Soil or sediment shrinkage is the decrease in volume because of water removal through evaporation. Shrinkage is a complex and poorly studied process typical for wet soils developed in different bioclimatic conditions. The study of the nature, conditions, and factors of shrinkage and swelling processes makes it possible to reinterpret many problems of soil genesis, to reveal their dependence on soil properties, and to predict the possible compaction of soils under irrigation.To study an important aspect of vertic soil genesis related to the alternation of wetting and drying processes, we investigated the relative shrinkage deformation with the objective of solving the following problems: (1) to study the nature and character of shrinkage in soils differing in genesis and density; (2) to elucidate the role of shrinkage in soil compaction and formation of vertic horizons; and (3) to determine the dependence of shrinkage on the particle-size composition, structure, moisture content, and consistence of soils. As shrinkage and swelling are two opposite ends of the same state of soil (wetting and drying), they can be characterized by the following main statements: (a) swelling soils (sediments) are subjected to seasonal volume deformations (swelling and shrinkage), which arereversible with respect to water; (b) the highest swelling is typical for soils (sediments) with maximum shrinkage values; (c) the maximum swelling of a specific soil (sediment) is possible only after the maximum shrinkage and vice versa (other conditions being equal); It is known that swelling processes are based on the action of adsorption, osmotic, and capillary forces, which determine the tension retaining the water in a structured system at different drying stages. These forces also affect shrinkage; However, there is no unified theory to explain the complex nature of swelling and shrinkage processes so far. According to the best-known Deryagin theory, soil swelling "results from the wedging action of the bound-water solvation shells formed amongclay minerals and organomineral particles in hydration conditions." Soil shrinkage, which involves a decrease in the thickness of water films between soil particles, is considered in the same terms. The strength and water resistance of structural bondsare important parameters for the characterization of soil swelling and shrinkage. Cracks are formed in the sites of the loosest bonds during shrinkage. Cracking is directly related to drying conditions and the relative shrinkage deformation, which depends on the soil texture. The finer the soil texure, the more significant are its compaction and shrinkage under drying. Soils with coarse textures are not compacted under drying because of their "rigid" skeleton composed of coarse partcles, which preventsthe soil to decrease its volume and to compress its consistence under drying.
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