Size Effects for Multi-Axial Tension Testing of HDPE and PVC Geomembranes
Geosynthetics may be subjected to multi-axial stress states from plane strain loading, such as dragdown along lined slopes and from out-of-plane loading conditions imposed by localized subsidence beneath waste containment cover and liner systems. The multiaxial tension test is useful in evaluating the performance of geosynthetics subjected to multi-axial stress states. However, widespread acceptance and use of this test has been slow due to the perceived requirement of a large-diameter test apparatus with its corresponding high cost. The development of a fully automated axisymmetric tension test apparatus and the results from laboratory testing of two geomembranes (HDPE and PVC) are presented in this paper. With this test apparatus, any one of four clamping rings of different diameters may be used to restrain the geomembrane during a particular test. Critical aspects of the test equipment and test procedures are described. The laboratory results indicate that use of a clamping ring with a significantly smaller diameter than currently required will provide comparable results provided that guidelines concerning the ratio of the clamping ring diameter to the material thickness are adopted and that stress-strain-time compatibility is maintained. This finding is supported by relevant membrane theory.