Figure 15-2.-Types of soil gradation. GRADATION The  size  and  shape  of  the  soil  particles  dis- cussed above deal with properties of the individual grains  in  a  soil  mass.  Gradation  describes  the distribution of the different size groups within a soil  sample.  The  soil  may  be  well  or  poorly graded. To  be  classified  as  WELL  GRADED,  a  soil must have a good range of all representative par- ticle sizes between the largest and the smallest. All sizes  must  be  represented,  and  no  one  size  should be  either  overabundant  or  missing  (fig.  15-2). Poorly  graded  soils  are  either  those  containing a narrow range of particle sizes or those with some intermediate sizes lacking (fig. 15-2). Soils with a  limited  range  of  particle  sizes  are  called UNIFORMLY  GRADED.  Soils  that  have  some intermediate  size  or  sizes  not  well  represented or  missing  are  called  GAP  GRADED,  STEP GRADED,  or  SKIP  GRADED. COMPACTNESS Compactness  refers  to  how  closely  a  mass of  soil  particles  are  packed  together;  the  closer the  packing,  the  greater  the  compactness  and the   larger   the   weight   of   soil   per   unit   vol- ume. The structure of a total mass of soil particles may be dense. In this case, the particles are closely packed and have a high degree of compactness. A  dense  structure  provides  interlocking  of  par- ticles  with  smaller  grains  filling  the  voids  be- tween the larger particles. When each particle is closely surrounded by other particles, the grain- to-grain contacts are increased. This lessens the tendency  for  displacement  of  the  individual  grains under  load,  and  the  soil  is  then  capable  of supporting   heavier   loads.   Well-graded   coarse materials usually are dense and have strength and stability  under  load. On  the  other  hand,  the  structure  may  be loose,  in  which  case  the  particles  are  not packed  as  closely  together  as  possible,  thereby lacking compactness. Loose, open structures have large voids, which will lead to settlement or dis- integration when foundation or traffic loads are applied. SPECIFIC  GRAVITY Specific  gravity  is  designated  by  the  sym- bol  Gs.  It  is  defined  as  the  ratio  between the  weight  per  unit  volume  of  the  material and  the  weight  per  unit  volume  of  water  at  a stated  temperature— usually   20°C.   If   you   use the  system  international  (SI)  (metric)  system,  you can  determine  specific  gravity  by  the  following formula: Specific gravity = weight of sample in air (g) weight of sample in air (g) – weight  of  sample  submerged  (g) Test procedures will be discussed in detail later in this chapter. The specific gravity of the solid substance of most inorganic soils varies between 2.60 and 2.80. Tropical iron-rich laterite, as well as  some  lateritic  soils,  generally  has  a  specific gravity of 3.0 or more. Sand particles composed of  quartz  have  a  specific  gravity  of  about  2.65. Clays can have values as high as 3.50. The solids of   soil   particles   are   composed   of   minerals. Generally, these minerals have a specific gravity greater   than   2.60.   Values   of   specific   gravity smaller than that are an indication of the possible presence  of  organic  matter. SOIL MOISTURE The moisture content of a soil mass is often the most important factor affecting the engineer- ing characteristics of the soil. The water may enter from the surface or may move through the sub- surface  layers  by  either  gravitational  pull,  capillary action, or absorption. This moisture is present in most cases. It influences various soils differently; it   probably   has   the   greatest   effect   upon   the behavior of the soil when the soil is subjected to loading. 15-4