30 June 2001
ponding of water, surface icing, and loss of control of aircraft or unnecessary stresses to the
aircraft or vehicles using the pavement. This objectionable and abrupt differential movement is
caused by the use of different material in the base and subbase and/or the use of different
thicknesses than existing material. Longitudinal abrupt differences have been noted where the keel
section has been replaced on airfields. Transverse abrupt differences have been noticed in newly
added taxiways where the total thickness of pavement, base, and subbase has been different from
that previously used. The differences are most pronounced when the pavement type is changed
from PCC to asphalt concrete. PCC pavements generally require smaller base and subbase
thicknesses than asphalt concrete pavements resulting in deeper frost penetration and potentially
greater frost heave. To minimize these abrupt differences in pavement elevation, pavement surface
elevation surveys should be conducted in the summer and again in the winter when frost
penetration is near its maximum depth. Both surveys should be completed before the new facility
is designed. The difference in the two surveys will indicate the potential for abrupt differences in
pavement surface elevation resulting from differing designs. The abrupt differences can be
eliminated or substantially reduced by using proper transitions, or by using the same materials
previously used. However, care must be taken if consideration is being given to the use of similar
materials that resulted in the initial distress. Materials which are frost susceptible and placed too
near the pavement surface can result in premature failure.
15. COMPACTION. Subgrade, subbase, and base-course materials must meet the applicable
compaction requirements for nonfrost materials.
16. FLEXIBLE PAVEMENT DESIGN EXAMPLES FOR SEASONAL FROST CONDITIONS.
a. Example 1. Design an Air Force heavy-load pavement type B traffic area. The design
freezing index at the site is 9,331-degree Celsius hours (700-degree Fahrenheit days). The highest
elevation of ground water is about 915 millimeters (3 feet) below the surface of the subgrade. The
subgrade is a lean clay (CL), with a plasticity index of 18. The average moisture content of the
subgrade is 18 percent. The nonfrost design CBR of the lean clay subgrade is 13. A high quality
crushed base-course material with a normal period CBR of 100 is to be used.
(1) Reduced subgrade strength design.
(a) The subgrade is classified as an F3 frost susceptible soil from Table 20-1. From
Table 20-3, the FASSI value for an F3 soil is 3.5.
(b) Use the FASSI value with Figure 10-19 as though it were a CBR. Locate the
value of 3.5 and move down to type B curve; the combined thickness of pavement required is
1.78 meters (70 inches).
(c) From Table 8-5, the minimum thickness of the surface course is 127 millimeters
(5 inches). Therefore, the required base and subbase course thickness is 1.65 meters (65 inches).
(d) Compare pavement thickness with the limited subgrade frost penetration design.
(2) Limited subgrade frost penetration design.
(a) The moisture content of the base course is 3 percent, and the density of the base
course is 2,400 kg/m3 (150 lb/ft3). From Table 8-5, a minimum thickness of a 127-millimeter