53
correct the in situ moduli calculated for nonlinear granular materials
and subgrade soils from a Dynaflect deflection basin. These conclusions
were based on stress analysis comparisons of a single-axle 18-kip design
load, FWD (9000-lb. peak force) and Dynaflect loadings simulated in the
ELSYM5 elastic-layer program. An algorithm to perform this equivalent
linear correction has been incorporated into the FPPE0D1 self-iterative
computer program (120). However, results reported by Nazarian et al.
(81) tend to contradict the conclusions of Uddin et al. (120). Their
study involving FWD tests at 5- and 15-kip loads indicated that non
linear behavior occurs at higher FWD loads, and is more predominant in
the base course layers than the subgrade.
These results and those from other research work indicate there is
disagreement as to what type of approach should be used when the effects
of nonlinearity and stress dependency are to be considered. There are
at least three schools of thought in this regard. The first group
believes that the use of an equivalent effective modulus in an elastic-
layer theory would provide reasonable response predictions. This
approach would eliminate the expense, time and complexity associated
with more rigorous methods such as finite element models (61). The
research works of Maree et al. (70), Roque (96), and Roque and Ruth (97)
on full-scale pavements tend to support this theory.
The second school of thought recommends that the nonlinear stress
dependent models (Equations 2.5 and 2.6) can be incorporated into an
elastic-layer program to predict reasonable response parameters. How
ever, the asphalt concrete layer is treated as linear elastic. This
theory is supported by Moni smith et al. (78), among others, and has been
used in iterative computer programs like OAF, ISSEM4, and IMD.