Panel Discussion: Geosynthetics 2015 Conference
February 16, 2015 | Portland, Ore.
Session moderator: Ian D. Peggs, I-Corp International Inc., USA
Do liners leak?
It is said that all liners leak, including geomembranes. They don’t. Double geomembrane liners do not leak into the subgrade. Or do they? What should probably be said is that all single geomembrane liners have the potential to leak, and properly designed and constructed double liner systems result in negligible leakage into the ground.
Good design should assume some leakage rate and evaluate the potential for subgrade (and further liner) damage. Therefore, the lining system should be designed to manage that amount of leakage. This is best done by using a double liner that enables the leakage to be removed so there is no constant head on the secondary liner, except perhaps in the secondary sump.
But what should be the maximum allowable—or action—leakage rate (ALR) through the primary liner? Is zero practical? What about one drip per minute? Or does it have to be a constant stream? Perhaps zero is not measureable. Two hundred liters/hectare/day has been used for landfill primary liners under a leachate head of 300mm. Even 10 lphd has been proposed. But can a leak that generates 10 or 200 lphd be detected and located under 600mm of soil cover?
Clearly the leak detection system (LDS) should be capable of managing the action leakage flow rate and larger; but how much larger? Ten times?
Several wastewater treatment plants have agreed on an ALR of 5000 lphd (500 gpad) under 2m of water in single lined lagoons. Is this reasonable or excessive considering that materials and installation procedures have improved? What is an acceptable ALR for a large deep mining evaporation pond? Should it be a function of maximum or average depth? What is average depth?
Moderated by Ian Peggs, a panel consisting of J.P. Giroud, Richard Thiel, Catrin Tarnowski, Kapila Bogoda, Robert Denis, and Glenn Darilek briefly presented their thoughts related to the specification of action leakage rates (ALRs) for landfill and pond primary liners.
The presentations prompted lively discussion between the audience and panel members and among panel members themselves pursuing these topics. Of particular interest, the subject of ensuring that a realistic leakage rate through the primary geomembrane is actually measured.
All liners leak
J.P. Giroud, JP Giroud Inc., USA
Civil engineering is not a perfect world. Mistakes happen in manufacturing, design, construction, and operations, and unforeseen events will happen in the future.
Therefore, liners leak. When I proclaimed 30 years ago that all liners leak, I did not mean all liners leak all the time. I meant all liners may leak anytime. I was not thinking primarily of geomembrane manufacturers and installers. I was thinking of designers, owners, and regulators. I did not want them to act as if liners never leak. Designers should take measures, owners should pay for the measures, and regulators should adopt rational regulations.
Zero leakage can be desired, but should not be expected. Measures should be taken to reduce leakage and to control leakage. Reducing and controlling are two different goals.
We know what to do to reduce leakage (more or less by order of decreasing importance): (1) have at least one geomembrane liner subjected to low head (i.e., leachate collection layer in landfills and leakage detection layer in double liner systems); (2) use one or more composite liners in a liner system; (3) use electric leak location surveys; (4) use heavy geotextile cushion
(>1000 g/m2); (5) eliminate extrusion seaming; (6) have adequate design details; (7) eliminate geomembrane wrinkles.
It is rare to use all these measures on a given construction site. All of these measures are expensive and pose technical problems. For example, a simple solution for eliminating wrinkles and extrusion seaming is the use of geomembrane materials other than polyethylene. This solution is expensive because polyethylene geomembranes are less costly than other geomembranes; this solution is technically challenging or impossible depending on the degree of chemical resistance required.
Reducing leakage is not sufficient to fully address the leakage issue. We also need to control leakage. Control means that we can measure leakage and are able to develop a design that ensures that the consequences of leakage are acceptable. The criteria for evaluation acceptability of leakage are site specific and project specific.
Unacceptable leakage includes: loss of precious liquid; geotechnical deterioration of the ground (such as cavity formation, slope instability); contamination of ground and ground water above a certain level; leakage causing liner deterioration, hence more of the above.
A double liner system is indispensable for leakage control. If there is no double liner, there is no leakage monitoring. Excellent installation should not lead to the conclusion that double liners are not necessary because such conclusion would eliminate monitoring and, therefore, encourage leakage. A double liner system works only if it is properly designed. Most well-designed double liners include composite liners, but composite liners can cause problems (geomembrane uplift) if not properly loaded. Therefore, designing a double liner system is not the same for landfills and for reservoirs. Good design is not limited to traditional leakage by advective flow. Good design also includes consideration of diffusion. GCLs are ineffective against diffusion. Therefore, composite liners with clay layers and silt layers for attenuation of diffusion should be considered when diffusion of chemical compounds is an issue.
Monitoring leakage is an essential part of leakage control. Leakage monitoring requires targets. Therefore, the concept of multilevel action leakage rates is appropriate. The lowest ALR level should not be too low. Excessively low ALR will trigger unnecessary repairs that could be harmful to the geomembrane liners. The lowest ALR level should take into account the state of practice, in particular the capability of electric leak surveys, and must take into account the site conditions (in particular the head on the liner).
We should not forget that the measured leakage is not leakage into the ground. A lower ALR does not mean lower rate of leakage into the ground. While the lowest ALR level should not be too low, the highest ALR level should not be too high: it should be less than the flow capacity of the leakage detection layer divided by a large factor (e.g., 10). Also, keep in mind that the ALR must at the same time provide a means of
evaluating the quality of the primary liner of a double liner and be linked to the flow capacity of the leakage detection layer.
Finally, it is essential to periodically check the functioning of the leakage detection layer. It would be inappropriate to rely on a leakage detection layer that does not effectively evaluate the liner system. No leakage detected may simply mean that the secondary liner leaks.
Liner systems are used in an imperfect world, and all liners will continue to leak. Monitoring will always be necessary. But who will monitor the monitors? Establishing a rational doctrine for ALR is also necessary.
Presumptive leakage rates for design
Richard Thiel, Thiel Engineering, USA
Zero leakage is a target toward which we strive, but an unrealistic expectation. What GE has to do to achieve “zero” leaks on its X-ray tubes for CT-Scanners: Pump the fittings down to 10-6 atm. and then spray helium around the welded joints. GE uses helium because it is the second smallest molecule on earth that is safe to use. If the weld is bad, the helium will leak through the weld and an alarm will sound on the leak checker. This is in a clean manufacturing environment. We are orders of magnitude away from these levels of quality in field liner installations.
The design community accepts many compromises in its designs that allow things to be less than perfect. We allow some unevenness in the subgrade; we allow some wrinkles in the geomembrane; we don’t install excessive cushions that would protect against strains caused by overlying gravels and pipes; we don’t use best available technology for welding; we do not do 100% CQA. Fundamentally we do a “pretty good job” in getting containment, as indicated by field performance over the past generation.
Designs go along with whatever is considered industry standard, as set by local and regional practice. For example, in the USA we have a certain tolerance for subgrade preparation and wrinkles. In Germany there is another standard, which is typically higher. In the minds of some, the industry standards are too high and in the minds of others, they are too low. These standards set a sort of norm against which we lean when we say or believe that we do a “pretty good job.” From this point of view, we could say that each region’s practices result in “presumptive” leakage rates that are consequential to the standard practices for that region.
Instead of berating what values of “presumptive leakage rates” might exist currently, or what they should be, let me point to a few design factors that could be implemented to improve the standard of practice.
I will give a few examples related to the context of “buried” liners, such as landfills and heap leach pads. The issues might be similar for projects such as exposed ponds or secondary containment, but the details and approaches are different compared to buried liners. The technical approach to each of the issues shown on this list will have a direct impact on short- and long-term leakage rates. This list is not exhaustive, but suited to this panel discussion:
1. Subgrade preparation—higher standards related to firmness and smoothness.
2. Liner selection—use thicker liners with enhanced additive package.
3. Require double-lined containment with leakage detection layers for all important facilities.
4. Put more emphasis on control of wrinkles for buried applications.
5. Require more substantial protection layers—geotextile or sand—vis-à-vis the nature of the overliner material including pipes.
6. Require mandatory use of electronic leak location (ELL) surveys.
7. Seaming protocol, equipment, QC/QA are largely carryovers from the 1980s and could be taken up a notch.
8. Who is responsible for the final performance of details such as batten bars and penetrations? Is it the recommendations of the manufacturer (which is often specified), is it the craftsmanship of the installer, or is it the designer who often is not familiar with the tricks and limitations of field installations?
Why not increase the standards for these items? A slide showing the cost impact on average household disposal rates is presented for some of these items.
In this room is a set of industry leaders who perhaps have more to do with setting the industry standard than the average designer. The notes taken at this meeting could in some small way contribute to some future change in those industry standards.
Action leakage rate in Victoria, Australia: How it works and its success
Kapila Bogoda, EPA, Australia
The action leakage rate (ALR) plays an important role in landfill designs in Victoria, Australia. ALR is specified as the maximum seepage rate for landfills and is mandated as a required outcome in the Best Practice Environmental Management (BPEM) guidelines.
As such, ALR is a legislative requirement. The BPEM specifies a maximum seepage rate of 10 L/ha/day for municipal waste landfills, and 1,000 L/ha/day for solid inert waste landfills. The BPEM also provides indicative designs in achieving these respective ALRs. A composite liner system (a clay layer and a geomembrane liner combined with a drainage layer) is proposed for municipal waste landfills. A single liner system (a clay layer combined with a drainage layer) is proposed for solid inert waste landfills.
The ALR has been included in the BPEM guidelines since its introduction in 2001. All Melbourne metropolitan landfills are now designed and constructed to BPEM liner design requirements. Similarly, most of the regional landfills are also being improved to meet these requirements. The implementation of significantly improved specifications and construction quality assurance requirements since 2010 and the application of the environmental auditor system have been very effective in meeting the BPEM requirements.
Not every liner has to leak
Catrin Tarnowski, GSE Lining
Technology GmbH, Germany
I started in the geosynthetics industry working in a third party office doing site inspection and lab testing mainly for lining of German landfill jobs. Sometimes I was not beloved by installers for searching for and finding small mistakes. I had the German view—100% perfection to be achieved. By the way, the leakage rates of a German landfill basal liner system are not being measured.
Starting in international business it was the first thing I heard—you have to leave the German view behind and think international. What did that mean to me? Change my view completely—all liners leak? No, but there is a high potential that liners leak and there are designs with ALRs considering that liners will leak. Installation rate is different, acceptance criterions are different, control procedures are different. OK, lessons learned.
I am still convinced that it is possible to install liners with high perfection, with no measurable or minimal leakage. Why do we want high-quality geomembranes if we later do not allow for adequate installation? There are some conditions to do so—a lot is in the design, in the budget, and site conditions. The German landfill lining concept combines a lot of conditions to achieve a “leak-free” barrier.
Leak-free liners: Achievable?
Robert Denis, Solmax Intl. Inc., Canada
Would anyone expect a new paint job for a spouse’s treasured 1953 first-generation Corvette to be flawless without proper body work prep? How about spray painting over dents, rust, scratches, bird droppings? Doing it in the open in a flooded parking lot? Starting in the winter? Watched by one’s in-laws with constant threats of liquidated inheritance—with minimum budget?
Liner work ALR is not an absolute and universal concept because it is obviously linked to design, project technical requirements, and especially site conditions as evidently displayed by the wide range of figures that have been suggested or claimed over the years within the scientific publication realm (actually anything from 0.007 to 800 gpad—e.g., a 5 order of magnitude span!). Not to mention that measurements are relative to the test method’s reliability and precision.
That’s two strikes out of three, folks. Now here’s the third strike coming at you, and it’s a stinking junk pitch. The blurry answer as to what’s really needed! And by all means, that’s the slimiest one to even chip out in foul territory, yet the most telling. Clearly our industry cannot officially adopt a maximum acceptable ALR based on a strikeout (in spite of the best “athletes” barely hitting .300).
Oh, but hitting .300 is a tremendous feat! Considering it’s being played out in the elements most of the time, with a poorly aerodynamically designed projectile hurled at you from a less than perfectly geometrical dirt mound into an equally uneven, trampled, and often soaked batter’s box, with a less than perfectly rounded third-class lever ash club, within an imaginary—hence, subjective—3-D strike zone, the sun blazing in your eyes, under the watchful stance of an equally blind umpire and thousands of libation-filled raucous onlookers.
Striking out 7 times out of 10 will actually get you the game’s greatest honors and recognition: your name etched in bronze in Cooperstown, your jersey retired, your rookie card sought after, parents naming their newborns after you, a postage stamp bearing your likeness, a scholastic honorary degree, a motion picture about you, perhaps even a Congressional seat, and last but not least, lavish endorsements from beer to denture adhesives.
But it works. And it’s been working for more than 150 years because of primary considerations for the pragmatic world in which we live. Now what’s even more extraordinary is that even though modern sport science, training methods, and performance-enhancing drugs have transformed the players into a supreme race of hefty bearded entertainment gladiators who have nothing in common with the forefathers’ improvised roster of barflies, ex-cons and weekend warriors.
It’s still a close call at second! How’s that for foresight! A standard irrelevant of technical advancements, based on unwavering universal laws of relativity, entropy, fractals, murphy’s, and uncertainty principles!
Give the installer an honestly designed and constructible challenge, an adequate workbench, and clement weather, as well as a proportional budget prior to arguing about theoretical conundrums, and I will agree to talk with you.
Theoretical factors that affect geomembrane leak detection
Glenn Darilek, Leak Location
Services Inc., USA
When discussing geomembrane leak detection, it is common to focus on specifying how small of a leak that can be detected from what distance. Leak detection does depend on the size of the leak and the distance to the leak, but there are far larger factors that affect leak detection.
A mathematical model has been developed to calculate the amplitudes of leak signals with any survey parameters on any arbitrary grid. All of the leak location survey parameters can be input into the model, including leak size, geomembrane thickness, source voltage, depth of overburden, dipole spacing, and three different values of earth resistivity for the materials above, in, and below the leak.
This theoretical model is used to examine the theoretical maximum signals that can be measured. Some examples are given of the leak signal vs. leak size, vs. the resistivity of the material in the leak, and vs. the resistivity of the subgrade. These results point to some of the factors that are much more important than leak size. Of course, the model gives the results one would obtain under perfect conditions. Some real-world factors are also identified.
Repairs: Increase leakage rate?
Ian Peggs, I-Corp International Inc., USA
In a double lining system when any water exits the leakage detection system (LDS) between the two geomembranes it is likely to be called “leakage” through the primary (upper) geomembrane. However, the significance of that leakage may be different to the owner, the project engineer, the general contractor, the installer, the CQA firm, and the regulator.
What the owner considers a leak, the installer considers a minor nuisance that should simply be recycled into the pond. These differences can lead to arguments, bad feelings, and irrational decisions. The most irrational decision may be for the owner to insist that the responsible leaks be found and repaired. This will probably require the pond to be dewatered, human and equipment traffic on the liner, additional extrusion welding, and refilling of the pond to again check for leakage.
This process will cause the geomembrane to expand/contract, to be subject to wind uplift stresses, to be susceptible to traffic punctures/tears, and to be stressed at all fixed points such as pipe penetrations; the practical result being that the leakage rate actually increases.
The solution is to specify an ALR agreeable to all parties and which is a reflection of the quality of the lining system, but below which it is not necessary to attempt liner repairs.