Figure 21. Typical backcalculated PCC modulus from ILLIBACK (OCA)

Figure 19. Change in base course modulus during spring thaw

Figure 22. Relationship between measured total basin area and calculated subgrade modulus

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Outagamie County Airport

Subgrade Modulus from WESDEF (MPa)

Figure 20. Relationship between subgrade moduli backcalculated by

WESDEF and ILLIBACK.

Outagamie County Airport

Figure 21. Typical backcalculated PCC modulus from ILLIBACK (OCA).

calculate the partial basin areas and found that none sig-

oped. Basin area results from ILLIBACK and WESDEF

nificantly increased the correlation with the calculated

are shown for both OCA and CWA in Figures 22 and

subgrade modulus.

23. In Figure 22, the total basin area was calculated us-

A similar relationship between ISM and subgrade

ing all seven deflections. In Figure 23, the basin area

modulus is shown in Figure 24. A second-order polyno-

was calculated using the third to seventh deflections.

mial function provides the best fit to the data. Again, a

Previously, we had determined that the thaw depth

better correlation is obtained using the results from IL-

under AC pavements could be related to the partial ba-

sin area calculated using the third to seventh deflec-

The results from OCA and CWA are combined in

tions (Janoo and Berg 1990). The current data indicate

Figures 25 and 26. In Figure 25a presents a relationship

that this partial basin area (A3-7) may be related to the

developed between the measured total basin area and

subgrade modulus. The figures show that as the total

subgrade modulus calculated from ILLIBACK for both

basin area or partial basin area increased, the subgrade

sites. An excellent correlation between total basin area

modulus decreased. A power relationship was found to

and subgrade modulus is found. In Figure 25b, the rela-

best fit the total basin area or partial basin area and the

tionship developed between the total basin area and

subgrade modulus. We also found that a better correla-

subgrade modulus calculated from WESDEF for both

tion existed with the subgrade modulus obtained from

sites is presented. There are two distinct trends. This is

ILLIBACK. The use of the partial basin area (A3-7)

probably attributable to the separation of the base and

(Fig. 23) did not significantly increase the correlation

subgrade in WESDEF. Again, a power curve seems to

at OCA. We used other combinations of deflections to

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