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The paper deals with the formation of limiting bulk densities of sandy soils of different origin against different values of humidity and varying structural features. The authors have identified that the optimum moisture content is typical for sands and clays exposed to mechanical compaction. The nature of this dependence is different from the one between the density and humidity of clay soils. These differences are driven by the peculiarities of formation of bound water shells in the event of low humidity. A linear dependence between the optimal humidity of sands and maximal molecular moisture capacity has been identified. The authors make a statement based on the proven de
pendence between the maximal molecular moisture capacity and the morphology of sands. Their statement is that the formation of bound water shells in the low humidity environment is dependent not only on the fineness of particles, but, to a higher extent, on the peculiarities of the shape and the nature of the surface of sand grains. Another important factor of impact on the density of sandy soils in the natural environment consists in their humidity.
Multiple researchers believe that the correlation between density and humidity of sands is to be the subject of research. It is noteworthy that limit densities of air-dried sands are to be assessed. Therefore, any sands have some particular bound water content, and the lower the intensity of treatment of sand particles, the higher the water content. The findings demonstrate that in most cases typical coagulatory and transitory contacts of non-saturated sands are to be considered in line with the ideas expressed by V.I. Osipov, as the above contacts determine the formation of effective stresses from the prospective of the physicochemical theory.

The authors examine the applicability of the physicochemical theory of effective stresses to
sandy soils. It is proven that the above theory may be furthered by the argument that all three
types of energy contacts, including coagulatory, transient and phase contacts, are present in sandy
soils. Coagulatory contact predominates over the other three types of contact. The authors provide
their findings in terms of morphological peculiarities attributable to genetic differences of quaternary
sands. The authors have completed an experimental research to identify the fixed water content in
sands and artificial mixtures that have a sand grain size, and they have also analyzed dimensions
of the per-unit surface of samples of natural sands and their fractions to identify correlation between
morphological peculiarities of sands and their maximal molecular water-absorbing capacity. The
authors have proven the presence of a correlation between the morphology of sand grains and their
number in the test sample exposed to the research undertaking, as it determines the number of
contacts between particles of sand.
The experimental data has proven that the principal provisions of the physicochemical theory
of effective stresses are applicable to sandy soils. Natural sands that have particles of different
shapes and surface nature demonstrate different types of contacts that prevent compliance of analytical
data with the results of experimental researches. Moreover, natural dimensions of coagulatory
contact areas of "sandy" systems substantially exceed those identified analytically. Areas of per-unit
surfaces of sands that contain particles with highly developed surfaces and clay films exceed those
of rounded particles the surfaces of which are smooth and polished as a result of their soft friction
in the water. This phenomenon boosts the thickness of adsorbed hydrated or solvated shells. As the
influence of physicochemical factors onto effective stresses in sands is not as prominent as in clays,
supplementary and more accurate experiments are to be held in respect of separate sand fractions
of monogene sands and in respect of samples of natural sands.