Cryospheric systems - Fundamentals of the Physical Environment - page 373

presence or absence of an organic layer, soil moisture
content, and drainage.

where T = freezing point ( K), T o = normal freezing point
with uniform pressure of ice and water, V 1 = specific
volume of water, and L w = latent heat of fusion of water.
For every atmosphere difference of pressure between the
water and the ice in the soil, the freezing point is lowered
by 0.08

SOIL FREEZING AND TYPES OF
GROUND ICE

C.
In the 1970s it became clear that unfrozen water in
frozen soils is able to migrate along a temperature
gradient, in the direction of lower temperatures. In this
way supercooled water moves to bodies of ground ice,
causing their further growth. The permeability of a frozen
soil is several orders of magnitude less than unfrozen soil.
Surprisingly, however, ice lenses in the frozen soil lying
across the path of water flow have an accretion of water
molecules as ice on the upstream side, while molecules
depart as water from the downstream side. Where
migrating water accumulates, the amount of water will no
longer be appropriate to the temperature, and there will
be a transfer of water to ice to restore the equilibrium. In
this way, bodies of ice in soil are able to grow quite quickly
(Williams and Smith 1989).
Much frozen ground consists of soil interspersed
with layers of pure ice ranging in thickness from a few
millimetres to over a metre. Ground ice may occur as
structure-forming ice that bonds the enclosing soil, or as
larger bodies of pure ice or massive ice . Structure-forming
ice includes pore ice, ice coatings on soil particles/stones,
ice veins, ice lenses, and intrusive ice. Massive ice occurs
as ice wedges , as the ice core in pingos and in massive beds
( Plate 15.17 ). Ice wedges are common in permafrost, and
occur where ice grows vertically in wedges three to five
metres deep, with sharp edges pointing downwards. They
are normally arranged in a polygonal pattern at the
ground surface, and take hundreds of years to form by an
annual contraction and vertical cracking of the upper few
metres of the soil under extreme cold. The cracks become
filled with more ice each winter, and the additions
accumulate year-on-year to reach widths of one to two
metres at the top ( Figure 15.18 ). Another type of massive
ice is injection ice , found in striking landforms such as
pingos produced by the growth of a large core of ice
where the groundwater pressures are very high. Massive
ice beds usually form by water migrating from warmer
unfrozen soil, and accumulating and freezing within the
frozen soil. This process is referred to as ice segregation .
Some massive ice beds may also be buried glacier ice. The
distribution of ground ice is strongly influenced by soil
texture; organic and fine-grained soils rich in silt and clay
contain much larger amounts of structure-forming ice
than coarse-grained sands and gravels.

Freezing of water when confined within the pores of soils
and sediments is a complex process, and not until the
1960s did a clearer understanding emerge. Although
permafrost is defined on the basis of temperature, water
and ice can coexist in frozen soils, and in the case of fine-
grained soils like silt and clay, appreciable amounts of
water may remain unfrozen at temperatures below 0 C.

Two theories may explain this. The first is that freezing
point depression is caused by solutes dissolved in soil water.
However, concentrations of dissolved salts are not usually
sufficient to explain the total amount of depression. The
second theory is that water coexists with ice precisely
because it is confined within small soil pores. Two forces
arise from the proximity of the soil mineral surfaces to the
water (p. 466). The first is capillarity , which causes water
to rise in tubes of small diameter, due to the lower pressure
in the water at the meniscus or air-water interface. Surface
tension is responsible, and the effect is greater the smaller
the diameter of the capillary, i.e. the more curved the
meniscus. In freezing soils there are similar effects at
ice-water interfaces. The second force is adsorption, the
attraction between water and the faces of clays and silts,
which modifies the density, viscosity and the freezing
point of the adsorbed water. The water in a frozen soil as
a whole has a lower potential or suction relative to that of
water in the adjacent unfrozen soil.
Williams (1968) reminds us of the relevance of the
Clausius-Clapeyron equations to freezing soils, especially
in silts and clays with small pores. An equation for the
pressure difference between a small spherical ice crystal
and the water in which it is submerged in soil is:

2 iw

P i - P w = ________
r iw

where P i = pressure of ice, P w = pressure of water, iw =

surface tension ice-water, and r iw = radius of curvature of
the ice-water interface. Thus, the pressure difference
between ice and water increases as the radius of curvature
of the interface decreases. As the pressure difference
increases, so the freezing point falls:

V 1 2 iw T o

T - T o = ________
r iw L w