stress, strain, and deflection at any point in the homogeneous mass for

any value of Poisson's ratio.

 Although most asphalt pavement structures cannot be regarded as

being homogeneous, the use of these solutions are generally applicable

for subgrade stress, strain and deflection studies when the modular

ratio of the pavement and subgrade is close to unity. This condition is

probably most exemplified by conventional flexible granular base/subbase

pavement structures having a thin asphalt concrete surface course (133).

Normally, in deflection studies for this type of pavement, it is assumed

that the pavement portion (above the subgrade) does not contribute any

partial deflection to the total surface deflection.

2.2.3 Two-Layer System

 Since Boussinesq's solution was limited to a one-layer system, a

need for a generalized multiple-layered system was recognized.

Moreover, typical flexible pavements are composed of layers such that

the moduli of elasticity decrease with depth (133). The effect is to

reduce stresses and deflections in the subgrade from those obtained for

the ideal homogeneous case.

 Burmister (18,19,20) established much of the ground work for the

solution of elastic layers on a semi-infinite elastic layer. Assuming a

continuous interface, he first developed solutions for two layers, and

he conceptually established the solution for three-layer systems. The

basic assumption made was full continuity between the layers, which

implies that there is no slippage between the layers. Thus, Burmister

assumed that the strain in the bottom of one layer is equal to the

strain at the top of the next layer, but the stress levels in the two