Three conditions must exist for ground ice to grow and become a substantial
component of a soil mass: a source of soil water, sufficiently cold air tempera-
tures to cause heat loss from a soil and subsequent freezing of soil water, and a
frost-susceptible soil (usually a silty soil) (Anderson et al. 1978).
Silty soils absorb water rapidly because they have particles small enough to
provide comparatively high capillary rise and large enough pore spaces to allow
quick flow of water through the silt (Jumikis 1962). These characteristics lead to
rapid increase in water content within soil voids upon freezing. More coarse- and
fine-grained soils do not absorb water as rapidly. Thus, silty soils with available
soil water are most susceptible to the substantial seasonal changes in soil strength
and erodibility caused by FT cycling. However, Janson (1963) reports that even
sand may become frost-susceptible if it is well compacted, and Chamberlain* has
observed needle ice in almost any soil type.
In addition to soil texture, frost susceptibility depends upon vegetative cover,
the depth and density of snow cover, initial soil temperature, air temperature
regime, exposure to the sun, the temperature gradient within the soil, the mobility
of soil water, the depth to the water table, overburden stress, and soil density
(Jumikis 1962, Chamberlain 1981). As ice crystals form within soil voids, soil
aggregates and particles are forced apart and ice pressure may compress or rup-
ture the aggregates. The net effect of ice formation on soil structure depends on
soil type, water content, and intensity of freezing.
The FT-induced, physical changes in a soil affect soil-particle cohesion, soil
density and strength, infiltration, runoff, and soil-surface geometry, which, in
turn, affect that soil's erodibility and the erosivity of subsequent surface runoff.
The magnitude of these effects varies with location. McCool (1990) reported that
major FT-soil runoff events occurred in nine of 40 years in Whitman County,
Washington, and that 41% of the total estimated soil loss in the 40-year period
occurred during these nine years. Zuzel et al. (1982) concluded that snowmelt
and/or frozen soil were responsible for 86% of the observed soil loss events in
the Pacific Northwest. In spite of this general regional sense of the importance of
FT cycling, Benoit and Voorhees (1990) and Kok and McCool (1990) reported
that soil FT effects are some of the least understood aspects of the soil erosion
process, even though FT processes have been investigated for years.
Personal communication, Edwin J. Chamberlain, Jr., Research Civil Engineer, U.S.
Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire.