1981). The predicted frost depth can be used as a
ture is sufficiently high for needles to form, and
measure of the thickness of FTC-weakened bank
minor slumping and sliding may occur. In the
soils in the spring. Aitken and Berg (1968) pro-
winter, once a bank is frozen, surface sediments
vide the figures and tables needed to determine
are usually stable and not highly erodible, but
some can slough off the bank face due to sublima-
the values for the parameters in the equation and
tion of ground ice within surface sediment, or
a computer program for the equation is available
due to shallow daily thawing of sediment that
via E-mail.* However, some parameters may have
has been frost heaved. In the spring, when thaw
to be modified to apply the equation to bank soils
predominates, soil slumps, flows and blocks slide
rather than uplands.
down a bank face due to the high soil moisture
and significantly reduced soil strength.
48 K n F (or I )
The sediment that accumulates at the base of a
x=λ
L
bank as a result of such seasonal soil failures must
be removed if bank recession is to occur. Other-
where x = depth of frost or thaw penetration (ft)
wise, these accumulations will build up and pro-
l = lambda coefficient (accounts for effect
tect the bank toe from waves and currents. This
of temperature changes in soil mass)
protection maintains the position of the bank toe,
which adds to the stability of the bank by allow-
hr F)
ing the slope of the upper bank to eventually
n = factor to convert an air index to a sur-
decrease as gravity moves sediments downslope.
face index
Thorne (1982) describes this as basal end-point
F = air freezing (degree-days F)
control. The slope of the entire bank would then
I = thawing index (degree-days F)
decrease to a stable angle and the bank face would
L = volumetric latent heat of fusion (Btu/
eventually revegetate. At this stage, instability of
cu ft)
the bank soils and erosion from the bank face
would be minimal and bank recession would be
The equation assumes (1) an isothermal soil
very slow.
system at the beginning of the freezing season, (2)
Thus, methods that protect the bank toe have
one-dimensional heat flow with the entire soil
been employed for many years, because by pro-
mass at its mean annual temperature prior to the
tecting the toe, they effectively stabilize the bank
start of the freezing season, (3) when the freezing
whether the bank is unstable due to freezethaw
season starts, the soil surface temperature changes
processes or other geotechnical processes that
suddenly from the mean annual temperature to a
move bank sediments to the bank toe. Efforts to
temperature below freezing at which it remains
develop low-cost and biotechnical toe protection
during the entire freezing season, (4) soil water
approaches using native vegetation that is toler-
must lose the latent heat of fusion before it turns
ant to shoreline conditions, combinations of
to ice, and (5) soil freezes at 32F. It cannot nor-
readily available materials, biodegradable mate-
mally be used to calculate thaw depths in sea-
rials and vegetation, and reshaping and reveg-
sonal frost areas or frost depths in permafrost
etating banks have been explored by the Water-
areas, and it cannot calculate frost penetration
ways Experiment Station (WES). Some of these
over part of a freezing season.
methods have proven successful and information
on them can be obtained from the Environmental
and Geotechnical Laboratories at WES.
REMEDIATING FOR
Because waves and currents transport the ac-
SOIL FREEZETHAW
cumulated sediment along a bank toe from a site
As I have described, the general types of soil
and remove the sediment on a bank face that has
failures resulting from FTC-weakening of soils
been weakened by FTC, reducing maximum wa-
can vary with season. In the fall, needle-ice often
ter level stage and duration in the spring would
displaces soil surface sediments when soil mois-
help to stabilize the banks at numerous sites. A
combination of toe protection and water level con-
trol may be the most effective approach to bank
stabilization.
* Personal communication with Richard L. Berg, Re-
Because excess soil water caused by thawing of
search Civil Engineer, Civil and Geotechnical Engi-
ground ice is the primary condition that leads to
neering Research Division, CRREL, 1995.
14
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