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