High density polyethylene (HDPE) has become the geomembrane material of choice. In fact the term "HDPE" is almost synonymous with the word "geomembrane". However,it is little recognized that in many respects specifying an "HDPE" geomembrane is a little like specifying a "steel" tank. There are many steels, and there are many HDPEs.

Specifying for durability

Thus HDPE geomembranes can and should be better specified to suit their specific applications,particularly in critical applications, when 1000 year lifetimes are required, when buried and when exposed in very hot sunny environments. For instance an exposed liner in a hot environment that is to contain hazardous liquid waste should not have the same specifications and performance characteristics as a decorative pond on a golf course - it should be much better.

In 1998, for five standard production HDPE geomembranes, it was found that the long term mechanical durability, expressed by stress cracking resistance (SCR), varied by a factor of almost 1000 - one product from one manufacturer broke after about 10 hr in the notched constant load (30% of yield strength) test, but another product lasted almost 10,000 hr. Which would you prefer for your hazardous liquid pond liner? Would you take a chance, as is mostly done at present, or would you specify the more durable one?

The Geosynthetic Research Institute (GRI) GM13 standard, typically used internationally for HDPE geomembranes, is a minimum consensus specification. It does not provide assurance that any HDPE geomembrane that meets GM13 specifications will perform adequately in all applications. Significant improvements can be made to achieve maximum durability. However, there are only four parameters that define the differences between HDPE

geomembranes: SCR, high pressure oxidative induction time (HP-OIT), oven (thermal) aging (TA),and UV resistance (UVR). These parameters are important because the end-of-lifetime (EOL) failure will be by loss of antioxidant (AO), oxidation, and stress cracking initiated on the oxidized surface. All other "conventional" mechanical and physical parameters are essentially the same, regardless of resin manufacturer and stabilizer formulation. Manufacturers use different resins and different formulations of AO additives that result in these durability differences.

Given GRI.GM13 there is little need for improvement if the liner will be covered and in a cool environment.If buried and hot, SCR and TA could be increased (UVR need not be). And if exposed in a Middle East desert, for example, all four parameters should be maximized, while being consistent with available or custom manufacturing capabilities. For instance, a 1000 yr lifetime was targeted for an industrial waste landfill closure cap. While it is clear that no-one can guarantee a 1000 yr lifetime the best available geomembrane was specified by increasing the GM13 specifications as follows:

• Stress cracking resistance from >300 hr to >1500 hr
• HP-OIT from >400 min to >525 min
• Thermal aging from 80% to 85% retained, and
• UV resistance from 50% to 80% retained 
• Several HDPE geomembrane manufacturers were not willing to guarantee these specifications, but two were, and thorough Manufacturing Quality Assurance
  confirmed that they were met. However, the manufacturer's QC and the MQA tests identified some deficiencies in the ASTM D5885 HP-OIT test procedure, but these are now being addressed by the ASTM D35 Geosynthetics committee. At least three measurements, rather than just one, were required to get meaningful results.

On-Site Handling

When a suitable liner is manufactured, it then has to be installed without significant damage. That it will be damaged should be assumed. Hence, the need to perform construction QA (CQA) by an independent,but knowledgeable third party. However, if that party has not previously worked on geomembranes the CQA will be ineffective and a waste of good money. Even then, it should be assumed that any single liner will leak some minimum amount. It should also be recognized that zero leakage is not a realistic option. It might occur but it probably will not. This might be difficult to accept! In the USA the maximum allowable leakage rate through a single primary liner of a landfill is 200 litres per hectare per day (lphd). Above this the leak must be located and repaired. Therefore, the liner design should include a subgrade drainage system that can handle such leakage without further damage to the subgrade and the liner.

To eliminate or minimize the number of leaks in a newly constructed liner the final stage of CQA should be a geoelectric liner integrity/leak location survey.Survey statistics show that, despite good CQA there can still be from 12 to 2 leaks per hectare depending on liner area and complexity. These holes can vary from a pinhole to a sizable gouge/puncture, created by an excavator bucket and its teeth. Most damage (74%) is done when the geomembrane is covered with soil. Surprisingly, most holes occur on the floor liner,not at penetrations and corners, and are, not surprisingly,made by stones (drainage) and heavy equipment.

In an exposed 635,000 m2 liner that had been installed and CQA'd by experienced companies a final geoelectric integrity survey found 131 leaks (~2 per hectare) from a pinhole to a 1 m lack of seam bonding.In this exposed liner most leaks (40%) were punctures between 2 and 10 mm in size. Clearly, it was beneficial to the owner to find and repair these leaks rather than to put the tailings pond into service expecting that no leaks were present.

In a much smaller decorative pond project in a park on top of a capped landfill a geoelectric survey was required to be performed by the liner installer,but at what stage was not specified. Since the installer was not responsible for placing the soil cover on the single HDPE liner but knew that this was the first liner project the earth mover had worked on, the installer elected to do the survey at the completion of geomembrane installation, before the soil cover was placed. Four small leaks were found and repaired.After the soil had been placed, with no CQA, it was noticed that there were large fragments of broken glass in the cover soil! The installer was correct to do the survey on his work before others damaged it.

However, the owner and regulator should obviously require the survey to be done after the soil cover has been placed and all possible damage has been done.

CQA: Geoelectric Leak Location

There is no question that geoelectric surveys are beneficial. In fact, New York State is about to require that surveys be performed on the floor of the secondary (lower) liner of their double lining systems as well as on the complete primary liner. However, not all lining systems are survey-able. Geoelectric surveys work because the geomembrane is an electric insulator.A DC electric potential is applied between the liner. The potential gradients are measured in the water, or on top of the soil above the liner, and the unique signal at a leak is sought. Leaks can be pinpointed in water and located within a few millimeters under 1 m of soil cover. However, to do this effectively requires four basic boundary conditions to be observed:

• A conductive medium immediately on top of the geomembrane
• A conductive medium through the holes being sought
• A conductive medium immediately under the geomembrane, and
• No electrical connection between the media above and below the liner

- all current should flow through the holes being sought. Thus, a single liner with a soil cover must have liner exposed around its periphery so that current does not flow over the edge of the liner from cover soil to sub-grade soil. Therefore, liner designers soil/water on top of the liner and the soil under the should consider these boundary conditions for lining systems that might require surveying as the final stage of CQA or during service. These boundary conditions also need serious consideration when there is an unacceptable leak in a liner that has to be found.These conditions will determine whether an effective survey can be performed, or what preparation needs to be done (such as digging an isolating channel around the edge) in order to perform a successful survey.

Long-Term Evaluation

In the USA and Europe HDPE geomembranes have been in exposed service for about 30 years, so there is increasing interest in just how long they will last before finally failing - by thermal and photooxidation and stress cracking. Some materials made in the mid 1990s in some applications have reached EOL after only 15 years. In other applications they would have lasted longer. Likewise, other materials in Geomembranes: Reliability and Durability HDPE Geomembranes: Maximizing Reliability and Durability Gayatri Polymers & Geosynthetics the same application would last longer, but it is not known how much longer. On the assumption that facility owners would rather have a few years notice of EOL rather than to be faced with an unexpected costly and environmentally damaging failure, the author has developed a protocol for periodically examining exposed HDPE liners to generate that advanced notice of EOL.

The protocol involves periodic on-site examination and sampling of a liner. A benchmark sample is obtained from as-manufactured archived material, or from material in an anchor trench that has not been exposed to elevated temperatures and UV radiation. Other benchmarks, though not as satisfactory, could be the manufacturer's specifications, the project specifications, the applicable standard at the time of manufacturing, or the test results from the first site sampling. Testing of sampled and benchmark materials includes only the relevant durability parameters listed above;
1) the change in HP-OIT that reflects the rate of loss of stabilizers providing protection against thermo- and photooxidation and the amount remaining for continued protection,
2) the reduction in SCR, and
3) whether or not oxidation of the surface is already occurring and, thus,whether stress cracking might already have been initiated.

It should be recognized, of course, that oxidation is an exposed surface effect whereas HP-OIT is typically measured on a specimen from the full thickness of a geomembrane sample. Thus, there can remain some AO in the center and underside of the geomembrane that provides an HP-OIT value even though the exposed surface has had all AO consumed (zero HP-OIT), and may have cracks initiated in it.Hence the reason to determine whether or not the surface has actually oxidized even though HP-OIT implies it has not. At the leading edge of this technology we are at present building a database to relate aged HP-OIT values to the likelihood of there being surface oxidation. At the same time we are building another database to determine how much surface oxidation is necessary for stress cracking to be initiated.

Better Design, Better Use Thus, HDPE geomembrane liner technology has developed to the point of being able to:

• specify liners with durabilities appropriate to the application;
• nondestructively evaluate100% of the area of complete new lining systems for the presence leaks before being placed in service;
• locate problem leaks in in-service lining systems covered by soil and water;
• obtain a few years warning of EOL of exposed liners.

Thus liner technology is moving from a "one-sizefits-all" philosophy to more mature and responsible subgrade stone protrusion Geomembrane liner being installed in quarry landfill l Liner exposed around pond periphery so that geoelectric survey can be performed on geomembrane under soil cover. Excavator bucket teeth holes through new landfill liner,found during a geoelectric integrity survey. Equipment track damage covered during liner construction. Triple liner (nine geosynthetic layers) being constructed under railroad refueling platform.

Dr. Ian D. Peggs is President of I-CORP INTERNATIONAL,Inc., providing geosynthetic materials performance consulting, failure analysis, and expert witness services to clients in more than 30 countries.He teaches leak location to construction professionals through the TRI-CORP Liner Integrity Center (T-CLIC). He can be reached by email at icorp@geosynthetic.com and on the internet at www.geosynthetic.com