Geology Reference
In-Depth Information
between frozen (or partially-frozen) ground and unfrozen ground. As such, it does not
necessarily coincide with the 0 °C isotherm (the cryofront). Under these conditions, water
is drawn upwards to the advancing freezing plane by cryosuction, fi rst to form pore ice
when it reaches the freezing plane, and subsequently to form segregated ice lenses. Thus,
the “frozen fringe” refers to the unstable conditions in the cryogenic zone immediately
adjacent to the frost line.
4.2.4. Frost Heave
Frost heave refers to the raising of the ground surface following formation of segregated
ice. The latter develops when soil moisture migrates through the frozen fringe to form
discrete layers or lenses. To understand this process, we need to consider the phase transi-
tion of substances. To recap, two phases of a single substance can coexist only when the
free energies of the phases are equal. In the case of water (H
2
O), this exists when water
is at the freezing (or melting) point. However, at temperatures below the freezing point,
the free energy of the liquid state exceeds that of the solid state. The result is an unstable
condition in which the phase of lower free energy is increased at the expense of the higher
energy phase. In other words, as temperature drops below 0 °C, water progressively changes
to ice (i.e. the solid phase).
It is also useful to distinguish between primary and secondary frost heave. Primary
heave usually refers to the heave that occurs near to the frost line while secondary heave
refers to heave that occurs within frozen layers at various temperatures. Primary heave
occurs predominantly in the autumn freeze-back period and is related to the volumetric
expansion of water as it freezes combined with the growth of segregational ice lenses.
Secondary heave is not so clearly understood, but occurs at temperatures below 0 °C and
at some distance behind the freezing front. Several experiments have demonstrated that
moisture migrates through frozen soil (Burt and Williams, 1976; Ershov et al., 1980;
Hoekstra, 1969; Vtyurina, 1974). The importance of secondary heave lies in the large
heaving pressures that may develop over time.
Two quantitative parameters are used to describe frost heave. The fi rst, the frost-heave
coeffi cient (
η
), is defi ned as the ratio of total heave (
∆
h
) to frost penetration (
h
):
η=∆
hh
(4.1)
A second parameter, the frost-heave strain (
), is often used in preference to the frost-
heave coeffi cient. This is defi ned as the ratio of total heave (
ε
∆
h
) to frost penetration (
h
)
minus the total heave (
∆
h
):
ε=
∆∆
hh
h
(4.2)
The reason why frost-heave strain (
ε
) is usually preferred to the frost-heave coeffi cient
(
) relates to the fact that it emphasizes relative heave and better refl ects frost-heave sus-
ceptibility and ice content. For example, consider a hypothetical frost-susceptible regolith
that is 35 cm deep before freezing and 47 cm deep after freezing. The magnitude of frost
heave (
η
∆
h
) is 47
−
35
=
12 cm, frost penetration (
h
) is 35 cm, and (
h
−
∆
h
) is 35
−
12
=
23 cm.
These give values of
52.2%. Real data are provided by the following
two examples. First, typical fi eld values of
η
=
34.3% and
ε
=
for saturated, fi ne-grained, and highly frost-
susceptible lake-bottom sediments in the Mackenzie Delta, Canada, range between 20%
and 60% (Burn, 1990b). Second, and by contrast, frost heave measurements on well-
drained slopes on the Tibet Plateau, China, indicate values of
ε
ε
that are only between 3%
