UFC 3-260-02
30 June 2001
CHAPTER 20
SEASONAL FROST CONDITIONS
1. GENERAL. This chapter presents criteria and procedures required for the design and
construction of pavements placed on subgrade materials subject to seasonal frost action. If frost
does not penetrate into the subgrade using thicknesses necessary for nonfrost design, pavement
design need not consider effects of frost action unless the base, subbase courses contain other
than NFS, PFS, S1, or S2 materials. The designer must select subbase materials which do not
allow pumping of subbase course or subgrade fines during periods of saturated or nearly saturated
conditions. The detrimental effects of frost action in frost susceptible subsurface materials are
manifested by nonuniform heave of pavements during the winter and/or loss of strength of affected
soils during the ensuing thaw periods. Studies have shown that the modulus of subgrade reaction
is reduced substantially during the thaw period. Application of load on thaw-weakened pavements
can lead to premature failure. Other effects of frost on pavements are possible loss of compaction,
pumping, increased pavement roughness, restriction of drainage by frozen layers, and cracking of
asphalt concrete pavements. In extreme conditions, these problems can cause hazardous operating
conditions, or Foreign Object Damage (FOD) to aircraft, and can lead to extensive maintenance of
the pavement surface. Except in cases where other criteria are specifically established, pavements
should be designed so that there will be no interruptions of traffic at any time due to differential
heave or to reduction in load-supporting capacity. Pavements should also be designed so that the
rate of deterioration during critical periods of thaw weakening and thermally induced cracking will
not be so high that the useful life of the pavement is less than that assumed as the design
objectives. For interior pavements which fall within a geographical area subject to subgrade frost
action, the "reduced subgrade strength" or the "limited subgrade frost penetration" pavement
design criteria should be used for all aircraft hangar pavements in heated or unheated areas.
2. FROST-SUSCEPTIBILITY CLASSIFICATION. For frost design purposes, soils are divided into
eight groups as shown in Table 20-1. Soils are listed in approximate order of increasing frost
susceptibility and decreasing bearing capacity during periods of thaw.
a. The frost susceptibility of the soils classified in Table 20-1, based on laboratory tests, are
shown in Table 20-2. The NFS, S1, and S2 groups are negligible to very low frost susceptible
soils. Based on laboratory tests, the heave rates range between 1 and 4 mm/day and the thawed
CBR ranges between 12 to 20 percent. These soils are considered to be suitable as base and
subbase course material. Soils categorized as F1, F2, F3, and F4 are unsuitable as base or subbase
materials.
b. Under special conditions the frost group classifications adopted for design may be
permitted to differ from that obtained by application of the above frost group definitions provided a
written waiver is obtained and a valid justification is presented in the design analysis. Such
justification may take into account special conditions of subgrade moisture or soil uniformity, in
addition to soil gradation and plasticity, and should include data on performance of existing
pavements near those proposed to be constructed. This will require the approval of HQUSACE
(CEMP-ET), the appropriate Air Force Major Command or the Naval Facilities Engineering Command.
3. METHODS OF THICKNESS DESIGN. Three methods are prescribed for determining the
thickness of a pavement that will have adequate resistance to distortion by frost heave, cracking
from differential frost heave and distortion under traffic load as affected by seasonal variation of
20-1
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