targeted during the iteration process, which is also limited to a

maximum of three loops. When applied to field measured deflections on

an airfield pavement in Florida (23), BISDEF predicted unreasonably high

values of the AC modulus for all the different NDT devices used in the

study. Also, Bush and Alexander (23) conceded that the program provides

the best results if the number of unknown layer moduli is three.

 The ISSEM4 computer program (106) incorporates the principles of

the method of equivalent thicknesses (MET) into the ELSYM5 multilayered

elastic program to determine the in situ stress-dependent elastic

moduli. The parameters for the nonlinear stress-dependent relationships

(see Section 2.4.3) are established from FWD tests performed at differ-

ent load levels. The iteration process is seeded with a set of E-values

(106). The ELMOD program (123) also utilizes the MET principle and the

iteration procedure. Both programs provide relatively good solutions if

the asphalt concrete modulus is known.

 ILLI-PAVE (45), the only program which utilizes a finite element

model, is specifically developed to handle Road Rater deflection data.

However, Road Rater deflection basins must be converted to equivalent

FWD deflection basins prior to being used in the program (45). Also,

the nonlinear stress-dependent material models incorporated into the

finite element method utilized relationships established from previous

laboratory material characterization procedures. It is also surprising

that the authors resulted to nomographs for specific applications of the

back-calculation model (45).

 Most of the iteration programs listed above require a set of ini-

tial moduli--seed moduli--and therefore are user-dependent. Therefore

computational times and cost can be prohibitive. Unique solutions