Title: Assuring the Quality of HDPE Geomembrane Liners: An International Perspective
By: Ian D. Peggs, I-CORP INTERNATIONAL, Inc.

ABSTRACT

The use of geomembranes for the containment of solid and liquid waste, for the collection, containment, and conveyance of water, for the pollution protection of groundwater, and for corrosion protection is growing rapidly internationally. The geomembrane of choice, also internationally, is high density polyethylene (HDPE). There is approximately 30 years of experience with HDPE, during which there have expectedly been a number of failures and problems, but from which much has been learned. Consequently, significant improvements in liner materials, installation practices, and testing have been made, which have led to the development of effective quality programs at different stages of a liner installation project. Such programs are necessary since geotechnical and civil engineers, regulators, and general contractors are not thoroughly familiar with the performance characteristics of viscoelastic engineering polymer products.

A series of comprehensive integrated Quality Control and Quality Assurance programs are outlined that potentially provide optimum assurance that a liner is adequately designed, specified, manufactured, installed, tested, and placed in service. With an integrated quality program there is no reason that the service period should not exceed several hundred years.

INTRODUCTION

With the success and popularity of high density polyethylene (HDPE) and other geomembrane lining systems to cost-effectively contain hazardous and municipal waste and valuable water resources for hundreds of years there is an increasing number of liner failures. Initially, in the early 1980s many of the failures were due to the use of inappropriate resins. After thorough investigations of the failures, resins were improved to the point that few failures are now due to the quality of the geomembrane material itself. However many materials are improperly used in specific applications and many design engineers are specifying incorrect tests, inappropriate values, requiring improper installation procedures, or omitting important design details. Therefore, there still remains a significant need for integrated quality programs through the HDPE geomembrane liner manufacturing, testing, installation, and liner testing phases of all lining projects. In this paper Quality Control and Quality Assurance programs for HDPE geomembranes, the predominantly used geomembrane liner around the world, will be outlined.

QUALITY PROGRAMS

Quality Control (QC) programs are performed by those manufacturing or working with the product to ensure that their work product meets their specifications. Quality Assurance (QA) programs are performed by another party to ensure that the work product received by them meets project and the manufacturer’s specifications. Therefore, with respect to an HDPE geomembrane;

• The HDPE resin manufacturer performs QC tests on its own manufactured resin.
• The geomembrane manufacturer performs QA tests on the incoming resin and carbon black, and performs QC tests on its manufactured geomembrane and welding rod.
• The liner installer performs QC tests during installation of the lining system.
• The owner of the facility contracts an independent party, or the design engineer, to perform QA from the beginning to the end of the project.

It is most important to note that the liner installer cannot perform a QA function for the owner, the engineer, or the regulator, since the philosophy of its own QC program is in direct conflict with the QA requirements of the owner. A borderline situation will be accepted by the manufacturer/installer but rejected by the owner.

It must also be noted by regulators and owners that QA is not a program that ensures that the lining system is perfectly installed – it is a program that ensures that what was designed is installed, but with the best possible workmanship within the constraints of the schedule and the budget. Therefore, at one extreme, QA will simply ensure that a poor design is what is installed – it will not make it a well-designed properly functioning installation.

LINER APPLICATIONS

Geomembrane applications include three primary classifications;

• Engineered protection of groundwater
• Collection, containment, and conveyance of water for irrigation
• Engineered sealing and corrosion protection

Whatever the application, all liners require adequately durable material, minimum stress after installation, effective seaming, installation without damage, and filling/covering for initial operation without damage. However, a more assured lining system is probably required for the containment of hazardous waste than for an irrigation canal liner. Therefore, on-site quality programs start with preparation of the subgrade and finish, not with completion of the geomembrane, but with the non-damaging placement of the layers covering the geomembrane.

LINER DESIGN

The quality program should be initiated at the same time as the design of the liner starts. The design engineer should be familiar with the performance of HDPE geomembranes or should seek the assistance of a knowledgeable QA person who does understand more about geomembrane materials, welding, and testing. The QA person will help the engineer achieve the desired performance and results.

Typically there will be a project specification document and a separate QA document. However, these need careful integration to ensure there are no conflicting overlaps nor gaps between them – hence the reason for the QA people to work with the design engineer from the start of the project. Both these documents should also be integrated with the QC programs of the geomembrane manufacturer and the installer. In effect, the QA document should be the glue that ties the project specifications to the QC programs and seamlessly integrates all of the quality programs. Therefore, it is proposed that the design engineer and QA people work together to produce a single document that includes both project specifications and the QA program and which will identify the monitoring that will be done and actions required in the event of any nonconformance.

In material specifications the most important parameters for HDPE are the stress cracking resistance (SCR), the oxidative induction time (OIT), and the seam properties. The SCR is the single most important property of HDPE that determines its long-term mechanical durability and its resistance to mechanical damage and overheating that might occur during installation. Under no circumstances should the ASTM D1693 (or equivalent) bent strip test be specified or performed to assess SCR. HDPE resins have improved to the extent that the bending stress relaxes completely before stress cracking starts – the specimens will not fail! (Hsuan et al.) The proper test is the ASTM D5397 notched constant tensile load test which is typically required (GRI.GM13) to exceed a single point break time of 300 hr.

The OIT (ASTM D3895) test evaluates the quality of the antioxidant additive package and the resistance of the geomembrane to exposed service. The GRI.GM13 standard requires that this should exceed 100 min. The test can be performed at a lower temperature in a pressurized oxygen atmosphere (ASTM D5886) if Hindered Amine Light Stabilizers (HALS) are present in the additive package, but the thermograms are often more difficult to interpret.

Seam properties (ASTM D6392) typically require measurement of shear and peel strengths but it is also very important that the specimens have adequate ductility in the shear test and do not separate in the peel test. It should also be required that all five shear and all five peel specimens meet specifications for acceptance of the complete seam specimen, although it is frequently allowed for 1 peel and 1 shear specimen, but not two of the same, to fail. However, it must then be recognized that this is a 20% failure rate.

RESIN MANUFACTURER

The resin manufacturer’s QC program will typically include measurement of density (representative of crystallinity), melt flow rate (representative of molecular weight), SCR, and OIT. For the measurement of SCR it is necessary to melt the resin and cast it into a thin sheet (plaque). The cooling rate after casting is critically important. One standard (ASTM D4703) suggests cooling rates of 15ºC/min, 5ºC/hr, and quenching. Quenching will not allow the development of a crystalline microstructure, thereby providing very optimistic results. The most appropriate cooling rate, most similar to that occurring during liner production, is 15ºC/min.

The antioxidant additive package will typically be blended into the resin by the manufacturer or may be added by the geomembrane manufacturer. This is a very proprietary package and, other than measurement of OIT, does not feature in the QC/QA programs.

QC certificates with these results will be sent to the geomembrane manufacturer with the resin.

GEOMEMBRANE MANUFACTURER

On receipt of the resin the geomembrane manufacturer will typically perform density and melt flow rate (MFR) QA measurements to confirm that what was ordered has been delivered.

The resin (dried) will then be uniformly blended with antioxidant and carbon black additives to provide the geomembrane with resistance to oxidation and ultraviolet radiation, then extruded either through a flat die or a round die (blown film). The latter is the more common manufacturing process producing sheet of 7 or 10 m in width and from 0.5 to 2.5 mm in thickness. Round die equipment is also capable of producing three layer coextruded material that enables the production of textured material for use on steeper slopes.

A textured surface can be added by thermal fusion bonding as a secondary process but this requires a compromise between sufficient heat to prevent the texture from being abraded off the surface but not too much heat that stress cracks can be introduced at the base of the texture element in the weld zone.

As the blown film tube of extruded material rises from the horizontal die it is supported by cooling air inside the “bubble”. At a height of about 20m the tube is flattened as it passes over a set of rollers to start its downward journey. The flattening develops two “folds” in the geomembrane. The center of one side of the flattened tube is then cut and the tube opened to form a flat sheet that has the two folds at the one and three quarter roll widths. This is a very important stage in the manufacturing process since the folds must not be too sharp and the opening process can induce several wrinkles and creases in the geomembrane. A slight tensioning and an annealing process may help in the opening process. It has been found that the folds do not typically weaken or decrease the performance of the geomembrane.

As the geomembrane approaches the roll up station the edges are trimmed to the required roll width. This edge trim material, provided it is kept clean, can be ground up and used to make new geomembrane, at concentrations up to about 10%. However, the manufacturer will choose whether or not to use this edge trim depending on the application of the finished product. Under no circumstances should commercial recycled material or any other “filler” be used in any HDPE geomembrane.

As the geomembrane is rolled and cut to length thickness measurements are made across its full width and a full roll width sample is taken for QC testing. Tests will at least include density, uniaxial tensile properties (most significantly break properties), carbon black content and dispersion, OIT, and SCR. All tests will not be performed on every roll, and each test will be performed at a different frequency (see GRI.GM13). A sample of material will be archived for reference purposes.

A QC certificate will be prepared for each roll, listing the roll number, the resin lot and all test results covering that roll. A copy of the certificate for each roll will be sent to the project engineer or the QA consultant for each project no later than delivery of the roll to the site.

Each roll will be identified with a label showing the product, the roll number, thickness, and length. Labels will be placed on the outside of the roll at one end and on the outside or inside of both ends of the core. This enables the roll to be easily identified when stacked on top of or under others.

LINER INSTALLER

Prior to the start of the project the installer will submit to the engineer or QA consultant a diagram outlining the planned orientation of rolls on the site for approval. The review will ensure that seams are oriented up and down slopes and that no transverse seam is within 2 m of the toes of slopes.

The installer’s QC program starts with trial seaming on fragments of geomembrane welded at a location typical of where production seams are about to be made. For instance, if welding is to occur on a steep wall then trial welding must be done under the same conditions. Logs will be kept of the number of the welding machine, the name of the operator, the temperature, speed, and nip roll pressure setting of the welding machine, time, liner (not ambient) temperature, and the results of peel and shear testing. If the seams meet specifications production welding will proceed without changing that machine/operator combination or any settings. If trial seams fail, then that equipment will not be used until adjustments have been made and passing trial seams made. Some QA programs require that two passing seams be welded if the first trial seam fails.

Trial seams are required every four or five hours or whenever the liner temperature changes by about 20ºC. Note that the only temperature of interest is the actual liner temperature, not the ambient temperature nor the temperature a specific distance above the liner.

Each seam will be marked with the time the seam was made, the welding machine number and operator, and often the machine settings. A log will also be kept of these parameters. Logs will also be kept of the 100% nondestructive air pressure, vacuum box, or spark testing of each seam. Again each seam will be marked with time, operator, testing details, and whether the seam passes or fails testing. If spark testing is used it is necessary to perform a calibration test to ensure that adequate voltage is used to signal a hole, should one be present.

Since nondestructive tests only assess the continuity of the seam and not bond strength it is considered necessary to periodically remove samples of production seams for destructive testing. The QA personnel generally dictate the locations of these samples. Since it is not desirable to put holes in seams, these samples are often removed from the anchor trench. These samples should be taken at the required frequency for every welding machine/operator combination. All sample holes should be repaired whether in the anchor trench or not. Seams should be continued into the anchor trench to the edge of the liner.

All destructive samples will be numbered and all repairs numbered. All liner panels will also be numbered, often with the roll number as well. Seams are identified by the panel numbers on each side. All of these details will be included on a record panel layout drawing of the liner.

The function of a geomembrane liner is to act as a barrier, not to be a load-bearing member of the system. Therefore there should be no puncturing stresses, no contraction stresses, no wrinkles that will fold over, and no bridging of corners or subgrade voids. The liner should be fully supported by the subgrade, especially when it is covered.

QUALITY ASSURANCE CONSULTANT

The QA consultant will initially ensure that all project documents are in order and integrated with each other. If they are not, a meeting should be called to identify overlaps, omissions, and conflicts so that all parties are in agreement with the final construction documents and understand the actions required in the event of any nonconformance.

Conformance testing of materials delivered to the site will then be performed to ensure they meet specifications. This is not an opportunity to repeat QC testing but should simply be a spot check of relevant properties. These might include thickness, density, uniaxial tensile properties, carbon black content and dispersion, OIT, and SCR. Again, these tests will not be done on every roll or at the same frequencies. For instance it will only be necessary to perform SCR on each type of resin used. OIT might be performed on each lot of resin. Note that specifications should be written to require that only one resin type be used to make geomembrane for the project, and that welding rod will be made from the same resin.

If a roll fails conformance testing the two rolls numbered either side of it will be sampled, and so on until the failing rolls are bounded by acceptable rolls. Clearly this can be a time consuming process if rolls have to be replaced. It is, therefore, often preferable for QA personnel to visit the manufacturer’s plant to audit manufacturing of the specific project material, to oversee QC testing, and to sample the material for conformance testing at that stage, before shipping material. Then, barring an accident during transportation, the material can be used immediately it arrives on site.

The QA personnel will monitor geomembrane deployment, trial seaming, nondestructive testing, and will identify the locations of samples for destructive testing. They will keep logs of all activities, welding parameters, personnel, sample numbers, and testing results, the objective being to generate sufficient data that should a problem occur in the future, the extent of that problem can be evaluated be it related to weather, specific equipment, specific operators, or specific roll numbers.

However, effective QA people will not only record comprehensive data, they will also proactively contribute to improving the quality of the installation through sensible discussion with the installer and project engineer. They will also ensure that a comprehensive record drawing is made of the installation.

Their final on-site responsibility is to ensure that the geomembrane is covered without damaging it. Electrical leak location surveys have shown (Nosko et al.) that about 19% of leaks occur at seams, but over 70% of leaks occur when the liner is covered by soil or stone, therefore covering is a very critical stage for a geomembrane.

Then a final QA report will be prepared that outlines all activities, modifications to the design that occurred during installation, and that include all field logs, and the record drawing. In the absence of a QA consultant the installer will provide a record drawing to the project engineer.

A final stage of CQA testing that is beginning to be required is electrical testing of the complete liner surface to locate any leaks (Peggs).

SUMMARY

HDPE geomembranes provide extremely efficient and cost-effective lining systems, but they are quite different to conventional construction materials. As such, and since few engineers are thoroughly familiar with the performance of viscoelastic materials, they need comprehensive integrated QC and QA programs at all stages of the project, from design through manufacturing, testing, installation, and covering.

With proper design, resin selection, manufacturing, installation, QA, testing, and operation, an HDPE geomembrane lining system should perform satisfactorily for a few hundred years.

REFERENCES

Hsuan, Y.G., Koerner, R.M., Lord, Jr., A.E., 1992, “The Notched Constant Tensile Load (NCTL) Test to Evaluate Stress Cracking Resistance”, 6th GRI Seminar, Philadelphia, PA, USA, pp 244-256.

Nosko, V., Andrezal.T., Gregor, T., & Ganier, P., 1996, “SENSOR Damage Detection System (DDS) – The Unique Geomembrane Testing Method”, Geosynthetics: Applications, Design and Construction, Balkema, Rotterdam, The Netherlands, pp 943-748.

Peggs, I.D., 1999, “Mobile Geoelectric Liner Integrity Surveys: Planning Ahead”, Geosynthetics 99 Conference – Specifying Geosynthetics and Developing Design Details, Boston, Massachusetts, IFAI, Roseville, MN, USA, pp 627 - 634.