System identification method for determining the anisotropic resilient properties of unbound aggregates

Unbound granular layers have been used as major structural components in pavements for decades. There are well-documented facts that the resilient properties of unbound granular materials are nonlinear and stress dependent. Recent developments in pavement materials research suggest that directional or anisotropic elastic modeling can reduce and even reverse horizontal tensile stresses predicted in unbound granular layers with isotropic elastic properties. A testing protocol has been developed using a triaxial set up rn which the five anisotropic elastic parameters are determined using the system identification method. This protocol assumes a hyperelastic material behavior and that the tangential elastic moduli are non-linear, smooth functions of the stress invariants as implemented in the Uzan model. The resilient axial and radial displacements were measured in a triaxial set up and implemented in a system identification scheme to backcalculate the five elastic properties at different stress states. Two base materials, crushed limestone and gravel, were tested in the laboratory to evaluate their directional elastic properties. The materials tested exhibited stiffer vertical moduli. Also, the vertical Poisson's ratio values measured were less than the horizontal Poisson's ratio values, but both were generally constant at all stress states. Regression analysis was used to compute the k-value stress dependent coefficients for each elastic parameter. These k-values can be used as material parameters in a finite element code such as CT-PAVE to predict the response of a pavement section with unbound aggregate layer(s) to traffic loads.