Over the past 30 years, geosynthetics made of high-density polyethylene (HDPE) have become the material of choice in dozens of applications ranging from landfill liners, to heap leach pads, and critical storage ponds to concrete protection membranes. The transition, which came on the heels of regulatory changes in 1982, introduced better technology that changed the purpose of landfill liners from a tool that minimizes leaks, to a tool that drastically reduces and ideally prevents them altogether (1).
“Prevention (using geomembranes), rather than minimization (using compacted clay liners), of leachate migration produces better environmental results in the case of landfills used to dispose of hazardous wastes. A liner that prevents rather than minimizes leachate migration provides added assurance that environmental contamination will not occur.” – EPA
We will review the history of HDPE as a liner, explore its benefits as a construction material, and discuss potential weaknesses that can be overcome with best practices and quality control.
A brief history of the HDPE liner
Since the early 1980s, HDPE has been the geomembrane of choice in landfills applications across the world—with EPA and the German equivalent, UBA, leading the research and development toward improved waste containment strategies (see Table 1).
Before the 1980s, landfill containment, if any, was mostly done using a compacted clay liner (CCL), often obtained from the chosen dump site. CCLs can be quite effective at achieving low permeability values, but came with two significant drawbacks: the sheer volume required and susceptibility to chemical reactions. For clay liners to work, they had to be between 600 mm and 1,500 mm thick—a significant amount of volume that could have been used toward containing the waste itself. Additionally, clay liners have been shown to be subject to chemical reactions and subsequent shrinkage (2). Bentonite clay, for example, can be ruined by low-confining pressure and substitution of the sodium ion with calcium or magnesium.
Clay’s chemical susceptibility ultimately prompted the EPA to impose changes, including a mandate that would require a geomembrane liner in all landfills (4).
Table 1. Current waste containment strategies at landfills in the United States and Germany, from reference 1.
|Single mineral type||Multiple mineral type|
|0.75 mm or 1.5 mm geomembrane||2.0 mm or 2.5 mm geomembrane|
|“Intimate contact” between GM and CCL||“Press fit” between GM and CCL|
|Performance drainage||Prescriptive drainage|
|Double liners with leak detection||Single composite liner|
HDPE benefits: The containment trifecta
Of the geomembranes known in the early 1980s, HDPE quickly found favor as the landfill liner of choice thanks to five driving properties: strength, flexibility, chemical and weather resistance, and ease of installation.
When you need to contain heterogeneous waste, there are no products on the market more reliable or more consistent than HDPE. That’s because HDPE can be made without the need of other chemical additives, it is relatively uniform in its inert composition, promoting broad chemical resistance (see Table 2).
“For the storage of liquids that are an unidentifiable or of an unknown variety (e.g., from industrial processes that are in the design stage and not yet on-stream) or for leachates of a very heterogeneous nature, extreme conservatism must be used… Because of its relative inertness with chemicals, HDPE will often be the material of choice.” – Koerner (1)
Table 2. General chemical resistance guidelines of some commonly used geomembranes (from reference 4).
|Crude Petroleum Solvents||✓||✓||✓||✓|
HDPE also possesses high tensile strength, with elongation yielding between 10% and 15% strain, with elongation failure exceeding 700%. Finally, its ease of installation comes from its lightweight properties and its ability to be integrally fusion-welded by thermal methods rather than by using solvents and adhesives.
These properties and decades of excellent performance have solidified HDPE’s role within the containment industry. Thanks to these years of study, the precise role of HDPE as a liner has been established at a global scale.
HDPE is considered the longest life span of all synthetic liners. Predictions for the life span (half-life predictions) of HDPE liners have been estimated to be over 400 years in covered applications, based upon the temperature of the operating environment.
Unmaking Waves: Using landfill construction quality assurance
The lining of a landfill has since become the foundation of these civil engineering structures. As such, great care must be taken through its construction. The process to ensure that the engineered design is implemented properly is called Landfill Construction Quality Assurance (CQA).
Both in Germany and in the United States, HDPE is required to be in intimate contact with the underlying layer (see Table 1). This requirement can be challenged if large waves form during placement, generally caused by unmitigated temperature fluctuations throughout the day. However, by following proper installation procedures and implementing good quality CQA, wave formation can be avoided.
Wave management techniques for HDPE installation include thermal management through synchronized deployment, whereby the liner is placed and seamed during strategic times of the day to counter act expansion and contraction forces. This allows the material to experience a full cycle of thermal heating and cooling. This coordinated installation method is favored in Germany and many countries worldwide as the primary means of satisfying the CQA requirements. Material placement is a careful process of utilizing low ground pressure equipment and techniques that allow the material to gently come in contact with the geosynthetics, versus pushed across like a typical soil placement project. Good quality construction and CQA oversight is important.
- Koerner, Robert M. (2012-01-16). Designing with Geosynthetics – 6Th Edition; Vol2 (Kindle Locations 1939-1949). Xlibris US. Kindle Edition.
- Peggs, Ian D. “Geomembranes in landfills: Overview of liners, caps and floating covers.” Waste Management World. (2008).
- Colten, Craig E.; Skinner, Peter N. (2010-06-28). The Road to Love Canal: Managing Industrial Waste before EPA (Kindle Locations 3350-3352). University of Texas Press. Kindle Edition.
- Vandervoort, J., The Use of Extruded Polymers in the Containment of Hazardous Wastes, The Woodlands, TX: Schlegel Lining Technology Inc.
- Beck, E. C. “The Love Canal Tragedy.” EPA Journal. (1979). Accessed online: https://archive.epa.gov/epa/aboutepa/love-canal-tragedy.html.