Page 243 - Geotechnical Engineering Soil and Foundation Principles and Practice

Pore Water Pressure, Capillary Water, and Frost Action
238 Geotechnical Engineering

11.10.2 Permafrost Active Zone
Seasonal changes in temperature from winter to summer cause thermal energy
waves that slowly penetrate and are damped out with depth as the heat flux from
each wave moves upward as well as downward. The result is a damped sinusoidal
curve for ground temperatures at any given time, shown by the curved line
between d and e in Fig. 11.18. These lines present an envelope of minimum and
maximum ground temperatures, and extend downward into the permanently
frozen zone.
The line for the maximum temperature defines the maximum depth of thawing,
which is called the active zone. Ice lenses generally are abundant below the active
zone and cause major ground subsidence if conditions change so that melting
proceeds downward. According to the theory for frost heave the lenses may be
‘‘paleo-ice’’ formed during initial freezing.

11.10.3 Permafrost Landforms

Thermal cycling in frozen soil below the active zone causes contraction tensile
cracks that fill with water trickling down from the thawed active zone. The water
immediately freezes, creating permanently frozen vertical wedges of ice. The
wedges then grow incrementally during subsequent thermal cyling, pushing the
ground up between adjacent wedges. The cracks often follow a hexagonal pattern
similar to a honeycomb, and create a characteristic feature of permafrost areas
called polygonal ground, shown in Fig. 11.19. If soil immediately next to the
wedges is pushed up, the result is ‘‘low-centered’’ polygons, or if the entire soil
mass between wedges is pushed up, the result is ‘‘high-centered’’ polygons.

Figure 11.19
Summer on the
tundra. Water is
trapped on the
surface of Arctic
tundra by
permafrost.
Vertical ice
wedges have
pushed up soil to
form low-centered
polygons that
pond water and
can cause partial
melting of the
permafrost to form
large lakes.

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