T = 3 h
t = 7 day
x= 7 h
t = 7 day
t = 724 h

Fig. 2.10. Transformation of macro-structure (a) and micro-structure (b) of frozen soil samples interacting with an NaCl solution over time x (according to Yu.P. Lebedenko and E.M. Chuvilin): I - soil samples prior to the experiment, z = 0; II - the samples after the experiment; 1 -kaolinite clay, deformed by mass exchange and ice segregation (NaCl solution 0.2N., t = -1.5°C); 2 - montmorillonite clay soaking and swelling (NaCl solution 0.4N., f = —1.5 °C); 3 polymineral sandy-clayey silt with the formation of rupture fractures (NaCl solution 0.4N., t = - 20°C).

crystals thus dissolve, passing into solution, while the frozen soil near the contact loses moisture. As the concentration of the solution in contact with the frozen soil decreases, the normal osmotic flow diminishes as well and falls to zero when a critical concentration is reached. At this critical solution concentration the total thermodynamic water potential in soil is equal to the osmotic water potential in the solution and a thermodynamic equilibrium is attained. The critical concentration value for the solution Ccr will depend on the composition and structure of the particular soil, and also on external thermodynamic conditions. The value Ccr goes up with finer grain-size and lower temperature. For sands interacting with a NaCl solution, Ccr is under 0.1 g mol 1"1 and for clays over 5 g mol 1"1

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