Geology Reference
In-Depth Information
The latter are dealt with in Part IV; here we deal with the geomorphic processes associ-
ated with repeated freezing of ground and the manner by which destruction of rock and
soil proceeds under cold-climate conditions.
4.2.1. The Freezing Process
The freezing process has attracted much attention from geotechnical engineers in Canada,
Norway, the USA, China, and Russia, and from government agencies such as the US Army
Cold Regions Research and Engineering Laboratory (CRREL), the US Highway Research
Board, the Norwegian Geotechnical Institute, and the National Research Council of
Canada. The detailed physics of the freezing process need not concern us; the text by
P. J. Williams and M. W. Smith (1989, pp. 1-8, 174-201) includes a good discussion.
However, a few comments are necessary to preface this section.
First, different soils cool at different rates depending upon heat conductivity and mois-
ture content, and thus freeze at different rates. Second, soils do not necessarily freeze
when their temperatures fall to 0 °C since they are known to exist in a supercooled state.
For example, a common condition is for saline groundwater to lower the temperature at
which soil freezes. Third, the duration and intensity of a temperature drop below 0 °C will
affect the rate and amount of soil freezing.
Bearing these comments in mind, it is also necessary to describe a few basic relation-
ships that relate to the freezing of soil and bedrock.
First, pure water freezes at 0 °C and in doing so expands by approximately 9% of its
volume. This phase transition, between liquid and solid, is fundamental to our understand-
ing of frozen and freezing soils. The most obvious result is the volume increase in soil;
this is commonly known as frost heave and has considerable geomorphic and practical
signifi cance, the latter by displacement of buildings, foundations, and road surfaces (see
Part IV), the former by heaving of bedrock, uplifting of stones and objects, and frost
sorting (see Chapter 8). However, it must be emphasized that the 9% expansion associated
with the change from water to ice is not what permafrost scientists normally regard as
frost heave. For soil to heave, ice must fi rst overcome the resistance to its expansion caused
by the strength of the overlying frozen soil. This usually occurs only when segregated ice
lenses form.
Second, not all water freezes at 0°C. This is because soil moisture, or water in
the ground, commonly contains dissolved salts. These result in depression of the
freezing point. This concept is illustrated in Figure 4.1. Usually, because the concen-
tration of dissolved salts is weak, the freezing-point depression is only 0.1°C or so
below 0°C.
A third physical process relates to the molecular forces that exist between phases when
the interface is confi ned. This is known as capillarity. In soils, the capillarity between soil
particles increases as the soil particles become smaller. Moreover, during soil freezing,
the formation of ice results in water being confi ned progressively in a smaller space.
Accordingly, the free energy of the water falls as freezing takes place and this is most
apparent in fi ne-grained sediments. This effect is termed cryosuction and is the cause of
water migration to the freezing zone.
Fourth, the amount of water which freezes at any one particular temperature will also
depend on the nature of the soil in question. This is because different mineral particle
surfaces have different adsorption properties. Adsorption refers to forces emanating from
the particle surface which lower the free energy of water in a thin layer near the surface
of the particle. For freezing to continue, this moisture needs to be converted to ice, and
consequently lower temperatures are required. Usually, a fi lm of water separates soil ice
