Page 270 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
Soil Consistency and Engineering Classification 265

plasticity index and liquid limit plot below the A-line on the plasticity chart.
Because of separation of coarse particles gradation is less important, and both
well-graded and poorly graded materials are included in these groups. Some sands
and gravels in these groups may have a binder composed of natural cementing
agents, so proportioned that the mixture shows negligible swelling or shrinkage.
Thus, the dry strength is provided either by a small amount of soil binder or by
cementation of calcareous materials or iron oxide. The fine fraction of
noncemented materials may be composed of silts or rock-flour types having
little or no plasticity, and the mixture will exhibit no dry strength.

GC and SC
These groups consist of gravelly or sandy soils with more than 12 percent fines that
can exhibit low to high plasticity. The plasticity index and liquid limit plot above
the A-line on the plasticity chart. Gradation of these materials is not important, as
the plasticity of the binder fraction has more influence on the behavior of the soils
than does variation in gradation. The fine fraction is generally composed of clays.

Borderline G and S Classifications
It will be seen that a gap exists between the GW, SW, GP, and SP groups, which
have less than 5 percent passing the No. 200 sieve, and GM, SM, GC, and SC
soils, which have more than 12 percent passing the No. 200 sieve. Soils containing
between 5 and 12 percent fines are considered as borderline and are designated by
a dual symbol such as GW-GM if the soil is a well-graded gravel with a silt
component, or GW-GC if well-graded with a clay component. Many other dual
symbols are possible, and the meaning should be evident from the symbol. For
example, SP-SC is a poorly graded sand with a clay component, too much clay to
be SP and not enough to be SC.

ML and MH
ML and MH soils include soils that are predominantly silts, and also include
micaceous or diatomaceous soils. An arbitrary division between ML and MH is
established where the liquid limit is 50. Soils in these groups are sandy silts, clayey
silts, or inorganic silts with relatively low plasticity. Also included are loessial soils
and rock flours.

Micaceous and diatomaceous soils generally fall within the MH group but may
extend into the ML group when their liquid limit is less than 50. The same is true
for certain types of kaolin clays and some illitic clays having relatively low
plasticity.

CL and CH
The CL and CH groups embrace clays with low and high liquid limits,
respectively. These are mainly inorganic clays. Low-plasticity clays are classified
as CL and are usually lean clays, sandy clays, or silty clays. The medium-plasticity
and high-plasticity clays are classified as CH. These include the fat clays, gumbo

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