30 June 2001
f. Subtract thickness of the surface and base from total thickness required above subgrade to
obtain the required thickness of subbase. If thickness of subbase is less than 150 millimeters (6 inches),
consider increasing thickness of base course.
ADDITIONAL CONSIDERATIONS FOR THICKNESS DESIGN.
a. CBR Values less than 3. Normally, sites which include large areas of the natural subgrade with
CBR values of less than 3 are not considered adequate for airfield construction. However, CBR values
of less than 3 are included on the flexible pavement design curves so that thickness requirements for
occasional isolated weak areas can be determined.
b. Frost Areas. Pavement sections in frost areas must be designed and constructed with nonfrost-
susceptible materials of such depth to prevent destructive frost penetration into underlying susceptible
materials. Design for frost areas in accordance with Chapter 20.
c. The thickness of the rapid-draining or open-graded material is determined from
AFJMAN 32-1016 and is substituted for an equivalent thickness of base or subbase according to design
d. Expansive Subgrade. Ensure that moisture condition of expansive subgrade is controlled and
e. Limited Subgrade Compaction. Where subgrade compaction must be limited for special
conditions, pavement thickness must be increased in conformance with reduced density and CBR of the
f. Rainfall and Water Table. In regions where the annual precipitation is less than 380 millimeters
(15 inches) and the water table (including perched water table) will be at least 4.6 meters (15 feet) below
the finished pavement surface, the potential for subgrade saturation is reduced. Where in-place tests on
similar construction in these regions indicate that the water content of the subgrade will not increase
above the optimum, the total pavement thickness, as determined by CBR tests on soaked samples, may
be reduced by as much as 20 percent. The reduction will be effected in the subbase course having the
lowest CBR value. When only limited rainfall records are available, or the annual precipitation is close to
the 380-millimeter (15-inch) criterion, careful consideration will be given to the sensitivity of the subgrade
to small increases in moisture content before any reduction in thickness is made. For assistance in
interpolating limited rainfall data, the USAF Environmental Technical Applications Center,
USAFETAC/ECE Scott AFB, IL 62225-5000, may be contacted.
a. Example 1.
(1) Design an Air Force heavy-load pavement type B traffic area. Design CBR of the lean
clay subgrade is 13; the natural in-place density of the clay is 87 percent extending to 3 meters (10 feet).
The analysis that follows assumes that subgrade does not require special treatment and frost
penetration is not a problem.
(2) Enter Figure 10-19 at a CBR equal to 13, move down to type B traffic area curve, then
move horizontally to the required total thickness of pavement above the subgrade, 735 millimeters