Figure 5-5.Compactive soil.
Another method, more commonly used, is to
express the gradation in terms of the TOTAL
PERCENTAGE that passes each sieve. This is
determined by adding the listed percentage that passed
each sieve and was retained on the next finer sieve to
the percentages listed beside each of the sieves below
For example, in the sample just analyzed, 24%
passed the 1/2-inch sieve and was retained on the
3/8-inch sieve. The total percentage that passed the
1/2-inch sieve is 24% + 20% + 15% + 19% + 14%
+ 8% = 100%
The total percentage that passed the 3/8-inch sieve
is 20% + 15% + 19% + 14% + 8% = 76%.
Continuing the calculation as indicated, the results of the
test in terms of total percentage passing each sieve are
Aggregate passing 1/2-inch
= 3,000 grams = 100%
Aggregate prosing 3/8-inch
= 2,280 grams = 76%
Aggregate passing No. 4
= 1,680 grams = 56%
Aggregate passing No. 10
= 1,230 grams = 41%
Aggregate passing No. 40
= 660 grams = 22%
Aggregate passing No. 200
240 grams = 8%
Figure 5-4 shows the aggregate gradation curve for
this particular example, plotted by using the above
For bituminous aggregate, anything that passes the
No. 4 sieve and is retained on the No. 200 sieve is
considered FINE aggregate. Anything retained on the
No. 4 sieve is considered COARSE aggregate.
Material that can pass a No. 200 sieve is sometimes
used in bituminous paving; this material is known as
MINERAL FILLER, DUST, OR FINES. It consists of
finely powdered rock, portland cement, hydrated lime,
or some other artificially or naturally powdered dust.
All testing sieves larger than three-sixteenths of an
inch are identified by the actual clear opening in inches.
All testing sieves smaller than three-sixteenth of an inch
are identified by mesh number. (See fig. 5-4.)
Compaction Qualities of Soil
Compaction is the process of physically densifying
or packing the soil. The strength of any given soil can
be increased by the densification obtained from
Three factors affect compaction: material
gradation, moisture content, and compactive effort.
Material gradation (fig. 5-1) refers to the
distribution of different sizes of particles within a given
soil sample. A well-graded sample contains a good, even
distribution of particle sizes. When the composition of
a soil sample is made up mostly of one size of particles,
this soil would be poorly graded. A well-graded soil will
compact more easily than one that is poorly graded. In
well-graded material, the smaller particles tend to fill the
empty spaces (voids) between the larger particles,
leaving fewer voids after compaction (fig. 5-5).
Moisture content is the amount of water present in
the soil. Because water acts as a lubricant that helps
particles slide into a denser position, the moisture
content of a soil is very important to compaction. Water
also helps bond clay particles, giving cohesive materials
their sticky qualities.