36
and is the most widely used correlation (133). Other correlations (79)
have been made between E and plate bearing subgrade modulus, K. It
should be recognized that the conditions of dynamic testing generally
yield moduli in the linear elastic range. Conventional tests such as
the CBR and plate bearing tests produce deformations that are not
completely recoverable and, therefore, are partly in the plastic range.
Thus, one would expect some variation in the correlation between E
modulus and pavement parameters, such as K and CBR.
Mechanistic analysis of NDT data is usually performed by one of the
following:
1. Direct relationship between deflection parameters and the
elastic moduli of the pavement layers.
2. Inverse application of a theoretical model by fitting a
measured deflection basin to a deflection basin using an
iterative procedure.
3. A combination of 1 and 2.
The above mechanistic methods employ deflection data from either vibra
tory or impulse loading equipment. While these devices are dynamic in
nature, most of the mechanistic solutions are based on elasto-static
(19,32,74) and visco-elasto-static (7) models. Recently, an elasto-
dynamic model (54) has been used to interpret NDT data (66,67,105).
However, the use of dynamic analyses for interpretation of NDT data can
be considered to be in the research stage. Another significant obser
vation is that almost all the mechanistic solutions available employ
layered theory or simplified versions of it. The only exception to this
is the use of a finite element model presented by Hoffman and Thompson
(45). A review of the various solutions is presented below.