Presented to
MICHIGAN DEPARTMENT OF NATURAL RESOURCES
By
Dr. I.D. Peggs
I-CORP INTERNATIONAL
Ocean Ridge, FL
THE PVC GEOMEMBRANE INSTITUTE
1.0 INTRODUCTION
It is not my intention in this document, and in the associated meeting, to show that PVC is a better geomembrane than HDPE, and that all lining and cover systems can be built with PVC. However, it is my objective to show that 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.
HDPE cannot arbitrarily be used in all lining systems. I am in the midst of a
failure analysis of an HDPE lining 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 most 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 confidence that the geomembrane users
place in HDPE is as unjustified as the lack of confidence they place in other materials
such as PVC. And this is somewhat strange since PVC has been so successfully used for
waste containment for over 36 years. I would like to dispel the myths of PVC and HDPE so
that they will both come out of the starting gate being considered by competent designers
on their 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.
In the first draft of Act 641 a single minimum thickness (60 mil) for any geomembrane used
in lining and cover systems was proposed. Such a requirement effectively defined that only
HDPE could be used as a geomembrane. The Department of Natural Resources is to be
commended for recognizing the inadequacies of this draft and for making appropriate,
professional allowance for the use of three specific materials and for leaving the door
open for new geomembranes as they become available. Such a regulatory approach is like a
breath of fresh air for designers and offers the potential for competent, durable lining
system designs.
There are two groups of people and institutions that always have, and always will lose
their battles: those who stick with the old, ignoring new developments, and those who
blindly jump to the new, not fully recognizing the problems of the new. Those who remain
flexible and take advantage of both will be the winners. The PVC geomembrane manufacturers
are recovering from the former. Those who have had HDPE failures are suffering from the
latter.
As you consider PVC and HDPE geomembranes 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
different PVCs formulated with different additives to meet different environmental
conditions. HDPEs also are different but they are different not so much by design but by
default, due to the different resin types. The factors responsible for the vastly variable
fundamental stress cracking resistance (mechanical durability) of HDPE are not yet fully
understood, and, therefore, cannot be intentionally engineered. Just because a failure has
occurred in a PVC or HDPE geomembrane does not automatically mean that all other PVC and
HDPE products will fail 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 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.
Michigan's Act 641 will certainly help achieve this goal in its second draft form.
2.0 PVC PERFORMANCE
I will attempt to dismiss some of the myths of PVC and put the performance
of HDPE into perspective. The following concerns are usually expressed about
PVC:
In assessing these features one should keep in mind that the major factor is whether the material concerned 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 performance. The fact that one changes more than another is irrelevant.
2.1 Pinholes
Since a geomembrane is intended to be an impermeable barrier, pinholes are not desirable features. However, no geomembrane is absolutely impermeable, and it is doubtful that any sizeable geomembrane will be installed without some construction induced damage and a few sections of inadequate seaming that might leak. After all, the premise of the double lining system is that a single liner cannot be made leak-free. This is, however, no reason to tolerate the existence of pinholes in any geomembrane, but it may put the existence of the one or two pinholes that may exist into practical perspective. In all CQA Plans, project specifications, and regulatory aspects, it is necessary to remember that ideals are unachievable and that the real world (practical) situations require some compromise.
PVC sheeting has been made in thin gages (as low as 4 mil) for vapor barriers, swimming pool liners, and other critical medical applications for many years without pinholes. With continuous in-plant backlighting and QC techniques, 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 thickness greater than 10 mil contain absolutely no pinholes. Alberta Environment has used many millions of square feet of 20 mil PVC geomembrane 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 non-existent. Improved methods of calendering have effectively eliminated pinholes in PVC sheet and geomembranes. Three or four separate rolling actions ensure there are no holes.
Alberta Environment did find a minor problem with gel defects (cold roll) under
startup conditions, but discussions with the manufacturers prevented such material being
delivered to the site. Similar (gel) problems can occur in HDPE when agglomerates of
decomposed HDPE find their way through the extruder into the geomembrane. They can
sometimes be identified as bumps, or solid blisters on the surface of the geomembrane.
They may also be confused with carbon black agglomerates when making carbon dispersion
measurements, if the carbon black is not properly dispersed. Agglomerates of carbon black
have been seen to initiate stress cracking.
The US Bureau of Reclamation has had 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 moving to 20 mil geomembrane, found it was 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 would, however, be foolish to state that PVC geomembrane contains absolutely no pinholes since one cannot be absolute. But the numbers that may escape manufacturing and fabrication QC procedures are very small, and are effectively zero when compared to other construction damage that can produce leakage.
It would be even more foolish to state that PVC contains pinholes and HDPE does
not. In my last visit to an HDPE geomembrane manufacturing plant I saw holes in
geomembrane from a few millimeters diameter to holes one could put one's fist through. In
a recent CQA project I.CORP found a 60 ft length of 60 mil HDPE geomembrane that contained
about 10 holes up to 80 mils in diameter.
Both HDPE and PVC geomembranes can, therefore, appear in the field with holes. However,
with appropriate specifications, and good in-plant QC and field CQA, holes in both PVC and
HDPE can be effectively eliminated, or be caught and repaired.
2.2 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". 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 that reach an equilibrium condition within certain limits occur, the geomembrane is considered to be compatible with the leachate.
For practical purposes it is immaterial whether one of two types of the same
material, or one of two different materials, degrades more than the other if both still,
and will continue to, adequately meet the project specifications.
There is no question that for any given chemical containment requirement there is more
chance that HDPE (but not all HDPE's), rather than PVC, will provide the better chemical
resistance. However, for most municipal waste leachate applications it is probable that,
although HDPE may be technically more resistant, PVC may provide more than adequate
resistance for the required service. The historical performance of PVC geomembrane in
Michigan and elsewhere has proven this. It could be an unreasonable trade off to use the
fundamentally more chemically resistant material if it is more expensive, and with poor
design, poor installation practice, and poor CQA could lead to other durability problems.
Most installers surveyed (Appendix A) for this project feel that PVC is much easier to
install correctly than HDPE. A European contact 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."
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 leachates.
None of the PVC geomembranes has failed the test. One of the tests in hazardous leachate
has 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 may still
provide adequate service. Two of the laboratories commented that if owners and regulatory
agencies were concerned about the chemical resistance of PVC, a move to Oil Resistant
grades of PVC should more than put those concerns to rest.
Out of a total of approximately 55 EPA Method 9090 tests recently performed by all three laboratories on PVC, only two displayed problems; with solvent seams. However, not all solvent seams were problematic in EPA Method 9090 testing. None of the dielectric seams tested gave problems. It Is clear, that to obtain the optimum combination of material and performance, it is essential to experimentally assess the 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 underway in France, in which PVC, HDPE, and
other geomembranes have been exposed to an MSW leachate for 1 year, has shown that both
HDPE and PVC have suffered loss of additives from only a 100 m
m surface layer. (Artieres, Sardinia 91 Landfill Conference).
The second draft of Act 641 specifies that resistance to leachate shall be assessed by
performing the EPA Method 9090 test. On this basis, of the 55 tests performed by the three
laboratories surveyed, PVC geomembranes were appropriate for containing 53 of the
leachates. PVC is unquestionably a stable material for containing landfill leachates, as
has been proven in Michigan.
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 failed after 12 to 18 months by environmental stress cracking. The nitric acid
pond 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 commented that municipal waste 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, therefore, possible that in Michigan the 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 that it can be done in such a way as to promote the
degradation of the municipal waste (Sardinia '91 Landfill Conference).
There is a large amount of evidence, not only from EPA Method 9090 testing, but
also from field experience, that PVC is satisfactorily containing municipal waste
leachates. There are more than approximately 30 PVC lined landfills in Michigan that are
performing satisfactorily. The few problems that have occurred are, as in most HDPE
failures, related to inadequate design, poor installation, and/or poor CQA. 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.
In summary, there is no justification for taking the following two approaches to
geomembrane chemical resistance; to dismiss PVC out of hand saying it does not have
adequate leachate resistance, and; to assume that HDPE will perform adequately without
appropriately testing it, even though the manufacturer may provide a written guarantee.
2.3 Weathering and Thermal Resistance
This is the most confusing area of concern with PVC, and may relate to our need to understand that there are different grades or PVC, since we, in North America, 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 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.
The US Bureau of Reclamation 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 might 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 its plasticizer during aging, HDPE can
degrade by consumption of antioxidant as it experiences temperatures as high as 80oC
or more in summer sunshine. In other words it is expected that these additives will be
lost during different phases of service, but that fact alone is immaterial provided
sufficient additive remains.
It has been reported that 40 mil PVC has successfully survived one year EMMAQUA (standard
sunlight exposure) testing and that 60 mil HDPE has failed this test. On the other hand it
is probable that there will be HDPEs that would survive this test and PVCs that would not.
In fact, it has been stated by the US Bureau of Reclamation that a one year EMMAQUA test
"may be too long, resulting in accelerated weathering conditions too severe for some
materials". All PVCs and all HDPEs are not created equal. Therefore, it is not a
matter of simply stating that PVC is better than HDPE (or vice versa) for a specific use;
one should also consider that one grade of HDPE may be better than another, or one type of
PVC may be better than another.
PVC geomembranes are now being successfully heat seamed in the US. 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 exposure to
sunlight for extended service periods, as proven on dams, roots, and canal banks.
For landfill uses most weathering/thermal problems are eliminated by the regulatory
requirement for covering the liner with various types of soil and other geosynthetic
layers. Such layers protect the geomembrane from ultraviolet and severe thermal effects.
Act 641 requires that geomembranes be covered by soil within 30 days.
Concern has been expressed about the potential problems of a landfill PVC cover material
being exposed to weathering when overlying soils are eroded. This should be considered to
be a problem with the design of the cover system rather than a potential problem with the
geomembrane. It is, surely, unacceptable for any soil cover, on any landfill cover system,
to erode to expose the geomembrane, whatever the geomembrane material. Proper separation,
filtration, and drainage layers (probably geosynthetics) as required in EPA recommended
designs will prevent soil erosion. Act 641 requires that erosion of soil in the cover
system shall be prevented.
2.4 Low Temperature Brittleness
PVC canal liners with an Alberta Environment brittleness temperature
specification of -20°C (ASTM D1790) have been successfully installed in Canadian winters.
When deployed, a covering layer of stones is dumped onto the geomembrane from a conveyor
belt. This is a severe cold impact test. National Sanitation Foundation International
Standard 54 "Flexible Membrane Liners" specifies a maximum brittleness
temperature of -29°C for the 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 would continue to provide adequate protection. If the geomembrane is only
subject to static loading it will be protective to much lower temperatures than -30 o
C, since the brittleness temperature is determined by an impact procedure (ASTM D1790).
The measured brittleness temperature of HDPE 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 also be noted that PVC is tested by impacting a bent
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 is 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.
A majority of designers and installers surveyed (Appendix A) for this project agree that
PVC has better mechanical property characteristics and is easier to install than HDPE.
HDPE is a problem because of the yield point in its stress/strain curve 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 a much
safer installation, or one with a lower factor of safety and, therefore, a wider range of
safe 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 little 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.
2.5 Plasticizer Leaching and Volatilization
This has been dealt with partially in Section 2.2. Plasticizer loss does occur,
but in most cases has been shown to be of no practical consequence. Similarly the
consumption of antioxidants in HDPE occurs and has been shown to be of little practical
consequence. When the fundamental chemical resistance of the geomembrane to the leachate
is correctly assessed, or materials with the correct additive packages are exposed to
UV/thermal radiation, both PVC and HDPE will provide adequate durability in landfill
liner/cover applications.
The Bureau of Reclamation has shown that 30% of the plasticizer in a 10 mil PVC
geomembrane may be lost within 4 years, but that after 19 years the geomembrane still
contains over 50% of its plasticizer. Such material is still very flexible. As with most
processes, the rate of loss of plasticizer decreases with time.
A number of studies have shown 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%. This value is still more than 10 times greater than the useful 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. For design purposes HDPE has failed at the yield point.
Plasticizer has been claimed as being a desirable food to rodents, which there by
chew holes in PVC liners. Where rodents have consumed PVC in the field they have only done
it to gain access to the warmth behind the liner, not because they find it good to eat. A
survey of users identified only one positive case where this has occurred. On the other
hand, HDPE is claimed to be inedible by rodents. However, at the Somerset Generating
Station of New York State Electric and Gas there is ample evidence, in the form of 1-in.
diameter holes, that vole-type rodents find HDPE extremely edible. Both these cases may be
the exceptions that prove the rule, but they also identify the danger of generic
statements and the resultant possibility of avoiding perfectly adequate materials based on
misstatements.
2.6 Quality Control and Construction Quality Assurance.
There is a feeling that the qualities of HDPE geomembranes and geomembrane
seams are higher than those of PVC since the HDPE industry has more visible QC
documentation, and CQA documentation is more extensive. Comprehensive CQA is absolutely
essential for HDPE, particularly in locations such as Michigan, where it can be very cold,
since its service performance is critically dependent upon proper installation - it is not
as forgiving a material as PVC. HDPE's window of seaming parameters is narrow, allowance
for expansion/contraction and wrinkling 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 CQC Plans.
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 A) of nationally
recognized designers and geomembrane installers (those who regularly install PVC and HDPE)
in the USA and Canada elicited the fact that most felt most comfortable working with PVC,
then VLDPE, and finally HDPE.
A number of significant comments returned with the survey were as follows:
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 each material on its own merits and not pre-select it
based on generalities.
With the promulgation of Subtitle D, and the development of Michigan's Act 641, all
geomembrane installations will require comprehensive CQA programs, 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 potentially problematic parameters for each material,
whether that material is PVC or HDPE.
2.7 Seams
All geomembranes require some field seaming. It is universally recognized that field prepared seams are potentially the most problematic feature 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 PVC may be 20% of that required in HDPE. It is, therefore, a matter of rationalizing whether, for any specific project, it is better to have fewer field seams with the potential problems that PVC might have, or more seams with the problems that HDPE might have. This is a decision only the designer can make. Such a point was concluded by the Bureau of Reclamation in their study on the chemical exposure and weathering of FML field seams, (EPA/600/S2-87/015): "Generic-type material specifications are not sufficient to ensure satisfactory performance of FML seams when used for hazardous waste containment applications".
2.8 Thickness
For many years EPA has recognized, as now has Michigan DNR, 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 semicrystalline 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
examining 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 durability of the geomembrane - other parameters must
be considered.
Other parameters must be considered for every type of traditional and novel geomembrane. 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 mil 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. If the geomembrane is HDPE it must be at least 60 mil thick. A higher thickness
for HDPE is required to make allowance for its problematic features; it is extremely
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. In fact, with a tolerance of +10% (NSF 54) on HDPE
geomembrane thickness and the supposed maximum grinding depth (EPA/530/SW-91/051) of 10%
of thickness, 60 mil HDPE geomembrane could be almost 40 mil thick adjacent to extruded
seams, patches, and penetrations. EPA has stated (EPA/530/SW-91/054): "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.
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 we have seen, a minimum thickness may not
be functional, even if that thickness is tailored for the specific material selected. The
second 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 in a letter dated 29 September 1989 to the Corps of Engineers in which he
stated:
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 recommendations to consultants that a minimum thickness of material type should be specified then let the consultants "design" the system. Our recommendations based on seamability, punctureabilty and installability is 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 seamed in the field. There is also concern that this is at the lower limit for creating conditions that encourages 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 Germans are now requiring PE thickness greater than 100 mil.
On the other hand, a minimum thickness is a preliminary regulatory means of
eliminating totally inadequate designs. However, it should be made perfectly clear that
this is only a guide, and that engineering calculations should be made to confirm the
thickness required for any liner or cover application.
In addition to allowing for different minimum thicknesses of geomembranes Michigan Act 641
requires that a "geomembrane shall be of sufficient tensile strength to withstand
anticipated stressed without failure". This provides an opportunity for the regulator
to confirm that a proper design has been done and that the related geomembrane thickness
is based on design calculations and not the regulations themselves.
3. CLOSURE
A number of geomembranes, including PVC geomembranes, have been used successfully, for many years, to contain municipal and hazardous wastes in Michigan and elsewhere. PVC does not suddenly become inappropriate because HDPE geomembranes have become available. HDPE has many positive attributes, but it also has a number of negative attributes that are not yet fully understood. New materials (e.g. polypropylene/EPDM alloys) that offer specific advantages and, hopefully, fewer disadvantages are appearing and will continue to appear. It is desirable to design geomembrane lining systems using the least expensive material that will best achieve the performance specifications. Regulations should be written, as in the second draft of Act 641, 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 designers and Michigan DNR recognize, optimum design cannot be achieved by regulating a single minimum 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. Fortunately, a minimum thickness of 30 mil is also acceptable to those producing elastomeric geomembranes and fully-crosslinked elastomeric alloys, two other classes of materials.
There is little doubt that HDPE has become the landfill liner geomembrane of choice, but it has achieved this position with a significant dose of effective marketing and regulatory support. At the same time that the emotive pendulum has swung in favor of HDPE it has unjustifiably swung away from PVC. At present the potential problems of HDPE are overlooked, as are the many advantages of PVC.
Michigan's Act 641 has now been written to include three values, rather than one value, of minimum thickness, and to stress the importance of thorough geomembrane design in order that the people of Michigan get optimum cost effective landfill waste containment from the complete range of geomembranes available. This is a responsible approach. A fair technical hearing, should now be given to PVC as well as HDPE. PVC has performed well in the past in Michigan 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. The second draft of Act 641 has opened the door for responsible design, DNR should consider each design on its site-specific, technical merits.
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