A Comparison of the Response of Geosynthetics in the Multi-Axial and Uniaxial Test Devices

The wide strip tension test imposes boundary conditions that vary from a uniaxial stress state near the middle of the specimen to a plane-strain, biaxial stress state at the clamps. The multi-axial tension test imposes boundary conditions that vary from a plane-strain, biaxial stress state at the restraining ring to a nearly isotropic, biaxial stress state at the center. To evaluate the influence of the stress state induced during testing on the stress-strain response of geomembranes, strain-controlled multi-axial and wide strip tests were performed on specimens of elastic latex, polyvinyl chloride (PVC), and high density polyethylene (HDPE). The ratio of the secant Young's modulus in the multi-axial test to that in the wide strip uniaxial test was approximately 1.2 for the nearly linear, elastic latex membrane, which is substantially less than the theoretically derived value of 2.0. This ratio was approximately 1.4 and 1.9 for PVC and HDPE geomembranes at 1% strain, respectively, indicating greater differences between measured multi-axial and uniaxial responses with materials exhibiting more nonlinearity. Strength values measured in the tests were similar, but the uniaxial test overestimated the ductility (i.e. failure strain) of the HDPE geomembrane. Wider use of the multi-axial test device is recommended for cases where the geomembrane deforms in a biaxial stress state, and material ductility and stiffness are important.