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Questions and answers from the GMA Techline

News, Q&A: GMA Techline | February 1, 2017 | By: ,

Spraying? Shear Strength? Hydraulics?

[A more] non-permeable GT?

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

A: 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 volatile organic compounds (VOCs) and radon. Such polymer sprays make the geotextile quite UV resistant—how much so requires testing. Let me know if you need more information.

Bob Koerner  |  GMA Techline

GM puncture?

Q: I am writing because a client of ours would like to spray Posi-Shell® directly onto an 80mil-thick high-density polyethylene (HDPE) geomembrane. My understanding is that the product is shot under pressure with a nozzle onto a surface. Our concern is puncturing of the geomembrane. Have geomembranes been tested under similar conditions or pressure?

A: Posi-Shell should pose no problem insofar as damaging (aka, puncturing) a geomembrane. I would consider a textured material of at least 1.5mm thick as being a conservative approach. That said, we have not tested it as such but there is lots of field experience. Most landfill owners have used it, particularly as a fire suppressant but also for odor control.

Bob Koerner  |  GMA Techline

GM grinding

Q: In our specifications we direct the contractor not to remove more than 4mils of material when grinding before extrusion welding. I am trying to determine where we came up with this requirement.

In your construction quality assurance (CQA) class you discuss light grinding to remove the oxidated layer that is about 1mil-thick. In an installer manual, it says do not remove more than 10% of the materials.

Any thoughts on the 4-mil limit or the 10% limit we specify? Do you know the source for these recommendations?

I assume that the remedy for over-grinding would be to patch the area that was over ground. Does that sound reasonable?

A: The depth of grinding for extrusion
welds is 5% of the sheet thickness, per the EPA’s technical guidance document, Inspection Techniques for Fabrication of Geomembrane Field Seams.  It is EPA/530/SW-91/051 May 1991. We will send you the appropriate section.

Bob Koerner  |  GMA Techline

Hydraulic conductivity

Q: We do want to keep E96 in the requirements. Would it be possible to also state the units as a hydraulic conductivity (cm³/cm²/s or just cm/s)?

The Railroad Commission of Texas has it in their specs and it keeps showing up on lightweight material specifications for other applications: Summary of Statewide Rule 8 / Additional requirements for non-commercial fluid recycling (NCFR) pits.

Pit design: Pits must be lined with a liner that has a hydraulic conductivity 1.0 x 1 0 -7 cm/s or less.

A: Yes, it is possible to convert diffusion permeability (via an E96 WVT test) into hydraulic conductivity, aka a Darcian hydraulic permeability. We will send the conversion to you.

As you will see, a 0.75mm PVC has a Darcian k-value of 0.5 x 10-11cm/sec. Incidentally, I keep including it in Designing with Geosynthetics and do get occasional comments in this regard.

Bob Koerner  |  GMA Techline

Internal shear strength

Q: I am hoping you could help with a question or point me in the correct direction. There is some concern with the internal shear strength of heavyweight geotextiles. After searching online, I did not come across studies or publications on this. Are you aware of anything that is available on the topic or if it has become a concern?

A: Your question is of interest but only for needle-punched nonwoven geotextiles of mass greater than about 18 ounces per square yard (osy). At this mass and thicker, a manufacturer cannot do a single pass at needling and then resort to needle-punching two 9 osy (or whatever gives the product the desired value) geotextiles together: It is a secondary needling if you will. Whether a more sensitive shear plane is then developed depends on the needling density of the two previously needle-punched products by themselves. The answer to your specific situation is to do the direct shear test to see if such a plane of weakness applies.

Bob Koerner  |  GMA Techline

GM under roads

Q: We are working on a design/permit application. We are requiring the installation of geomembrane under the access roadway on the closure.

We have done some initial veneer slope stability calculations (looking at 8–10% grades).

Have you previously evaluated the use of geomembranes under roads on closed landfills/exposed geomembrane covers (EGCs)? Any insight or references to papers on the subject would be greatly appreciated.

A: Your situation is similar to a ramp for trash trucks going down into a below-grade landfill after the liner system has been completed. A complete roadway system must be designed, including geotextile protection layer, gravel drainage layer, temporary wearing surface, and gravel where the ramp ties into the upgrade sideslope. Depending on the size of the vehicles, including dynamic loads, the design is like any other unpaved road.

That said, we have been involved with four such failures: sliding on a geosynthetic clay liner (GCL), damage to the upper geomembrane, and two washouts from water coming off of the upgrade sideslope. Depending on the traffic at your site, you will likely end up with at least 18 inches of base course plus the geosynthetics.

Bob & George Koerner  |  GMA Techline

Recycling question

Q: I am looking for information regarding what is our industry allowance for using recycled polymer in either the resin or the manufactured sheet. I have been reading that no recycled polymer is allowed in the resin pellets but some percent (2–5%?) of regrind or trimmed materials during manufacturing process (not from reclaimed used sheet) is allowed. What is the GRI position on this? Maybe you have some guidance?

A: The opinion you state was likely ours to begin with—i.e., no recycle (postconsumer) but 5–10% rework (of the same formulation) was allowable. It somehow seems to be cut back to 2–5%.

Bob Koerner  |  GMA Techline

Transition strips?

Q: Years ago in our landfill, I recall us using a manufactured Hypalon to PVC transition strip to connect Hypalon on the slope to PVC on the floor.

Fast forward to today. We are currently transitioning from PVC to HDPE geomembrane in our base liner system. Our consultant came up with a very interesting methodology—an elaborate shingling design—to connect the two differing liner systems. But the process would consume a lot of airspace and would involve a lot of costs for liner materials too.

So, I was wondering if you have seen or are familiar with any known transition strips to connect two dissimilar materials? Any information you can provide would be most helpful.

A: PVC to Hypalon seams are doable since both have similar stiffness and thickness. Furthermore, their melting temperatures are such that there is a window at which thermal fusion (usually hot air) can get a watertight bond, but not much tensile strength.

PVC to HDPE is yuk! All three items above are greatly dissimilar between the two materials. As a result, I would look into adhesive tapes. The ethylene propylene diene monomer (EPDM) materials are regularly seamed in this manner and there are also tapes advertised for HDPE joining specifically. We tested one and it performed excellently in the lab, but it pulled apart in the field under exposed conditions. So be careful in this regard.

Lastly, I don’t have a grasp on your “elaborate shingling system.”

Bob Koerner  |  GMA Techline

Unintentional reinforcement

Q: In your veneer stability formula (Koerner and Soong 1998), if there is no geomembrane layer, do we consider entering passive cohesion for cover soil and set zero for active adhesion value since there is no real “interface” between the cover soil and the foundation soil? Or can we use a different spreadsheet altogether?

A: We use the uppermost layer interface shear strength in the cross section, which is generally a geotextile. Also cohesion/adhesion is generally omitted since dryness or saturation discounts its use. That said, if the cross section has multiple layers with a low interface layer within, then there are several issues to consider. The paper calls this unintentional reinforcement and it deals with this situation accordingly.

Bob Koerner  |  GMA Techline

Stormwater basin

Q: I am involved in a project where, as a minor component of the works, there is a proposal to construct a temporary stormwater detention basin for the short-term storage of water. There is a concern that the water is likely to be impacted with per- and poly-fluorinated alkyl substances (PFAS or PFCs) and other compounds. The concern is that either the PFAS compounds might have a detrimental effect on the geosynthetic or that they might diffuse through more readily than desirable.

I’ve been trying to find any recent literature looking at the suitability of synthetics such as HDPE and LLDPE for use in a composite liner where PFAS-impacted water is likely to be in contact with the liner, possibly in landfill scenarios, but have been unable to find any documented studies on this matter.

A: Your concerns are well-founded. The concentration of the chemical is critical and can only be assessed with actual site-specific testing. We recommend EPA 9090 type tested per either ASTM 05322 (lab) or ASTM 05496 (field) immersion. Subsequent to the 90-day immersion in site-specific leachate, the geomembrane should be tested per the ASTM 05747 schedule.

George R. Koerner  |  GMA Techline

Resin controls

Q: If you could, please provide your view on resin controls to ensure the manufactured materials is as intended in the specification.

My issue of interest is if it is acceptable to mix resins for different manufacturing sources when manufacturing sheet HDPE? Geomembrane #13 doesn’t prevent this.

Does a formulation need to define a specific resin source, or is it acceptable to use resin from one of several sources?

There are comments in technical specifications attempting to ensure a consistent product, but there appears not to be any industry definitions around this issue of resin from different sources. Many thanks in anticipation.

A: Thank you for your email which includes some dicey details … as you undoubtedly know!

The test controls on the resin are density and melt flow index. For the latter, I also like a flow rate ratio using 21.6 and 2.16 weights. The ratio is a nice indicator of the molecular weight standard deviation. The Designing with Geosynthetics book gives details.

How much variation of these three variables is allowed between different resin producers is unknown, at least to me! I suggest that the resin producers know quite accurately.

Now for the master batch containing carbon black, antioxidants, and the carrier resin. The requisite tests are oxidative induction time (OIT–standard and high pressure), CB content (and particle size), and carrier resin density and melt flow index. I think (?) there is much more variation in different master batches than in the pure resin. This is obviously an opinion.

Bob Koerner  |  GMA Techline 

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