CEMP-E
TI 850-02
AFMAN 32-1125(I)
1 MARCH 2000
(1) This procedure may only be used for fine grained soils of relatively uniform composition at
different depths.
(2) Determine the Air Freezing Index - the number of degree days (above and below 32 F) between
the highest and lowest points on the cumulative degree day curve for a freezing season. For the design
air freezing index, use the 3 coldest years in the last 30 years of record, or use the average index and
multiply it by 1.75. (If fewer than 30 years of data are available, the air freezing index for the coldest year
in the last 10-year period may be used). If local records are unavailable, use the values directly from
either figure 6-15 or the PCASE WORLD INDEX program.
Figure 6-15. Design Air Freezing indexes in North America
(3) Determine the frost depth using figure 6-16. The gravel thickness in this figure is the total depth
of ballast and sub-ballast. Multiply the frost penetration value by 0.6. If the resulting number is smaller
than the gravel thickness, then there is an adequate amount of ballast and sub-ballast in the cross
section. If it is larger, additional ballast and/or sub-ballast should be added and the thickness
recalculated.
6-22
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