Unbound granular material is used in the pavement structure and usually comprises the bulk
of the structural and foundation layers of a typical South African pavement. The term unbound
granular material refers to the classification of natural material, which has not been modified
in any way. Unbound granular material is classified from a G1 to G10 quality according to its
fundamental behaviour and strength characteristics.
Young’s modulus and Poisson’s ratio are theoretical concepts of linear elasticity that can at
best approximate experimental results of actual material elastic response. In their basic linear
elastic form, Young’s modulus and Poisson’s ratio are rather poor approximations of actual
unbound granular material behaviour. The non-linear, stress-dependent behaviour of unbound
granular material can, however, be simulated using the linear elastic model as a basis, but with a
proper constitutive material model that adheres to the observed material behaviour.
The objective of this paper is to utilise a chord modulus model and calibrate it for a range
of unbound granular material classifications. The model was calibrated for five bulk material
samples, ranging from G2 to G8. The calibration process included linking variables of the
model to mathematical functions that approximate the trends observed when variables were
considered against degree of saturation. A parametric analysis indicated that the saturationand
stress-dependent chord modulus model realistically predict material behaviour. The trends
depict the stress-dependent behaviour of unbound granular material, where an increase in
initial modulus is observed for increasing confinement pressure, as well as initial stress softening
with increasing stress ratio followed by stress stiffening.
It can be concluded from the results presented in this paper that a saturation- and stressdependent
chord modulus model could be refined and calibrated for crushed and natural
unbound granular material. This refinement did not negatively influence the accuracy or ability to realistically predict the material behaviour. The preliminary conclusions reported in this paper
indicate that the chord model formulation yield satisfactory predictions, especially when the
model is calibrated for each individual material type.