30 June 2001
(d) Compare pavement thickness with the limited subgrade frost penetration design.
(2) Limited subgrade frost penetration design.
(a) The moisture content of the base course is assumed to be 3 percent. The density
of the base course is assumed to be 2,403 kg/m3 (150 lb/ft3). From Table 8-3, a minimum
thickness of a 51-millimeter (2-inch) asphalt concrete layer is required. The frost penetration a
from Figure 20-5 is 3.6 meters (142 inches).
(b) The required base thickness c for zero frost penetration from Equation 20-8 is:
c = a - p = 142 - 2 = 140 inches in English units
= 3,600 - 51 = 3,549 millimeters in SI units
(c) In this case the base thickness of 3,549 millimeters (140 inches) is more than the
thickness of 339 millimeters (13 inches) from the reduced subgrade design. Therefore, the
thickness from reduced subgrade design is more economical than from the limiting subgrade frost
penetration design. Also, the thickness from the reduced subgrade design is greater than that
obtained from the nonfrost design. Therefore, the combined thickness of 330 millimeters
(13.0 inches) will be used as the design thickness. The pavement structure could be made up of
51 millimeters (2 inches) of surface course, 100 millimeters (4 inches) of an NFS drainage layer
beneath the surface course, and 178 millimeters (7.0 inches) of NFS base over a separation layer.
A geotextile fabric could be placed between the subgrade and base course as a separation layer.
Subgrade preparation is required to a depth of 610 millimeters (24 inches) based on the subgrade
preparation criteria described in Paragraph 10a.
17. RIGID PAVEMENT DESIGN EXAMPLES FOR SEASONAL FROST CONDITIONS.
a. Example 1. Design an Air Force medium-load pavement. The design air freezing index at
the site is 9,330-degree Celsius hours (700-degree Fahrenheit days). The highest elevation of
groundwater is about 914 millimeters (3 feet) below the surface of the subgrade. The subgrade is
a silty sand with 20 percent finer than 0.02 mm by weight. The average moisture content of the
subgrade is 15 percent. The nonfrost design modulus of soil reaction k is 54 MN/m3 (200 lb/in.3).
The 90-day concrete flexural strength R is 4.8 MPa (700 psi).
(1) Reduced subgrade strength design.
(a) From Table 20-1, the subgrade is classified as a F3 frost susceptible soil.
(b) Select several combined base thicknesses and obtain FAIR values from
Figure 20-5 or from Equation 20-1. For example: