Figure 16-3.-Plasticity chart.
plasticity index are used in specifying the laboratory
criteria for these groups. Reference also is made to the
plasticity chart shown in figure 16-3 that is based upon
established relationships between the liquid limit and
plasticity index for many different fine-grained soils.
The symbol M is used to indicate that the material
passing the No. 40 sieve is silty in character. An M
usually designates a fine-grained soil of little or no
plasticity. The symbol C is used to indicate that the
binder soil is predominately clayey in character.
GM AND SM GROUPS.— Typical of the soils
included in the GM group are silty gravels and
gravel-sand-silt mixtures. Similarly, the SM group
contains silty sands and sand-silt mixtures. For both of
these groups, the Atterberg limits must plot below the
A-line of the plasticity chart (fig. 16-3). The plasticity
index must be less than 4.
GC AND SC GROUPS.— The GC group includes
clayey gravels and gravel-sand-clay mixtures.
Similarly, SC includes clayey sands and sand-clay
mixtures. For both of these groups, the Atterberg limits
must plot above the A-line with a plasticity index for
more than 7.
Borderline Soils
Coarse-grained soils that contain between 5 and 12
percent of material passing the No. 200 sieve are classed
as border line and are given a dual symbol, such as
GW-GM. Similarly, coarse-grained soils that contain
more than 12 percent of material passing the No. 200
sieve, and for which the limits plot in the shaded portion
of the plasticity chart (fig. 16-3), are classed as border
line and require dual symbols, such as SM-SC. It is
possible in rare instances for a soil to fall into more than
one borderline zone. In this case, if appropriate symbols
were used for each possible classification, the result
would be a multiple designation consisting of three or
more symbols. This approach is unnecessarily
complicated. It is considered best to use only a double
symbol in these cases. You should select the two that
you believe to be most representative of the probable
behavior of the soil. In cases of doubt, the symbols
representing the poorer of the possible groupings should
be used. For example, a well-graded sandy soil with 8
percent passing the No. 200 sieve, an LL of 28 and a PI
of 9 would be designated as SW-SC. If the Atterberg
limits of this soil are such as to plot in the shaded portion
16-12
