configurations (which result in higher stresses) are applied to flexible
highway pavements. For this reason, some of the NDT back-calculation
procedures have accounted for the stress dependency effect by incorpo-
rating Equations 2.5 and 2.6 into their algorithms (45,65,106). How-
ever, the problem of determining the material constants, A, B, K and
1
K still remains, especially when NDT deflection basins are used to
2
characterize the pavement. The most common approach is to use labora-
tory resilient moduli and regression analysis to determine these para-
meters (45,65,72). Thus, the material parameters will depend upon
sample preparation procedures, disturbance, prestress-strain conditions,
etc.
Other researchers (93,106,121) have suggested determining the mate-
rial constants from FWD tests conducted at three or more load levels.
However, it is not clear how viable this procedure is since the resul-
tant load-deflection response of a pavement is a combined effect of the
behavior of the individual layers. The relative contribution of each
layer is not clearly known. It is even more complex since the asphalt
concrete layer is dependent on the temperature and age-hardening
characteristics of the asphalt cement. Moreover, contrary to previous
belief, Thompson (116) has found that the material parameters are not
independent-of each other, especially for granular bases and subbases.
Uddin et al. (118,119,120) have applied the concepts of equivalent
linear analysis developed in soil dynamics and geotechnical earthquake
engineering to evaluate the nonlinear moduli. They concluded that the
in situ moduli derived from an FWD deflection basin (at 9000-1b. peak
force) are the effective nonlinear moduli and need no further correc-
tion. However, an equivalent linear analysis has to be performed to