Geology Reference
In-Depth Information
Several other terms need immediate defi nition. They are illustrated in Figure 5.1B. The
permafrost table is the upper surface of the permafrost, and the ground above the per-
mafrost table is called the supra-permafrost layer. The active layer is that part of the
supra-permafrost layer that freezes in winter and thaws during summer - that is, it is sea-
sonally-frozen ground. Although seasonal frost usually penetrates to the permafrost table
in most areas, in some areas where permafrost is relict (see Section 5.5), an unfrozen zone
exists between the bottom of seasonal frost and the permafrost table. This unfrozen zone
is called a talik. Unfrozen zones within and below the permafrost table are also termed
taliks.
5.1.2. Moisture and Ice within Permafrost
The terminological subtleties described above are more than mere semantics. Many
important problems posed by permafrost are related, either directly or indirectly, to the
water and/or ice content of permafrost. These may be summarized under three main
categories.
First, the freezing of water results in ice segregation, as explained in Chapter 4. The
magnitude of frost heave varies according to the amount and availability of moisture.
Poorly-drained silty soils usually possess some of the highest ice or water contents
and are termed “frost-susceptible” while coarse, free-draining sediments are termed
“non-frost-susceptible.” The nature of primary and secondary heave is explained in
Chapter 4.
Second, ground ice is a major component of permafrost, particularly in unconsolidated
sediments (see Chapter 7). Frequently, the amount of ice held within the ground in a
frozen state exceeds the natural water content of that sediment in its thawed state. There-
fore, if permafrost thaws, ground subsidence is the result. Thaw consolidation occurs as
thawed sediment compacts and settles under its own weight. High pore-water pressures
generated in the process may favor soil instability and mass movement. The various proc-
esses associated with permafrost degradation are discussed more fully in Chapter 8. A
related problem is that the physical properties of frozen ground, especially where soil
particles are cemented together by pore ice, may be quite different to those in the same
material but in an unfrozen state (Tsytovich, 1973). For example, in unconsolidated and/or
soft sediments there is often a signifi cant loss of bearing strength upon thawing. This is
discussed further in this chapter.
Third, the hydrologic and groundwater characteristics of permafrost terrain are differ-
ent from those of non-permafrost terrain (Hopkins et al., 1955; van Everdingen, 1990).
For example, the presence of both perennially- and seasonally-frozen ground prevents the
infi ltration of water into the ground or, at best, confi nes it to the active layer. At the same
time, subsurface fl ow is restricted to unfrozen zones or taliks. A high degree of minerali-
zation in subsurface permafrost waters is often typical, caused by the restricted circulation
imposed by the permafrost and the concentration of dissolved solids in the taliks. These
characteristics are examined in Chapter 6.
5.2. THERMAL AND PHYSICAL PROPERTIES
An understanding of the properties of frozen ground is necessary to understand
the surface features of permafrost terrain. While mechanical properties are important
to geotechnical engineering (see Chapter 14), physical properties are more useful to
periglacial geomorphology and geocryology.
