4. This process is repeated until the two deflection basins are vir-

 tually the same. The process may have to be repeated several times

 before a reasonable fit is obtained.

 Because of the time consuming nature (49) of the trial-and-error

method, many researchers have developed computer programs to perform the

iteration. Table 2.2 lists some of the self-iterative computer pro-

grams. The major differences among the various programs are the differ-

ent models, algorithms and tolerance levels used in the iteration pro-

cess. A few of these will be discussed here.

 Anani (6) developed expressions for surface deflections in terms of

the modulus values of a four-layer pavement. However, he could not

obtain direct solutions to determine the moduli. Therefore he used an

iterative procedure to obtain the moduli from Road Rater deflection

basins. The computer programs reported by Tenison (114) and Mamlouk

(66) followed the successive approximation method of Anani (6). In the

overlay design program called OAF, Majidzadeh and Ilves (65) employed a

deflection matching technique for determining the in situ layer stiff-

nesses. While using field data to substantiate the applicability of the

procedure, they experienced difficulties and commented,

 . the computed asphalt layer stiffness shows a
 large variation, and in a few cases the asphalt is
 stiffer than steel; nevertheless the values are
 reasonable in a great majority of the cases . .

 (65, p. 85)

 The BISDEF computer program (23) is an improvement over the CHEVDEF

(22) to handle multiple loads and variable interface conditions. The

number of layers with unknown modulus values cannot exceed the number of

measured deflections. However, a maximum of four deflections are