Geology Reference
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and 6%. These refl ect the moisture-poor nature of the Tibet Plateau (Wang and French,
1994).
In spite of the fact that frost-heave pressures are relatively easily explained, prediction
of their amounts appears diffi cult and controversial, as in the case of chilled gas pipelines
(Konrad and Morgenstern, 1984; Williams, 1986). Frost-heave pressures as high as 100-
300 kPa may develop relatively quickly (Williams and Smith, 1989, p. 197) and there is
still no widespread agreement upon the basis for quantitative predictions of heave.
4.3. FREEZING AND THAWING
The frequency of freezing and thawing is of particular signifi cance with respect to frost
wedging and rock shattering. While the calculation of thawing and freezing degree-days,
as described in Chapter 3, provides an index of the severity of the climate and the magni-
tude of the thaw and freezing periods, these data give no indication of the frequency at
which temperatures oscillate above and below the freezing point. Yet the frequency of
freezing and thawing (“freeze-thaw cycles”), and the formation of pore and segregated
ice, is signifi cant with respect to frost wedging (Matsuoka, 2001a, b) and rock shattering
(Douglas et al., 1983; Mackay, 1999).
Unfortunately, several problems limit the usefulness of freeze-thaw cycles as a measure
of frost-action effectiveness. First, there is the basic diffi culty of defi ning the exact point
of freezing across which the oscillations should be measured. Second, the use of air tem-
peratures to defi ne cycles is not satisfactory because, as demonstrated in Chapter 3, sig-
nifi cant differences exist between air and ground temperatures. Third, even when direct
ground temperature measurements are available, just what constitutes a freeze-thaw cycle
is debatable. Each occasion when water either freezes or melts requires a different degree
of heating and cooling dependent upon such factors as the ground temperature, the mois-
ture and/or unfrozen water content, and the nature of the soil or bedrock. Surface condi-
tions, such as the intensity of solar radiation and the character and depth of any snow
cover, are also relevant. Fourth, cycles can be of different intensities (i.e. different tem-
perature ranges) and this makes any comparison of cycle frequency diffi cult. Finally, the
duration of a cycle can range from seconds to several days (Hall and André, 2001), and
it is probably unwise to assign equal signifi cance to the various cycles.
Numerous studies conducted in many different cold-climate environments now seri-
ously question the assumption that numerous freeze-thaw cycles occur. Direct ground
surface measurements indicate the number is surprisingly few (Table 4.1). The greatest
occur at the ground surface and these are twice as numerous as air cycles. With depth,
however, there is a rapid drop in frequency such that beneath 5.0-10.0 cm, only the annual
cycle takes place.
Cold oceanic climates of low annual temperature range (see Chapter 3) are tradition-
ally assumed to be the most suited for freeze-thaw processes. Even here, where there is
a relative abundance of moisture for ice segregation, the number of freeze-thaw cycles is
low. For example, on Signy Island, only 19 and 14 freeze-thaw cycles were recorded at the
1.0 and 5.0 cm depths, respectively (Chambers, 1966), and at depths in excess of 10.0 cm
only the annual cycle occurred (Table 4.1). Mid-latitude alpine climates are also regarded
as being particularly suited to numerous freeze-thaw cycles since they experience marked
diurnal temperature fl uctuations. In the Japanese Alps, an annual average of 88 cycles
occurred at a depth of 1.5 cm over a 5-year period, while in the Colorado Front Range,
USA, over 50 fl uctuations across the 0 °C threshold were monitored within 24 hours at
two sites over one year.
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