Lesson 3/Learning Event 3
Reduced Subgrade Strength.
Thickness design may also be based on the reduction in strength of the subgrade which occurs during
thawing of soils which have been affected by frost action. This design method normally permits less
thickness of pavement and base than that needed for limited subgrade frost penetration. The method
may be used for flexible pavements on F1, F2, and F3 soils when the subgrade is horizontally uniform
(or slightly variable for flexible pavements) such that significant or objectionable differential heaving
and resultant cracking of pavements will not occur. The method may also be used for flexible type
pavements on F1 through F4, horizontally variable subgrades when the pavements are of a minor-slow-
speed, and non-critical character in which heave and its effects can be tolerated. When the reduced
subgrade strength method is used for F4 subgrade soils, the combined pavement and base thicknesses
should be determined using the design curves for F3 soils in Figures 13-15 through 13-21 in the
Appendix. When a thickness determined by the reduced subgrade strength method exceeds that
determined for limited subgrade frost penetration or for complete protection, the applicable smaller
value shall be used, provided it is at least equal to the thickness required for non-frost conditions.
In situations where use of the reduced subgrade strength method might result in objectionable surface
roughness or pavement cracking caused by frost heave, but use of the limited subgrade frost penetration
design method is not considered necessary, intermediate design thicknesses may be used, as necessary,
to prevent objectionable heaving, provided justification is offered on basis of frost heaving experience
developed from existing airfield and highway pavements where climatic and soil conditions are
comparable.
Flexible pavements. In the reduced subgrade strength method of design, the curves in Figures 13-
15 through 13-21 in the Appendix should be used to determine the combined thickness of flexible
pavement required for aircraft wheel loads and wheel assemblies. Figure 13-22 should be used for
highway design. The curves for highways require greater combined thicknesses than the curves for
equivalent single-wheel aircraft loadings because of the higher frequency of load applications. General
field data and experience indicate that on the relatively narrow embankments of highways, reduction in
strength of subgrades during frost melting may be less in substantial fills than in cuts because of better
drainage conditions and less intense ice segregation. If local field data and experience show this to be
the case, then a reduction in combined thickness of pavement and base of up to 10 percent may be
permitted for highways on substantial fills. In no case shall the combined thickness of pavement and
nonfrost-susceptible base be less than 9 inches where frost action is a consideration.
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