30 June 2001
(5-inch) asphalt concrete layer is required. The frost penetration a from Figure 20-5 is
1,143 millimeters (45 inches).
(b) The required base thickness c for zero frost penetration from Equation 20-8 is:
c = a - p = 1,145-127 = 1,016 millimeters (40 inches)
p = asphalt concrete thickness of 127 millimeters (5 inches)
(c) Compute water content ratio r from Equation 20-13. r = 18/3 = 6. Since
r > 2.0 for type B traffic area, use r = 2.0 with Figure 20-6. From Figure 20-6, the allowable
subgrade frost penetration is approximately 178 millimeters (7 inches). Again, from Figure 20-6,
the design base thickness b as determined by the limited subgrade frost penetration method is
685 millimeters (27 inches).
(d) The base thickness of 685 millimeters (27 inches) is less than the thickness of
1,651 millimeters (65 inches) from the reduced subgrade design. In this case, the thickness from
limiting subgrade frost penetration design is more economical than from the reduced subgrade
design. Also, the thickness from the limited subgrade frost penetration is greater than that
obtained from the nonfrost design (see Paragraph 5a, Example 1, Chapter 12). Therefore, the
combined thickness (combined asphalt plus base and subbase material) of 813 millimeters
(32 inches) will be used as the design thickness.
(e) The pavement structure could be made up of 127 millimeters (5 inches) of
surface course, 102 millimeters (4 inches) of a NFS drainage layer beneath the surface course,
254 millimeters (10 inches) of NFS base course, 228 millimeters (9 inches) of S1 or S2 subbase,
102 millimeters (4 inches) of a NFS drainage layer. A geotextile fabric separation layer shall be
placed between the subgrade and the drainage layer.
(f) No subgrade preparation is required because the 813-millimeter (32-inch)
combined thickness of pavement and base exceeds two-thirds of the design frost penetration depth
of 762 millimeters (30 inches).
b. Example 2. Design a type A traffic area on an Army Class III airfield as defined in
Paragraph 4c, Chapter 2. The design freezing index of the area is 39,990-degree Celsius hours
(3,000-degree Fahrenheit days). The subgrade is a mixture of poorly graded gravely sand and silty
sand with a fine content of about 9 percent. The average moisture content of the subgrade is
9 percent. The nonfrost design CBR of the subgrade is 16.
(1) Reduced subgrade strength design.
(a) The subgrade can be classified as SP-SM soil. It also classifies as an F2 frost
susceptible soil from Table 20-1. From Table 20-3, the FASSI value for an F2 soil is 6.5.
(b) From Figure 10-3 with a FASSI (CBR) value of 6.5, the combined thickness of
pavement required is 330 millimeters (13.0 inches).
(c) From Table 8-3, the minimum thickness of the surface course is 51 millimeters
(2 inches). Therefore, the required base and subbase course thickness is 279 millimeters