In-plane shear behavior of geosynthetics from bias biaxial tests using digital image correlation

Abstract

Geosynthetics are polymeric membranes used for structural reinforcement of soils in a variety of roadway and foundation applications, many of which create biaxial loading on the geosynthetic. Orthotropic linear elastic models have been used to represent geosynthetic behavior at working load levels for engineering design purposes. Typically, the models rely on index parameters obtained from test methods that do not represent the biaxial field loading conditions. Proper calibration of these models requires load-strain data obtained from tests that have controlled stress and strain boundaries such as biaxial tension tests. Previously at Montana State University, Haselton (2018) successfully used a custom biaxial device to perform biaxial tension tests on cruciform shaped geosynthetic specimens, producing a partial set of resilient elastic constants for two woven geotextiles and six biaxial geogrids. To complete the set of elastic constants by determination of the in-plane shear modulus, another mode of loading was necessary. Literature from biaxial shear tests of architectural membranes suggested cutting the cruciform shaped samples with the principal material directions on a 45-degree bias, which causes the sample to shear when the cruciform axes are unequally loaded. This test mode was successfully implemented with the existing biaxial device to determine the resilient in-plane shear modulus using an orthotropic linear elastic model. Full-field strain measurements were captured using digital imaging correlation (DIC) software available at Montana State University. DIC was shown to produce equivalent strain measurements to the mechanical instrumentation (LVDTs) used by Haselton, enabling a combined dataset. The full-field DIC strain measurements were then used to validate Haselton's assumption regarding the region of uniform strain and to identify the region of uniform strain for data collection in this thesis. DIC also showed reasonably pure biaxial tension in the cruciform samples, validating the elastic constant derivations for both Haselton and this thesis.