Long-Term Assessment of a Layered-Geotextile Protection Layer for Geomembranes

The effectiveness of layered-geotextile protection layers comprised of combinations of nonwoven needle-punched, woven slit-film, and nonwoven heat-bonded geotextiles to minimize strains in landfill geomembranes is examined. Results from physical experiments are reported where a sustained 700-N force was applied to a 28-mm-diameter machined steel probe on top of the protection layer, which was above a 60-mm-diameter, 1.5-mm-thick high-density polyethylene geomembrane and a 50-mm-thick compressible clay layer. The experiments are intended to simulate the physical conditions in a medium-sized landfill with an average vertical stress of 250 kPa and to capture the mean response with nominal 50-mm coarse gravel above the geomembrane. Screening tests were first conducted for up to 100 h at temperatures up to 55°C to evaluate three different combinations of layered geotextiles. Of those examined, the combination with a low-slack, heat-bonded geotextile above and below a thick, nonwoven, needle-punched geotextile as its central core was found to provide the lowest strains. A time-temperature superposition method was then developed and validated as a means to predict the long-term effectiveness of the most promising layered-geotextile composite. Last, long-term predictions of tensile strain were made and compared with proposed allowable limits. Despite the encouraging results from the short-term screening tests, even the most promising layered-geotextile composite is not recommended as a protection layer to limit long-term geomembrane strains for the particular force, particle size, and materials examined because the predicted strain after 100 years at 22–55°C of ∼10% exceeds the range of currently proposed limits of 3–8%.