TM 5-822-5/AFM 88-7, Chap. 1
when the average daily temperature is below 32
latest 30 years of record. If 30 years of record are
degrees Fahrenheit (freezing degree-days) and plus
not available, the air freezing index for the coldest
when above (thawing degree-days). Figure 18-1
winter in the latest 10-year period may be used. To
shows curves obtained by plotting cumulative
avoid the necessity of adopting a new and only
degree-days against time.
slightly different freezing index each year, the
(4) Freezing index is the number of degree-
design freezing index at a site with continuing con-
days between the highest and lowest points on a
struction need not be changed more than once in 5
curve of cumulative degree-days versus time for
years unless the more recent temperature records
one freezing season. It is used as a measure of the
indicate a significant change in thickness design
combined duration and magnitude of below-freezing
requirements for frost. The design freezing index is
temperatures occurring during any given freezing
illustrated in figure 18-1.
season. The index determined for air temperature
(6) Mean freezing index is the freezing
approximately 4.5 feet above the ground is
index determined on the basis of mean
commonly designated as the air freezing index,
temperatures. The period of record over which
while that determined for temperatures immediately
temperatures are averaged is usually a minimum of
below a surface is known as the surface freezing
10 years, preferably 30, and should be the latest
index.
available. The mean freezing index is illustrated in
(5) Design freezing index is the average air
figure 18-1.
freezing index of the three coldest winters in the
groups are generally suitable for base course and
18-3. Frost-Susceptibility Classification.
subbase course materials, and any of the eight
For frost design purposes, soils are divided into
groups may be encountered as subgrade soils. Soils
eight groups as shown in table 18-2. The first four
18-4
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