Granulometry

also granulometric analysis, or mechanical analysis, the set of procedures for determining the granulometric composition of loose rocks, soils, and artificial materials.

Rock fragments (such as pebbles, gravel, and sand), clay, and other clayey rocks and soils consist of mineral and organic particles of different sizes. These particles are divided on the basis of size into definite complexes or fractions. The separation of large-grain materials is done by means of a set of sieves (sifters). The separation of sand fractions (with particle dimensions from hundredths of a millimeter to 2–3 mm) is done by sifting (with washing or without it) through a set of sieves with appropriate openings (so-called sieve analysis). Separation of smaller particles is done by hydraulic methods based on either the difference in the speed of sedimentation of particles of different sizes in placid water or on the ability of jets of water flowing at different speeds to draw off particles of different sizes. There are also other methods, such as dispersion analysis and sedimentation analysis. The precision of these methods is not more than 1 percent. To a large extent results depend on the method of preparing the mixture for analysis. Sometimes the samples being analyzed are simply soaked in water in order to avoid disaggregation of the aggregates of very small particles present in them. In other cases, on the contrary, an effort is made to break these aggregates down as far as possible by boiling in advance and treating the samples with various agents. Furthermore, an essential condition for correct analysis is preventing coagulation of the suspension during the analysis. In order to avoid this, substances (so-called stabilizers) that prevent coagulation are added to the suspension being analyzed.

The results of the analysis are represented graphically or in the form of numerical tables. The most common charts are histograms, cumulative curves, and distribution curves. Distribution curves are constructed so that the dimensions of the fractions are laid out on the x-axis and their content in percentages is measured on the y-axis. For the cumulative curve, unlike the distribution curve, cumulative percentages are laid out on the y-axis. The analysis can also be represented in the form of a point on a triangle. (The closer the point is to a certain apex of the triangle the more there is in the given rock of the fraction corresponding to this apex.)