Fig. 4.13. Types of contact in frozen soils: 1 - organic-mineral particles; 2 - ice; 3 - bound water; 4 - semi-bound water; 5 - organic-mineral cementing matter; I - III - varieties of contact (I - mineral, II - mineral-ice. III - mineral-cement).

afion and extraction of a new phase from the pore solution. This process is associated with over-saturation of the pore solutions during diagenesis (including soil freezing, which increases their concentration) and the separating out from them of cementing matter, which forms strong 'bridges' between ground particles forming a rigid porous structure of the mineral component. Moreover, in soils strong cementation can appear not only as a result of crystallization of salts, but with polymerizing compounds as well, such as gels of silicic acids and hydrates of sesquioxides and humic or other organic compounds transient from sol to gel.

When the temperature is sufficiently low in soil (below —100 to — 150°C), then the mineral-ice contacts may appear. This will occur if practically the entire bound water is frozen out, or migrates into other parts of the soil and there are formed 'mineral-ice' contacts similar to the dry ones. They may reverse during thawing and are the least stable of all dry contacts. Their strength is determined by cohesion forces in ice.

A typical feature of water contacts is the presence of films of unfrozen (bound) water between the interacting elements of frozen soil. Usually close coagulation (coagulation proper) and distant coagulation (aggregation) contacts can be distinguished. Aggregation contacts are formed mostly as a result of long-range molecular forces, in some cases due to magnetic and dipole (Coulomb) interactions. Coagulation contacts are determined mostly by the action of molecular and ion-electrostatic forces. The latter forces have a middle radius of action and appear at distances between particles of a few nanometres. Their effect greatly increases the strength of soils and is particularly obvious during dehydration (drying or freezing of bound water).

Aggregations, recognizing these points, should be most typical of mixed silty sandy materials and clays in the range of high negative temperatures (0 to — 5°C). The strength of aggregation contacts is determined by the cohesion forces or 'glue forces' between molecules of semi-bound (unfrozen) water. This is caused by the circumstance that the cohesion forces (agglutination) between interacting components (mineral surface - bound water, ice bound water) and the cohesion forces in ice and mineral are much greater than the cohesion forces in semi-bound water. With allowance for the area of the contact interaction, we may expect a greater strength from area aggregation contacts (typical of clay-rich frozen soil) compared with the point contacts (dominant in frozen sands). The maximum strength should belong to volumetric aggregation contacts, when an armour of ice (enveloping the mineral particles) is formed. This kind of contact is most frequent in soils with high ice content (water-saturated) with basal cryogenic texture.

An analysis of the character of structural relations and contacts in the ground provides an explanation for the sudden growth of strength (almost by an order of magnitude) of soils in passing from the unfrozen into the frozen state. This increase of strength is a result of freezing of a part of the soil water and thinning of bound water films when a new solid ice component and a new surface boundary: 'ice - bound (unfrozen) water' is formed. This process is, accordingly, associated with a considerable growth (due to smaller distances) of the energy of interaction of the contacting elements (mineral - mineral and mineral - ice). The distance between these elements in frozen soils even at high negative temperatures usually does not exceed hundreds of nanometres, whereas in unfrozen soils it may amount to thousands of nanometres.

Therefore, in soils the character, size and shape of ground and ice particles, the degree of water saturation, the negative temperatures, and the conditions of freezing between particles can serve as causes for the appearance of a great variety of types and kinds of contacts which, as a result of external effects, can transform one to another and which determine the structural relations between the elements of the frozen material.

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