2000

WESDEF

OCA

ILLIBACK

WESDEF

CWA

1600

ILLIBACK

y = 0.0003 ISM 2 0.0365 ISM + 72.57

2

R = 0.73

1200

y = 0.0004 ISM  2 + 0.1378 ISM + 41.57

R2 = 0.96

800

400

Figure 27. Relationship between ISM and

0

400

800

1200

1600

2000

subgrade modulus at OCA and CWA.

ISM (MN/m)

best fit the data, so for estimation, a single power

In Figure 27, the relationship of ISM and subgrade

trend was applied to the data as shown in Figure 25c.

modulus for both OCA and CWA is shown. Again, a

Figure 26 shows the relationship between the meas-

single power curve can be fitted to the data.

ured total basin area and the coefficient of subgrade

Current criteria for PCC pavements state that failure

reaction (k) computed from ILLIBACK for both sites

occurs when the horizontal tensile stress at the bottom

The following equations have been developed from

of the PCC layer is equal to or greater than the flexural

the ILLIBACK results and can be used to estimate the

strength of the slab. The flexural strength reported at

subgrade modulus and the coefficient of subgrade re-

OCA and CWA was 4.5 MPa. The two types of aircraft

action (k)

used for these simulations were the MD-DC9 and the