Gravel contacts and geomembrane strains for a GM/CCL composite liner

A method to record the shape, size, and spacing of gravel contacts that act on a geomembrane from an overlying granular drainage layer is presented. The gravel contacts acting on a 1.5-mm thick, high-density polyethylene geomembrane are then quantified for two poorly graded, angular gravels (GP1 and GP2 with nominal grain sizes of 50 and 25 mm) with compacted clay beneath the geomembrane and when subjected to an applied pressure of 250 kPa. The geomembrane indentations and strains are also reported. Five types of contacts were defined: point, edge, area, perimeter and composite. Point contacts were the most frequent and, along with edge contacts, caused the steepest indentations and the largest strains. The average spacings between gravel contacts were found to be 55 mm for GP1 and 37 mm for GP2. Without a protection layer, the largest tensile strains in the geomembrane were 32 and 16% for GP1 and GP2, respectively. A nonwoven needle-punched geotextile was found to reduce the contact pressure acting on and the resulting strains in the geomembrane. However, none of the geotextiles tested (with masses up to 2240 g/m2) were able to limit the tensile strains below proposed allowable levels for long-term strain even for the short-term conditions examined.