Therefore, it is in the SP (poorly graded sands, gravelly
sands, little or no fines) category.
SAMPLE CLASSIFICATION PROBLEMS
shows a
The following soil classification problems are
presented to show you how the soil classification chart
(table AV-1, appendix V) is used to classify soils.
Sample Problem 1. From a sieve analysis, a soil
of 20 and a
gravel, 88-percent sand, and no fines (smaller than No.
200). When you are classifying this soil, the first
question is whether the soil is coarse-grained or
fine-grained. To be Coarse-grained, a soil must have less
than 50-percent fines. This soil contains no fines;
therefore, it is a coerse-grained soil with the first letter
in the symbol either G (gravel) or S (sand). Since it
contains more sand (88 percent) than gravel (12
percent), the first letter in the symbol must be S.
The next task is to determine the second letter in the
symbol. Since the soil contains no fines, it has no
plasticity characteristics; therefore, the second letter of
the symbol must be either W (well graded) or P (poorly
graded). Since the soil has a Cu greater than 6 and a Cc
between 1 and 3, it must be well-graded. Therefore, the
symbol for the soil is SW, meaning “well-graded sand.”
Sample Problem 2. A sieve analysis shows that a
soil contains (50-Percent gravel, 20-percent sand, and
20-percent fines. Plasticity tests show that the portion
passing the No. 40 sieve has an LL of 35 and a PI of 8.
Since the soil contains less than 50-percent fines, it is a
coarse-grained soil. The first letter is therefore either G
(gravel) or S (sand). Since gravel predominates over
sand, the first letter is G.
The next questions are (1) does the soil contain less
than 12-percent fines and (2) is it nonplastic? The
answer to both questions is negative, since the sieve
analysis shows 20-percent fines, and an LL and PI have
been obtained. It follows that the second letter in the
symbol must be either C (clay) or M (silt). If you plot
LL 35 and PI 8 on the plasticity chart (fig. 16-3), you
will find that the plotted point lies below the A-line.
Therefore, the complete symbol is GM, meaning “silty
gravel.”
Sample Problem 3. A sieve analysis shows that a
soil contains 10-percent sand and 75-percent fines.
Plasticity tests show that the portion passing the No. 40
sieve has an LL of 40 and a PI of 20. Since the soil
contains more than 50-percent fines, it is a fine-grained
soil; therefore, the first letter in the symbol is either O
(organic), M (silt), or C (clay). Assume that the soil
shows no indication of being organic (principal
indications are black color and musty odor); it follows
that the first letter must be either M or C.
If you plot an LL of 40 and a PI of 20 on the plasticity
chart, you find that the plotted point lies above the
A-line; therefore, the first letter in the symbol is C. Since
the liquid limit is less than 50 (which brings the plotted
point to the left of the B-line), the second letter of the
symbol is L (low plasticity or compressibility). The
complete symbol is CL, meaning “clay with low
compressibility.”
of 1.3 and contains 12-percent
FIELD IDENTIFICATION
Sometimes the lack of time and facilities makes
laboratory soil testing impossible in military
construction. Even when laboratory tests are to follow,
field identification tests must be made during the soil
exploration. Soil types need to be identified so that
duplicate samples for laboratory testing are held to a
minimum. Several simple tests used in field
identification are described in this section. Each test
may be performed with a minimum of time and
equipment. However, the classification derived from
these tests should be considered an approximation. The
number of tests used depends on the type of soil and the
experience of the individual using them. Experience is
the greatest asset in field identification; learning the
technique from an experienced technician is the best
method of acquiring the skill. If assistance is not
available, you can gain experience by getting the “feel”
of the soil during laboratory testing. An approximate
identification can be made by spreading a dry sample on
a flat surface and examining it. All lumps should be
pulverized until individual grains are exposed but not
broken; breaking changes the grain size and the
character of the soil. A rubber-faced or wooden pestle
are recommended. For an approximate identification,
however, you can mash a sample underfoot on a smooth
surface.
Field tests may be performed with little or no
equipment other than a small amount of water. However,
accuracy and uniformity of results is greatly increased
by the proper use of certain items of equipment. For
testing purposes, the following equipment or
accessories may be used:
. SIEVES. A No. 40 U.S. standard sieve is perhaps
the most useful item of equipment. Any screen with
about 40 openings per lineal inch could be used. An
approximate separation may be made by sorting the
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