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of the Dynaflect to FWD E4 values ranges from 0.55 to 1.54. In only 6
out of 17 cases did the Dynaflect predict higher E4 values than the
FWD. Some researchers argue that the light loading produced by the
Dynaflect would result in higher modulus values than the moduli obtained
with the heavy loads used in the FWD. The findings from this study tend
to contradict this argument in the case of the subgrade layer. Also,
this is the only layer which has unique relationship between the modulus
and the farthest sensor deflection(s) for each NDT device. The subgrade
modulus comparison does not indicate any effect of nonlinearity in the
subgrade soil materials. Therefore, the differences in the moduli pre
dictions could be attributed to the inherent differences between the two
NDT devices; viz, vibratory loading in the Dynaflect versus impulse
loading in the FWD.
Regression analysis of Dynaflect E4 values to FWD E4 values
resulted in the following equation:
E4 (Dynaflect) = 0.933 E4 (FWD) Eqn. 6.6
(R2 = 0.929, N = 17)
Tables 6.17 to 6.19 illustrate the comparison of the moduli ob
tained from plate loading tests and the NDT devices for the base, sub
base, and subgrade layers, respectively. Tables 6.17 and 6.18 indicate
that the plate bearing E2 and E3 values are closer to the FWD than the
Dynaflect predictions. This seems to support the argument that FWD
deflection response is influenced by the rigid plate effects. For
example, I-10A which was very stiff produced similar E2 values using the