Soil  Gradation

If a soil sample passes the 3-inch sieve but does not pass the No. 4 sieve, the larger particle size is less than 3 inches and the smallest size is larger than 1/16 inch. This soil is classified as gravel. Soils that pass the No. 4 sieve but are retained on the No. 200 sieve are classified as sands. Sands are further  broken  down  as  coarse  sand  or  fine  sands. Coarse sand passes the No. 4 sieve and is retained on the No. 40 sieve. Fine sand passes the No. 40 sieve and is retained on the No. 200 sieve. Any soil, passing the No. 200 sieve, is classified as  fine-grained. Soil  Gradation Gradation describes the distribution of different size groups within a soil sample. A  well-graded soil (fig.  15-60)  is  a  soil  sample  that  has  all  sizes  of material present from the No. 4 sieve to the No. 200 sieve. Poorly graded soil may be  uniformed-graded (fig. 15-61) or  gap-graded   (fig. 15-62). If a soil is uniformed-graded,  most  of  its  particles  are  about  the same size. An example of this is a sieve analysis in which sand size No. 20 is the only size present. If a soil is gap-graded, at least one particle size is missing.  An  example  of  gap-graded  soil  is  one  in which  a  sieve  analysis  reveals  that  sand  sizes  No.  10 and No. 40 are missing. All other sizes are present. Soil  Compaction Compaction is pressing together soil particles to form  a  consolidated  mass  with  increased  stability. Figure 15-60.—Well-graded soil. Figure 15-61.—Uniform-graded soil. Figure 15-62.—Gap-graded soil. 15-36


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