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 as follows:

COARSE-GRAINED SOILS are those in which at least half of the material, by weight, is larger than (retained on) a No. 200 sieve. This division is further divided in 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 (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.

The above is by no means a thorough description of the USCS and the methods used to classify soils; nor is it intended to be. However, the results of certain tests (sieve analysis and Atterberg limits) that you will be performing as an EA3 will be used for the purpose of soil classification. The pre-ceding discussion is presented so that you have an understanding of why you perform the tests, what the results are used for, and the importance of ensuring that your test results are correct and reliable. A full discussion of the test procedures will be presented later in this chapter, Should you desire to learn more about the USCS and soils classification, you may refer to the EA1 TRAMAN, to NAVFAC MO-330, Testing, publications on soil mechanics.

In the planning and execution of construction operations, it is vital to know as much information of engineering significance as possible about the subsurface conditions in the construction area. That information includes not only the location, extent, and condition of the soil layers but also the elevation of the groundwater table and bedrock; the drainage characteristics of the surface and subsurface soils; and the location of possible borrow areas from which soil and other mineral-product materials may be "borrowed" for a construction operation. Soil surveys are con-ducted to gather (explore) this information. These are multifaceted surveys that consist of the following:  gathering soil samples; soil testing by either laboratory or field procedures, or both; soil classification; and the development of soil profiles.

In the full scope of soil surveying, your primary concern, as an EA3, is gathering soil samples and conducting certain of the laboratory soils tests. Should you desire to learn more about soil survey-ing, an excellent source is NAVFAC MO-330, Materials Testing.