Fig. 2.7. Distribution with depth h of total moisture content as ice sublimation proceeds, in samples of soils of different granulometric and mineral composition: 1 - fine and clayey-silty sand; 2 - light sandy silty-clay mixed material; 3-5 - clay (3 - polymineral, 4 - hydrous mica, 5 - kaolinite).

where Psat is the partial pressure of saturated vapour of the ice at the sublimation front; Pga is the partial pressure of vapour at the 'ground - air' interface; Wuni = W'„ is the moisture content by weight, corresponding to the unfrozen water amount at a given temperature at the sublimation front; WCI is the equilibrium moisture content at the 'ground-air' interface: is the thickness of the zone of desiccation. Equation (2.14) expresses the steady-state flow in the frost-desiccated soil on account of vapour transfer (first term) and liquid (second term) separately.

The velocity of the sublimation front in frozen soil, recorded visually from the changed hue of the desiccated soil and from the first inflection in the curve of the sample's total moisture content distribution with depth, is directly proportional to the value Is and inversely proportional to the density of the soil matrix yD and the initial total moisture content (Win = Wnat) of soil. The thickness of the layer being desiccated ¿;s can be calculated from the equation:

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