According to Y.A. Kudryavtsev, seasonal thawing is thawing from the top, of frozen ground having a mean annual temperature below 0°C. The layer of seasonal thawing is always underlain by permafrost and its thickness is determined by heat cycles in the layer that extend to positive temperatures of the ground. Seasonal freezing is the process of freezing from the top of unfrozen ground having a mean annual temperature above 0°C. The layer of seasonal freezing is underlain by unfrozen ground and its thickness is determined by heat cycles that extend to negative temperatures of the ground.
The main features of the layer of seasonal thawing from the viewpoint of thermal physics are as follows. With the mean annual temperature being negative the layer is frozen for the greater part of a year. However, in summer the surface temperature becomes positive, for which an indispensable condition is > ICmiJ- The beginning of the process of ground thawing is associated with this progression of negative temperature to positive values.
Seasonal thawing of ground presents a complicated process of thermal physics with phase transitions, migration and seepage of moisture in thawed and frozen zones of the layer, ground subsidence and the like. This process goes on, as long as sufficient heat arrives on account of positive heat cycles, at the front of thawing to continue the process of transition of the frozen soil moisture into the liquid state. The depth of seasonal thawing reaches its maximum by the end of summer. The direction of the heat cycles that go through the ground surface changes with transition from its heating to cooling. The soils of the seasonally thawed layer begin to cool, and the thawing process slackens and stops altogether (Fig. 11.1a). Subsequently the thawed layer starts to freeze, and depending on particular conditions freezing can begin either at its bottom boundary, or from the ground surface.
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.
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