(MPa)
(ksi)
face. Such a condition could occur where snow
piled along a bank has insulated the lower bank
104
Thaw Weakening
3
face and the soil beneath that snow is still fro-
10
zen, while above the snow, the bank face has
3
10
2
thawed.
Thawing
10
2
10
Soil failures
101
Numerous investigations show that bank sedi-
Frozen
Recovering
Frozen
1
ments weakened by freezethaw can fail as slabs,
10
100
blocks (Fig. 11) or slides; can creep or slough
down the bank face as grains or aggregates
100
(Harrison 1970); or, if lubricated with sufficient
moisture, can flow down the bank. McRoberts
(1978) described the various forms of soil mass
Figure 9. Change in the resilient modulus of a low plastic-
failure resulting from slope instability in cold
ity silt (Johnson et al. 1978).
regions where spring thaw causes a saturated
layer of soil on a slope, and Reid (1984) reported
that slides, flows and slumps result from thaw-
Annual soil strength drops substantially to its
induced slope instability (Fig. 12).
lowest annual quantity during and after thaw
Even if the thaw-weakened sediments don't
and recovers as excess soil water drains out and
move down a bank, they are more erodible than
soil friction and cohesion return. Bredyuk and
before they were frozen and can be easily de-
Mikhaylov (1970) reported that thawed soil re-
tached by raindrop impacts, overland flows, and
gained shear strength in 15 to 45 days, depending
waves and currents if river or lake water levels
on soil water draining rates and the thickness of
rise sufficiently due to snowmelt or spring rains.
the thawed zone. Usually a thicker zone has a
Lawler (1992) calls this "freezethaw precondi-
slower draining rate.
tioning." Consequently, snowmelt floods in the
While in this weakened state, thawed surface
spring often erode significantly more bank soils
soil can easily fail by shear along the surface of
than floods of equal magnitude later in the year,
frozen soils below. This is especially likely due to
after the soils have regained their strength. Slavin
the increased unit weight of the thawed soil re-
(1977) and Wolman (1959) concluded that most
sulting from its excess water content. In addition,
bank erosion occurs in the spring. In addition,
as the water draining from the thawed soils en-
Leopold (1973) reported that small rises in river
counters the surface of the still-frozen sublayer,
flow that are separated by freezethaw periods
the water will be directed downslope along this
can cause channel migrations because the rises
surface (Fig. 10), and will increase seepage pres-
can effectively erode bank sediments that have
sures, possibly causing piping along the unfrozen
been loosened by the ground freezing processes.
and frozen soil interface if it intersects the bank
Hill (1973), Reid (1985), Sterrett
(1980), and Thorne and Lewin
(1979) report that twenty to ninety
Water Flows Along
Frozen Zone Surface
percent of the erosion along river
and lake banks in various regions
Thawed Zone
where seasonal frost occurs was
Depth of Frost
due to freezethaw and ground/
Frozen Soil
ice processes and to freezethaw
failures in the winter and spring.
Possible Water Piping
Depth of Thaw
Twenty to thirty percent of the bank
recession along Lake Sakakawea in
North Dakota is due to frost rup-
ture and thaw failure along the
bank face and along cracks land-
ward of the bank face (Reid et al.
1988, Reid and Dorough 1989, Reid
Figure 10. Reduced subsurface soil water drainage due to frozen subsoil.
1992). Freezethaw action and thaw
11
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