Classifying Soils

The Unified Soil Classification System (USCS) is a common soil classification reference or system that has a universal interpretation. In this system, all soils are divided into three major divisions, including coarse-grained soils, fine-grained soils, and highly organic soils.

Coarse-grained soils are those in which at least half of the material, by weight, is larger than, or retained on, a No. 200 sieve. This division is further divided into gravels and sands. If more than half of the coarse fraction, by weight, is retained on a No. 4 sieve, it is classified as a gravel. If less than half is retained on a No. 4 sieve, then it is a sand. Gravels and sands are further subdivided into additional categories dependent upon the amount and characteristics of any plastic fines the soil sample contains.

Fine-grained soils are those in which more than half of the material, by weight, is smaller than, or passes, a No. 200 sieve. The fine-grained soils are not classified on the basis of grain size distribution but according to plasticity and compressibility.

Highly organic soils are those organic soils, such as peat, that have too many undesirable characteristics from the standpoint of their behavior as foundations and their use as construction materials. A special classification is reserved for these soils, and no laboratory criteria are established for them. Highly organic soils can generally be readily identified in the field by their distinctive color and odor, spongy feel, and frequently fibrous textures. Particles of leaves, grass, branches, or other fibrous vegetable matter are common components of these soils.