Fig. 11.3. Typical cryogenic structure of the soils of layers of seasonal freezing ¿;fr (a) and thawing ¿;tha (£>): 1-4 - cryogenic textures (1 - massive, 2 - laminated, 3 - netted, 4 - ataxite).

structure. Only if ground water is nearby are schlieren textures and higher ice content found in the lower part (and rarely in the middle part).

Regarding the layer of seasonal thawing in fine-grained soils, ice distribution in the section is dependent on the ratio of freezing from the top to freezing from below, pre-winter moisture content and the possibility of infiltration of overlying water from the surface. Most often a three-part cryogenic structure arises here with maximum ice content typical of the lower and upper parts of the layer. The middle part appears relatively desiccated. For the upper part of the layer rather fine laminated and netted cryostructures are typical, while for the middle one, massive and rarefied fine lens-like cryostructures are usual (Fig. 11.3). For the lower part of the layer, thickened laminated, netted and ataxitic cryostructures are typical and often horizontal ice schlieren seem to follow the outlines of the base of the seasonally thawed layer. The ice content of this part of the layer is usually equal to or exceeds total moisture capacity. It is practically independent of pre-winter moisture content and is formed in the course of slow freezing of the layer from below, in the most favourable conditions of moisture migration towards the front of freezing because the migrational and gravitational directions of moisture movement coincide. As shown by V.A. Kudryavtsev (17), freezing of the seasonally thawed layer from below is conditioned by half-yearly heat cycles that go through its base:

Assuming that all these heat cycles supply only phase transitions of moisture, i.e. formation of ice layers, one may easily calculate the maximum possible total thickness of such ice layers in the bottom of the layer of seasonal thawing. With average values of X and C for the frozen soils the value Q is equal to |tmean|6285kJm~2. Each lOOOkJm-2 on freezing from below can give an ice layer 0.3 cm thick. Based on this, the maximum thickness of the ice layer at imean = — 10°C can hypothetically be as large as 15-20 cm , at tmean = -5°C, 7.5-10 cm, while at tmean = - 1°C it does not exceed 1-1.5 cm.

If in the case of freezing of the seasonally thawed layer there is inflow of suprapermafrost water, then ice is uniformly distributed over the section and there is no horizon of soil losing water. The maximum heaving of the soils of the seasonally thawed layer is observed in this case, with formation of frost mounds persistent for a year. In conditions of maritime climate, when freezing from the top occurs relatively slowly, the most favourable setting for ice segregation arises in the lower horizon of the seasonally thawed layer. In the case of sharp, continental climate, when the rate of freezing from the top is high, freezing from below is insignificant. Accordingly less favourable conditions are created for ice schlieren segregation in the seasonally thawed layer, and the ice content of the lower horizon is reduced.

0 0

Post a comment