Moderated by George R. Koerner
BIOLOGICAL DEGRADATION OF PE AND PP
Q: Can you please enlighten me on the biological degradation of polyolefin polyethylene (PE) and polypropylene (PP)?
A: Within the various plant forms of biological life, i.e., bacteria, actinomycetes, fungi and algae, polymer degradation is essentially nonexistent due to high molecular weight of the common resins used in PE geomembranes. In order for such degradation to occur, the chain ends must be accessible, and this is highly unlikely for molecular weights greater than 1,000, let alone 10,000 to 30,000, which is common for geomembrane resins such as polyethylene or polypropylene (Koerner et al. 2007, Rowe and Sangam 2002, Restrepo-Flórez et al. 2014). Biological degradation might be possible for plasticizers or additives compounded within polyvinyl chloride (PVC), but information is not authoritative on this subject and many current plasticizers can be made biologically resistant (Rollin et al. 2005, ASTM G160 and D3082, ISO or EN 12225).
Within the higher forms of biological life, i.e., protozoa, spiders, insects, moles, rats and small mammals, polymers are not a food source and thus are unlikely to be consumed. It is possible, however, that an animal may try to penetrate the geosynthetic for access to the opposite side. In this case, hardness of the predator’s teeth enamel versus the geomembrane’s hardness is the key comparison. While such events are possible, authoritative information is not known by the author.
Verification of biological resistance is confirmed by soil, sewage or sludge burial tests (ASTM G160 and D3082, ISO or EN 12225). It is usually carried out for long exposure times, at nearly neutral pH and at elevated temperature. The test specimens are periodically removed from the soil and tested for changes in properties. The extent of the degradation is also examined by way of surface microscopy and various fingerprinting techniques.
LINER FOR CONCRETE WASHOUTS
Q: I am a specifications engineer for the Roadway Design Division at the Oklahoma Department of Transportation. I am in the process of updating some of our stormwater control standards. One of these is the concrete washout system. In all or most of the standards and specifications I’ve read online from other state DOTs, not one seems to go into detail as to the specification or minimum criteria needed for the “plastic liner.” There are phrases such as “minimum of 10 mil thick” and “impermeable and leak-proof,” etc., but I don’t see anything that would refer to a specification or other federal agency’s (EPA, ASTM, AASHTO, etc.) document.
Can you help me with criteria you think DOTs should follow when using a liner for concrete washouts in their transportation projects?
A: Nice to hear from you, and I hope this email finds you well. 10-mil “plastic sheet” is covered under federal specifications by the Defense Logistics Agency. The Commercial Item Description is A-A-3174 Plastic Sheet, Polyolefin, and different Type, Class, Grade and Finishes are defined in the spec. In addition, these materials are also covered by the following ASTM specifications:
ASTM C171, Standard Specification for Sheet Materials Used for Curing Concrete, and ASTM D2103, Specification for Polyethylene Film and Sheeting.
I hope this information helps.
P.S. Given the choice between clear and black, the black generally lasts longer (i.e., more durable).
GEOMEMBRANE THICKNESS IMPACTS ON WRINKLE DEVELOPMENT
Q: As some states continue to review and “improve” their regulations governing base liner design, the use of 80-mil geomembrane in both primary and secondary liner systems is being considered. At the same time, the maximum height of wrinkles may also be regulated. The “stiffer” 80-mil product should help with wrinkles but comes with more thermal mass, which may be an issue for black product. Does GSI have any information on geomembrane thickness impacts on wrinkle development or propagation?
A: In regard to your question on high-density polyethylene (HDPE) wrinkles as they relate to thickness, I have attached two pages from Te-Yang Soong’s 1996 Drexel University Ph.D. thesis entitled “Behavior of Waves in HDPE Geomembranes.” I think they answer your question directly and well. Unfortunately, thickness is not a major factor affecting wrinkles.
To achieve intimate contact with the subgrade (i.e., reduce wrinkles), we recommend the following for HDPE:
- Fixing berms or advance soil in piles with a track hoe or gradall excavator with articulating bucket
- White geomembranes or light color geotextiles to reduce surface temperature
- Textured or scrim-reinforced sheet
- Morning- or night-controlled backfilling when temperatures are low
TESTING PEC MATERIALS
Q: I have a question regarding product testing. We have a product called PEC that is a combination of nonwoven as a backing sheet and knitted with polyester yarn in both directions. To capture the true movement, we remove the yarn of other direction when conducting the test. You may find more details from the attached file.
FYI, the yarn is having higher strength compared to the nonwoven. The strength and elongation are mainly contributed from polyester yarn where their elongation of raw yarn is less than 10%. So, we remove the yarn of other direction when testing, as the effect is negligible.
We would appreciate it if you could provide your opinion and guidance about this practice used during testing.
A: This is not so simple. By cutting the principal yarns free of the confining cross machine webbing, one stops a very nasty artifact of the wide width tensile (WWT) test, which is unique to testing PEC materials. Namely, the webbing knifing through the principal direction yarns and prematurely breaking the PEC material is far lower than the anticipated ultimate tensile strength (from single yarn testing and Pearce equation extrapolation).
Unfortunately, this phenomenon is real, and I have seen it happen in the field. I do not think you have the liberty to remove the webbing during setup of the WWT test without express written consent of the engineer-owner. WWT are performance tests for design, not index tests for saturated process control or conformance use.
USING POSTCONSUMER WASTE PE IN GEOMEMBRANES
Q: Do you think it is possible to include other material besides high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE) in the cross section of a multilayer geomembrane? Suppose we can run on three layers cast line, then put recycled material in the middle. This may be good for a sustainability purpose, if we can include industrial waste, or later postconsumer waste in the middle layer . . . for certain geomembrane grade, perhaps the less sensitive one.
A: In response to your question of utilizing postconsumer waste polyethylene in geomembranes, I do not think this is a good idea. Geomembranes are critical barrier materials used in containment systems with very harsh geoenvironmental conditions. They need to last hundreds of years in a flawless manner. Durability is a critical property, which has to be proven with statistical process control data for both base resin and masterbatch.
That written, there are always off-specification products that are used for noncritical projects. One needs to be very careful in regard to marketing these products.
Furthermore, we are seeing advances in polymer cleaning and pelletizing systems, which can sometimes yield a very good product; however, consistency and traceability are issues.
INSTALLING CCR LINER LOW TO HIGH?
Q: I am an engineer for a coal combustion residual site, and I want the installer to install the liner system starting at the sump and progress to higher elevations. Please advise.
A: What you are recommending is unusual, and I am surprised that an installer would agree to it. I have seen most, if not all, liner systems installed from high to low gradient. There are many stakeholders involved in the panel layout and sequencing of geomembrane installation. The installer, general contractor, regulator, designer and owner all have a schedule and ideas on the matter. The decision must consider the tasks of subgrade preparation, approvals, access, geometry, facility size, weather and groundwater. Know that if you install a liner system low to high and have a precipitation event, you are going to have a mess on your hands because water will get under the liner system. Furthermore, if the liner system is a composite one (i.e., GM/GCL), the clay component will saturate, hydrate, swell and need to be reworked prior to sign-off and approval.
ASTM D3083. Specification for Flexible Poly (Vinyl Chloride) Plastic Sheeting for Pond, Canal, and Reservoir Lining (Withdrawn 1998).
ASTM G160. Standard Practice for Evaluating Microbial Susceptibility of Nonmetallic Materials by Laboratory Soil Burial.
ISO, EN 12225:2000. “Geotextiles and geotextile-related products: Method for determining the microbiological resistance by a soil burial test.”
Koerner, G. R., Hsuan, Y. G., and Koerner, R. M. (2007). “Durability of geosynthetics,” chapter 3, Geosynthetics in civil engineering, ed. by R. W. Sarsby. Woodhead Publishing, CRC Press, Cambridge, England, pp. 36–61.
Müller, W. (2007). HDPE geomembranes in geotechnics. Springer-Verlag, Berlin, Germany, p. 384.
Restrepo-Flórez, J. M., Bassi, A., and Thompson, M. (2014). “Microbial degradation and deterioration of polyethylene—A review.” International Biodeterioration and Biodegradation, 88, pp. 83–90.
Rollin, A., Pierson, A., Lambert, M., and Christopher, B. (2005). “Geomembranes: A guide to material selection.” Proc., Geo-Frontiers 2005, Austin, Texas, ASCE Press, pp. 313–327.
Rowe, R. K., and Sangam, H. P. (2002). “Durability of HDPE geomembranes.” Jour. of Geotextiles and Geomembranes, 20, pp. 77–95.