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Fig. 11.1. Dynamics of the depth of seasonal thawing (a) and freezing (b) of the ground: 1-4 - stages: 1 - of rapid thawing (freezing); 2, 3 - respectively, of slower penetration and of relatively stable position of thawing (freezing) front; 4 - freezing (thawing) from below; tj - moment of joining.

It can be tentatively assumed (3) that freezing from below of the thawed layer starts later than from the top (at imean of ground from — 0.5 to — 4°C), or simultaneously from the top and from below (at imean of ground from —4 to — 5°C) or it can occur earlier than that from the top (with imean of the permafrost below — 5 °C). With lower mean annual temperatures of the ground and higher temperature of freezing at the surface a greater portion of the seasonally thawed layer freezes from below. In natural conditions, at imean ranging from - 7 to - 10°C usually 15-20% of the seasonally thawed layer freezes from below. Freezing from the top starts from the moment of stable negative temperature over the surface. Thus, freezing of the seasonally thawed layer is as a rule, characterized by development of two fronts or boundaries dividing 'frozen-unfrozen ground' resulting from the development of the freezing process both from the top and from below. Such a pattern of freezing leads to moisture migration to both fronts, schlieren ice formation in the freezing zones, the arising of tensions and deformations in the soil etc. Freezing of the thawed layer continues until the fronts join together (see Fig. 11.1a). From this moment (which is called the moment of joining, Tj) and on to the new development in early summer of the process of seasonal thawing, the ground is found in the frozen state; although there can be processes of moisture migration, phase transitions and ice segregation, provided there are corresponding temperature gradients, which can bring about re-structuring of the cryogenic structure of this layer.

In seasonal freezing many aspects of the formation and development of the seasonally frozen layer are qualitatively distinct. Thus, freezing of this layer occurs only from the top, i.e. there is only one boundary dividing 'frozen-unfrozen ground'. The processes of moisture migration and phase transitions that occur in the seasonally frozen layer give migrational-seg-regational interlayers of ice; there is vertical heaving of the ground surface, deformations and tensions of shrinkage and swelling, etc. With deepening of the freezing front the rate slows to a complete stop, usually observed in the late winter (see Fig. 11.1b). Then, after the surface temperature has risen above 0°C the process of thawing of the seasonally frozen layer both from the top and from below begins. Thawing from below can occur simultaneously, later or earlier than that from the top, determined by the value of heat flow from the underlying thawed (unfrozen) ground, i.e. by the value of the positive mean annual temperature of the ground. The higher the temperature, the greater the portion of the seasonally frozen layer that is thawed from below. In the southern and snow-abundant regions, in particular, the island of Sakhalin, the south of the Ukraine, Caucasus, Crimea and Central Asia, the seasonally frozen layer of the ground mainly thaws from below. Thus, two fronts of thawing are formed during thawing of the seasonally frozen layer, i.e. two 'thawed-frozen ground' boundaries. Complete thawing of this layer occurs when the two fronts join, which is usually observed in the late spring-early summer (see Fig. 11.1b).

The rate of movement of the fronts during freezing and thawing is dependent on a variety of factors. The rate of thawing £tha and of freezing tfr from the top is first of all dictated by the temperature regime of the ground surface. The more rapid the temperature increase (for ctha) or drop (for tfr) and the greater the amplitude of these fluctuations, the quicker and deeper the front moves from the top downwards. This rate is much dependent on the thermal-physical properties of the soils and most of all on the value of <2ph- The greater is <2ph (i.e. with greater moisture content of the freezing-thawing soils) the slower is the movement of the front £. The rate of advance of the freezing or thawing boundary from below is mainly determined by the mean annual temperature of ground at the bottom of the layer

The maximum depth of both seasonal freezing and thawing, considered over a long-term period, varies within a wide range. Annual deviations of the thawing depth from the mean long-term value at the same place increase southwards while those of freezing depth diminish southwards. Thus, in the north of the permafrost zone at mean annual temperatures of the ground below — 5°C, variations of the seasonally thawing layer depth at a point do not exceed 10 cm. In the Moscow-city region, for example, within the limits of the same site, for the period of 25 years seasonal freezing depth varied from several centimetres to 1 m, i.e. varied by an order of magnitude.

The dynamics of seasonal freezing and thawing depths is studied at weather stations and posts with the help of cryopedometers of various design. Under field conditions depth of freezing or thawing may be studied directly in bore holes, outcrops, etc., by geophysical methods (vertical electric sounding, electric prospecting, thermometry and the like). All the

Fig. 11.2. Curves of thawing intensity for the ground in different regions of permafrost: 1 -Srednekolymsk; 2 - Yakutsk; 3 - Noril'sk; 4 - Anadyr; 5 - Salekhard.

data on depth of thawing over time obtained during field studies indicate depths of maximum seasonal thawing £tha at the same points in accordance with the method of V.F. Tumel (for seasonal freezing - according to the method of L.D. Pikulevich). Relative seasonal thawing (freezing) versus time r is plotted for this purpose (7tha = £r/£tha x 100). Despite the fact that there are annual variations of depth the intensity or rate of thawing (freezing) remains more or less constant. The availability of such empirical dependencies for different types of terrain makes possible tentative estimation of maximum depth of seasonal thawing (freezing) by a single measurement ct. As an example, Fig. 11.2 shows curves of soil thawing intensity obtained for the different regions of permafrost development.

Cyclic recurrence of seasonal freezing and thawing that occurs in a relatively thin stratum of the ground, the annual passage of positive and negative temperatures accompanied with phase transitions, to a great extent determines the specific composition and cryogenic structure of the seasonally thawing and seasonally freezing layers. The process of repeated freezing and thawing leads to the formation of primary silty particles. At the same time physical-mechanical processes that occur in the freezing of clay-rich soils cause coagulation of clay and colloid particles and formation of secondary silty micro-aggregates. As a result, the soils of seasonal thawing and freezing layers are characterized by a higher silt content.

The dynamics of the seasonal freezing and thawing processes in the permafrost zone leads to a differentiation in the composition of the profile of these layers. Thus, in the layer of seasonal thawing three horizons may be distinguished: a lower one that freezes either at contact with the permafrost (at low negative imean of the ground), or from the surface (at high negative fmean) which is subject to seasonal fluctuations of temperature; the middle one that freezes from above and is subject to both seasonal fluctuations and fluctuations over periods of days; and the upper one subject to seasonal fluctuations and those over periods of days, and diurnally. The deposits of the lower and, especially, upper horizons have, as a rule, increased fine-grain content. In the profile of the seasonally freezing layer, as regards grain size often only two horizons are discerned.

Despite the fact that chemical and microbiological processes in the seasonally thawing layer are much reduced compared with the layer of seasonal freezing, the soils of the permafrost zone have a rather large absorbing complex. They contain secondary minerals and there is a process of gleying that causes peptization of the previously formed micro-aggregates. Gleying is accompanied by the formation of considerable amounts of hydrophilic organic and mineral colloids that promote a thixotropic structure of the ground.

The features of the cryogenic structure of the seasonally frozen and seasonally thawed layers are determined by the composition, texture, thermal-physical and water properties of the soils, by the initial (pre-winter) moisture content and its distribution over the section, the depth to and regime of the ground water relative to the base of the seasonally frozen layer, and by the distribution of the permafrost and overlying water in the seasonally thawed layer and its regime, with the dynamics of winter freezing and the temperature regime of the freezing soils. Ice distribution through the section is different in the case of seasonal freezing and in the case of seasonal thawing.

In the case of seasonal freezing massive cryogenic texture is, as a rule, formed in the uppermost part of the layer, the freezing of which occurs under large gradients of temperature. Then, lower in the layer where the freezing rate diminishes there are more favourable conditions created for migra-tional ice segregation and here schlieren cryogenic structures are formed in which ice layers become somewhat thicker and more spaced with depth (see Fig. 8.3). In general the ice content and thickness of the ice layers are dependent on the depth to the ground water level. In conditions of an arid climate, where seasonally frozen soils occur, massive cryogenic structure prevails over the whole profile. Northwards, where the climate becomes less arid, pre-winter humidity is greater and mean annual temperatures of the soils are lower, there is intense re-distribution of moisture in the layers. In the fine-grained soils of the upper part of the seasonally frozen layer (below a thin horizon without schlieren) laminated, netted, cellular and other cryos-tructures are formed, while in the lower part there is frequently massive

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