CHEMICAL PROPERTIES OF SOILS, Solubility of soils, its main...


Chemical properties of soils characterize the chemical changes occurring in them, their ability to participate in chemical interactions with various substances. They are manifested in all kinds of chemical reactions occurring in soils, but the most important of these are the solubility of soils, the features of acid-base properties, and their chemical aggressiveness.

Soil solubility, its main characteristics and methods for their determination

The solubility of soils is their ability to form solutions with other substances, that is, homogeneous mixtures of variable composition. Quantitatively, the solubility is characterized by the maximum amount of soil (or mineral) that can dissolve in a given solvent for certain P and T, ie, the concentration of a saturated or equilibrium solution, measured, for example, in mg/l, in molar (mol/m) or molal (kmol/1000 kg) forms. The solubility of minerals and substances in soils is also conveniently characterized by the product of solubility (PR), the product of the molar concentrations (activities) of cations and anions of the mineral or substance in its saturated solution [50].

In engineering-geological purposes it is important to know the most soluble differences, which include, first of all, halide soils containing halite, silvin, some types of carbonate soils (limestone, dolomite, chalk, marl), sulfate soils containing gypsum, anhydrite , as well as saline soils, etc. In all cases, the admixture in soil of water-soluble minerals of the class of simple salts with ionic type of bonds and possessing comparatively low energies of crystalline lattices increases their solubility.

To soils should be attributed soils, in which the content of easily and medially insoluble (water-soluble) salts exceeds the values ​​indicated in Table. 7.1.

Table 7.1

The minimum content of readily and medially insoluble salts in soils [111]

Name of saline soils

The minimum total content of salts in% of the weight of air-dry soil

Large clastic:

With a Sand content of 40 % and more


if the aggregate content is in the form of loam 30 % and more


if the content of aggregate in the form of sandy loam is 30% or more




Sandy loam




Easily soluble salts include: chloride NaCl, KCl, CaCl, MgCb; bicarbonates: NaHCC> 3, Ca (HCO3), Mg (HC (> 3), sodium carbonate NaiCCb, magnesium sulfate and sodium MgSOa, NaiSOj. <> The medium-insoluble salts include gypsum CaSOj • 2HCb and anhydrite CaSCb .

Saline soils are confined to desert and semidesert, less often to steppe zones, that is, to areas with negative water balance, as well as to areas located in hypergenesis zones of rocks containing unstable components (sulphate, galloid, etc.). They form solonchaks, solods, solonetzes, takyrs, which differ in the composition and content of readily soluble salts and are formed on lowered relief elements: trails of slopes, lowlands, shores of saline lakes and estuaries, in valleys on floodplains, in bottoms of steppe saucers of suffosive origin, where mineralized waters stand close to the earth's surface (1 ... 3 m).

The content of water-soluble salts in the soils of the aeration zone depends on many factors, primarily climatic conditions, which determine the amount of precipitation that can wash out salts. The process of salinization of soils manifests itself under the following conditions:

• with horizontal migration of salts and their deposition from groundwater in the Jurassic and foothill regions, in subaerial deltas and foothill plains;

• as a result of vertical migration of salts during the evaporation of pore solutions; due to weathering of rocks containing unstable components (carbonate, sulfate, galloidic rocks);

• for filtration through the soil of liquid waste from sludge collectors, salt collectors, solution-bearing communications of industrial enterprises, etc.

Saline soils lying at the base of structures on the continental shelf, as a rule, do not have a negative effect on the stability of structures, since under sea conditions it is impossible to desalinize them and cause deformations of suffosive compression.

Saline soils are characterized by the following indicators [111]:

• the degree of solubility in water (q $ r )

• degree of salinity (Dsal);

• absolute suffosive compression (Δhsf );

• relative suffusion compression (esf);

• initial pressure of suffusion compression (psf);

• degree of leaching of salts (β).

The degree of solubility in water q sr , g/l, is a characteristic reflecting the ability of soils to dissolve in water and expressed in the amount of water-soluble salts. According to the degree of solubility, the soils are subdivided according to Table. 7.2.

Table 7.2

Classification of soils by degree of solubility in water

The type of soil

The amount of water-soluble salts


q "<0.01

Hardly soluble

0.01 & lt; & lt; qg & lt; 1


1 <& lt; q '- & lt; 10

Easily soluble


Strongly soluble

q "& gt; 100

The degree of salinity Dsal%, is the ratio of the mass of water-soluble salts in a certain volume of soil to the mass of dry soil of a given volume. According to the degree of salinity D, soils are subdivided according to Table. 7.3 (113).

The degree of leaching of salts β is the ratio of the mass of leached salts from the soil to their initial mass.

Absolute suffosion compression Δ h φ mm, is the decrease in the initial height of the soil sample due to chemical suffusion constant vertical pressure and continuous filtration of water or solutions, the filtration of which is possible at the base of the structure.

Relative suffusion compression e s / is the ratio of absolute suffosive compression to the height of a soil sample of natural moisture at natural pressure.

The initial pressure of suffusion compression pd, MPa, is the minimum pressure at which suffusion compression of the soil is manifested.

Table 7.3

Soil classification by degree of salinity [34]



Degree of soil salinity D, %

easily soluble salts

medium-soluble (gypsum, anhydrite) salts

Chloride sulfate-chloride salinity


Chloride -




sandy loam



& lt; 0.5

& lt; 0.5

D sal 5 5

D sal 5 5


Lightly saline

0.5 < D sal & lt; 2.0

0.5 < D sal & lt; 1.0

5 & lt; D sal & lt; 10

5 & lt; D sal & lt; 10


Among the fond of salt

2.0 & lt; D /& lt; 5.0

1.0 & lt; D sal & lt; 3.0

10 & lt; D sal 5 20

10 & lt; D sal & lt; 20


Strongly saline 11th

5,0 & lt; D sal & lt; 10.0

3.0 & lt; D sal & lt; 8.0

20 < D sal & lt; 35

20 < D sal & lt; 30

10 & lt; D sal & lt; 15



A a /> 10.0

D sal & gt; 8.0

D & gt; 35

D sal> 30

D sal & gt; 15

The last three characteristics (Δh, ef /, p s j) are determined from the results of tests of soil samples in compression-filtration devices (see , excluding the possibility of lateral expansion of the soil sample when loaded with a vertical load. For detailed study of individual sections of the construction site, the indices are determined by field tests with a static load with a long soaking base. If there are results of field tests and construction experience in similar engineering-geological conditions, these characteristics can be determined only by laboratory methods. The normative values ​​of characteristics of saline soils x / and p st are calculated as the mean values ​​of the results of their definitions. The calculated values ​​can be assumed to be equal to the normative (y g = 1).

When conducting engineering and geological surveys in areas where saline soils are distributed, the qualitative composition and the quantitative content of water-soluble salts and their dissolution capacity should be established.

Determination of the content of salts in soils . The content of readily soluble salts should be determined with the help of aqueous extract, and the medium-soluble salts with the help of hydrochloric acid extract.

Water extract. Select an average sample of soil (300 ... 500 g), grind and sift through a sieve with holes of 1 mm. Determine the hygroscopic moisture of the soil. Select the average analytical sample 50 or 100 g (depending on the qualitative-quantitative test on SG and S0 4 '"). Add to the sample a five-fold (1: 5) amount of distilled water, devoid of CO: (if there is a large amount of sodium sulfate in the soil, it is better to prepare the extract 1:10). The mixture is shaken for 5 minutes, after which the extract is completely filtered through a filter of thick paper. The analysis of aqueous extract is carried out according to generally accepted procedures, with the determination of the dry residue, pH and the content of C03, HCO3, C0, S0 4 , Ca, Mg, NaK in mg-eq per 100 g of rock or in percentage to the mass of the rock.

Hydrochloric acid extract. From the air-dry ground, sifted through a 0.25 mm sieve, take a sample of 2.5 g based on an absolutely dry mass. Destroy the carbonates with strong hydrochloric acid (1: 1). Soak the sample in 125 cm hydrochloric acid 0.2 N concentration (ratio of soil to acid 1:50), mix thoroughly and leave for 12 hours. Then the solution is filtered into a volumetric flask (250 ml). The filter cake is washed with hydrochloric acid (0.2 N) to a negative reaction to Ca 'and S0 4 . The filter with the precipitate is calcined in a crucible and the silicate part of the soil is determined. The filtrate in the flask is added to the mark with distilled water and used for further determinations. Based on the analysis of the hydrochloric acid extract, the content of gypsum, as well as sulfate, calcium and magnesium ions is determined as a percentage of the mass of absolutely dry soil.

The composition and content of readily soluble salts should be determined by the results of analyzes of water extracts according to the following standards: dry residue and pH - according to GOST 26423, carbonate and bicarbonate ions - according to GOST 26424, chloride ion - by argentometric method or ionometric titration according to GOST 26425, ion sulfate - by weight method according to GOST 26426, calcium and magnesium ions - by the complexometric method in accordance with GOST 26428, potassium and sodium ions - according to GOST 26427 with the use of a flame photometer.

To study the composition of pore water , they should be isolated from the soil by the following methods (depending on the moisture content, consistency and granulometric composition of soils): centrifugation, squeezing under pressure, replacing the pore solution with a neutral liquid, etc.

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