Civil and Building Engineering.

The methods used to design foundations do not consider the size of the soil particles (also called grains) as a primary factor. However, the terminology used to describe grain sizes is useful when describing the type of soil.

Soil consists of three main types—clays, silts, and sands or gravels depending on the size of the grain. The commonly used separation sizes are 2 m to separate clay and silt, and the #200 sieve (0.074 mm) to separate silt and sand. In general, grain sizes are measured in two ways. One way is to pass the soils through a stack of wire sieves; the other way is to measure the rate of sedimentation in a hydrometer test and then infer the size of the soil grains. Measurements of grain size using sieves are used primarily for cohesionless soils such as sands and gravels. The hydrometer sedimentation test is used primarily for fine-grained soils such as silts and clays.

In a sieve analysis, the soil is passed through a stack of sieves progressing from large to small openings. The ranges in grain size are defined by the sizes of openings in testing sieves through which a soil particle can and cannotpass.

A hydrometer is a device for the measurement of fluid density. In the hydrometer sedimentation test, the density of a fluid is measured against time for a mixture of soil in water. As sedimentation occurs with the passage of time, the density of the fluid decreases because the suspended soils drop from suspension. The grain-size distribution is determined from Stokes’ law1 for the settling velocity of spherical bodies in a fluid of known density. The types of soil with the smallest particles are clays, which may have particles smaller than 2 m, 2 and the clay particles may have a variety of shapes. Usually, natural soils are composed of mixtures of several different minerals, though often one type of mineral is dominant. The behavior of clay is influenced significantly by the properties of the minerals present in the specimen, but the types of minerals are identified only in special circumstances using specialized tests. In practice, various types of field and laboratory tests on soils from natural deposits are employed to define the soil properties used in design.
For comparison, here are the dimensions of various items:

1 George G. Stokes (1819–1903), Fellow of the Royal Society and Lucasian Professor of Mathematics at Cambridge University.

2 The identification of soil according to size has been proposed by a number of agencies; the sizes given here are from the American Society for Testing and Materials, D-2487-90 (1992).