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
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
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.
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.