ASSESSMENT OF LONG-TERM TENSILE STRAINS IN HDPE GEOMEMBRANES FROM GRAVEL INDENTATIONS
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The physical response of composite landfill liners, consisting of a high-density polyethylene geomembrane overlying a compacted clay liner, was examined with an emphasis on quantifying the magnitude and possible implications of sustained tensile strains that may develop in the geomembrane from gravel particles either present on top of or underneath the geomembrane. This involved development of new experimental apparatus to study the effects of time, temperature and chemical action on geomembrane strains. A method based on the concept of time-temperature superposition was developed to predict long-term lower temperature geomembrane strains from short-term higher temperature data. It was found that the short-term response is dominated by loading strains while in the long-term, creep is more significant. The predictions from this method suggest that the protection provided by a nonwoven needle-punched geotextile is not sufficient to limit long-term tensile strains, but a geocomposite may be able to limit geomembrane strains below proposed allowable limits, depending on the applied pressure and landfill temperature. Experiments were also conducted with the new apparatus to examine the implications of sustained tensile strains. It was found that sustained tensile strains resulted in brittle rupture of a notched geomembrane, despite stress relaxation for a geomembrane under a single gravel particle and constant vertical force. The time to rupture depended upon the maximum applied force as well as the stress crack resistance of the geomembrane. This represents the first compelling evidence that long-term geomembrane strains from gravel indentations should be limited. However, even when using an excellent protection layer from overlying gravel particles, gravel particles in the underlying compacted clay liner were identified as another possible source of tension in the geomembrane that for very large landfills may exceed tensile strains limits. For such cases, compacting the top most layer of the clay liner at the lower bound of its acceptable water content combined with careful site inspection such that visible gravel particles are removed from the clay surface will minimize the tensile stains that may develop.