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Questions and answers from the GMA Techline

August 1st, 2016 / By: , / GMA Techline, Resources

Geosynthetic filter failure

Q: I am interested in your upcoming webinar on geosynthetic filters and was hoping to get a little clarification on the content. I’m most interested in how geotextile fabrics are used with riprap but I also understand these fabrics are perhaps more commonly associated with roadway construction. How much does this webinar address riprap filters?

I have come across such a wide range of opinions on the efficacy of these types of filters and have been looking for some better guidance than what I have access to currently.


A: The webinar covers about 70 geotextile (GT) filter failures and a few of them are, indeed, beneath riprap. They seem to be uplifts of the GT into the voids of the riprap and then scour beneath the GT followed by rock deformation into the scour hole.

That said, the webinar is not a design presentation per se. It is a picture show of problems with commentary but far less than a forensic investigation.

Bob Koerner | GMA Techline


Welding geomembrane to pipe

Q: I am designing a final cover system for a landfill. We are placing a 40-mil LLDPE membrane composite system. There are several HDPE riser pipes along the toe of the slope. So the questions:

Is it possible to extrusion weld LLDPE membrane to HDPE piping?

If so, can you suggest some standard construction detail callout language we can use?

If not, can you suggest an alternate sealing method between the two materials?

Thanks for your help!


A: Thanks for your question and let me proceed in the order you asked. No, it is not possible since you will either melt the LLDPE or not melt the HDPE pipe, particularly since the pipe is true HDPE (whereas HDPE geomembranes are really MDPE).

Factory-manufactured pipe boots are commonplace. Your manufacturer has such details. That said, the toe of the slope is “dicey” to make such a connection.

I suggest you re-examine your design and get risers off of the floor of the landfill. Again, the design seems awkward to me.

Good luck.


Question–follow-up: Thank you. Unfortunately, I cannot get the risers off the floor. This is an already filled landfill so the pipes are embedded where they are. I will see if I can get some suggestions on a suitable pipe boot.

Answer–follow-up: OK, but I sense you will have leakage issues going forward.

Bob Koerner | GMA Techline


Geotextile filters

Q: I have a question regarding current criteria for geotextile filters, particularly for use with very fine clay soils. Several authoritative sets of criteria give rather different answers, and I would like to get your opinion on which is most appropriate to use.


A: The Luettich chart is what we use regularly. It is very well thought out and presented. ASSHTO M288 is more generalized but in this case agrees with [other] charts. The FHWA is much too broad and frankly I have not used or referenced it.

Hope this helps.

Bob Koerner | GMA Techline


Welding question

Q: Have you seen or heard of anyone welding a reinforced scrim product to a smooth geomembrane? If so, how would you test the seam for peel and shear?


A: This is a new one on me. I have never heard of anyone welding a reinforced scrim product to a smooth geomembrane. I would test it, strip and grab in shear first, to see what happens. Then I would move on to the peel test looking for continuity near the scrim.

George Koerner | GMA Techline


Geomembrane puncture calculations

Q: We are reviewing some old calculations for geomembrane puncture and the calculations referenced in [your] Designing with Geosynthetics–4th Edition, equation 5.33. In that equation there is factor for long-term chemical/biological degradation (RFCBD).

There are three values listed (mild leachate = 1.1, moderate leachate = 1.3, and harsh leachate = 1.5), but I don’t see a definition for mild, moderate, or harsh leachate. Can you tell me how to define the mild, moderate, and harsh leachate?

We have the 6th edition of Designing with Geosynthetics, but I don’t see that equation in that book.

Thanks in advance for your help.


A: When it comes to accelerated degradation of geosynthetics, I think mainly of hydrocarbons that cause swelling and subsequent chain scission of the crystalline structure of the polymer. That said, at what level of the site-specific leachate contains either aliphatic or aromatic hydrocarbons is unknown to me and, as a result, I didn’t list any limits.

In short, I chickened out! Sorry about that. I suspect that commercial testing labs that do EPA 9090 testing have information, but it is generally proprietary.

Bob Koerner | GMA Techline


HDPE temps

Q: I am trying to find some literature by the GRI that states clearly the maximum recommended temperature HDPE can be exposed to or in contact with if reasonable durability is required (beyond 25 years).

Can you please guide me in the right direction?


A: For a 25-year lifetime, HDPE can withstand enormous temperatures. Our data shows that at 40°C the halflife is 93 years, so don’t worry about the material; rather worry about the installers. Our U.S. EPA Technical Guidance Document gives no upper limit and a 0°C lower limit for installation.

Bob Koerner | GMA Techline


Techline archive?

Q: While reading the GMA Techline in Geosynthetics magazine, I was wondering if a compilation of all the questions and answers exists somewhere. I’m hoping that you will be able to direct me to a link on the website or have a report available.

However, if one doesn’t exist, I’d like to pose the thought that such a compilation would be extremely helpful to many.

Just an idea.


A: We are delighted that you read the results of this ongoing task. It is actually enjoyable to read the questions and to give the best answers that we can without making them research projects. That said, a report that we are preparing will have every one of the most difficult questions and answers (Level #1) presented accordingly. There are about 120 in this category.

Otherwise, the other 2,880 of them are in a huge filing cabinet incubating for the time when someone wishes to do “data mining.”

Bob Koerner | GMA Techline


Non-permeable geotextile

Q: I have received an inquiry for a non-permeable geotextile that is UV-resistant and will be installed on a long slope for erosion control. Could you recommend a product that would meet these requirements or a solution for this application? Your assistance with this will be much appreciated.


A: You ask an interesting question since geotextiles are known for their excellent permeability, not the other way around. That said, by applying a polymeric spray to almost any geotextile, you have made it impermeable. There are a number of spray products available that, when applied, create a spray-applied fabric barrier to liquids and gases.

Perhaps the most common application in this regard is basement waterproofing against VOCs and radon. Such polymer sprays make the geotextile quite UV resistant; how much so requires testing.

Bob Koerner | GMA Techline


Shear strength

Q: On a recent project, the shear strength was determined on 2in.-wide coupons of fPP (scrim-reinforced) seams. It wasn’t until I got the test results in before I realized that 4in.-wide coupons were not tested. The shear results are much lower than the specified shear strength due to the smaller coupon size. Is there an equation to convert the strength determined on one coupon size to the strength that would be anticipated for a different size coupon?

I’m not sure if it’s as easy as just doubling the strength I got on a 2in.-wide coupon to get the strength on a 4-in. coupon. I wanted to check and see if it is as simple as that or that there is some equation to use. Please let me know. Thanks!


A: I assume that you are testing seams in shear and furthermore they are grab tensile tests, which should be conducted on 4in.-wide specimens gripped in the central 1in. If that is correct, the difference between 2in. and 4in. widths should be quite similar since only the central 1in. is gripped.
In one case you have 0.5in. non-gripped material on each side, and the other you have 1.5in. of non-gripped material on each side of the stressed material. You might get a nominal increase with the 4in. specimen. In no case will the 4in. be twice the 2in. with this type of grab test configuration.

If, however, the test is a strip tensile, then your doubling might be reasonably close. The ultimate answer is to do both tests and observe the differences.

Bob Koerner | GMA Techline

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