Test method and strain calculation effects on geomembrane strain from gravel indentations

The effects of test method and strain calculation method on strains from nominal 25 mm coarse gravel indentations are examined for a 1.5 mm thick HDPE geomembrane with full-scale physical modeling. Maximum principal strains were calculated using thin plate theory that considers lateral displacement effects and bending strain. Strains from index tests with no subgrade were found to be twice as large as those from performance tests with clay, while strains from index tests with rubber as the subgrade were only 40% of those with clay; neither index test is suitable for selecting protection layers to limit geomembrane strain. Strains from past index tests with idealized single-point loading need to be multiplied by a factor of at least 1.8 to reproduce the maximum strain from performance tests with coarse gravel. Limiting the average membrane strain to 0.25% was found to limit the maximum principal strain to less than 6%, but not to 3% as originally intended by the German standard. The maximum result of membrane plus bending strain of 3% was shown to be closer to a maximum principal strain of 4–6% because of large-displacement and three-dimensional effects. The geotextile protection layers tested (nonwoven, needle-punched, 1500 and 1800 g/m²) were only able to limit the strain to 6% at a vertical pressure of 250 kPa and were unable to limit strain below 3%.