Geology Reference
In-Depth Information
monitored (Mackay, 1997; Mackay and Burn, 2002). During the last 25 years, downward
freezing has started on the exposed lake-bottom while upward freezing has developed
from the permafrost beneath the talik. It is important to stress that the unfrozen sediments
in the drained lake-basin were water-saturated sand and that the surrounding terrain was
underlain by cold permafrost. Thus, freezing took place in a closed system. Figure 5.5
summarizes schematically what happens to a saturated unfrozen zone when sandwiched
between downward- and upward-aggrading permafrost. As explained earlier in Chapter
4, the progressive freezing of a non-frost-susceptible, water-saturated soil can result in
pore-water expulsion and the development of artesian pressures in the unfrozen zone. By
contrast, in a frost-susceptible soil, the growth of ice lenses may result in water loss and
consolidation of the unfrozen zone.
Measurements made in the years that followed drainage have included the detailed
leveling of benchmarks inserted in the lake bottom, the recording of ground temperatures,
pore-water salinities, and electrical freezing potentials, and analyses of surface water
quality (Mackay, 1982, 1983a, 1984b, 1986d, 1997, 2000, pp. 62-63; Mackay and Burn,
2002; Parameswaran and Mackay, 1983). Collectively, they indicate uplift (heave) of the
lake bottom, fl uctuations in the amount of heave relative to a fi xed datum (indicating
artesian pressures), increasing values of pore-water salinity and freezing-point depression,
a decrease in active-layer thickness in association with the establishment of a vegetation
cover, and the growth of ice at the top of newly-formed permafrost. In April 1995, a small
ice mound, approximately 1 m in height and 30 m in diameter, formed, indicating ground-
water fl ow that previous winter. All these phenomena result primarily from permafrost
Non-
frost-susceptible
soil
Frost-susceptible
soil
Temperature
-
0
+
Active layer
Permafrost
Permafrost
Unfrozen
Talik
(Unfrozen)
A
B
Permafrost
Permafrost
Direction of
upfreezing or
down freezing
Pore-water
expulsion
Direction of
water movement
Figure 5.5.
Diagram illustrating downward and upward freezing in a closed system in frost-
susceptible and non-frost-susceptible soils. In frost-susceptible soil (A), the growth of ice lenses
results in water loss and consolidation of the unfrozen zone. In non-frost-susceptible soil (B), pore-
water expulsion results in water gain and artesian pressures in the unfrozen zone. Modifi ed with
permission from Mackay (1984a).