A multi-axial tension test for geotextiles

The use of geotextiles as reinforcement is well-established in geotechnical applications. However, some uses of these materials occur in geotechnical systems where the in situ loading and boundary conditions on the geosynthetic vary greatly from laboratory testing conditions that are used to characterize their constitutive behavior. In this work, the development of a new largediameter experimental device capable of applying multi-axial, out-of-plane loading to relatively large geosynthetic specimens (48 cm diameter) is presented. Although similar in concept to the types of apparatuses typically used for the established Multi-Axial Tension Test for Geosynthetics, this newly developed device is unique in that load is directly applied to the circular specimen using a rubber membrane, thus allowing testing of pervious materials such as geotextiles. A key advantage of the device is that it mimics the in-service loading conditions of geosynthetics used in a range of design applications including the spanning of subsurface voids and geosyntheticreinforced pile-supported embankments. Constant strain rate, multi-axial tension tests were completed on a range of seven geotextiles that varied in mass per unit area, resin type, anisotropy, fiber type, fiber density, and weave. Two methods for interpreting the results from the multi-axial test, the constant-thickness and constant volume methods, are derived and compared. Uniaxial and fiber tension tests were also performed to provide a better understanding of the multi-axial test results. Three dimensional models, constructed using photogrammetry, were created to evaluate the assumptions that are used in the interpretation of the multi-axial test results and to provide insight into the micro-level behavior of geotextiles in multi-axial tension. The constant strain rate, multi-axial test results indicate that there is a significant deviation in the response of geotextiles in multi-axial tension compared to their response in uniaxial tension. Although ultimate strength values were found to be comparable, the ratio of secant modulus values from the multi-axial test over the uniaxial tension test at 2%, 5%, and 10% strain are consistently on the order of 0.6 - 0.9. The implications of using uniaxial test parameters in analytical and numerical models where multi-axial stress is present is discussed.