Geomembranelinerdurability: Contributing factors and the status quo

Abstract

Regulators and engineers have sufficient confidence in the durability and long-term integrity of geomembrane lining systems to require their use as barriers between potential contaminants and groundwater. Yet experience with such lining systems covers only about 30 years. However, in that period adequate performance has been demonstrated. But how long will such geomembrane materials last before ultimate degradation or failure? In the case of municipal solid waste landfills chemical dissolution and degradation of the typical high density polyethylene (HDPE) geomembrane is considered to be a non-issue. Ultimate durability will be a function of the stress cracking resistance of the specific HDPE resin used, the effectiveness of its antioxidation additives, the stresses generated in the geomembrane during installation and landfill operation, and the stress relaxation rate. The potential influences of each of these phenomena individually, and synergistically, on the lifetime of geomembranes are considered. Introduction It is interesting to note that environmentalists frequently claim that the plastic bags that float around in the oceans are a peril to wildlife for ever, yet they also claim that specially formulated and designed plastic based landfill lining systems are bound to fail in a relatively short time! In our technical world the lifetimes of HDPE geomembranes in landfill lining systems have been variously estimated to be between 200 and 750 years. At the other end of the scale installed HDPE lining systems in other applications, typically exposed pond liners or cast-in concrete liners, have not lasted 6 months without failing. “Failing” is practically defined as developing a leak. Of the many HDPE geomembrane liners that have “failed” in the past 20 years, all have failed in a very limited number of ways, but none have just “worn-out” or generally degraded to nothing, nor is it expected that they will. However, our practical experience with HDPE geomembranes is limited to about 25 years. Polyvinyl Chloride (PVC) has been evaluated after 30 years, and polypropylene (PP) is quite young at about 10 years. North American municipal solid waste (MSW) leachate is typically quite benign, as shown by the model for a standard testing leachate in Appendix A, to the extent that in the USA chemical resistance tests of HDPE are now rarely required. Many EPA 9090 “Compatibility Test for Wastes and Membrane Liners” tests have been performed with MSW leachates and none have been shown to damage the geomembrane – the degradative effect of MSW leachate on HDPE can practically be ignored. HDPE liners in landfills and other applications fail or are made to fail as follows: • Inadequate welding and attachment to structures • Imposed stresses during construction • Mechanical damage during construction • Stress cracking at stress points • Service stresses that separate welds