UFC 3-260-03
15 Apr 01
(3) Accept the results of the WESDEF run realizing that the predicted values are outside the
typical range for a particular material.
j.
The following guidelines may be helpful in determining layer modulus values using WESDEF:
(1) Do not attempt to compute the modulus values for more than three layers in a single
WESDEF run. Limit the number of computed layer moduli to two if possible (particularly for rigid
pavements).
(2) Do not attempt to compute the modulus of layers less than 3 inches thick. The modulus of
a thin layer should be fixed based on material type, temperature, etc.; or else a thin layer should be com-
bined with an adjacent layer and a composite modulus determined.
(3) When computing the modulus of a PCC layer, it may be necessary to combine a base or
subbase layer with the subgrade layer and determine a composite modulus for the material beneath the
PCC slab.
(4) Exercise caution when using modulus values outside the default ranges. Because the
ranges are quite broad, values outside these limits may be unrealistic.
(5) For NDT devices with circular loaded areas, the offset distance to the first measured deflec-
tion is input to WESDEF as one-half the radius of the loading plate to approximate the deflection at one-
half the radius of a uniformly distributed circular loaded area.
7.
STEP 4 (DETERMINE DESIGN TRAFFIC).
a. The total number of passes of each aircraft type that the pavement will be expected to
support over its design life must be projected. The normal design life for airfield pavements is 20 years.
The Navy and Marine Corps use 10 years for evaluation purposes. For a runway, passes are determined
by the number of aircraft movements across an imaginary transverse line placed within 500 feet of the end
of the runway. Touch-and-go aircraft operations are typically not counted as passes. In some cases, and
for Navy and Marine Corps evaluations, they may be counted for large aircraft (which may produce
significant damage to the pavement), or for the case of outlying airfields (which receive essentially touch-
and-go operations). For taxiways and aprons, passes are determined by the number of aircraft move-
ments across a line on the primary taxiway that connects the runway and the parking apron. The designer
should strive to obtain data for a specific airfield facility under evaluation to forecast aircraft traffic opera-
tions over the design life of the pavement. For a given projected aircraft mixture, the critical aircraft and
design pass level must be determined for the evaluation. The critical aircraft is that aircraft from the mix-
ture which requires the greatest pavement thickness to support its projected passes. The number of
passes of the critical aircraft required to produce an equivalent effect on the pavement as the mixture of
traffic is the design pass level. The computer program TRAFFIC will determine the critical aircraft and
compute equivalent passes of the critical aircraft. The procedures incorporated in TRAFFIC are as
follows:
(1) Determine the total pavement thickness required for each individual aircraft at its projected pass
level using current criteria. Thicknesses should be computed using a representative subgrade modulus
for the airfield or pavement feature. The aircraft requiring the greatest thickness is designated as the criti-
cal aircraft.
(2) Determine the allowable number of passes for each individual aircraft for the maximum required
thickness.
4-6
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