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Fig. 2.5. Temperature (1), unfrozen water content (2), the total initial (3) and final (4) moisture contents along the length of frozen polymineral clay with adjacent ice {Willj is the final water content in the ice volume after the water outflow into freezing soil).

in the adfrozen ice will have a lower bond energy, and its thermodynamic potential value will be higher than that of the unfrozen water in the frozen soil sample. This variation of values of potential fia has to result in unfrozen water migration from adfrozen ice to frozen soil. Consequently this ice will first of all replenish the unfrozen water storage in the warmer part of the sample, which is being expended in developing the segregated ice layer. The adfrozen ice loses mass and becomes loose, porous and light in colour (see Fig. 2.4c).

The above experimental results were obtained in less than a month. In field conditions where frozen ground is many thousands of years old and characterized by an intricate structure with alternating layers of segregated ice and minerals, it is natural to find as well some ice layers growing (with sufficient grad t available) at the expense of the others. In other words, ice layers formed under a lower freezing temperature will grow at the expense of higher-temperature ice layers due to the ice content redistribution. As a result, the original cryogenic structure of permafrost can change. Such a process is very likely in seasonally frozen soil layers, which have high temperature gradients. The segregated ice layers can change their thickness and new, less-thick ice layers may develop.

Water transfer in frozen ground and its interaction with air With soils which are losing moisture under frost action, the external driving force of unfrozen water transfer is the difference of partial pressure of water vapour between the atmosphere Pm and the soil-air interface, Psurf(13).

As ice sublimates in frozen soil and water is transferred into the air, the soil gets a desiccated zone, recognized visually by its lighter colour (as compared with the lower soil layer not affected by sublimation). The front of sublimation (and the thickness of the desiccated zone) (£s = £d) is most clear-cut in ice-saturated sands and not blurred as, for instance, in the case of clay soils. The ice sublimation intensity in soils (Is = /(t)) is not constant in time; it naturally diminishes as the sublimation front goes deeper. Sublimation increases with an increasing proportion of clay and fine silt particles, of minerals of the montmorillonite group, of multivalent cations and with the degree of salinization.

This is associated with the increased Wun{ towards the sublimation front, and consequently, with grad Wun{ and the intensity of unfrozen water flow V^ (Figs. 2.6 and 2.7). The total water distribution in samples of soils is characteristically substantially different between sands and clays, which attests to the difference in the water transfer mechanism in coarse grained and fine grained soils (see Fig. 2.7).

In sands, which have practically no unfrozen water, moisture transfer occurs completely in the form of vapour transfer. In fine grained soils, which have a considerable amount of unfrozen water, evaporation is at work throughout all the volume of the zone of desiccation which can be subdivided into layers with small or large gradients of total moisture content. Gradients of water content present in clay soils attest to the importance of an internal moisture transfer by unfrozen water migration towards the surface. The component of unfrozen water movement in clays can account for a considerable quantity, up to 50-70%, of the total moisture flow (vapour plus water). The curves of the distribution with depth of the total moisture in clay soil clearly display two distinctive water content points (see Fig. 2.7): near the surface of the sample (WJ) which is some percent higher than the value of hygroscopic soil moisture at the temperature of the experiment; and on the sublimation boundary between soil layers with small and large gradients of the total moisture (Wcr"). These moisture values (Wct' and Wcr") remain practically constant when external conditions for the desiccation due to freezing of the clay soil remain unchanged. Wun{ was found to be correlated with moisture content at the sublimation front (Wct") at given temperature, in soils of various composition, structure and properties. This finding has proved to be the basis of a new method (the sublimation method) of estimating unfrozen water content of soils (9).

Ice sublimation intensity is limited under natural conditions not by exter-

Fig. 2.6. Influence of granulometric (a) and chemical-mineral (b,c) soil composition on sublimation intensity Is and the movement of the front of desiccation id: 1 - fine sand; 2 - medium clay-silt; 3-11 - clays of montmorillonite (3), hydromica (4), kaolinite (5), and polymineral (6-11) composition with differing concentration of CaCl2 solution (6-8 - without CaCl2; 9 - IN sat., 10 - 2N sat., 11 - 3N sat.).

Fig. 2.6. Influence of granulometric (a) and chemical-mineral (b,c) soil composition on sublimation intensity Is and the movement of the front of desiccation id: 1 - fine sand; 2 - medium clay-silt; 3-11 - clays of montmorillonite (3), hydromica (4), kaolinite (5), and polymineral (6-11) composition with differing concentration of CaCl2 solution (6-8 - without CaCl2; 9 - IN sat., 10 - 2N sat., 11 - 3N sat.).

nal moisture exchange but by the internal. Thus one should treat the problem of moisture transfer independently and separately from the problem of heat transfer, in the desiccated soil layer. An equation for the intensity of the frost desiccation (ice sublimation) of soils is:

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