Short-Term Local Tensile Strains in HDPE Heap Leach Geomembranes from Coarse Overliner Materials

Local tensile strains in a 1.5-mm-thick high-density polyethylene (HDPE) geomembrane induced from coarse overliner soil materials intended to simulate the physical conditions at the base of a heap leach mineral extraction pad under very deep burial are reported. They were obtained from physical experiments conducted in a 590-mm-diameter pressure vessel at applied vertical pressures up to 3,000 kPa for 100 h and a temperature of 21°C. An applied pressure of 3,000 kPa corresponds to a heap leach depth of around 150 m. Three different coarse-grained overliner materials placed directly above the geomembrane and a silty sand underliner beneath the geomembrane were examined. Both the grain size and grain size distribution of the overliner affected the maximum tensile strain in the geomembrane. At an applied pressure of 3,000 kPa, the largest strain of 27% recorded for the coarsest overliner tested, which had a maximum particle size of 50 mm and 20% sand, well exceeded one proposed maximum allowable strain limit of 6%. Finer overliners with both a smaller maximum particle size (25 mm) and much more sand (35 and 55%) reduced the geomembrane strains, but the maximum values still exceeded 6% by a factor of 2, even for the finest overliner examined. At these high pressures, a 150-mm-thick silty sand protection layer between the geomembrane and even the coarsest overliner examined was found to be very effective at reducing local indentations in the geomembrane from overliner particles and was able to reduce the tensile strain in the geomembrane to 2%.