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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