30 June 2001
c = thickness of unbound base, millimeters (inches)
a = thickness for complete frost protection, millimeters (inches)
p = thickness of asphalt or concrete for nonfrost design
d. Step Four. For limited frost penetration into the subgrade, determine the average moisture
content of the subgrade prior to freezing. Compute water content ratio r.
moisture content of the base = same as that assumed for nonfrost base material in step 2.
If the computed r exceeds 2.0, use 2.0 for types A, B, and primary traffic areas. If r exceeds 3.0,
use 3.0 for all pavements other than those in types A, B, or primary traffic areas.
e. Step Five. Enter Figure 20-6, with c (from step 3) as the abscissa and, at the applicable
value of r, find the design combined base thickness b on the left scale and the allowable frost
penetration into the subgrade s on the right scale or use Equations 20-10 and 20-11. This
procedure will result in a sufficient thickness of material between the frost susceptible subgrade
and the pavement so that for average field conditions subgrade frost penetration of the amount s
should not cause excessive differential heave of the pavement surface during the design freezing
b = design combined base thickness
c = combined base thickness for zero penetration
s = limited subgrade frost penetration depth
f and g = factors from the following tabulation