Nonlinear Elasticity

In addition to confinement effect, the unbound layers for some of the Heavy Vehicle Simulator (HVS) tests also showed typical non-linearity, with the stiffness of granular layers increasing with increasing bulk stress and the modulus of cohesive materials decreasing with increasing deviator stress. Because of the variation in modulus given by the equation for confinement (bending stiffness) these non-linearities had to be treated as functions of the wheel load rather than as functions of the stress condition:

Modulus of unbound material as a function of hub load.

where EP is the stiffness at hub load P in kN,

 E_40kN is the stiffness at a hub load of 40 kN, and

 α is a constant (positive for granular materials and negative for cohesive).

Effects of seasonal variations on unbound material stiffness is currently disabled based on recent research that finds no such effect in unbound layers in California highway (Curran, 2018).

Curran, H., J. T. Harvey and R. Wu (2018). Guidance for Selection of Unbound Pavement Layer Seasonal Stiffnesses University of California Pavement Research Center UC Davis, UC Berkeley.