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an application of the analytical solution of a vertically loaded elastic
plate floating on a heavy fluid. The solution to this problem was
presented by Hertz in 1884 and was first applied to concrete pavement
analysis by Westergaard in 1926 (79). The Hogg model consists of an
infinite plate on an elastic subgrade. The subgrade can be either of
infinite extent or underlain by a perfectly rigid rough horizontal
bottom at a finite depth. Analysis of this model was reported by A.H.A.
Hogg in 1938 and 1944 (131). In both methods, the flexural rigidity of
the composite pavement which will best fit a measured deflection basin
is calculated.
Lytton et al. (62) and Alam and Little (5) have developed another
method based on elastic-layer theory for prediction of layer moduli from
surface deflections. This method makes use of the explicit expression
for deflection originally postulated by Vlasov and Leont'ev (126). The
major drawback of this technique is the need to develop several con
stants, five in all, for which no analytical or test method exists as
yet. In applying this method, the authors (5,62) resorted to the use of
regression analyses and computer iterative solutions.
Cogill (28) presented a method which provides an estimate of the
stiffness of the pavement-layer materials. The method essentially is a
graphical presentation in which the deflections measured over a parti
cular range of load spacing can be related to the stiffness of the pave
ment material at a certain depth. The relationship is an approximate
one and is expressed with the aid of Boussinesq's formula.
All the methods presented above use deflection measurements
obtained from vibratory loading equipments--Dynaflect and Road Rater.
The only approach for the direct estimation of layer moduli from impulse