HDPE Liner Waves
by George Yazdani, MSc., P.E.

HDPE liner waves were the hottest topic of discussion at the 11th Geosynthetic Research Institute Conference held during December 9 - 10, 1997, in Philadelphia. Several papers were presented regarding this topic: two papers by researchers, two by HDPE liner manufacturers/installers, and one by regulators. The first presenter expressed the following areas of concern for HDPE waves:

• Intimate contact with the subgrade soils cannot be completely achieved.
• Waves create mini dams, which may restrict flow of leachate over the liner.
• Residual stresses in the wave area may affect the long-term performance of the liner.
• Changes in ambient temperature of entrapped air under a wave could possibly result in desiccation of underlying clay soils.

George R. Koerner of GRI presented the second paper² on properties of HDPE field waves. In this study samples were removed from a 60-mil smooth HDPE liner near the sump area of an 8-year old landfill. The samples consisted of a number of wrinkles with different shapes (i.e. prayer shape, "S" shape, and mushroom shape). The samples were tested in GRI's laboratory for evaluation of short-term tensile properties and environmental stress crack resistance. Wide-width tensile test results indicated that the tensile strength and elongation at yield were not reduced for the wrinkled material as compared to flat material. The environmental stress crack test results by the notched constant tensile load test (NCTL test) indicated a reduction of up to 55% for the wrinkled material. However, the residual stresses (up to 22% according to Dr. Soong's study) in the wrinkled samples were not taken into account for application of the tensile load for the NCTL test. In other words, due to the residual stresses in the wrinkled samples, these samples may have been tested at higher tensile loads than the non-wrinkled samples, resulting in lower values.

The third paper³ was presented on the advantages of white-surface HDPE liner. This was a case history on a municipal solid waste landfill in Arizona, which used 60-mil single-sided textured and double-sided textured white-surface HDPE liner. This paper indicates that white-surface liner does not heat up as much as the black liner, therefore it can be installed during hot summer days without excessive wrinkles.

The forth paper⁴ discussed field implications of requiring flat geomembranes. An interesting dilemma was brought up: "Should waves be allowed and even artificially created for future thermal expansion and contraction as usually required in North America, or should they be ironed out for maximum intimate contact as usually required, for example, in Germany". The presenter indicated that flat liner installation is possible at higher cost and longer installation period, or in his own words "...So give me a hard body as a subgrade i.e. perfectly flat, solid and dry, a 3-mm (120 mil) liner to install, a fully cooperative general contractor, an extended installation period, and tons of cash, and I swear you'll have your flat geomembrane; as flat as it gets."

The fifth paper⁵ was by Robert J. Phaneuf of N.Y. Department of Environmental Conservation, who presented a regulatory perspective on HDPE liner waves. This paper discussed the areas of concern with waves and their implications. The paper concluded a need for desired design and installation standards to achieve as "flat" geomembrane installation as is practically attainable, even at higher installation costs.

The main point of these presentations was that HDPE liners should ideally be installed so that they are free of all waves. At the same time they should also be free of tensile stresses, which is associated with a liner with no slack. Practically, it is difficult to achieve this delicate balance to install a stress-free liner, which is at 100% intimate contact with subgrade. The absence of all waves is most likely an indication of a stressed liner. If a wave-free geomembrane is needed, then the liner system designer and installer should implement site-specific design and installation techniques to achieve this goal. The Germans often do install wave-free geomembranes, but at much lower installation production rates and much higher costs. My understanding of the entire presentation was that although there are no reports or studies that indicate HDPE wrinkles are the direct cause of any liner system failure, the HDPE geomembrane industry should minimize, if not eliminate, waves in HDPE liners by better design and installation practices.

Poly-Flex would appreciate your input and any insights you could provide based on your experience regarding this topic.

References:

1. "Behavior of Waves in High Density Polyethylene Geomembranes", pp. 128 - 151 Proceedings ofthe 11th GRI Conference.
2. "Properties of Exhumed HDPE Field Waves and Selected Aspects of Wave Management", pp. 152 - 162 Proceedings of the 11th GRI Conference.
3. "Installation of a White Surface High Density Polyethylene Geomembranes", pp. 163 - 169 Proceedings of the 11th GRI Conference.
4. "Field Implications of Requiring Flat Geomembranes", pp. 170 - 171 Proceedings of the 11th GRI Conference.
5. "A Regulatory Perspective on HDPE Waves in Environmental Containment Systems", pp. 172 - 178 Proceedings of the 11th GRI Conference.