Page 309 - Planning and Design of Airports

Structural Design of Airport Pavements 267

Air temperature
below freezing Frozen Water in large void space
Surface course freezes into ice crystals
along plane of freezing
Ice crystals
temperature.
Base
Unfrozen
Frozen subgrade

Plane of freezing Frozen
temperature Ice crystals attract water
from adjacent voids,
which freezes on contact
and forms larger crystals.
Moving water
Capillary water Unfrozen
Unfrozen subgrade

Crystals continue to
Frozen
grow and join, fed
Frost mostly by capillary
heaving Ice lens water, forming ice lens.
Vertical pressure exerted
by ice lens heaves
Moving water
Unfrozen surface.
FIGURE 7-3 The process of ice segregation (http:// www.pavementinteractive.org).

During periods of thaw, the ice lenses begin to melt, and the
water which is released cannot drain through the still-frozen soil at
greater depths. Thus, lack of drainage results in loss of strength in
the subgrade. It is also possible that a reduction in stiffness will occur
in subgrade soils during the thaw period, even though ice lenses
may not have formed.
Originally developed by the U.S. Army Corps of Engineers, the
FAA categorizes soils into four “frost groups.” Soils in frost group 1
are least susceptible to frost and associated soil weakening, while soils
in frost group 4 are most susceptible. As illustrated in Table 7-4, those
soils with larger particle sizes, such as the gravelly soils, are found in
frost group 1, while very fine soils are found in frost group 4.
The design of pavements, both flexible and rigid, is modified
slightly depending on the propensity of the soil to encounter frost
and the depth of the frost, known as frost penetration. These consid-
erations are described in further detail later in this chapter.

Subgrade Stabilization
In addition to frost, factors such as poor drainage, adverse surface
drainage, or merely variations in soil depths, contribute to reductions