UFC 3-260-03
15 Apr 01
area B, the required total base thickness b that would hold subgrade frost penetration within the allow-
able limit is 1.4 meters (55 inches). In this case, the 76-millimeter- (30-inch-) thick section of pavement
and base does not provide adequate protection against frost action, and evaluation for frost and nonfrost
conditions are required.
f.
Evaluate for Reduced Subgrade Strength.
(1) Determine the allowable load and PCN during thaw-weakened period. The soil is classified
as F4 frost susceptible soil. From table 7-3, the FASSI value is 3.5. The figure 5-16 or from APE, for
26,000 passes, the allowable load during the thaw-weakened period is 186,800 pounds. The PCN is
39/F/D/W/T.
(2) Determine the period of Thaw Weakening. From WORLDINDEX, the design beginning of
freezing is Oct 11th and ends in April 16th. The average length of the freezing season is 188 days. The
thaw weakened period after the end of winter from table 7-4 is estimated to be 3 months. Since the
airfield is located in an area with a DFI of 6487 EF days, an additional 1 month (to cover for intermediate
thaw period during the freezing period) is added to the 3 months to obtain the total weakening period of
4 months. The mean thaw-weakened period is from March 17th to July 15th.
7-14
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