Geology Reference
In-Depth Information
governed by a frictional resistance factor. Since a considerable amount of unfrozen water
was present in the permafrost, the frictional resistance of the frozen clay was signifi cantly
reduced and failure occurred.
9.7.3. Stability of Thawing Slopes
Permafrost slopes commonly experience failure at angles considerably lower than the
equilibrium angle predicted by standard geotechnical analysis. Table 9.7 lists a number
of studies where analysis indicates that the failures have occurred at angles below their
predicted equilibrium angle. The table also includes data from several studies under-
taken in non-permafrost environments where low-angled slopes have experienced unusual
failure. These failures have been attributed to frozen ground conditions in the
Pleistocene.
The geotechnical details of slope stability analysis are beyond the mandate of this text.
However, so-called “back-analyses,” involving assumptions as to the depth of thaw, and
the bulk density and water content values that might have been involved in a particular
slope failure, permit identifi cation of the conditions necessary to initiate failure. For
example, on Ellesmere Island, C. Harris and A. G. Lewkowicz (2000, pp. 457-460) con-
cluded that, even taking residual shear strength of 25°, pore-water pressures associated
with thaw and a progressive reduction in shear strength at the base of the active layer from
gelifl uction movement were suffi cient to initiate failure on a 12° slope.
Similar slope failures have been described from the permafrost regions of western
Siberia (Liebman, 1996; Liebman et al., 2003). In the boreal forest and taiga zones, it is
not uncommon for failure to occur following the destruction of vegetation by fi re (Zoltai
and Pettapiece, 1973). Where substantial bodies of ground ice are present, an initial slope
failure may be the trigger mechanism for subsequent retrogressive-thaw slumps (see
Chapter 8).
With predicted global climate warming, it must be anticipated that permafrost slopes
will experience substantial slope instability and thermokarst modifi cation. This will be
especially pronounced in areas of warm permafrost and at the southern (i.e. warm)
Table 9.7.
Summary data on reported failures in thawing slopes.
Locality
Lithology
Soil residual
Predicted
Failed angle
Source
strength ø
angle
(degrees)
(degrees)
(degrees)
Active slope failures in permafrost:
Svalbard
Sandy clayey silt
36
20
6-12
Chandler (1972)
Mackenzie
Clay
23
12.5
3-9
McRoberts and
Va l ley, N W T
Mogenster n (19 74)
Pleistocene slope failures:
England
Solifl ucted clay
12.4-15.5
6.8-8.1
3-7
Weeks (1969)
(Weald,
Gault, and
London clays)
England
Sandy clay
23
12.0
6.8
Chandler (1970a)
Sandy clayey silt
16
8.8
4.0
Chandler (1970b)
