Geology Reference
In-Depth Information
9.7.2. Thermokarst and Thaw Consolidation
An important cause of slope instability in permafrost environments is associated with the
time-dependent compression of frozen ground that results from thaw and subsequent
drainage of pore water. This process, which occurs annually in the active layer, is termed
thaw consolidation (Morgenstern and Nixon, 1971). Many geotechnical engineers, working
either in permafrost regions (McRoberts and Morgenstern, 1974) or upon slope stability
problems of strata previously frozen (Hutchinson, 1974; Hutchinson and Gostelow, 1976),
regard gelifl uction as one form of thaw consolidation. In all probability, a process contin-
uum exists between slow mass wasting and certain rapid mass movements with thaw con-
solidation being the central, common, mechanism.
The signifi cance of thaw consolidation is only beginning to be appreciated by geomor-
phologists working in permafrost regions, and some additional words of explanation are
appropriate.
During thaw, the fl ow of water from a thawing soil may be unimpeded. In this situation,
the variation of settlement with time is controlled solely by the position of the thawing
front. However, if fl ow is impeded, as is often the case in fi ne-grained sediments, the rate
of settlement with time is controlled by the compressibility and permeability of the thawed
ground. If the rate of thaw is suffi ciently fast, water is released at a rate that exceeds that
at which it fl ows from the soil. As a result, pore-water pressures increase in the thawed
material and, if these exceed shear strength (i.e. the cohesion of the soil), instability will
result.
The thaw-consolidation ratio,
R
, is commonly used to describe the relative rates of
generation and expulsion of excess pore fl uids during thaw (Morgenstern and Nixon, 1971;
Nixon and McRoberts, 1973):
R
=α
2
Cv
and
α=
dt
where
Cv
is the coeffi cient of consolidation, and
d
is the depth of thaw in time
t
. Because
the depth of slippage is limited by the frost table (i.e. top of permafrost), slope failures in
permafrost terrain are generally planar in nature. The obvious illustration of slope failures
in permafrost that can be explained in terms of thaw consolidation is the active-layer
detachment described earlier in this chapter.
According to the thaw-consolidation ratio, a value greater than unity predicts the
danger of sustained substantial pore pressure at the thawing front, and hence the possibil-
ity of instability due to reduction of shear strength at that plane. Usually, the permafrost
table acts as a lubricated slip plane and controls the depth of the failure plane. Previously,
such failures were colloquially termed “skin fl ows” (Capps, 1919). In all instances, the
active layer and its vegetation mat detaches from the underlying permafrost surface (see
Figure 9.13).
In areas of discontinuous permafrost, thawing of permafrost frequently involves
adjacent non-permafrost sediments. The interactions are complex. For example, in the
Mackenzie Valley, NWT, Canada, failure can occur through frozen soil with the base of
the slide in unfrozen clay (McRoberts and Morgenstern, 1974). The permafrost tempera-
tures were between
4 °C and high pore-water pressures controlled the available
shear strength in the unfrozen clay. The long-term strength of the permafrost soil was
−
2 °C and
−
