1.5 to 6.0 in. had an R2 of 0.958 with 192 number of cases analyzed.

Error analysis indicated that only 9 out of the 192 total cases had

predictive errors above 15 percent but all did not exceed 20 percent.

These occurred in pavements with E4 value of 40 ksi and ti of 1.5 or 6.0

in.

 Because of the high reliability of Equation 4.23, the database was

expanded to include other moduli and thickness combinations. Analysis

of the large data set (N = 400) resulted in Equation 4.24 with an R2 of

0.935. In 62 out of 400 cases, predictive errors were more than 20 per-

cent. Three pavements had 40 percent or more prediction errors with one

of the them being 55.6 percent.

 Additional evaluation of the practical limits of Equation 4.24

 indicated that prediction accuracy was satisfactory even when the E

 range was expanded to 600 ksi. Errors were generally less than 30 per-

 cent and only 7 out of 80 cases had predictive errors between 30 and 43

 percent using El = 600 ksi. Overall assessment of this E3 prediction

 suggests that the best accuracy is obtained from 3 to 6 in. of asphalt

 concrete.

 4.4.2.4 Subgrade Modulus, E Five equations were derived to pre-

 dict subgrade modulus from the 9-kip FWD load deflections. The first

 three equations (4.25 to 4.27) were essentially dependent upon individ-

 ual sensor deflections, D6 to D8, respectively. The other equations

 (4.28 and 4.29) included two sensor deflections in each equation. All

 five equations had R2 values greater than 0.997 for N = 400.

 Considering the high R2 values obtained for Equations 4.25 through

 4.29, their prediction accuracy is expected to be good. Also, a high

 degree of accuracy is required for E4 prediction since small changes