UFC 3-260-03
15 Apr 01
Mean freezing length = 129 days
Mean start of freezing occurs on November 11.
Mean end of freezing occurs on March 19
b. Determine the Depth of Frost Penetration. For complete frost protection the depth of frost pene-
tration (d) estimated from MODBERG program is 1,422 mm (56 inches).
c. Determine if the Base/Subbase and/or Subgrade is Frost-Susceptible. From table 7-1, the sub-
grade is classified as a F3 frost-susceptible soil.
d. Evaluate for Complete Frost Penetration. With a 127-millimeter- (5-inch-) thick pavement, the
thickness of base course (c) for zero penetration of the subgrade is 1,422 - 127 = 1,295 millimeters
(56 - 5 = 51 inches). The thickness of the pavement, base, and subbase layer (x) is 660 millimeters
(26 inches). Since x # c, then the pavement structure was not designed for complete frost protection.
e. Evaluate for Limited Subgrade Frost Penetration. The ratio of subgrade to base-course water
content r = 24/3 = 8. From figure 7-5, using the maximum permissible ratio r of 2.0 applicable to traffic
area A, the required total base thickness b that would hold subgrade frost penetration within the allowable
limit is 813 millimeters (32 inches). In this case, the 660-millimeter- (26-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 allowable load and PCN during thaw-weakened period. The soil is classified as
a F3 frost-susceptible soil. From table 7-3, the FASSI value is 3.5. Use either figure 5-27 and the
procedure outlined in chapter 5 or the computer program Airfield Pavement Evaluation (APE) to determine
the allowable load for the traffic area during the thaw-weakened period using the FASSI value of 3.5. The
allowable loads for pass intensity levels I to IV and the respective PCN are tabulated below.
Pass Intensity Level
No. of
Allowable
for C-17 Aircraft
Passes
Load kg (lb)
PCN
I
15,000
97,296
19/F/D/W/T
(214,500)
II
3,000
114,623
27/F/D/W/T
(252,700)
III
500
146,692
40/F/D/W/T
(323,400)
IV
100
201,667
64/F/D/W/T
(444,600)
(2) Determine the Period of Thaw Weakening. From WORLDINDEX, the design beginning of
freezing is Nov 11th and ends in March 19th. The average length of the freezing season is 129 days. The
thaw weakened period after the end of winter from table 7-4 is estimated to be 2 months. Since the
airfield is located in an area with a DFI of 2903 EF days, an additional 1 month (to cover for intermediate
thaw periods during the freezing period) is added to the 2 months to obtain the total weakening period of 3
months. The mean thaw weakened period is from Feb 17th to May 18th.
7-11
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