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predictions could be considered to be in error. These will be verified
when the FWD deflection basins are modeled using BISAR in Section 6.5.
The subgrade modulus, E4, computed from the three applicable equa
tions tends to be in favorable agreement for most of the test pavement
sections. The agreement in the three equation predictions of E4 could
be attributed to the high degree of accuracy of the developed equations.
It can also be considered as an indicator of the homogeneity of the sub
grade soils. Where they differed, for example, SR 26C, US 301, and
I-10A, it is possible that the stiffness or strength of the underlying
soils vary with depth. The lack of D8 measurements prevented means of
assessing the equivalence of E4 predictions from the deflections at
varying radial distances. It is postulated that knowledge of Dg, Dy,
and D could assist in indicating the variability of the properties of
O
the subgrade soils with depth.
Tables 6.2 and 6.4 suggest that small changes in deflections
greatly affected the predicted moduli. This occurred on SR 26C, SR 15A,
SR 715, and SR 15C test sections in which two adjacent deflection
mneasurements were interpreted. However, the sensitivity analysis of
Section 4.2 had indicated that large changes (50 and 100 percent) in E^,
E2, and E3 did not have a large effect on FWD deflections. This was
assessed by changing one variable while keeping the others fixed. It
was not possible to assess the combined effect of the various layers on
the deflections. However, the equations developed to predict Ej, E2,
and E3 were dependent on almost all sensor deflection measurements. In
this case, any changes in one or more sensor deflections would have a
significant effect on the predicted modulus value. Therefore, the