SOIL CLASSIFICATION

SOIL CLASSIFICATION The principal objective of soil classification is the prediction of engineering properties and behavior of a soil based on a few simple laboratory or field tests. The results of these tests are then used to identify the soil and put it into a group of soils that have similar engineering characteristics. Although there are several different methods of soil classification, the method adopted for use by the military is the Unified Soil Classification System (USCS). Soils seldom exist in nature separately as sand, gravel, or any other single component. Soils usually form mixtures with varying proportions of different size particles. Each component contributes to the characteristics of the mixture. The USCS is based on the textural or plasticity-compressibility characteristics that indicate how a soil will behave as a construction material. In the USCS, all soils are divided into three major divisions: (1) coarse grained, (2) fine grained, and (3) highly organic. As you know from your previous studies, coarse-grained and fine-grained soils are distinguished by the amount of material that is either retained on or that passes a No. 200 sieve. If 50 percent or more of the soil by weight is retained on a No. 200 sieve, then the soil is coarse-grained. It is fine-grained if more than 50 percent passes the No. 200 sieve. Highly organic soils can generally be identified by visual examination. The major divisions are further subdivided into soil groups. The USCS uses 15 groups and each group is distinguished by a descriptive name and letter symbol, as shown in table AV-1 of appendix V. The letter symbols are derived either from the terms descriptive of the soil fractions, the relative value of the liquid limit (high or low), or the relative gradation of the soil (well graded or poorly graded). The letters that are used in combination to form the 15 soil groups areas follows: COARSE-GRAINED SOILS Coarse-grained soils are divided into two major divisions: gravels and sands. If more than half of the coarse fraction by weight is retained on a No. 4 sieve, the soil is a gravel. It is classed as a sand if more than half of the coarse fraction is smaller than a No. 4 sieve. In general practice there is no clear-cut boundary between gravelly and sandy soils, and as far as behavior is concerned, the exact point of division is relatively unimportant. Where a mixture occurs, the primary name is the predominant fraction and the minor fraction is used as an adjective. For example, a sandy gravel is a mixture containing more gravel than sand by weight. For the purpose of systematizing the discussion, it is desirable to further divide coarse-grained soils into three groups on the basis of the amount of fines (materials passing a No. 200 sieve) they contain. GW, GP, SW, and SP Groups Coarse-grained soils with less than 5-percent nonplastic fines may fall into the groups GW, GP, SW, or SP. The shape of the grain size distribution curve determines the second letter of the symbol. GW AND SW GROUPS.— The GW groups contain well-graded gravels and gravel-sand mixtures that contain little or no nonplastic fines. The presence of the fines must not noticeably change the strength characteristics of the coarse-grained fraction or interfere with its free-draining characteristics. The SW groups contain well-graded sands and gravelly sands with little or no plastic fines. GP AND SP GROUPS.— The GP group includes poorly graded gravels and gravel-sand mixtures containing little or no nonplastic fines. The SP group contains poorly graded sands and gravelly sands with little or no nonplastic fines. These soils will not meet the gradation requirements established for the GW and SW groups. GM, GC, SM, and SC Groups Coarse-grained soils containing more than 12-percent fines may fall into the groups designated GM, GC, SM, and SC. The use of the symbols M and C is based upon the plasticity characteristics of the material passing the No. 40 sieve. The liquid limit and 16-11