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Fig. 7.6. Dependence of the unfrozen water content on temperature in soils of different chemical-mineral composition: 1-3 - respectively, Na-, Ca- and Fe-kaolin; 4-6 - respectively, Ca-, Fe-, and Na-bentonite.

— 2°C to higher temperatures, whereas the 'indistinct' pattern of pore distribution by size as seen on differential porosity curves for hydromica clay and, especially, for montmorillonite clays, is reflected in a smooth curve of Wvnf (t) typical of these clays' mineral composition.

The influence of exchange cations on the phase composition of moisture depends on soil composition and temperature. This influence manifests itself only in clays that include minerals of the montmorillonite group. As is shown by the experimental data presented in Fig. 7.6, the role of exchangeable cations in the formation of unfrozen water increases with saturation of bentonite with univalent cations. Within the temperature interval 0 to

— 10°C the Na+-bentonite contains the greatest amount of unfrozen water, while Fe3+ and Ca2 +-bentonite contains the smallest. This is explained by the fact that Na +-bentonite is almost completely represented by microaggregates of colloid size. The fine granular structure of Na +-bentonite brings about a small size of inter-aggregate pores approaching that between particles.

Salinity of the frozen soil, along with its chemical-mineral composition and dispersion, is a main characteristic that exerts a substantial influence on the content of unfrozen water and ice. The effect of salinity on the phase composition of the moisture in freezing soils depends on the concentration and type of salts.

Fig. 7.7 shows the dependence of the unfrozen water content on tempera-

Fig. 7.7. (a) Dependence of unfrozen water content on temperature in heavy silty clay (solid lines) and kaolin (dotted lines) with different CaCl2 concentration in pore solution (1 and 5 - 0.0N, 2 and 6 - 0.1N, 3 and 7 - 0.5N, 4 and 8 - 1.0N), and (b) intensity of unfrozen water content variation depending on salinization of medium-grained silty clay at — 2°C temperature (b) 1 - NaCl; 2 - FeCl3; 3 - Ca(N03)2.

Fig. 7.7. (a) Dependence of unfrozen water content on temperature in heavy silty clay (solid lines) and kaolin (dotted lines) with different CaCl2 concentration in pore solution (1 and 5 - 0.0N, 2 and 6 - 0.1N, 3 and 7 - 0.5N, 4 and 8 - 1.0N), and (b) intensity of unfrozen water content variation depending on salinization of medium-grained silty clay at — 2°C temperature (b) 1 - NaCl; 2 - FeCl3; 3 - Ca(N03)2.

ture and on CaCl2 concentration in the frozen heavy silty clay and in kaolin. Salinization of kaolinite clay with CaCl2 leads to a more drastic increase of the unfrozen water content than with polymineral heavy silty clay. The influence of salinity on the phase composition is dependent on the liquid phase content of the frozen soil; the smaller its initial content, the greater is the influence.

It should also be emphasized that the freezing point of a highly salinized soil is often similar to that of a free solution of the same concentration, since pore solution does not have in this case a physico-chemical bond with the soil skeleton. The experimental data (Fig. 7.7) show the significance of both concentration and type of salts. Thus, the addition of 1 % NaCl into the frozen nonsaline silty clay is sufficient to change the unfrozen water content twofold, while addition of the same amount of Ca(N03)2 has practically no effect on the content of unfrozen water.

Phase composition of moisture in frozen soils under natural conditions is defined by the existing thermodynamic conditions as well as by the petro-graphic characteristics of materials. Therefore, there is an intimate connection between Wun[ and the geologic-genetic types of the frozen soils. Within the framework of each genetic complex, differences in phase composition of moisture are functions of the composition and structure of the soils. A large component of unfrozen water in soils of marine origin is associated with high salinity, fine-grained material, with availability of Na+ within the ion-exchange complex and the fine-pored structure of these deposits (Fig. 7.8). There is a distinct difference in the phase composition of continental

Fig. 7.8. Dependence of unfrozen water content on temperature in soils of alluvial (1-8), talus (9-12), eluvial (13-14), glacial-marine (15-20), glacial (21-22), marine (23-28), alluvial-marshy (29-30) and organic (31-34) origin. Soil composition: 3-14,19-22 - sandy clayey-silt; 1,2,15-18, 25-27 - clay-rich silty sand; 23, 24, 28 - clays; 29, 30 - respectively, poorly and moderately degraded peat.

Fig. 7.8. Dependence of unfrozen water content on temperature in soils of alluvial (1-8), talus (9-12), eluvial (13-14), glacial-marine (15-20), glacial (21-22), marine (23-28), alluvial-marshy (29-30) and organic (31-34) origin. Soil composition: 3-14,19-22 - sandy clayey-silt; 1,2,15-18, 25-27 - clay-rich silty sand; 23, 24, 28 - clays; 29, 30 - respectively, poorly and moderately degraded peat.

deposits of alluvial and eluvial origin which is likely to be associated with the presence of a large amount of hydrophilic organic matter and dissolved compounds of bivalent cations and active anions Cl~ in the alluvial sediments.

In unweathered water-saturated strongly consolidated rocks such as sandstones, aleurolites and argillites, phase transition of moisture into ice may occur at temperatures as low as — 10°C, which is caused by the small size of pores and substantial density of the materials. Weathering leads to higher temperature of freezing owing to the relevant transformations of structure (Fig. 7.9).

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