resistance. This is not so, since stress cracking resistance is a fundamental material property. In fact, the use of a thicker material with the same surface scratches and defects as a thinner material may increase the susceptibility of the liner to stress cracking;the applied stress must be lower and closer to the brittle fracture range. As a constant stress on an HDPE geomembrane decreases from the yield stress, the break mode changes from ductile to the brittle (stress cracking) mode. In a given situation, a thicker HDPE geomembrane may be more inappropriate than a thinner one.
One disadvantage, therefore, of specifying a minimum thickness is that it may eliminate excellent candidate geomembranes. Even the best available geomembrane, that would perform adequately at lower thicknesses, may be eliminated on the basis of cost at higher thicknesses. The intent of regulations is to provide safer waste containment, not simply more expensive waste containment. And, as discussed, a minimum thickness may not be functional, even if that thickness is tailored for the specific material selected. Another disadvantage of specifying a minimum thickness, whether or not there are one or two values to suit different materials, is the danger that some 'designers' will read it as a specification, i.e. a geomembrane of the minimum thickness will perform the job, and no other factors need be considered.
The same sentiment was expressed by Bob Landreth, chief of EPA's Landfill Technology Section(17):
"Thickness of materials should be a function of design which implies site specific information and considerations. Although other thicknesses, 30 and 60 mils, are allowed (page v), this approach, we believe, ties the hands of the designer and will force the use of generic designs and could lead to increases in overall project costs. We also strongly believe and as part of our recommendation to consultants (sic) that a minimum thickness of material type should be specified then let the consultants "design" the system. Our recommendations based on seamability, punctureability and installability is (sic) as follows:
Type Min Thickness (mil)
PVC 20-30 (30 is very tough)
CPE 30
CSPE-R 36
Polyethylene* 60
*Polyethylene is set as a 60 mil minimum primarily from a seamability standpoint. It has not been clearly demonstrated to us that PE products less than 60 mil can be constantly (sic) seamed in the field. 'Mere is also concern that this is at the lower limit for creating conditions that encourages (sic) stress cracking. While stress cracking is still under review we are starting to see improvements in seaming techniques. It is interesting to note that the West German are now requiring PE thickness greater than 100 mil.'
CLOSURE
A number of geomembranes, including PVC geomembranes, have apparently been used successfully, for many years, to contain municipal and hazardous wastes. PVC does not suddenly become inappropriate because HDPE geomembranes have become available. It is desirable to design geomembrane lining systems using the most cost effective material that will best achieve the performance specifications. Regulations should be written to accommodate all candidate materials that will adequately perform the required function, and the performance of that function should be decided by proper engineering design, not by a regulatory recipe. In no way can a regulation provide a satisfactory design.
In the September/October 1990 issue of Geotechnical Fabrics Report, Bob Landreth of EPA's Risk Reduction Engineering Laboratory stated:
"The modifications (to chemical analysis techniques and control of wastes) should increase the number of geomembrane compositions available for use. The increased number of geomembrane compositions should now allow the designer to develop innovative designs. We (EPA) believe innovative designs will be more economical, technically viable, and be more reliable."
As knowledgeable designers and regulators recognize, optimum design cannot be achieved by regulating a single minimum geomembrane thickness. At least two minimum thicknesses are required to accommodate two fundamental types of materials; amorphous thermoplastics (such as PVC) and semi-crystalline thermoplastics (such as HDPE). The minimum thicknesses, (30 mil and 60 mil respectively), specified in Subtitle D are acceptable to producers of both these classes of materials.
It is hoped that the information presented in this paper will assist users, designers, and regulators in being open to the use of PVC on the basis of its technical merits. There is a wide range of geomembrane materials available with many interesting and useful properties, but no one material is a panacea appropriate for all applications. The positive attributes of each material should be compared to the prime requirements of each project in order to make the primary selection of candidate materials. The negative attributes of each material should then be assessed to determine which ones can most easily be accommodated. The optimum selection of geomembrane material can then be made.
PVC has performed well in the past and, as it continues to be improved, it will perform well in municipal solid waste landfills in the future. Designers and regulators can achieve more, and widen the window of environmental protection, by taking advantage of the many unique properties of PVC.
REFERENCES1. Private Communication. Alberta Environment, October, 1991.
Private Communication. US Bureau of Reclamation, October, 1991.
EPA Method 9090. "Compatibility Test for Waste and Membrane Liners". U.S. Environmental Protection Agency, September, 1986.
Artieres, 0. Goussé,F., and Prigent, E. "Laboratory-Ageing of Geomembranes in Municipal Landfill Leachates".
Proc. 3rd. International Landfill Symposium, CISA. Cagliari. October, 1991, pp. 587-603.
Pohland, F. G. "Fundamental Principles and Management Strategies for Landfill Codisposal Practices".
Ibid, pp. 1445-1460.
Taylor, F. "Lycoming County Landfill Protected with Geomembrane". Geotechnical Fabrics Report.
IFAI, July/Aug, 1991, pp. 22-25.
7. Private Communication. P. L. Sembenelli, October, 1991.
8. Private Communication. Alberto Scuero, October, 1991.
Morrison, W. R. and Starbuck, J.G. "Performance of Plastic Canal Linings". REC ERC-84-1.
US Bureau of Reclamation. January, 1984.
10. Steiniger, F. "The Effect of Burrower Attack on Dike Liners." Wasser Und Boden, 1968.
11. Einbrödt, H. "Testing of Schlegel Sheet for Rodent Resistance." Private communication. April, 1978.
Zitcher, F. F. "Resistance to Microorganisms and Rodents." Plastics in Water Engineering. Wilhelm
Ernst & Sohne, 1971.
13. NFS International Standard 54. "Flexible Membrane Liners". National Sanitation Foundation. May, 1991.
Morrison, W. R. and Parkhill, L. D. "Evaluation of Flexible Membrane Liner Seams After Chemical Exposure
and Simulated Weathering". EPA/600/2-87/015. US Environmental Protection Agency. February, 1987.
Landreth, R. E. and Carson, D. A. "Technical Guidance Document: Inspection Techniques for the Fabrication of Geomembrane Field Seams". EPA/530/SW-91/051. US Environmental Protection Agency.
May, 1991.
Hartley, R. P. 'Technical Resource Document: Design, Construction, and Operation of Hazardous and Non-
hazardous Waste Surface Impoundments'. EPA/530/SW-91/051. US Environmental Protection Agency.
June, 1991.
17. Landreth, R. E. Letter to US Army Corps of Engineers. September, 1989
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