51
invariant is generally used to characterize granular base materials.
The relationship is of the form
K
E = K^e 2 Eqn. 2.5
where
E = granular base/subbase modulus,
0 = first stress invariant or bulk stress, and
K K = material constants
1 2
The subgrade stiffness, on the otherhand, has been found to be a
function of the deviator stress (stress difference). For fine-grained
soils, resilient modulus decreases with increase in stress difference
(78). The mathematical representation of the subgrade stiffness is of
the form
E
Eqn. 2.6
where
E = subgrade modulus
a = stress difference, and
A, B = material constants for the subgrade
The constant B(slope) is less than zero for the stress-softening model,
while for the stress-stiffening model, the slope is greater than zero.
The stress-dependency approach of characterizing pavement materials
is of great importance for high traffic loadings. Situations in which
high traffic loadings occur are larger aircraft loadings in the case of
airfield pavements, and when heavy wheel loads and/or single tire