CWUBt 23Wcom Wunt Wnat

where Wnat is natural humidity of the ground at the moment of the beginning of freezing or thawing; WUBf is the amount of unfrozen water; and Wcom is total moisture capacity.

Each type of seasonal freezing and thawing distinguished in Kudryavtsev's classification has the name that reflects its main classification features, for example: arctic, sharply-continental, sandy, deep; stable, continental, clayey-silt, fine; etc. The number of different combinations of the main parameters can be large. Therefore, there is a great number of different values of seasonal freezing and thawing depth. At one and the same time, the same depth in different regions is often produced by a different combination of conditions, i.e. with different conditions of formation, the values of seasonal freezing or thawing will be the same. The change of the same parameter or factor of the environment by a similar amount but in different regions will result in widely different depths of seasonal freezing (thawing).

Kudryavtsev's classification makes it possible to map the types of seasonal freezing and thawing of ground, i.e. the conditions determining depth, at a required scale. For each type shown in the map the particular depth values £ can be given, obtained after processing of the field observation data and calculated according to equations based on using parameters A0, fmean, and taking account of soil composition and humidity. In its turn, analysis of such maps makes possible the identification of the general and regional conditions controlling ground temperature at depth £ and the pattern of variation of the thickness of seasonally thawed or seasonally frozen layers in their dependence on geological-geographical conditions. In other words such maps can show the occurrence of particular characteristics of the seasonally thawed or frozen layers at given sites. Consequently, with knowledge of the variation of natural conditions and of the changes of the main (four) classification parameters, one can predict depth as well as exercise control over the depths of freezing and thawing of soils. This offers the opportunity of predicting the ground temperature fluctuations and conditions of formation of the layers of seasonal freezing and thawing under various field conditions caused either by the natural-historical development of landscapes or the economic development of the area.

The main principles in the variation of seasonal freezing and thawing depths in the most typical soil conditions are distinctly traced in plots of the dependence of £ on mean annual temperature of the soils, amplitudes of surface temperature fluctuations and composition and humidity of materials (Fig. 11.5). The families of curves presented can be used for the tentative assessment of the variation of seasonal freezing and thawing depths. The qualitative effect of the classification indices (imean and A0) on the values can be plotted in a convenient form (Fig. 11.6). It is shown in the diagram presented that: 1) higher positive or lower negative values of fmean from ij to t2 (at a constant value of A0) lead to the reduced £ value (from to £2); 2) with higher amplitude of surface temperature fluctuations A0, the depth of seasonal freezing or thawing £ increases, with lower amplitude it diminishes. The diagram shows these dependences by the sequence of curves of variation of depth of seasonal freezing and thawing that correspond to the

Types with respect to tmean long term stable stable —

southern (tmean)

Types with respect ■fco soil composition with respect tc moisture peaty, fine grained sandy-silty; sllty-ciayey; more seldom"andy and ruaaceous shallow (167600)

middle (104750)

Types

with respect ■fco soil composition with respect tc moisture

maritime

moderate maritime

moderate continental

continental

rather -highly continental

sharply continental

extremely sharply continen-

peaty, fine grained sandy-silty; sllty-ciayey; more seldom"andy and ruaaceous shallow (167600)

middle (104750)

sandy and rudace' ous; more seldom sandy-silty and-sllty-clayey deep (41900)

Types with respect to tmean transitional seii-trais-itlonal long term stable stable —

southern (tmean)

Fig. 11.5. Approximate depths of seasonal thawing (freezing) of ground with the most typical soil conditions at different imean and A,

Fig. 11.6. Variation of seasonal freezing £fr and thawing £tha depth depending on the mean annual temperature fmean and amplitude A0 of temperature at the ground surface.

increasing ground surface temperature amplitudes, i.e. at A3> A2> Al and any fixed value of f (for example, f3) it is apparent that the depth of thawing £3 > > The diagram also shows that the same depth £ can be developed at quite different combinations of fmean and A0.

Changes in lithological composition of materials leads to changes of their thermal-physical properties: thermal conductivity X and heat capacity C. As follows from V.A. Kudryavtsev's equation for the calculation of £ (1.35), this value is proportional to J~X and has a more complicated dependence on C, increasing in general with lower C. It is known that finer-grained soils lead to reduced X. Therefore, with other conditions being equal, the greatest depths of seasonal freezing (thawing) are encountered in rudaceous and coarse-grained materials (for instance, in sands) and the shallowest ones in the fine-grained soils (for instance in clays). Changes in soil humidity affect the £ value in two ways: firstly through change in thermal-physical properties of soils (X and C) and, secondly, through the amount of phase transition which, as a rule, has a more substantial impact. The greater the moisture content, the greater the amount of heat consumed for phase transition and the smaller is the depth of seasonal freezing and thawing. Soil composition and moisture content also have a considerable influence over such an indicator of seasonal freezing (thawing) as t( - mean annual temperature at the bottom of this layer, and through this quantity affect £ - the depth of freezing (thawing). This influence is associated with development of the thermal offset, the value of which is determined by composition and moisture content of the seasonally thawed (seasonally frozen) layers since it is proportional to the difference between square roots of thermal conductivities of the frozen and thawed ground, as well as to the value of the annual heat cycle.

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