E. M. PART IV
In order to use the minimum base-course thickness required to restrict the seasonal thaw to pavement and base
depth, consideration should be given to using relatively high moisture retaining non-frost-susceptible soils, such as
uniform sands in the lower base. After initial freezing such soils provide considerable resistance to thaw penetration
because of the high latent heat required to melt them. The use of frost-susceptible soils of groups F1 and F2 in the lower
base is also permissible if some heaving may be tolerated and these soils are covered with a thickness of non-frost-
susceptible base determined by use of the reduced strength of subgrade method. The depth of thaw penetration into such
a layered base may be estimated by methods presented in chapter 6. For pavements of lesser importance where
considerable heaving is allowable, the use of a group F3 or F4 soil in the lower base is permissible in extreme instances
where more suitable materials are not available to prevent initial thawing of a high ice content subgrade with large ice
concentration. In such cases, the minimum thickness of the non-frost-susceptible base course over group F3 or F4 soils
shall be determined by the reduction in strength method and some, possibly objectionable, heaving should be
a. Design to restrict seasonal thaw to pavement and base depth. In arctic regions a design which will keep the
seasonal thaw within the base-course depth will keep the subgrade frozen and prevent frost heaving or damaging surface
settling. The required gravel base thickness may be determined from figure 3, utilizing the thawing index of the
pavement surface and the 5 percent base moisture content for which the curve is computed, unless information on base
materials indicates a different moisture content can be maintained. For base courses at other moisture contents, the
thicknesses shown in figure 3 may be multiplied by the appropriate factor given in the note. Observations have shown
that a base consisting of non-frost-susceptible material normally has a moisture content between 3 and b percent. A
completely saturated gravel base will only rarely contain more than 9 percent moisture. The surface thawing index may
be computed by multiplying the thawing index based on air temperatures by a correction factor for the type of surface.
This correction factor may be taken as 1.6 for bituminous pavements and 1.5 for portland cement concrete pavements.
In each individual case the air thawing index should be computed with specific site data; however, order-of-magnitude
values may be estimated from figures 6 and 8 in chapter 1.
The thickness determined for figure 3 is the thickness of the non-frost-susceptible base only and is not the combined
thickness of pavement and base. Because of the fact that the surface course generally has a very small or negligible
moisture content, it follows that little heat is lost in melting the frozen moisture therein. Thus, for determinations of this
type, the thermal resistance of surface courses 6 inches thick, or less, may be neglected.
It will be noted from figure 3 that for a base thickness of 6 feet, the surface thawing index must be about 500 or less,
to restrict thawing to the gravel base course.
b. Design based on reduced strength of subgrade. Since the combined pavement and base thicknesses required
to prevent thawing of the subgrade are commonly greater than 6 feet, except in extremely frigid areas, design must
usually be based on the assumption that thawing and freezing will occur in the subgrade.
This method may be used for flexible pavements on subgrade soils of groups F1, F2, and F3 and rigid pavements
over group F1 and F2 soils, when subgrade conditions are sufficiently uniform to assure that objectional differential
heaving or subsidence will not occur, or where subgrade variations are correctible to this condition by removal and
replacement of pockets of more highly frost-susceptible or high ice content soils. The method may also be used for
design of flexible pavements where appreciable non-uniform heave or subsidence can be tolerated in pavements of
lesser importance, used for slow speed traffic, including pavements designed over group F4 subgrades.
When the reduction in subgrade strength method is used for design of flexible pavements over subgrade soils of group
F4, the combined pavement and base thickness should be determined using design curves for F3 and F4 soils shown on
figures 4 through 7. The use of the reduction in strength method