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size distribution and particle shape, stress history and moisture content. From the
literature it is known that the material stress state is certainly the most significant factor
that can affect the resilient modulus property of unbound materials (Hicks and
Monismith, 1971; Rada and Witczak, 1981; Uzan, 1985; Kolisoja, 1997). Many studies
on the mechanical response of unbound materials have shown that the resilient
modulus property is highly dependent on the confinement pressure and sum of the
principal stresses. An increase in confining pressure and the sum of the principal
stresses result in a considerable increase in the resilient modulus. However, the deviator
or the shear stress has a much less significant influence on the material stiffness. In
coarse-grained granular materials, change in deviator stress had no or insignificant
influence on the stiffness of the material (Lekarp et al., 2000). The effect of the deviator
stress on the resilient modulus seemed to be very much dependent on the material type,
compaction and the deviator stress level itself. In a study conducted by Hicks and
Monismith (1971) minor softening behaviour of the material was observed at low stress
levels while minor hardening behaviour was observed at higher stress levels. Hicks
(1970) stated that the resilient modulus is in practice independent of the deviator stress
level if no plastic deformation is experienced. This implies that the resilient modulus
stress-dependent behaviour of coarse-grained granular materials can sufficiently be
captured by only using the first stress invariant.
In fine-grained materials and subgrade soils, the resilient modulus usually decreases with
increase in the deviator stress. The softening behaviour in the material due to increase
in the deviator stress level is assumed to be related to increase in the shear stress, which
softens the material and thus yields to a lower resilient modulus (Drumm et al., 1990; Li
and Selig, 1994; Muhanna et al., 1999).
The significant impact of the stress state on the resilient modulus of unbound materials
has resulted in the development of a number of constitutive models that mathematically
describe the material stress-strain relationship. Most of these models describe the
resilient modulus stress dependency using different stress state variables (Lekarp et al.,
2000) and are mainly developed through curve fitting and nonlinear least square
regression methods using RLT test data. A comprehensive analysis and evaluation of
the resilient modulus models can be found in Andrei (2003), Lekarp et al. (2000), Rada
and Witczak (1981) and Kolisoja (1997).
Among all the available stress dependent resilient modulus models, the generalized
constitutive model proposed in the
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