Table 2. Hypothesized and generalized sequence of bank soil freezethaw with associated
effects.
Fall
Winter
Spring
In early fall, bank surface
The zone of frozen soil
Strength of bank soils is
soils cycle between unfrozen
deepens as the average air
significantly reduced upon
and frozen as air temperature
temperature drops; soil
thaw because of excessive
fluctuates above and below
water migrates to the
soil water from melted
32F.
thickening frozen zone.
ground ice.
Later in the fall, as the daily
Ground ice forms in the
Thawed soils fail en masse.
average air temperature de-
frozen zone if sufficient
creases, the lower part of the
soil water is available.
frozen surface soils remains
frozen when the upper part
Ground ice can sublimate
High river and lake
thaws during the day when
at a bank face, resulting in
water during spring removes
the air temperature rises
sloughing of surface soil
weakened or failed bank soils.
above 32F.
particles.
Soil strength is reduced
Continued ground ice
Bank soil slowly recovers
in the freezing and thawing
growth can heave the bank
its strength but is more
zone due to the soil structure
soil surface and displace
susceptible to removal or
being disrupted by ice
soil grains downslope.
failure during recovery.
formation and swelling and
shrinking of the soil mass.
* Intermediate
* Minimum
* Maximum
* Comparative state of soil erodibility and instability resulting from freezing and thawing.
The freeze periods are usually not sufficiently
nitude and types of the soil changes caused by
long or intense in the fall to cause frost to pen-
FTC are dependent on initial soil conditions, i.e.,
etrate very deeply into the soil; however, needle
soil type (Chamberlain, 1989), aggregate size and
ice or near-surface ice lenses are very likely to
bulk density (Benoit and Voorhees 1990), on soil
grow. Conversely, solar input and the air tempera-
and weather conditions during the FTC, and on
tures are usually sufficiently high for most of the
the number of FTC. Thus, investigators have
frozen soil to thaw during subsequent thaw peri-
found that a variety of complex effects result from
ods. Even if segregated ice lenses or needles do
the intensity and number of FTC and that the
not form, the ice that forms in soil pores during
degree of soil weakening varies with site-specific
freezing and melts during thaw often causes the
soils.
soil to swell and shrink, which changes soil struc-
Aoyama and others (1985) found a reduction
ture and particle bonding (Anderson et al. 1978).
in soil cohesion that increased as freezing tem-
As the average daily air temperature drops
peratures dropped but found little change in the
and solar insolation lessens later in the fall, bank
soil friction angle after the FTC. Although Ogata
soils lose enough heat during the daily freezing
and others (1985) observed large differences on
period for the frozen layer to get sufficiently thick
the strengths of undisturbed alluvial soil and con-
that its lower portion remains frozen even after
solidated kaolin, they found in both soils that
its upper part thaws during the daily thaw pe-
FTC reduced cohesion and increased the angle of
riod. More interstitial ice may form now during
internal friction. Othman and Benson (1993) ob-
freezing than earlier in the fall because the inten-
served a network of cracks formed by ice lensing
sity of the freezing has increased sufficiently for
and soil shrinkage that increased the hydraulic
more of the soil moisture to freeze and for addi-
conductivity in clay soil after one FTC, and found
tional soil water to migrate to the freezing front
that new lenses and cracks formed with addi-
and freeze.
tional FTC. However, they determined that after
three cycles new lenses are negligible and a fur-
Soil weakening and failures
ther increase in the hydraulic conductivity ceased.
Van Klaveren (1987) and Kok and McCool
Yong and others (1985) observed that signifi-
(1990) show that the critical shear strength of a
cant changes in the liquid limit and the undrained
soil subjected to FTC is less than that of the same
shear strength of a clay from Matagami, Quebec,
soil that has not been frozen. However, the mag-
occur after one FTC (Fig. 2) and that additional
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