UFC 3-260-03
15 Apr 01
where
E = subgrade modulus, pounds per square inch
D72 = deflection measured at a distance of 72 inches from an applied NDT
loading normalized to 25,000 pounds
A range for the subgrade modulus is then established as the predicted value plus and minus 5,000 psi.
This relationship is not valid for the case where bedrock is present near the pavement surface (<20 feet),
and the default values should be used if this situation is encountered.
f. Typically, the modulus of any surface layer can and should be computed with WESDEF. How-
ever, in some instances it may be necessary to assign a modulus value to the AC or PCC layer (i.e.,
WESDEF yields unrealistic values or the surface layer is very thin). If assigned, the value will be based on
the type of material or properties of the material at the time of testing. For flexible pavements, the surface
temperature at the time of testing is added to the previous 5-day mean air temperature, and the mean
pavement temperature is determined from figure 3-2. The assigned AC modulus is obtained using fig-
ure 4-6 and the loading frequency for the NDT device. The FWD device normally produces a load fre-
quency at or near 20 Hz. The curves in figure 4-6 are extrapolated from laboratory relationships for new
AC mixes; therefore, predicted values may not always agree with actual field values. A modulus of
5,000,000 psi is recommended for a PCC layer in good condition.
g. WESDEF incorporates a layer of infinite thickness having a modulus of elasticity of 1,000,000 psi
and Poisson's ratio of 0.5 below the subgrade layer. This stiff layer should be located at a depth of 20 feet
unless soil profiles indicate the need for some other representation (i.e., shallow rock).
h. WESDEF is capable of handling both multiple loads and variable interface conditions. For a
given layer (n) and underlying layer (n + 1), the interface value should be set at 1 for complete adhesion
between the layers or 1,000 for almost frictionless slip between the layers. Values between 1 and 1,000
may be input to simulate varying degrees of friction. Almost frictionless slip is usually assumed at the
bottom of a PCC layer and full adhesion is generally assumed for most other pavement materials.
i. WESDEF provides a tool with which modulus values can be predicted. Normally three iterations
within the program produce a set of modulus values that yield a deflection basin that is within an average
of 3 percent of each of the measured deflections. In analyzing the results from the WESDEF program, it
is important to check the predicted modulus for each layer and determine if any of the predicted modulus
is against the limits. If the modulus is outside a limit, engineering judgment is required to select one of the
following:
(1) Rerun WESDEF computing modulus values for fewer layers. Some options to be con-
sidered are as follows:
(a) Fix the modulus of an AC or PCC surface layer based on material type and condition
at the time of testing rather than computing the modulus.
(b) Combine base and subbase into one layer and compute a composite modulus or
divide the base course into two layers.
(c) Fix the subgrade modulus based on results of a preliminary run. In some cases,
subdividing the subgrade into two layers may be warranted.
(2) Rerun WESDEF with modified limits to include the predicted E disregarding boundary
conditions. (Values outside default ranges may be unrealistic.)
4-5
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