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The third school of thought contends that layered elastic theory,
when used with certain combinations of pavement moduli, predicts tensile
stresses in granular base layers, even if gravity stresses are also
considered (16,45,112). Instead of using a layered approach, this group
prefers a finite element model in which the nonlinear responses of the
granular and subgrade materials are accurately characterized. Again,
the asphalt concrete layer is considered to be linear elastic. The
ILLI-PAVE finite element back-calculation program (45) is a classic
application of this theory.
In the finite element approach discussed above, researchers have
used, with limited success, various failure criteria and in some cases
arbitrary procedures to overcome the problem of tensile stresses
(16,112). For example, Brown and Pappin (16) used a finite element
program called SENOL with a K-0 contour model and found it to be capable
of determining surface deflections and asphalt tensile stresses but
unable to determine the stress conditions within the granular layer.
The asphalt layer was characterized as elastic with an equivalent linear
modulus. They therefore concluded that the simplest approach for design
calculations involves the use of a linear elastic-layered system pro
vided adequate equivalent stiffnesses are used in the analysis. This
conclusion is shared by other investigators (10,61,96,97) and is the
philosophy behind the work presented in this dissertation.