Cross-Anisotropic Characterization of Unbound Granular Materials

The mechanical properties of granular materials are nonlinear and stress dependent. Several researchers have identified cross-anisotropy as a better model for characterizing granular materials. However, determining cross-anisotropic material properties from the conventional triaxial setup has been extremely difficult. A laboratory testing protocol has been developed using theories of elasticity and the system identification method. The testing protocol is efficient and precise. Material properties determined with the testing protocol satisfy elasticity requirements. The cross-anisotropic material properties of four granular materials were determined. The material properties were entered into a finite element program, which was modified to accept these properties, to determine pavement response. Nonlinear crossanisotropic modeling was observed to eliminate tension zones predicted by isotropic models in granular layers. This is an important step because the presence of such tensile zones has required adjustments in layered pavement models to predict pavement performance on the basis of response parameters.