Page 242 - Geotechnical Engineering Soil and Foundation Principles and Practice

Pore Water Pressure, Capillary Water, and Frost Action
Pore Water Pressure, Capillary Water, and Frost Action 237

11.10 PERMAFROST

11.10.1 Overview

Permanently frozen ground, termed ‘‘permafrost,’’ underlies nearly one-fifth
of the total land area of the world. Permafrost is most widespread in the
northern hemisphere in the treeless high Arctic, referred to as tundra, but also
can extend south of the tree line and as far south as the 50th parallel. The
southern boundary is irregular, and patches of permafrost occur sporadically
beyond the boundary. As may be expected, extensive permafrost occurs in
Antarctica.

Permafrost occurs where the mean ground surface temperature is below 08C
(328F), which in turn is influenced not only by the air temperature but also by
ground cover, topography, and local climatic and soil conditions. The depth to
which permafrost penetrates is limited by the geothermal gradient, as shown in
Fig. 11.18. The thickness of permafrost therefore varies from a few meters to
several hundred meters, in general being thicker along the coast of the Arctic
Ocean, and diminishing in thickness farther south.

Figure 11.18
Permafrost and
the geothermal
gradient. The
active zone
undergoes annual
changes in
temperature, and
the permafrost
thickness is limited
by the geothermal
gradient.

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