UFC 3-260-03
15 Apr 01
(1) If DFI# 13,330 EC hours (1,000-EF days), one-half of the length of the freezing season will
be included in the total period of weakening recommended in table 7-5.
(2) If DFI > 13,300 EC hours (1,000-EF days), a month will be added to the thaw-weakening
period recommended in table 7-5.
Table 7-5
Length of End-of-Winter Thaw-Weakened Period for Evaluation Purposes
Frost-Susceptible Soil Classification
End-of-Winter Thaw-Weakening Period (months)
F1
1
F2
1
F3 and F4 (Noncohesive)
2
F3 and F4 (Cohesive)
3
(3) The MODBERG program and the WORLDINDEX database (both programs are available
from the PCASE Bulletin Board) contain information on the length of the freezing season (winter) and the
mean date of the start of the freezing season and can be used to estimate these dates and length of the
freezing season.
(4) A more accurate estimate of the total period of weakening can be obtained by applying the
FROST program using measured air temperatures from, or near, the site for a 10- to 30-year period. The
CRREL technical staff can provide additional guidance on the use of this program.
b. Computations. Evaluation of airfield pavements in seasonal frost areas involves calculation of
allowable aircraft loads for a given number of passes or allowable number of passes of a given load and
PCN values that may be applied to a pavement during the total period of weakening. For calculating the
allowable load during the total period of weakening, it is assumed that the thaw-weakening period is over a
1-year period. The allowable traffic over this time period is the design traffic divided by the design year.
Using the APE program or LEEP for reduced modulus determines the allowable load and PCN for the
controlling aircraft for the pavement profile and properties at the site during the total period of weakening.
16. EXAMPLE 1. Evaluate an Air Force flexible pavement type A traffic area consisting of 127 milli-
meters (5 inches) of asphalt concrete, 229 millimeters (9 inches) of crushed stone base (CBR = 100), and
305 millimeters (12 inches) of subbase (CBR = 30) over a silt subgrade. The pavement is to be evaluated
for the C-17 aircraft. The pavement surface is in good condition. The subgrade has dry density of
1,762 kilogram/cubic meter (110 pounds per cubic foot) and an average water content of 24 percent. The
nonfrost CBR of the subgrade is 13. The base/ subbase-course material is a nonfrost-susceptible sandy
gravel (GW) with an average dry unit weight: 2,163 kilograms/meter (135 pounds per cubic foot) and
average water content after drainage of 3 percent. The highest ground water is 610 millimeters (2 feet)
below subgrade surface. For this example, the airfield is located in Bismarck, ND.
a. From WORLDINDEX:
DFI = 38,712 EC hours (2,903 EF days)
Mean annual temperature = 5EC (41EF)
7-10
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