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The GMA Techline

April 1st, 2020 / By: / GMA Techline

Moderated by George R. Koerner and Y. “Grace” Hsuan

Welding Rod

Q: This question is in regard to the welding rod intended for welding of geomembranes (both high-density polyethylene [HDPE] and linear low-density polyethylene [LLDPE]).

Are there any GRI GM13- or GM17-based requirements for welding rods or are there any white papers or specifications? I am mainly interested in the parameters “Carbon Black Content” and the “Carbon Black Dispersion.”

Shall the welding rod also comply with the GM13 and GM17 specification for these two parameters (2%–3% content and the 1, 2 or 3 categories for the dispersion)? 

A: You are correct, “adequate extrudate welding rods or pellets, of the same composition as the geomembrane, must be used” when welding polyethylene. This is clearly written in the EPA Technical Guidance Document 30SW-91-051. See attached for details. It is also written into the BAM (Germany) regulations and most construction quality assurance plans. We think that it is an important consideration and is often unchecked or overlooked on projects. 

Welding nonhomogeneous geomembranes 

Q: I would like to know what your opinion is about welding conductive
HDPE geomembranes.

The current situation is that the resistance for the shear test meets the specs of the GM19 in all tests; however, there are problems with the peel test: the strength in a peel test on-site does not meet the minimum value required by GRI GM19 for sheets of 60 mils (1.5 mm) (91 pounds/inch [15,936 N/m]). They have obtained values of 80 pounds/inch (14,010 N/m) on average.

Strangely, we are doing welding trials in the plant with a liner of the same production batch and, with a certain range of welding parameters (650ºF–750ºF [343ºC–399ºC] and 6–8 feet/minute [1.8–2.4 m/min]), we are easily obtaining 100+ pounds/inch (17,513 N/m) in peel. However, apparently the installer is not obtaining the same results on-site even after trying different temperatures and speeds.

I would like to know your opinion on the welding of nonhomogeneous polyethylene geomembranes. Do you think they must meet the same strength as homogeneous geomembranes?

A: We have much historical data surrounding GM19. This includes information on nonhomogeneous geomembranes (multilayered, conductive, colored, textured, etc.).  The values in the specification are low (i.e., liberal). I am surprised that an installer cannot meet the values of the specification for both homogeneous and nonhomogeneous (multilayered) geomembranes.

 With that said, there are tricks with nonhomogeneous geomembranes. In short, much mixing is required with wedge welding. Setting the parameters of speed, temperature and pressure are not the same as with smooth, black homogeneous geomembranes. Both Leister and DemTech acknowledge this with their automated welder setups. An additional item to be considered is that the BAM allows for a 20% thickness reduction with nonhomogeneous geomembranes. Again, the point of more mixing at the area to be bonded is clear. If I were to guess at a weld parameter to modify, it would be speed (slow the welder down). In addition, several installers have different-shaped wedges for nonhomogeneous geomembranes. They are wider and longer than conventional wedges. They also have four heaters ported in the wedge for more even temperature distribution.

It must be clearly stated that we are not installers! There is a science/act to this task that changes with environmental factors. As engineers, we humbly check the end product and should be cautious about instructing installation contractors (a noble profession) how to do the work.

Failing DS test

Q: At our project we tested nine seams for destructive seam (DS) tests but one of them has failed. So, what is your suggestion? It’s a seam about 82 feet (25 m) long. Do we need to make a new welding or make extrusion welding full of seam (we don’t think it’s suitable because there may be another fail seam)? Do you have any documents about DS tests when they fail?

A: I would suggest GRI GM19a “Seam Strength and Related Properties of Thermally Bonded Homogeneous Polyolefin Geomembranes/Barriers” as a reference document. It is attached for your convenience. What you are describing is not a failure unless the outlier is very low. When a seam fails a destruct set of tests, an attempt to isolate the failed section is made. This technique is described in EPA/530/SW-91/051 Tech Guidance Document: Inspection Techniques for the Fabrication of Geomembrane Field Seams or Daniel and Koerner’s Waste Containment Facilities textbook.

Daniel, D. E., and Koerner, R. M. (2006). Waste containment facilities: Guidance for construction, quality assurance and quality control of liner and cover systems, 2nd edition, ASCE Press, New York, N.Y. 

Daniel, D. E., and Koerner, R. M. (1993). U.S. EPA Technical Guidance Document: “Quality assurance and quality control for waste containment facilities.” EPA/600/R-93/182, Washington, D.C., Office of R & D. 

Geocomposite/geomembrane final landfill cover?

Q: For a landfill final cover, I’m thinking we could use a geocomposite below the geomembrane and connect it to gas lateral pipes that connect to a gas header pipe and then to a blower/flare system. This approach would be in lieu of installing vertical gas wells that penetrate the final cover system. We could also connect the gas blower/flare system to the leachate collection system piping in case gas moved downward into it. I see several advantages to this approach, including not having numerous vertical gas wells that protrude through the final cover. 

A: Your question is complex and does not have a simple answer. Much depends on the nature of the waste (how degraded, MSW, CCR, C&D, age, moisture content, etc.) and the thickness of the new landfill. If the new landfill is shallow, 15–20 feet (4.65–6.1 m) deep, a geocomposite alone under the geomembrane cap might work. However, shallow landfills are not the norm. U.S. EPA has a very nice Landfill Gas Emission Model (LAND GEM) that we would recommend calculating gas generation rate estimates. After you have this information and know the geometry of the landfill, you will probably need both vertical gas wells and a gas vent under the geomembrane cover for deep fills and a geocomposite only for shallow ones. Sorry.

Smooth versus textured geomembrane elongation

Q: According to the U.S. specification, we must use smooth geomembrane at the base or the bottom of tailing storage facilities and textured geomembranes on slopes with 79 mils (2 mm) HDPE thickness. In the specification of our country, there is a standard method called TS EN ISO 527 for geomembranes break elongation that is accepted up to 700% no separated, smooth or textured. We use the 700% limit if we follow ISO 527. But when we look at the GRI GM13 standard specific test method and frequency of smooth and textured HDPE, there is the ASTM D6693 standard for smooth geomembrane the parameter of break elongation is 700% and for textured the parameter is 100%.

My question is, why is there a different percentage break elongation between smooth and textured geomembranes? Why has the current standard been determined that the break elongation of the textured geomembrane is less? If you have these standards and other documents about this topic, please forward.

A: With most things in life, there are trade-offs. If you want high friction (i.e., textured geomembrane), then you will have lower elongation at break (i.e., texturing causes notching and irregularities). When GM13 was written, a compromise was struck to establish elongation at break of 100%. Please note, most designs for polyethylene are based on yield not break properties.

Service life of geomembrane

Q: We have a client asking us the following question regarding OIT results: “With the report of the OIT being 30 minutes versus the GRI GM13 spec of 100 minutes, would you think the liner is past its service life?” What would be the best answer? I have also attached the OIT report/results.

A: The ratio of old to new is one-third; therefore, you have had a 67% drop in standard OIT ASTM D3895 in 15 years. This is not unheard of and should be contrasted with other properties of the polyethylene. Specifically, elongation at break, which I see is still between 700% and 800%. Based on your elongation results at break, your geomembrane still has service life. 

Accuracy, precision and bias

Q: I am going through a few items for the accreditation and there has always been one thing that I have never been too sure of in the standard when it talks about precision and bias. 

I am never sure how to use this information and have always simply calculated the coefficient of variation and if that falls below the 10%, then results are acceptable. I am beginning to doubt myself and wanted to check whether you see it the same way or if there is a different way.

A: First, a few definitions are in order. Accuracy refers to the closeness of a measured value to a standard or known value. Precision refers to the closeness of two or more measurements to each other. Using the following example, if you weigh a given substance five times, and get 7.1 pounds (3.2 kg) each time, then your measure is very precise. However, if one was actually weighing a 6.6-pound (3.0-kg) mass, you are not very accurate. Both accuracy and precision are determined in a single lab and generally are characterized numerically by the term “repeatability.” Repeatability equals the standard deviation divided by the average written as a percent for a single data set in one given lab.

Bias on the other hand is a systematic distortion of the relationship between a treatment, risk factor, or exposure and laboratory outcomes. Several types of bias can be distinguished: information, equipment, personnel, selection, confounding, etc. These types of bias and their potential solutions are discussed at great length in ASTM E9. Bias is determined in multiple labs and generally are characterized numerically by the term “reproducibility.” Reproducibility equals the standard deviation divided by the average written as a percent for multiple data sets
by several laboratories.

One needs both precision and bias (repeatability and reproducibility) to calculate the ultimate goal: Uncertainty. Unfortunately, there is not a right way (i.e., one) to calculate uncertainty. GAI-LAP has its favorite way, as shown in Equation 1.

Equation 1

To further complicate matters, one can calculate uncertainty based on equipment, personnel, time, location and material variability. GAI-LAP annually calculates uncertainty based on its internal numbers (Sr=repeatability) and the external number from the GAI-LAP proficiency tests (SR=reproducibility). A summary paper on this subject is attached.

Now for my public service announcement (i.e., why are we requiring all of this?). Knowledge of testing result uncertainty is fundamentally important for laboratories; their clients and stakeholders use these results for comparative purposes. Uncertainty of measurement is a requirement of ISO 17025 and a key to mitigating risks, improving quality and reducing costs. Despite the established requirements of a test standard or norm, no measurement is exact. No matter how carefully the result is obtained, every measurement result contains an independent amount of uncertainty. Therefore, if measurement is important, then measurement uncertainty is equally important. 

According to both ASTM and ISO, no measurement is complete without an accompanied statement of the associated amount of uncertainty. Creating awareness for the importance of measurement uncertainty is the key to ensuring that the geosynthetic industry has a focus on measurement quality.

Chemical resistance of HDPE to cyanide

Q: Hope you are doing well. See if you can help us. An agency in Mexico is worried about 79-mil (2-mm) HDPE performance with cyanide (I guess they are worried about degradation). Do you happen to have a paper that we can use to demonstrate and build a case?

A: There are numerous citations of the chemical resistance of HDPE to cyanide. Please also note that millions of square meters of HDPE have been used in gold mines throughout the world in the application of heap leach mining. See attached reference list.

Geosynthetics and CCR

Q: As geosynthetics are becoming more ubiquitous for coal combustion residual (CCR) disposal, do you have any recent articles detailing chemical resistance or longevity of geosynthetics in these applications?

A: I am writing to you in response to your inquiry of the virtues of HDPE geomembrane in regard to its chemical compatibility rating against CCR waste.

HDPE has a very good compatibility rating with most chemicals and is resistant to strong acids and bases, as well as gentle oxidants and reducing agents. For landfill liner 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 73°F and 122°F (23°C 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.

There is little doubt that for any given chemical containment requirement, there is more chance that HDPE will provide better chemical resistance than other geomembrane polymer types.  

In addition, there is a large amount of field evidence that HDPE satisfactorily contains Subtitle D waste. For instance, samples of HDPE geomembrane liner removed from the sump of Lakeview Landfill, Erie, Pa., and from the Cumberland County Improvement Authority (CCIA) landfill, Millville, N.J., after exposure to leachate for more than 20 years are still very flexible and show little signs of degradation.

There is some justification for assuming that HDPE will perform adequately in advance of appropriately conducting 9090 testing. However, we believe that all sites have specific factors affecting them and that the engineer has the right to challenge all geosynthetic products used for containment on a case-by-case basis.