Soils are divided into groups based on the size of the particle grains in the soil mass. The EA identifies 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 larger ones to be retained on the sieve. When sieves of different sizes are stacked so that the largest screen openings are at the top and the smallest at the bottom, soil can be separated into particle groups based on size. The amount remain-ing on each sieve is measured and described as a percentage by weight of the entire sample. Table 15-1 shows size groups as used in the Unified Soil Classification System. Particles passing the No. 200 sieve but larger than 0.002 mm to 0.005 mm are called silt, and those finer are clays.

The shape of the particles influences the strength and stability of a soil. Two general shapes are normally recognized: BULKY (fig. 15- 1) and PLATY.

Cobbles, gravel, sand, and silt particles cover a large range of sizes; however, they are all bulky in shape. The term bulky is confined to particles that are relatively large in all three dimensions, as contrasted to platy particles, in which one dimension is small as compared to the other two. The bulky shape has the following four subdivisions listed in descending order of desirability for construction:

ANGULAR particles are those that have been recently broken up and are characterized by jagged projections, sharp ridges, and flat surfaces. Angular gravels and sands are generally the best materials for construction because of their interlocking characteristics. Such particles are seldom found in nature, however, because the weathering process does not generally produce them. Angular material must usually be produced artificially, by crushing.

SUBANGULAR particles are those that have been weathered to the extent that the sharper points and ridges have been worn off.

SUBROUNDED particles are those that have been weathered to a further degree than subangular particles. They are still some-what irregular in shape but have no sharp corners and few flat areas. Materials with this shape are frequently found in stream beds. If composed of hard, durable particles, subrounded material is adequate for most construction needs.

ROUNDED particles are those on which all projections have been removed, with few irregularities in shape remaining. The particles resemble spheres and are of varying sizes. Rounded particles are usually found in or near stream beds or beaches.

Platy (or flaky) particles are those that have flat, platelike grains. Clay is a common example. Because of their shape, these flaky particles have a greater contact area for moisture and are un-desirable for construction purposes.

The size and shape of the soil particles discussed above deal with properties of the individual grains in a soil mass. Gradation describes the distribution of the different size groups within a soil sample. The soil may be well or poorly graded.

To be classified as WELL GRADED, a soil must have a good range of all representative par-ticle sizes between the largest and the smallest. All sizes must be represented, and no one size should be either overabundant or missing (fig. 15-2). Poorly graded soils are either those containing a narrow range of particle sizes or those with some intermediate sizes lacking (fig. 15-2). Soils with a limited range of particle sizes are called UNIFORMLY GRADED. Soils that have some intermediate size or sizes not well represented or missing are called GAP GRADED, STEP GRADED, or SKIP GRADED.