Liquid component of soils, Distribution, classification, composition...

Liquid component of soils

Distribution, classification, composition and properties of the liquid component of soils

The liquid component is the most important component of most soils. Spatially, water and other liquids are in the ground due to the presence in them of all kinds of voids (cracks, pores, channels, etc.) that are occupied by water or other liquids due to their high mobility. It is established that below the groundwater level to depths of about 4 ... 5 km and more, the actual weight of the rock cavity (with the exception of hydrocarbon deposits) is filled with aqueous solutions that form within the lithosphere the regional inseparable macroscopic hydrosphere systems. Underground waters amount to 60 million km, huge amounts of water (13 ... 15 billion km) are concentrated in deeper bowels of the earth's mantle. The annual intake of water from the mantle and magmatic foci is about 1 km. In the earth's crust, significant amounts of water are in a bound state, forming part of some minerals and rocks (gypsum, hydrated silica, hydrosilicates, etc.).

The chemical composition of the liquid can be divided into inorganic, organic and mixed, including emulsions, according to the formation conditions, groundwater is released: leaching, sedimentation, regenerated, etc., and by the prevailing ingredients of the chemical composition - hydrocarbonate, sulfate, chloride, etc.

The quantitative content of the liquid in the soil can be estimated by the characteristics of physical properties: bulk and by weight moisture, in clay soils by consistency indices, and also by other parameters given in the section Physical Properties. Some indicators that characterize the moisture content in soils are classifiable. Soils are subdivided into varieties according to [34] (Tables 2.2 and 2.3):

• by the degree of softening A ' L ()/;

• for the water saturation coefficient Sy,

• in terms of plasticity number/";

• in terms of yield index /.

Fig. 2.33. The structure of the water molecule [66]: a - structure: 6 - model of electronic orbitals; c is the distribution of the charges (the z-bond length of H-O is 1.41 10 μm, I is the length of H-O bonds equal to 0.96 × 10, and is the angle of H-O-H equal to/(MS ')

The water features a molecular structure - water consists of two hydrogen atoms and one oxygen atom, three nuclei in the molecule form an isosceles triangle with protons at the base and oxygen at the apex (Figure 2.33, a). c). Due to this polarity, the water has a high dipole moment (1.86 D), and the four pole poles allow each molecule to form four hydrogen bonds with neighboring (same) molecules (for example, in ice crystals). As a result, each molecule participates in the formation of four hydrogen bonds with the neighboring four molecules of water: with two acting as a donor, and with the other two - as an acceptor. In contrast to the structure of the water molecule, the structure of liquid water has not yet been completely clarified. The best agreement with experiment is provided by the so-called continual water models, which presuppose the existence of a three-dimensional sufficiently loose continuous "skeleton" from water molecules connected by hydrogen bonds in approximately tetrahedral coordination.

The water of the liquid component in the ground is energetically non-uniform: the water molecules in the immediate vicinity of the mineral surface experience attraction forces that distort its structure. In addition, a large contribution to the binding water molecules are introduced by hydratable exchange cations contained in the soil.

In soil science, water in soils is divided into free, bound and water of a transitional type. In Table. 2.23 is a classification of groundwater RI. Zlochevskaya [50].

Free water (Table 2.24), which has physical properties of ordinary water, is divided into two types in soils [66]:

closed water (immobilized) in large pores of the rock and therefore not involved in the filtration and movement of groundwater;

flowable free water (groundwater water), which moves under the action of gravity or head.

Free water can move in soils along large pores, cracks by filtration under the action of gravity or pressure, it forms the horizons of groundwater and has natural physical properties and undistorted structure.

Table 2.24

Classification of groundwater [50/

Water category (type)

Species and varieties of water

Free Water

Closed in large pores Flowing

Water of a transitional type

Osmotic-absorbed water

Capillary water (capillary condensation and capillary absorption)

Bound water

The water of the crystal lattice of minerals (constitutional, crystallization-bound)

Adsorption water (monomolecular and poly molecular adsorption)

The first ideas about bound water arose almost a hundred years ago, but its systematic studies began only in the 20-30-ies. The development of the theory of bound water was greatly contributed by such scientists as B.V. Deryagin, A.V. Dumansky, P.A. Rebinder, N.V. Churaev, V. Drost-Hansen, and its properties in soils were comprehensively studied by A.F. Lebedev, S. Mattson, A.A. Rode, V.A. Deviations, E.M. Sergeev, F.D. Ovcharenko, A.K. Larionov, Yu.I. Tarasevich, R.I. Zlochevskaya, V.A. Korolev, LI Kulchitsky, A.D. Voronin et al.

Bound water is retained in the rock by chemical and physical bonding forces (with an energy of 0.1 ... 800 kJ/mol) acting from the surface of the minerals and changing the structure and properties of water. The total content of bound water in the earth's lithosphere is OD 1 ... OD5 billion km, ie about 42% of the total amount of water in the earth's crust.

Associated water is of two kinds. The first type is water, which is a part of the crystal lattices of various minerals. This is a constitutional, non-molecular form of water type OH groups, the crystallization water of various crystalline hydrates (if they exist in the given rock), as well as water "bound" coordinatively unsaturated atoms and ions of the crystal lattice of minerals. To the second type is the adsorption water formed due to adsorption "attraction" of water molecules to the active adsorption centers of the surface of minerals.

Fig. 2.34. Orientation of water molecules on the surface of a mineral [66]

Bound water forms adsorption films with a thickness of one or more molecular layers and is contained in soils in burrows or microcracks of less than 0.001 μm in size. Among her, two varieties are distinguished: with the greatest energy of attraction to the surface (about

40 ... 120 kJ/mol) - water of island or monomolecular adsorption, with lower binding energy (less than 40 kJ/mol) - water of polymolecular (multilayer) adsorption. In this type of water, physical properties are most different from free ones.

Solid surfaces of minerals have hydrophilic properties, which causes the orientation of water molecules that are dipoles (Figure 2.34). It is established that water molecules are oriented normally to a solid surface. The orientation of molecules is caused by the action of electrostatic attraction and manifests itself macroscopically in the form of a decrease in the tangential mobility of molecules in a layer several nanometers thick.

Anomalous features of bound water have been established for such properties as density, viscosity, dielectric permeability, etc. [66]. It has been established that the density of bound water in thin films about 5 nm thick (nanometers) is increased by 1.5% compared to free water and averages about 1.02 g/cm. It was previously assumed that the density of bound water is 1.2 .. .1,4 g/cm (according to some data, 1.8 ... 2.4 g/cm).

Direct measurements of the viscosity of water in very gently quartz capillaries and porous glasses showed that the viscosity of bound water increases with a film thickness lower than 1 μm: at a film thickness of 0.2 ... 0.3 μm, its viscosity is increased compared to free water 1.1 times, with a thickness of 10 nm increased by 1.6 times (Figure 2.35).

Fig. 2.35. Dependence of the relative viscosity of bound water (tj/tjo) on the thickness of the water film (I):

} The viscosity of bound water: tjo The viscosity of free water [66]

Fig. 2.36. Thickness of the film (I) of unfrozen water as a function of temperature:

1 kaolinite clay: 2 - montmorillonite clay (66)

The structural effects of the anomalous properties of bound water are well traced in experiments on the study of their temperature dependence. Thus, as the temperature increases, the viscosity of the bound water decreases and at 65 ... 70 ° C it becomes the same as in free water, that is, when the water is heated, thermal destruction of the bound water structure occurs, a decrease in the thickness of its boundary phase with a distorted structure

and the transition to free water. When the temperature is lowered, on the contrary, the opposite phenomenon occurs: the structuring of bound water.

It is known that the water-ice phase transition occurs at O ​​° C (273 K). However, in films of bound structured water, it occurs at lower negative temperatures, and the thinner the film of water, the lower it freezes. In Fig. 2.36 shows the temperature dependence of the film thickness of unfrozen bound water on the surface of such widespread clay minerals as kaolinite and montmorillonite. The main reason for the lowering of the freezing point of bound water is its interaction with a solid mineral surface, more precisely, with its active centers. The energy of interaction of water molecules with the active centers of the surface of minerals, as well as with ions in the nerve solution, is greater than the energy of interaction of water molecules with each other. This leads to the fact that the active center breaks the grid of hydrogen bonds in water, and the phase transition occurs only at a lower temperature. As a result, a layer of unfrozen water may exist in the dispersed rock at the boundary between the particle and the ice (Figure 2.37), and its total content depends on the temperature (Figure 2.36).

Fig. 2.37. The state of unfrozen water in soils [66]

No less interesting property of bound water in soils is its lowered dissolution capacity in comparison with free water; the first is capable of dissolving fewer salts than ordinary (free) water. This is also a consequence of the changed structure of bound water. The theory of a non-dissolving volume, explaining this anomalous property of bound water, was developed by B.V. Deryagin, and the phenomenon itself found many practical applications, including one of the direct methods for determining the amount of bound water in soils based on it.

Another anomalous property of bound water is a decrease by several times in comparison with free water of its dielectric constant. If for ordinary water the permittivity is 81, then for the bound water it decreases depending on the thickness of the water film to 3 ... 40. According to the latest data, layers of bound water with a thickness of 0.5 ... 0.6 nm have a dielectric constant of only 3 ... 4.

Structural changes in bound water cause a change in its thermal diffusivity. The decrease in the thermal diffusivity of bound water in comparison with free water begins to appear in aqueous films and interlayers with a thickness of less than 1 μm. The thinner the layer of bound water, the more its thermal diffusivity is reduced. In interlayers with a thickness of 0.03 μm, the thermal diffusivity is reduced by approximately 30% compared to the free one.

To remove and move bound water, especially located closer to the surface of solids, significant force impacts are required. However, if two identical neighboring soil particles have bound water of different thickness, then the water from the thick film moves into a thin film until the thickness of the films becomes the same. Therefore, in the case of drying of the upper layers of the soil and, as a consequence, a local decrease in the thickness of the films around the particles, the migration of moisture from the lower layers of clay soil containing more water to the upper layers is observed in nature. The molecules of the outer zones of the layer of bound water can be torn off by a flow of filtering free water, and also squeezed out of contacts between the solid particles when the load is applied. As a result, two particles pressed against each other by an external load have a reduced thickness of bound water films in the contact zone.

In soils where there is bound water, and it almost always takes place, it is necessary to use the law of Archimedes very carefully. The buoyancy action of water is determined by Archimedes for open free water. In soils where there is no clearly expressed level of groundwater, everything becomes more complicated. In loams and especially in clays, the weighing effect of water in an overwhelming number of cases does not manifest itself to the full.

Water of a transitional type (from bound to free) is less exposed to the action of surface forces, it is retained near the surface of minerals due to weaker bonds. Therefore, its structure is less distorted, and the differences in physical properties compared to free water are less significant. Within this type two types of water are distinguished: osmotically absorbed and capillary.

The first type - osmotically absorbed water - is formed in the ground by selective diffusion of water molecules towards the mineral surface, due to the presence of the last "ionic atmosphere", the so-called double electric layer consisting of usually from pore cation cations, concentrating negative charge of mineral particles.

The double electrical layer (Figure 2.38) has two parts: the internal one, called the adsorption layer (c), and the outer one - the diffuse layer (d). The cation concentration exponentially increases along the normal to the mineral surface, and this causes the presence of a concentration gradient that causes osmotic movement of water molecules from the volume of a free-flowing solution (e) to the limits of a double, electrical layer (

Fig. 2.38. Formation of osmotically absorbed water near a negatively charged mineral particle [66]

The second type of water in the transition state, capillary water, is formed in pores of a capillary size (diameter from KG to 10 μm) due to capillary pressure and is retained in the rock by the capillary forces of water menisci (surface tension forces) , formed at the boundary of the water-air-solid surface. In this case, menisci are formed at each boundary of the gas with water, which cause tensile stresses (negative pressures) in the water and compressive stresses in the solid phase of the soil, the intensity of which depends on the curvature of the menisci, i.e., to a large extent on the size of the pores or particles of the soil. As a result, dry free-flowing sand, with its slight moistening, becomes connected and can hold albeit comparatively small, but vertical slopes. When the drying or considerable moistening of the meniscus and the strength of the internal capillary pressure disappear and the sand again becomes loose. Capillary forces practically do not change the structure of water, and therefore capillary water does not differ from the free physical properties by the basic physical properties.

Fig. 2.39. The state of capillary water in the ground: a - capillary-condensed water, b - capillary water proper {66}

The height of the capillary rise in the ground L s is the height of the water column, which can retain the capillary forces (surface tension developing in the pores of the rock at the water-air interface) . . The height of the capillary rise is proportional to the diameter of the capillaries of the soil: A c = ± 2Tcosa/rpg, where T is the surface tension, a - wetting angle, p is the density of the liquid.

The soils are distinguished by the height of the capillary uplift:

• with a low altitude A c & lt; 1.0;

• with an average height of 1.0 & lt; L with & lt; 2.5;

• with a large altitude A with & gt; 2.5.

The height of the capillary rise for some soils is shown in Table. 2.25.

Capillary water in the ground can be formed [66]:

1) due to the phenomenon of capillary condensation, when the water molecules gradually condense on the surface of the film of adsorbed moisture enveloping the rock particles, and, merging at the points of contact (at the junction of the particles), form water menisci (Figure 2.39, a );

2) due to the capillary absorption of water under the influence of surface tension forces along communicating pores, cracks and channels when the rock meets free water (Figure 2.39, b).

Table 2.25

Capillary rise height for disperse soils


Capillary rise, cm

Coarse-grained sand

2.0 ... 3.5

Medium-grained sand

12.0 ... 35.0

Fine-grained sand

35.0 ... 120.0

Sandy loam

120.0 ... 350.0


350.0 ... 650.0

Clay is light

650.0 ... 1200.0

Loess-like soils

400.0 or more

Silt, peat

Up to 2500.0 or more

The water of capillary absorption is sometimes divided by its position in the soil mass into two categories:

1) proper capillary (rising up from the groundwater table and forming the so-called capillary rim);

2) capillary-suspended , which is formed, for example, in the infiltration of sediments, has no contact with the surface of groundwater and spreads in all directions from the source of moistening [50, 66].

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