Effect of underliner on geomembrane strains in heap leach applications

A review of 92 heap leach projects from 15 countries provides a starting point for a series of experiments, at 22 °C and a vertical pressure of 2000 kPa, to examine short-term puncturing and the development of geomembrane strains that could affect longer-term performance. Underliners of gravel with some sand or those of gravel and sand caused significant puncturing and excessive strains in the geomembrane for the conditions examined. The shape of the underliner grading curve had a much greater effect on the potential for puncturing and the magnitude of the strains in the geomembrane than just the maximum particle size. Of the six granular underliners examined, the best performance was for the well graded gravelly sand with some silt which offered sufficient support to minimize the strains in the geomembrane due to the overliner while not inducing significant strains directly from the underliner. Nevertheless even in this case the maximum strain of 11% is almost double the maximum recommended in the literature for ensuring good long-term performance of the geomembrane. Consideration of composite liners with GCLs and compacted clay liners shows that the more deformable the foundation, the larger are the indentations and strains induced in the geomembrane by a given overliner. For the specific conditions examined, it is shown that there was no apparent improvement in performance for an LLDPE geomembrane versus the HDPE geomembrane tested. A 540 g/m2 geotextile protection layer above the geomembrane was also found to be insufficient to prevent significant strains in the geomembrane due to the overliner examined.