Geology Reference
In-Depth Information
The main resistance to heave is offered by the weight and shear resistance of any over-
lying ice-bonded rock mass. The deformation behavior is regarded as plastic in nature.
Laboratory experiments (Michaud and Dyke, 1990) suggest that the vertical displacement
of bedrock blocks is characterized by progressive and slow movement on a monthly time
scale.
6.6.2. Needle Ice
An interesting, but small-scale, heave phenomenon produced by diurnal one-sided freez-
ing at or just beneath the ground surface is needle ice. Delicate vertical ice crystals grow
upwards in the direction of heat loss and range in length from a few millimeters to several
centimeters. Occasionally, they may lift small pebbles or, more commonly, soil particles.
Needle-ice formation is particularly common in wet, silty, and frost-susceptible soil. The
importance of needle ice as a soil-disruptive agent has probably been underestimated,
especially if it exposes soil to wind action, defl ation, and cryoturbation activity. It may
also damage plants by causing mechanical stresses within the root zone (Brink et al.,
1967). In the coast range of British Columbia, needle ice occurs in oriented stripes
(Mackay and Mathews, 1974), and both wind direction and solar radiation have been sug-
gested as explanations. However, it is not clear whether oriented needle ice is primarily a
shadow effect, developed by thawing, or a freezing effect. In the French Alps, certain
micro-forms of patterned ground are thought to result from processes associated with the
thawing and collapse of needle ice (Pissart, 1977). These include frost sorting, frost creep,
and the differential down-slope movement of fi ne and coarse material.
6.6.3. Cryoturbation and Frost Heave
Cryoturbation is a collective term used to describe all soil movements due to frost action.
The term is also used in the plural sense to refer to irregular structures formed in soils by
deep frost penetration and frost-action processes. It is the fi rst (singular) meaning that is
used here. Thus, cryoturbation includes frost heave, thaw settlement, and all differential
movements that include contraction and expansion due to temperature changes and the
growth and disappearance of (segregated) ice bodies. According to ACGR (1988), the
water-ice phase change is necessary for cryoturbation, and this distinguishes cryoturba-
tion from other soil-movement processes. Therefore, frost sorting is part of this complex
process by which migrating particles are sorted into uniform particle sizes. The process
is still not fully understood.
Frost heaving on an annual basis is intimately associated with the freezing of moisture
in the active layer. Annual ground displacements of several centimeters (Table 6.3) with
cyclic differential ground pressures of many kilopascals per square centimeter are common.
Two types of heave are recognized: primary (i.e. capillary) and secondary heave. These
are discussed in Chapter 4. The engineering hazards associated with these displacements
and pressures, together with the adverse effects of segregated ice formation, are frequently
encountered in the construction and maintenance of roads, buildings, and pipelines in
cold environments. These aspects are considered in Chapter 14.
The progressive upward movement of stone and objects is the direct result of frost
heaving in the active layer. This movement, especially common in heterogeneous uncon-
solidated sediments, is called upfreezing. The mechanics are not fully understood. At least
two different hypotheses are suggested. The “frost-pull” mechanism assumes that the top
of a pebble or coarser particle is gripped by the advancing freezing plane and raised in
