Moderated by George R. Koerner
Minimum sediment thickness
Q: We are going to construct some evaporation ponds on existing sediment on top of a Hypalon geomembrane. The sediment looks to be predominantly fine. The subgrade condition below the liner is not known for sure. In such conditions, can you recommend a minimum sediment thickness that should be maintained between the chlorosulfonated polyethylene (CSPE) and heavy equipment?
We are still considering a testing program on the CSPE, but the client has not yet decided.
A: Most construction plans and specifications that I see call for 12 inches (30 cm) of cover soil over any geosynthetic before allowing vehicular traffic. Note the U.S. Army Corps of Engineers requires 18 inches (46 cm) of cover soil. This is based on the following references.
Wilson-Fahmy, R., Narejo, D., and Koerner, R. (1996, 1997 and 1998). “Puncture protection of geomembranes: Parts I, II and III.” Geosynthetics International, 3(5), 605–628.
Guglielmetti, J. I., Sprague, C. J., and Coyle, M. J. (1997). “Geomembrane installation and construction survivability.” Proc., Geosynthetics ’97, IFAI, St. Paul, Minn., 236–252.
Q: We are looking for a geosynthetic that would discourage a contractor from digging up contaminated soil. Do you have a suggestion?
A: With government policy leading to an increase in the development of brownfield sites, the decision is often made to contain hazards underground instead of removing or neutralizing them. In these cases, it’s vital that adequate warning is put into place to protect against future excavations.
There are several such products on the market. Most seem to be bright vivid orange and act as a warning barrier/separator. See the following references for details of the contamination indicator providing long-lasting deterrent at Rocky Mountain Arsenal near Denver, Colo.
Zornberg, J. G. (2010). “Geosynthetic capillary barriers.” Invited keynote lecture, Proc., First International GSI-Asia Geosynthetics Conf., Taichung, Taiwan, 13–28.
Kiel, R. et al. (2018). “Design of evapotranspirative (ET) covers at the Rocky Mountain Arsenal.” Proc., SWANA Conf., Denver, Colo.
Q: I just spoke with Andrew Aho at the Geosynthetic Materials Association (GMA) regarding a large water feature we are going to install at Edgewood Lodge in South Lake Tahoe, Calif. He suggested sending you an email for technical assistance. We are proposing to construct a water feature with a width and length of 25 × 30 feet (7.6 × 9.1 m) composed of large boulders piled on top of each other, having a weight of 35 to 40 tons (32 to 36 tonnes). Normally we use just a pond liner covered with boulders as the base. However, we want this to last far beyond the expected warranty of 15 to 20 years. Therefore, I have researched using a geomembrane with a geosynthetic clay liner and a geogrid.
It would be appreciated if you give us advice on the design. Keep in mind Tahoe is at 6,200 feet (1,890 m) elevation in the Sierra Nevada Mountains. Average low temperatures are 19˚F (-7˚C) in the winter and average highs are 78˚F (26˚C) in July. However, the water will be heated and kept at a constant temperature no less than about 50˚F (10˚C).
A: I agree that a composite liner (geomembrane/geosynthetic clay liner) sounds like a good idea for your application. It is like a belts-and-suspenders approach with redundancy. I would suggest a puncture protection geotextile over the geomembrane (heavy needlepunched nonwoven 12–16 ounces per square yard) instead of the geogrid. You are not looking for reinforcement. You want to protect the geomembrane against puncture.
Good news! Such a liner system, if designed and installed correctly, should last much longer than 20 years, possibly into the 100-plus range.
Ethylene vinyl alcohol copolymer (EVOH)
Q: We’re designing a landfill to manage soils affected by petroleum for a major oil company. The typical petroleum concentration in soil is expected to be on the order of 10,000 ppm (10,000 mg/kg). However, petroleum is only expected to contact the high-density polyethylene (HDPE) when dissolved in leachate. That being said, our local regulators have expressed concern about the potential for the HDPE to degrade when exposed to the petroleum in this situation. I would appreciate if you could provide, or point me toward, research that has been performed on this topic.
A: Existing geomembranes made of polymers, such as HDPE, linear low density polyethylene (LLDPE) and flexible polypropylene (fPP), offer excellent hydraulic resistance and act as a barrier to heavy metals; however, the barrier to many other contaminants can be significantly improved through use of ethylene vinyl alcohol copolymer (EVOH), a random copolymer of ethylene and vinyl alcohol, which has extremely high resistance to the migration of greenhouse gases, hydrocarbons and organic solvents.
EVAL EVOH is manufactured by Kuraray America Inc., Pasadena, Texas. I would search out the following two references for its effectiveness and justification for use in your project.
Armstrong S. (2012). “Performance of model geosynthetics for containment of volatile organic compounds in landfill applications.” Proc., Global Waste Symposium, Phoenix, Ariz.
McWatters, R. S., and Rowe, R. K. (2015). “Permeation of volatile organic compound through EVOH thin film membranes and coextruded LLDPE/EVOH/LLDPE geomembranes.” J. Geotech. Geoenviron. Eng., 141, 14. doi: 10.1061/(ASCE)GT. 1943-5606.0001209.
Life cycle assessment of HDPE
Q: Has a life cycle assessment comparison on the performance and other metrics of an HDPE cap and synthetic cap been conducted and documented?
I am looking at past questions and answers on the website https://tinyurl.com/GMATechline but have not seen anything and was hoping that you could provide a direct answer.
A: In response to your GMA Techline email inquiry, attached are GRI White Papers #6 “Geomembrane Lifetime Prediction” and #41 “Relatively Sustainability (i.e., Embodied Carbon) Calculations with Respect to Applications Using Traditional Materials versus Geosynthetics.” GRI White Paper #41 has a worked-out example of geosynthetics used in a landfill closure that is directly applicable to your inquiry. In addition, it may be of value to consult the following two international standards on the subject.
International Standard (ISO); Norme Européenne (CEN): Environmental Management—Life cycle assessment: Principles and Framework. ISO EN 14040 (2006)
International Standard (ISO); Norme Européenne (CEN): Environmental Management—Life cycle assessment: Requirements and Guidelines. ISO EN 14044 (2006)
Scrap or recycled polymer
Q: The Delaware Department of Natural Resources and Environmental Control has expressed concern that up to 2% rework/regrind is permitted in the manufacture of new geomembrane sheets. Understanding the industry standard is 10%, and the large number of completed projects in which the 10% spec has been followed with acceptable performance, we like your input on the issue.
A: Please realize that we need to get our wording consistent and precise. In section 4, Material Classification and Formulation, of GRI-GM13 specification for HDPE, section 4.3 states that “The resin shall be virgin material with no more than 10% rework. If rework is used, it must be a similar HDPE as the parent material.” Furthermore, section 4.4 states that “No post-consumer resin (PCR) of any type shall be added to the formulation.”
For starters, regrind is not discussed here and is too vague for an ASTM or ISO definition. Rework to most in the industry is edge trim, which is ground and then reintroduced into the hopper of the extruder. In most cases, it never leaves the factory and has never touched the floor. Regrind has enough processing aid-antioxidant package in it to survive several passes through the extruder without significant thermal degradation. We did an extensive study when GRI-GM13 was originally written to arrive at this number. It appears that the test of time (30 years) has corroborated this decision.
In addition to your question there often come inquiries like, “If we re-pelletize HDPE geomembrane scrap and use it at max 10% back into manufacturing of the same parent geomembrane, would we be in compliance with section 4.3 of the specification?” o this we answer, “no.” The intent of section 4.3 of the GRI-GM13 when written was edge trim. If one re-pelletizes HDPE geomembrane “scrap” and uses it back into manufacturing of HDPE geomembrane, the formulation could be off due to out-of-conformance “scrap,” traceability issues, dirt, moisture and many other noncompliant factors. We realize that re-pelletize equipment has made strides in efficiency and performance over the last several years. However, we think that geomembranes are used in critical applications and need to last a long time. This is no place for scrap or recycled polymer.