39
load-deflection response (such as an FWD deflection basin) is the
concept of equivalent layer thickness (121,122) in which the layered
pavement system is transformed into an equivalent Boussinesq (13)
system. This concept, originally proposed by Odemark (87), is based on
the assumption that the stresses, strains, and deflections below a given
layer interface depend on the stiffness and thickness of the layers
above that interface. Although this approach obtains an explicit
solution for the subgrade modulus (121), it relies on estimates of the
asphalt concrete layer modulus and also employs certain modular ratios
to obtain the moduli of the various layers above the subgrade (25). The
method of equivalent thicknesses (MET) has also been incorporated into
some iterative computer programs which are discussed in the next
section.
Several investigators have obtained equations to directly determine
the subgrade modulus from one or more sensor deflections. For example,
Figure 2.8 shows the relationship between the subgrade modulus and the
Dynaflect fifth sensor deflection as summarized by Way et al. (128).
Keyser and Ruth (55) developed a prediction equation from five test road
sections in the Province of Quebec, Canada, by using the BISAR elastic-
layer program to match measured Dynaflect deflection basins. The
equation is of the form
-1.0006
E = 5.3966(D ) Eqn. 2.4
4 5
where E^ is subgrade modulus in psi, and D5 is Dynaflect fifth sensor
deflection in inches. This equation had an R2 of 0.997 (55), and is
similar to that of Ullidtz (see Figure 2.8). Godwin and Miley (41) have