The granulometric composition of the soil. Classification and methods for determining the particle size distribution

In the list of geodetic works, one can sometimes find such a service as determining the composition of the soil. This procedure is performed to obtain information on the content of particles in the soil in a specific locality. In construction works, the definition of such a composition is not required often, but in agriculture and geological exploration activities it is indispensable. The granulometric composition can be determined by different methods. The choice of one of them depends on many factors and conditions.

General information about the particle size distribution

Under the granulometric composition is understood the presence of mechanical elements in the soil. And in this case the soil can be considered as a general designation of the soil, which can also be artificial. As for particles, they can have different characteristics and origins. There are also different kinds of formulations. For example, the granulometric composition of sand will be more or less homogeneous, even in terms of the content of particles of a certain fraction. Specialists note that the minimum size of elements that are able to detect the practiced techniques of this analysis is only 0.001 mm.

In accordance with GOST, six names of fractions are distinguished: the same sandy particles, blocks, gravel, clay, etc. Each fraction has not only its own range of sizes, but also biological origin. At the same time, one should not think that only the content of fine particles is characterized by the granulometric composition. GOST under number 12536-79 also notes that the maximum size of the fraction, which is taken into account as an integral part of the soil, reaches 200 mm. These are predominantly boulder elements, which can have large dimensions. The smallest fraction is clay, although sand particles can compete with it in this index.

Classification of the granulometric composition

In addition to the fractional gradation of soils, there are other principles of classification. One of them provides for a separation based on the clay particle content. In this case, the nature of the soil formation is also taken into account and the dominant fraction is revealed. An alternative classification is to determine the type of composition through the presence of elements of sand, dust and the same clay. That is, in some way such a granulometric composition will be determined by a combined principle with a complex representation of information on the elements included in it. It is important to note that because of the similarity between the two approaches to the classification of formulations, it is rather difficult to distinguish them in practice.

Direct methods for determining the composition

There are two fundamentally different groups of methods for determining the mechanical composition of the soil. One of them is indirect and designed to reveal patterns of soil formation in a specific locality, and the other is a segment of direct methods based on technical means of analysis. In particular, a group of direct methods can use special devices, devices and devices that allow you to determine the parameters of particles with a high degree of accuracy. In particular, electronic and optical microscopes can be used that realize a micrometric study. The direct method makes it possible to more accurately determine the granulometric composition of the soil, however, because of the complexity of the technical organization of the process and the high cost of use is extremely rare.

Indirect methods for determining the composition

To this group of methods for determining the composition, methods are usually referred to, which are based on the application of different regularities in the structure of the test mixture. In particular, dependencies between the elements of an array can be detected, but complex analysis is most often assumed. That is, in the process of comparison, other characteristics of the soil are also taken into account, including moisture, suspension properties, sedimentation dynamics, etc. Indirect methods for determining the granulometric composition also involve optical and areometric methods of recording physical qualities. In addition, the latest technologies allow the use of modeling of natural sedimentation. If we compare this line of analysis with direct methods, then its shortcomings include low accuracy. Therefore, if you want to do a one-time study on a particular site, then a straightforward method will still be preferable. But in large-scale and regular works, only indirect methods justify themselves economically.

Hydrometric method

This is a highly specialized, although popular technique, which is based on the principles of displaced fluid. Actually, this is how the hydrometer used in the analysis process works. The very principle operates according to the rule that the volume of the displaced liquid will be equivalent to the mass replaced by the new body. Only in the case of the practice of applying hydrometric techniques, the granulometric composition of the soil is determined through the collected suspension. In particular, the expert also checks the deviations from the data obtained earlier by immersing the particles in water. Usually such an analysis is performed serially, and in each case the work is conducted on the determination of one characteristic - density. Again, based on the relationship of particles and the conditions of their sojourn in the soil, fractional and mechanical composition can thus be determined.

The pipetting method

In this case, a fluid medium is also used, which makes it possible to distinguish individual particles according to their characteristics. The taken sample is immersed in water, after which the rate of fall of the elements of the composition is fixed. After a certain period of time, the analysis is completed, and the settled particles are removed. Then the sample is dried, measured and a report is generated based on the results of the test. As a rule, the determination of the granulometric composition by this technique is used in the analysis of clay soils. This is due to the fact that the particles in such a soil have a fine fraction, which can be analyzed by the rate of incidence in liquid media.

Rutkovsky's method

Like all indirect methods of composition analysis, this technique is not very accurate and gives only a general idea of the elements contained in the mass. The very principle of determining the characteristics of particles by the Rutkowski method is based on two parameters. First of all, this is the same rate of incidence of the element in the liquid medium. But in this case the dependence is not traced between the velocity and the origin of the particle, but with respect to the dynamics of immersion to size. And the second parameter, which makes it possible to determine the granulometric composition of the soil using this technique, is based on the ability of the particles to swell in the same aqueous medium. In this part of the analysis, both physical and, in some way, chemical qualities of the mass are revealed.

The sieve method

This is one of the oldest and most common methods for determining soil composition. It is based on the use of special sets of screens that pass fractions of the same size, and do not miss particles with larger parameters. The method is simple and affordable in use, so it is often used in the construction industry, where it is not possible to organize complex methods of indirect analysis. However, the verification of the composition through a sieve can not be reliably attributed to direct methods. Nevertheless, such analysis will not allow to determine, for example, the granulometric composition of rocks with the same degree of accuracy as a micrometric study will do. True, accuracy in many respects will depend on the analysis tool - that is, the set of sieves. There are two categories of these devices. One of them is oriented to work with sieving without washing. In this case, the cells have a size of 0.5 to 10 mm. Another group is a sieve having a passing fraction of 0.1 to 10 mm.

How does the particle size distribution affect plants?

Both the fraction and the representation by different minerals affect the agrarian and technical properties of the soil. In particular, the composition can determine the water-air environment of the soil, its propensity to erosion processes, aggregation, density, biological and chemical qualities. So, for example, sandy and clay soils cause the weakness of the environment in terms of air and moisture exchange. This is detrimental to most plants - especially those grown within agricultural lands, where the character of cultivation also affects the fertile layer. But the granulometric composition is important for vegetation, not even so much in terms of structure and density, but as a content of useful elements. Sometimes the presence of magnesium, phosphorus and salts in itself provides an optimal layer of nutrient base, eliminating the need for additional fertilizers.

Conclusion

An example of technological approaches to soil analysis for the particle size distribution shows how the latest measuring instruments are not competitive with research methods using the application of elementary physical rules and regularities. Of course, it can not be said that the determination of the granulometric composition of the soil by micrometric analysis loses indirect methods in qualitative working indices. But in terms of practicality it is the second group that proves to be more effective. In this case, the very concept of using high-precision technical means is not at all abolished. The most promising methods just suggest the combination of the two research principles.