UFC 3-260-03
15 Apr 01
17. EXAMPLE 2. Evaluate an Air Force rigid pavement type B traffic area consisting of 508 millimeters
(20 inches) of PCC and 102 mm (4 inches) of base on a clay subgrade. The flexural strength of the
concrete is 4.5 MPa (650 psi). Visual inspection of the pavement shows it to be in good condition. The
pavement is to be evaluated for the C-17. The aircraft traffic is applied uniformly throughout the year. The
subgrade is a clay with a PI of 10, a dry density of 1,602 kilograms per cubic meter (100 pcf), and an
average water content of 18 percent. From field tests, the subgrade k during the normal period was
33 KPa/mm (125 pci). The base material is a nonfrost-susceptible sandy gravel (GW) with a normal k
value of 122 kPa/mm (450 pci). The average dry unit weight and average water content of the subbase
layer are 2,163 kilograms/cubic meter (135 pcf) of 5 percent, respectively. The highest groundwater is at
the subgrade surface. For this example, the airfield is located in Fairbanks, AK.
a. From WORLDINDEX:
DFI = 86,496 EC hours (6,487 EF days)
Mean annual temperature = -2.6 EC (27.3 EF)
Mean freezing length = 188 days
Mean start of freezing occurs on October 11
Mean end of freezing occurs on April 16
b. Determine if Base/Subbase and/or Subgrade is Frost Susceptible. From table 7-1, the subgrade
is classified as a F4 frost-susceptible soil.
c. Determine Depth of Frost Penetration. The depth of frost penetration (d) estimated from the
MODBERG program, assuming coarse grained material below the PCC layer is 2,565 mm (101 inches).
d. Evaluate for Complete Frost Penetration. The combined base thickness of pavement and base
to prevent any freezing of the subgrade in the design freezing index year (complete protection) is
2,565 millimeters (101 inches). With a 508-millimeter- (20-inch-) thick pavement, the thickness of base
course (c) for zero penetration of the subgrade is 2,057 millimeters (81 inches). The thickness of the
pavement, base, and subbase layer (x) is 1,016 millimeters (40 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 = 18/5 = 3.6. From figure 7-5, using the maximum permissible ratio r of 2.0 applicable to type B
traffic area, the required total base thickness b that would hold subgrade frost penetration within the
allowable limit is 1,321 millimeters (52 inches). In this case, the 610-millimeter- (24-inch-) thick section of
pavement and base does not provide adequate protection against frost action, and evaluation for thawing
and normal 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 F4 frost-susceptible soil. From figure 7-7, the FAIR value is 50 kPa/mm (185 pci). The design load for
the C-17 is 263,084 kilograms (580,000 pounds). Using figure 6-17, the procedure outlined in chapter 6,
or the computer program APE, evaluate the allowable load for the traffic area during the thaw-weakened
period using the FAIR value of 50 kPa/mm (185 pci). The allowable loads for pass intensity levels I to IV,
and the respective PCN are tabulated below.
7-12
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