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The GMA Techline answers all

News, Q&A: GMA Techline | August 1, 2018 | By: , ,

UV Weathering of Geotextiles

Q: My firm is providing construction quality assurance services for a landfill cell expansion. We are having an issue with our contractor and are going to be in a position of having 8 ounces/square yard (271 g/m2) of nonwoven geotextile exposed for a period of time, which exceeds the seven-day limit in the specification. Is there research or a white paper that addresses this issue, and is there a justification for leaving the materials uncovered for approximately 30 days?

A: The specification you are working toward citing seven days as the limit for leaving geotextiles exposed is extremely conservative according to our data. In our paper published in the proceedings from the GeoAmericas 2016 Conference, we presented exposed lifetime data for four polypropylene nonwoven-needlepunched (NWNP) geotextiles exposed to ultraviolet weathering devices in the laboratory. We had 6 and 10 ounces/square yard (203 and 339 g/m2) data, and I will give you the 6 ounces/square yard data, since that geotextile is a bit lighter than yours. For half-life, we got 0.29 year in strength and 0.21 year in elongation. As extension, we project this information to Phoenix, Ariz.—a city with one of the highest ultraviolet radiation levels in an urban area in America—to a half-life of 0.42 year in strength and 0.55 year in elongation.

These half-life data points indicate that there is still material remaining, but they have been compromised insofar as original properties are concerned. In summary, even your suggested time of 30 days should be quite doable for this material.

Robert M. Koerner / GMA Techline

Patching Standard for Geomembranes?

Q: Our target solid waste regulations require construction quality assurance plans to address “the upper limit on the amount of patching any geomembrane panel can receive.” Does the Geosynthetic Institute have guidelines addressing this type of requirement, or are you aware of an industry standard? I appreciate your anticipated help in this matter.

A: Some time ago, George Koerner and I put together a draft guide on factory blemishes and small imperfections in geomembranes, which are usually repaired by putting a bead of extrudate over the damaged area. We tentatively set a limit of 20 such repairs per panel, but it was rejected by pretty much everyone as I recall. The somewhat related issue is geomembranes that have been wind displaced and ripped, thereby requiring a cap strip or extrusion weld, but, here again, the maximum linear dimension of such repairs per panel could not be agreed upon.

Sorry, but we have nothing definitive in this regard. Also, I have never read anything in the open literature on this.

Robert M. Koerner / GMA Techline

Why Are PVDs Called ‘Wick Drains’?

Q: I attended your webinar on prefabricated vertical drains (PVDs) yesterday and found it highly edifying and entertaining. However, there is one thing about it that confused me: Why are these drains also referred to as “wick drains”? As I understand the word “wick,” it means to draw moisture away by capillarity, which, manifestly, the PVDs do not. If you could explain the mistake in my logic, I would greatly appreciate it.

A: Thanks for taking part yesterday and remember that I told participants that I really like doing webinars.

Regarding your question as to the name of “wick drains,” I suspect that one viewing a completed field of installed PVDs with their ends protruding from the ground surface may think they are seeing a bunch of candlewicks, but that is strictly a guess. Incidentally, some like yourself feel the name “PVDs” does not represent the mechanism of hydraulic expulsion that is occurring.

One last thought is that when PVDs are installed at angles and not vertically, such as for drainage outlets on slopes, does the term “vertical” still hold or do they become PHDs? Semantics are quite often troublesome and rarely universal.

Robert M. Koerner / GMA Techline

Green Uses for Filtering Geotextiles

Q: I have been asked about a couple of applications where manufactured soils with organic matter would be placed on top of our dimpled drainage geocomposite. The applications are green roofs and rain gardens, bioswales, bioretentions, and dry stormwater ponds. What is the best type of fabric for these blended soils designed to be light and high in organics? Would there be harm in using a woven so that the small, organic fines pass? Is a nonwoven-needlepunched (NWNP) geotextile likely to clog, and do you have any white papers on it?

A: Since all filtration design models are based on soil particles (size, shape and density), organics, i.e., their individual “pieces,” remain atypical. I do not know of a single design model in this regard. Yet, biological clogging is indeed reported.

As a result, your idea of opening up the geotextile filter is good, certainly in light of the readily controlled porometry of a woven fabric. The next question is “How open?” The answer probably depends on how much the downstream drainage system can take without having a clogging issue. Your call in this regard, since it is site-specific.

Robert M. Koerner / GMA Techline

Creep and Modulus with HDPE Geomembranes

Q: I am working on a landfill analysis and wanted to see if you know of any papers that have to do with the deformation of a high-density polyethylene (HDPE) geomembrane over time due to creep and any papers on how the modulus of elasticity would change or vary depending on the temperature? (We are making a model that will be tested in ambient-air conditions, and we want to see how the model would vary in a landfill environment where the temperatures are higher.)

A: All polymers creep over time depending upon stress level, type and, particularly, temperature. Likewise, the modulus changes with fluctuating temperature. Creep and modulus are readily measured in the laboratory.

That said, we have no specific papers on the topic.

Robert M. Koerner / GMA Techline

Stacking Geotextile Tubes for a Seawall

Q: I have watched your webinar on the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE) website about walls made from geotextile fabric. As I am currently investigating a project for a 39-foot (12-m) high seawall, I am wondering if I can use them for land reclamation? Thanks in advance.

A: Geotextile tubes are excellent when using granular soils, and there are numerous references available. Geotextile tubes can be stacked, thereby forming a wall, but stacking to a height of 39 feet (12 m) in an ocean environment is asking a lot. That said, with polypropylene resins used to make the geotextiles, there is no issue with seawater degradation.

Robert M. Koerner / GMA Techline

Variations in Interface Shear Strength Measurements

Q: Do you know of a reference that discusses the expected or tolerated variation in interface shear strength measurements? That is, if you perform two tests for the same interface under the same conditions, what variation would be acceptable?

A: The latest data that George Koerner collected from his Laboratory Accreditation Program proficiency testing for direct shear per ASTM D5321 for sand on a textured geomembrane is 43° peak with a standard deviation of 5°, and 36° residual with a standard deviation also of 5°. Although we have never tested municipal solid waste (MSW) with sludge additive, I suspect values would be considerably lower and with much more variation. We will send more information on MSW.

Robert M. Koerner / GMA Techline

Updated GRI Seam Standard Specifications

Q: Can you please confirm if the Geosynthetic Research Institute (GRI) GM19 seam standard specification for thermally bonded polyolefin geomembranes has changed and now states that when you are doing peel and shear tests, all five samples for peel and shear should pass? I know the GRI GM19 (2015) standard specification states that four out of five should pass. If the test method did change, where can I find the latest revision?

A: When we originally released our GRI GM19 seam standard specification in 2002, we used relatively low values for shear and peel strengths, yet we retained four out of five as being acceptable. Over the years, people (mainly regulators) complained that with low strength values why not five out of five? We let sleeping dogs lie for a long time. In 2017 we reconfigured GM19 into GM19a for homogeneous geomembranes and finished, also in 2017, a new GM19b for scrim-reinforced geomembranes. Thus, we felt that the time to act is upon us, and we went to five out of five for both GM19a and GM19b. To review the 2017 standard specifications, visit and

Robert M. Koerner / GMA Techline

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