Lesson 1/Learning Event 1
PARTICLE GRAIN SIZE
Soils are divided into groups based on the size of the particle grains. The standard size groups employed
in the Unified Soil Classification System are shown in Table 1.
TABLE 1. STANDARD SIZE GROUPS
Common practice distinguishes the sizes through the use of sieves. A sieve is a screen attached across
the end of a shallow cylindrical frame. The screen permits particles smaller than the openings to fall
through and retains the larger particles. All standard sieves fall into two categories:
The larger sized sieves, 3 inches to 1/4 inch, are designated by the size of the actual opening in
the wire mesh.
The smaller sized sieves, #4 to #200, are designated by the number of openings per linear inch.
Sieves used by the military engineer have square openings and are designated as 2-, 1 1/2-, 1-, 3/4-, and
1/4 inch sieves and US Standard #4, 10, 20, 40, 60 and 200 sieves. A #4 sieve has four openings per
linear inch or 16 openings per square inch, and so on.
To perform a sieve analysis of the distribution of grain sizes in a given soil sample, use a nest of sieves
like the one shown in Figure 2. Then measure the amount remaining on each sieve and describe each
amount as a percentage by weight of the entire sample.