Unified Soil Classification System
Explanation
SOIL CATEGORIES
Soil rarely exists in nature as sand alone, gravel alone, or any other single
component. Usually, soil occurs as mixtures with varying proportions of particles of
different sizes. Each component contributes its characteristics to the mixture. The
Unified Soil Classification System is based on the characteristics of the soil that
indicate how it will behave as a construction material. There are many indicators and
descriptors for soil.
Particle size
Gravels range from about 3 inches down to the size of peas. Sands start just below
this size and decrease until the individual grains are just distinguishable by the
naked eye. The eye can normally see individual grains about 0.07 millimeter in size,
or about the size of the No. 200 sieve. Silt and clay particles, which are smaller than
sands, are indistinguishable as individual particles.
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Size Group
Boulders
Cobbles
Gravels
Sands
Fines (clay or silt)
Sieve Size
Passing
No maximum size
12 inches
3 inches
No. 4
No. 200
Retained On
12 inches
3 inches
No. 4
No. 200
No minimum size
Categories
In the USCS, all soils are placed into one of three major categories. They are
 Coarse-grained
 Fine-grained
 Highly organic
The USCS further divides soils that have been classified into the major soil
categories by letter symbols, such as
 S for sand
 G for gravel.
 M for silt.
 C for clay.
A soil that meets the criteria for clayey sand would be designated (SC). There are
cases of borderline soils that cannot be classified by a single dual symbol, such as
GM for silty gravel. These soils may require a dual symbol for a full description. For
example, (SM-SC) describes sand that contains appreciable amounts of silt and
clay.
Grain Shape
When a sample is examined for grain sizes, the shapes of the visible particles can
be determined. Sharp edges and flat surfaces indicate an angular shape; smooth,
curved surfaces indicate a rounded shape. Particles may not be completely angular
or completely rounded. These particles are called sub-angular or sub-rounded,
depending on which shape predominates. “Platy” is a term used to describe those
particles which have one relatively small dimension compared to the other two.
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Clays generally fall into this
category.
 Well-Graded Soils. A well-graded soil is defined as having a good
representation of all particle sizes from the largest to the smallest.

Poorly-Graded Soils. There are two types of poorly-graded soils.
o A uniformly graded soil consists primarily of particles of nearly the
same size.
o A gap-graded soil contains various particle sizes, but the gradation
continuity is broken by the absence of some particle sizes.
Coarse-Grained Soils
Coarse-grained soils are defined as those in which at least half the material is
retained on a No. 200 sieve. They are divided into two major categories, which are
 Gravel
 Sand
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Soil Classification
A coarse-grained soil is classed as gravel if more than half the coarse fraction by
weight is retained on a No. 4 sieve. The symbol G is used to denote gravel and the
symbol S to denote sand. Where a mixture occurs, the primary name is the
predominant soil portion and the minor soil portion is used as an adjective. For
example, a sandy gravel is a soil mixture containing more gravel than sand, by
weight. Additionally, gravels are further separated into either coarse gravel or fine
gravel with the 3/4-inch sieve as the dividing line. Sands are divided into coarse,
medium, or fine with the No. 10 and No. 40 sieves acting as the dividing line. The
coarse-grained soils may also be further divided into three groups on the basis of the
amount of fines (materials passing a No. 200 sieve) they contain. These amounts
are:
 Less than 5 percent.
 More than 12 percent.
 Between 5 and 12 percent.
Coarse-grained soils with less than 5 percent passing the No. 200 sieve may fall into
the following groups:
 GW represents well-graded gravels and gravel-sand mixtures with little or no
fines. The presence of the fines must not notably change the strength
characteristics of the coarse-grained soil portion and must not interfere with
its free-draining characteristics.
 SW represents well-graded sands and gravelly sands with little or no fines.
 GP represents poorly-graded gravels and sandy gravel mixtures with little or
no fines.
 SP represents poorly-graded sands and gravelly sands with little or no fines.
Coarse-grained soils containing more than 12 percent passing the No. 200 sieve are
classified using the following groups:
 GM represents silty gravel and poorly graded gravel/sand-silt mixtures.
 SM represents silty sands and poorly graded sand-silt mixtures.
 GC represents clayey gravels and poorly-graded gravel-sand-clay mixtures.
 SC represents clayey sands and poorly-graded sand-clay mixtures.
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The use of the symbols M and C is based on the plasticity characteristics of the
material passing the No. 40 sieve. Atterberg limits, which include the liquid limit (LL)
and plastic limit (PL), are used in determining the plasticity of the fine materials. The
symbol M is used to indicate that the material passing the No. 40 sieve is silty in
character. M usually designates a fine-grained soil of little or no plasticity. The
symbol C is used to indicate that the binder soil is clayey in character.
Liquid Limit. The LL (or wL) is defined as the minimum moisture content at which a
soil will flow upon application of a very small shearing force. With only a small
amount of energy input, the soil will flow under its own weight.
Plastic Limit. The PL (or wp) is arbitrarily defined as the lowest moisture content at
which a soil can be rolled into a thread 1/8 inch in diameter without crushing or
breaking. If a cohesive soil has a moisture content above the PL, a thread may be
rolled to less than 1/8 inch in diameter without breaking. If the moisture content is
below the PL, the soil will crumble when attempts are made to roll it into 1/8-inch
threads. When the moisture content is equal to the PL, a thread can be rolled out by
hand to 1/8 inch in diameter; then it will crumble or break into pieces 1/8 to 3/8 inch
long when further rolling is attempted. Some soils (for example, clean sands) are
non-plastic and the PL cannot be determined. A clean sand or gravel will progress
immediately from the semisolid to the liquid state.
Coarse-grained soils containing more than 12 percent of material passing the No.
200 sieve that are borderline between silt and clay are classified using a dual symbol
(SM-SC or GM-GC).
Course-grained soils with between 5 and 12 percent of material passing the No. 200
sieve also require a dual symbol based on gradation and plasticity.
 GW-GM
 GP-GM
 GW-GC
 GP-GC
 SW-SC
 SW-SM
 SP-SC
 SP-SM
Plasticity Index
The PI (or Ip) of a soil is the numerical difference between the LL and the PL.
PI  LL  PL
For example, if a soil has a liquid limit of 57 and a plastic limit of 23, then the
plasticity index equals 34 (PI = LL – PL = 57 - 23). Sandy soils and silts have
characteristically low plasticity indices, while the plasticity indices of most clays are
larger. Soils that have high PI values are highly plastic and are generally highly
compressible and highly cohesive. The plasticity is inversely proportional to the
permeability of a soil; so, the higher a soil’s plasticity index the lower the
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permeability. Soils that do not have a plastic limit, such as clean sands, are reported
as having a plasticity index of zero.
Fine-Grained Soils
Plasticity Index (PI)
Fine-grained soils are those in which more than half the material passes a No. 200
sieve. Fine-grained soils are not classified by grain size but according to plasticity
and compressibility. Laboratory classification criteria are based on the relationship
between the liquid limit and the plasticity index, determined from a plasticity chart.
The chart indicates two major groupings of fine-grained soils. These are
 The L groups, which have LL < 50.
 The H groups, which have LL 50.
The symbols L and H represent low and high plasticity, respectively. Remember that
the liquid limit and plastic limit are found on the fraction of the soil that passes the
No. 40 sieve and therefore may contain fine sands. The ML group includes
 Very fine sands.
 Rock flours.
 Silty or clayey fine sands with slight plasticity.
Plastic silts fall into the MH group.
CL and CH represent clays with low and high compressibility, respectively. These
soils plot above the A-line and are principally inorganic clays. The CL group includes
gravelly clays, sandy clays, silty clays, and lean clays. The CH group includes
inorganic clays of high plasticity, such as fat clays, the gumbo clays of the southern
United States, volcanic clays, and bentonite. The glacial clays of the northern United
States cover a wide band in the CL and CH groups.
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Soils in the OL and OH groups are characterized by the presence of organic matter,
indicated by the symbol O. Organic silts and organic silt clays of low plasticity fall
into the OL group, while organic clays plot in the OH zone of the plasticity chart.
Many organic silts, silt-clays, and clays deposited by rivers along the lower reaches
of the Atlantic seaboard have liquid limits between 40 and 100. Peaty soils may have
liquid limits of several hundred percent.
Fine-grained soils that exhibit properties of two groups can be given dual symbols
(for example, CL-ML). Several soil types exhibiting low plasticity plot in this general
region on the chart and no definite boundary between silty and clayey soils exists.
Color
Color helps in distinguishing between soil types and aids in identifying a specific soil
type. Color may also indicate the presence of certain chemicals or impurities and
often varies with the soil's moisture content. Thus, the moisture content at the time of
color identification should be noted. Some of the more familiar color properties are
stated below. Colors in general become darker as the moisture content increases
and lighter as the soil dries. Some fine-grained soil (falling into the OL and OH
categories on the chart) with dark, drab shades of brown or gray (including almost
black) contain organic colloidal matter. In contrast, clean, bright shades of gray,
olive-green, brown, red, yellow, and white are associated with inorganic soils. Grayblue or gray-yellow mottled colors frequently result from poor drainage. Red, yellow,
and yellowish-brown colors result from the presence of iron oxides. White to pink
may indicate considerable silica, calcium carbonate, or aluminum compounds.
Highly Organic Soils
A special classification, Pt, is reserved for the highly organic soils, such as peat,
which have many undesirable engineering characteristics. No laboratory criteria are
established for these soils, as they generally can be easily identified in the field by
their distinctive color and odor, spongy feel, and frequently fibrous texture. Particles
of leaves, grass, branches, or other fibrous vegetable matter are common
components of these soils.
Categorization Factors
These same properties can also be considered in field identification. The table
below provides characteristics that can be used in the field to classify soils.
What to look for
Visual
appearance and
feel
Movement of
water in the
spaces
Granular soils, fine
sands and silts
Coarse grains can be
seen; feels gritty when
rubbed between
fingers
When a small quantity
is shaken in the palm
of the hand, water will
Plastic (cohesive)
soils, clay
Grains cannot be
seen by the naked
eye; feels smooth and
greasy when rubbed
between fingers
When a small quantity
is shaken in the palm
of the hand, it shows
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Plasticity when
moist
Cohesion in dry
state
appear on the surface
of the sample. When
shaking is stopped,
water will gradually
disappear.
Very little or no
plasticity
Little or no cohesive
strength in dry state;
will crumble readily
no sign of water
moving out of the
voids.
Plastic and sticky;
sample can be rolled
High dried strength;
crumbles with difficulty
and disintegrates
slowly in water
Other characteristics observed should also be included in describing the soil,
whether the identification is made by field or laboratory methods.
Properties normally included in a description of a soil are
 Color
 Grain size, including estimated maximum grain size and estimated percent by
weight of fines (material passing the No. 200 sieve)
 Gradation
 Grain shape
 Plasticity
 Predominant type
 Secondary components
 Classification symbol
 Other remarks, such as organic, chemical, or metallic content; compactness;
consistency; cohesiveness near PL; dry strength; and source (i.e., residual or
transported)
An example of a soil description using the sequence and considering the properties
referred to above might be
 Dark brown to white
 Coarse-grained soil, maximum particle size 2 3/4 inches, estimating 60
percent gravel, 36 percent sand, and 4 percent passing the No. 200 sieve
 Poorly-graded (insufficient fine gravel, gap-graded).
 Gravel particles sub-rounded to rounded
 Nonplastic
 Predominantly gravel
 Considerable sand and a small amount of nonplastic fines (silt)
 (GP)
 Slightly calcareous, no dry strength, dense in the undisturbed state
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