developed correlations between the base, subbase and subgrade moduli and

the second, third, and fourth Dynaflect sensor deflections, respec-

tively. However, the modulus values used in the correlation were the

surface moduli from plate bearing tests which suffers from the problem

of incorporating plastic and nonrecoverable deformations.

 An approach using regression equations to estimate layer moduli has

been attempted by other investigators (83,120,132). This approach

usually involves analysis of computer-simulated NDT data using a theo-

retical model (usually layered elastic theory). The various investi-

gators reported success in the case of the subgrade modulus. To obtain

good correlations for the other layers (surface, base, subbase), certain

assumptions had to be made, such as the base course modulus being

greater than the modulus of the subgrade (83), or they had to resort to

computer-iteration programs (83,120).

 2.3.3.3 Back-Calculation Methods. The method of iteratively

changing the layer moduli in a theoretical model to match the theoreti-

cal deflection basin to a measured basin is currently called back-

calculation in the literature. Initial developments of this procedure

utilized a trial-and-error approach (49,72) using the following steps:

1. Pavement-layer thicknesses, initial estimates of the pavement-layer

 moduli, and the loading and deflection measurement configuration are

 input into the model (usually a multilayer elastic computer

 program).

2. The computed deflections at the geophone positions are compared with

 those actually measured in the field.

3. The layer moduli used in the computer program are then adjusted to

 improve the fit between the predicted and actual deflection basins.