pre-frozen water content of those soils (Nixon
and Ladanyi 1978); the excessive water can nei-
ther be reabsorbed into available soil voids (Cham-
berlain 1981) nor be adequately drained into the
soil below, prolonging the time for the thawed
soil to regain its pre-frozen strength through
Intergrain friction and cohesion that were re-
duced when ice disrupted grain interlocking and
packing remain low until the soil particles have
reoriented (recompacted) to a pre-frozen condi-
tion, which usually cannot occur until after drain-
age of the excess water. Even under drained con-
ditions, the voids and weakened planes left in the
soil after the ground ice melts often cause the soil
to depress and collapse under its own weight
(Burdick et al. 1978; Gifford 1984; Nixon and La-
danyi 1978).
Bredyuk and Mikhaylov (1970) reported that the
resistance of thawed soil to shear, as measured with
a shear vane, was 1.2 to 7 times less than the re-
sistance of the unfrozen soil layer below the frost
line, and that the maximum strength reduction
occurred in a soil that heaved the most during
freezing. They also concluded that the maximum
thaw weakening occurred along embankment
shoulders and slopes.
Figure 8. Sediment sloughed off a bank due to ground
Although these weakened, thawed soils usu-
ice sublimation, Orwell Reservoir, Minnesota (Reid
ally regain their pre-frozen soil strength (Chamber-
1985).
lain 1981), the time it takes varies with site condi-
tions. This timing may be reflected in the pattern
Generally, soils thaw more quickly from the
of change of the resilient modulus (Fig. 9). This
ground surface down than up from the bottom of
modulus, the ratio of total stress to recoverable
the frozen zone (Jumikis 1962), thereby creating a
strain, is a measure of the dynamic modulus of
layer of nearly impermeable frozen soil under
elasticity, but it does not have a one-to-one linear
thawed soil. Bank surface soils that contained
relationship with soil strength. However, a low-
ground ice will often have an amount of melt-
strength soil usually has a low resilient modulus.
water upon thawing that considerably exceeds the
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