Geology Reference
In-Depth Information
supersaturated sediment and supernatant water were 300 cm and 500 cm respectively, then
the excess ice value would be 62.5%. The advantage of this index is that it provides some
indication of the potential morphological change, or volumetric ground loss, consequent
upon thaw.
Visual estimates of the amount of excess ice present in frozen soil are deceptive
because sediment may not necessarily contain excess ice even though it may contain
visible ice lenses. This is because the sediment between the lenses may not be saturated
with ice and, upon thawing, all water is retained in the voids. Sediments that contain
excess ice are referred to as “ice-rich” or “icy.” In unconsolidated materials, excess
ice values of between 15% and 50% are common although exceptionally icy sediments
may have values as high as 70-80%. Frozen sediments containing excess ice are “thaw-
sensitive” and may be contrasted with “thaw-stable” materials which contain no excess
ice. The latter are not subject to thaw settlement and retain much of their mechanical
strength when thawed.
The terms “massive ice” and “massive-icy bodies” are usually reserved for relatively
pure ice bodies whose ice content averages at least 250% for a thickness of several
meters.
7.2. CLASSIFICATION
Ground ice may be either epigenetic (i.e. develops after the enclosing sediment has been
deposited) or syngenetic (i.e. forms at, or almost at, the same time as the enclosing sedi-
ments are deposited).
Many attempts have been made to classify ground ice. The earliest were developed in
Russia (Shumskii, 1959; Shumskii and Vtyurin, 1966; Solomatin, 1986; Vtyurin, 1975).
Although these systems are not widely adopted in North America, several comments are
appropriate. First, as many as 20 different ground-ice types are recognized by Russian
scientists. P. A. Shumskii (1959) lists segregated ice, injection ice, vein ice, recurrent-vein
ice, cave ice, thermokarst-cave ice, karst cave ice, and buried ice as the major types, each
with several subtypes. Second, Russian classifi cations often include the burial of surface
ice as a ground-ice category. Surface ice includes the following: (i) river, lake, and sea ice,
(ii) bottom (anchor) ice, and (iii) surface icings. It also includes snow banks and glacier
ice, both of which experience recrystallization and other changes following burial. Third,
many Russian scientists consider the composition, genesis, and initial moisture content on
freezing (syngenetic or epigenetic) (Kudryavtsev, 1978; Melnikov and Spesivtsev, 2000).
For example, A. I. Popov (1973) identifi ed ground ice as existing within three “permafrost
zones”: for simplicity, these are described as follows: (i) the “horizon of discontinuous
(seasonal) cryohypergenesis” (i.e. the active layer), (ii) the “horizon of active cryodiagen-
esis” (i.e. the permafrost that exists above the depth of zero-annual amplitude), and (iii)
the “horizon of passive cryodiagenesis” (i.e. the permafrost below the depth of zero-
annual amplitude). This three-fold division is useful in that it focuses attention upon the
movement of unfrozen moisture within frozen ground in response to the temperature
gradient (see Chapter 4).
In North America, the approach is simpler and more general. For example, an early
non-genetic classifi cation was developed by fi eld engineers in Canada in order to describe
ground ice conditions (Pihlainen and Johnston, 1963). Ice was classifi ed as being in one
of three categories: either “not visible,” “visible - less than 1” thick,” or “visible - greater
than 1” thick.” Each category was further subdivided and given a symbol designation for
easy use. The scheme is presented in Table 7.1.
