coarser  soils  have  larger  void  openings,  and,  as a  rule,  drain  more  rapidly.  Capillarity  is  prac- tically nonexistent in gravels and in sands contain- ing little fines. These soils, if they are above the groundwater table, will not usually retain large amounts of water. A second reason is that since the  particles  in  gravelly  and  sandy  soils  are relatively large (in comparison to clay and silt par- ticles), they are, by weight, heavy in comparison to the films of moisture that might surround them. On  the  other  hand,  the  small  (sometimes microscopic)  particles  of  fine-grained  soil  weigh so little that water in the voids has considerable effect.  It  is  not  unusual,  for  example,  for  clays to undergo large volume changes with variations in  moisture  content,  as  witness  the  shrinkage cracks in a dry lake bed. Consequently, unpaved clay roads, though hard enough when sun-baked, often  lose  stability  and  turn  into  mud  in  rainy weather. Not only do clays swell and lose stability when they become wet, but they also, because of their flat, platelike grain shapes and small size, retard the  drainage  of  water.  Since  drainage  is  of  the greatest  importance  in  (for  example)  the  construc- tion of airfield pavement, design engineers must know   whether   or   not   subsurface   clay   exists. Plasticity  is,  as  you  know,  the  characteristic  by which  clay  is  primarily  identified. ORGANIC  SOILS Soils  of  organic  origin  are  formed  either  by the growth and subsequent decay of plant life or by   the   accumulation   of   inorganic   particles   of skeletons or shells of organisms. The term  organic soil,  though,  refers  to  soils  containing  mineral grains and a more or less conspicuous admixture of  vegetable  matter.  An  organic  soil  may  be  an organic  silt  or  clay,  or  it  may  be  a  HIGHLY ORGANIC  soil,  such  as  peat  or  meadow  mat. Organic  soils  are  most  often  black  in  color, and  usually  have  a  characteristic  musty  odor. These soils are usually compressible and have poor load-maintaining  properties. EFFECTS OF SOIL CHARACTERISTICS In  summary,  soil  characteristics  area  measure of the suitability of the soil to serve some intended purpose.  Generally,  a  dense,  solid  soil  withstands greater   applied   loads   (has   greater   bearing capacity)  than  a  loose  soil.  Particle  size  has  a definite relation to this capacity. From empirical tests, it has been found that well-graded, coarse- grained  soils  generally  can  be  compacted  to  a greater  density  than  fine-grained  soils.  This  is because the smaller particles tend to fill the spaces between the larger ones. The shape of the grains also affects the bearing capacity. Angular particles tend to interlock, forma denser mass, and become more  stable  than  the  rounded  particles,  which  can roll  or  slide  past  one  another.  Poorly  graded  soils, with their lack of one or more sizes, leave more or greater voids and comprise a less dense mass. Moisture  content  and  the  consistency  limits  aid in describing the suitability of the soil. A coarse- grained sandy or gravelly soil generally has good drainage  characteristics  and  may  be  used  in  its natural  state.  A  fine-grained  clayey  soil  with  a high  plasticity  index  may  require  considerable treatment, especially if used in a moist location. SOIL  CLASSIFICATION As can be inferred from the previous discus- sions  in  this  chapter,  soil  types  are  important  fac- tors   to   consider   when   selecting   the   proper location  on  which  to  construct  any  structure  or facility.  With  the  soil  accurately  identified  and described, its suitability for supporting traffic as a subgrade, base, or foundation material or as an aggregate, a filler, or a binder for a mixture can be evaluated. CLASSIFYING SOILS The   UNIFIED   SOIL   CLASSIFICATION SYSTEM (USCS) is a common soil classification reference  or  system  that  has  a  universal  interpreta- tion. 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 char- acteristics  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 15-6