41
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.