Types of seasonal freezing and thawing of the ground

As early as the first years of the 1950s, V.A. Kudryavtsev pointed out that the processes of freezing and thawing are, on one hand, thermal-physical resulting from thermal interaction of the atmosphere and surrounding space with the lithosphere, and on the other geological-geographical, since the thermal processes take place in the particular geological medium and a complicated geographical setting. It was convincingly proved that determination of the features of seasonal freezing and thawing should be based on thorough study of each factor and condition of the given process, taking into consideration that the depth of freezing or thawing is an extremely variable and unsteady parameter. The characteristics of this depth and its variation can be determined on the basis of consideration of thermal-physical and geological-geographical aspects of the process of seasonal freezing and thawing. Consequently, the thermal-physical relations of depth of seasonal freezing and thawing should be represented by parameters which take a full and comprehensive account of the influence of geological, geographical conditions and their dynamics in the course of natural developments and man's activity. In other words, the types of seasonal freezing and thawing of soils determined by the particular combinations of contributing factors should be identified and mapped, and not only the depths that vary from year to year. Based on the above V.A. Kudryavtsev elaborated the classification of the types of seasonal freezing and thawing according to four dominating attributes: 1) mean annual temperature of ground imean; 2) annual amplitude of temperature fluctuation at ground surface A0; 3) composition of materials of the seasonally frozen and seasonally thawed layers; 4) moisture content of the materials. A specific feature of Kudryavtsev's classification is distinguishing the types of seasonal freezing and thawing by the conditions of their formation, and not by the depth. Depths can be obtained by using the field observational data and calculations for mean perennial or any extreme conditions. The criterion of their validity is comparison with actual data for the period identical to the studied one.

On one hand, the two first attributes are geographical; on the other, they characterize the thermodynamic conditions for existence of seasonally and perennially frozen ground. These indices are easily studied in natural conditions and may be mapped both at large and small scales. The last two attributes are geological ones and reflect the results of seasonal and perennial freezing of the ground under the established thermodynamic conditions at the surface.

The boundary that divides the phenomenon of seasonal freezing from that of thawing is assumed in the classification to be a mean annual ground temperature of 0°C. Further division of seasonal freezing and seasonal thawing in the classification of V.A. Kudryavtsev is based on stability of the process with regard to the transition from one into the other confirmed by the value of the mean annual temperature and its deviation from 0°C. Due to short-term fluctuations of climate and accidental deviations of mean annual temperature from its mean long-term value, to the south of the permafrost zone there are periodic transitions of mean annual temperature around 0°C of +1 and — 1°C and episodic ones within +2 and —2°C. Therefore there are transitional and semitransitional types of seasonal freezing and thawing of rocks in the classification based on mean annual temperature (Fig. 11.4). Transition of mean annual temperature from the range of + 2 to + 5 °C into the negative or from the range of — 2 to — 5 °C into the positive is associated with drastic and deep changes in heat exchange over the Earth's surface. Consequently, within these intervals of mean annual temperatures persistent types of seasonal freezing and thawing are distinguished. In the temperature interval of +5 to + 10°C and —5 to — 10°C the transition of one process into another is associated with long periods of climate change, and, therefore, steady types are distinguished, of both seasonal freezing and seasonal thawing of the ground. In the case of temperature being higher than + 10°C and lower than — 10°C transition of one process into another is practically impossible. Therefore, the type of

Fig. 11.4. Classification of types of seasonal freezing and thawing according to mean annual temperatures and amplitudes of surface temperatures (according to V.A. Kudryavtsev): 1-4 - types of seasonal freezing and thawing arising episodically (1) and periodically (2); disappearing periodically (3) and episodically (4).

Fig. 11.4. Classification of types of seasonal freezing and thawing according to mean annual temperatures and amplitudes of surface temperatures (according to V.A. Kudryavtsev): 1-4 - types of seasonal freezing and thawing arising episodically (1) and periodically (2); disappearing periodically (3) and episodically (4).

seasonal freezing with imean from +10 to +15°C is called southern, with fmean from +15 to +20°C - subtropical, and over +20°C - tropical, whereas the type of seasonal thawing with tmean from —10 to — 15°C is called arctic and from —15 to — 20°C - polar.

The following types of seasonal freezing and thawing are distinguished by the amplitude of ground surface temperature fluctuations the value of which is dependent on continentality of climate: 1) maritime type with amplitude less than 7.5°C typical of the sea coast in temperate latitudes; 2) moderatemaritime with amplitude ranging from 7.5 to 11°C typical of the northern seas coast; 3) continental with amplitudes ranging between 11 and 13.5°C, encountered in the European part of the former USSR; 4) continental, with amplitude from 13.5 to 17°C, typical, for example, of the West-Siberian lowland; 5) somewhat more continental with amplitude from 17 to 21 °C, typical, for example,of the Central Siberian tableland; 6) sharply continental with amplitude from 21 to 24°C (in the internal regions of the Middle Siberian tableland and in the north of Kazakhstan); 7) extremely sharply continental with amplitude over 24°C encountered in depressions of Russian North-East and eastwards of lake Baikal.

In total 85 geographical types of seasonal freezing and thawing of ground are distinguished with respect to fmean and A0, which are subordinated to latitudinal zonation of heat exchange at the ground surface. If the eight types of fluctuation are subdivided with respect to climate continentality, this number will become 133.

Regional characteristics of the processes of seasonal freezing and thawing are reflected in Kudryavtsev's classification for material composition and moisture content. The following main types of materials are distinguished by composition: 1) consolidated and semi-consolidated rocks; 2) gravel-pebble and rock waste; 3) sands; 4) silty-clay sandy material; 5) silty clay materials; 6) clays; 7) peat, as well as various combinations of these depending on regional conditions.

Four gradations are distinguished in the classification with respect to moisture content depending on the amount of moisture that participates in phase transitions in the course of freezing and thawing. The first gradation at ^nat < ^unf is characterized by the absence of phase transitions of moisture on cooling of the material below 0°C (there will be no freezing or thawing). In the remaining three gradations phase transitions increase from 0 at Wnat = Wunf to a maximum value at Wnat = Weom. The greater the moisture content of the material, the greater the amount of heat consumed for phase transitions of moisture, and, accordingly, the smaller is the depth of freezing or thawing. Therefore, depending on the value of moisture content different types are distinguished of a) deep, b) medium and c) shallow seasonal freezing (thawing):

0 0

Post a comment