1. Soil Classification
3. SOIL CLASSIFICATION
3.1 Field Identification of Soils
The methods of field identification of soils can conveniently be discussed under the headings of
coarse-grained and fine-grained soil materials.
Coarse-Grained Soil Materials
The coarse-grained soil materials are mineral fragments that may be identified primarily on the
basis of grain size. The different constituents of coarse-grained materials are sand and gravel. As
described in the earlier sections, the size of sand varies from 0.075 mm to 4.75 mm and that of
gravel from 4.75 mm to 80 mm. Sand can further be classified as coarse, medium and fine. The
engineer should have an idea of the relative sizes of the grains in order to identify the various
fractions. A small magnifying glass can be used to identify the small fragments of shale or mica.
The properties of a coarse grained material mass depend also on the uniformity of the sizes of the
grains. A well-graded sand is more stable for a foundation base as compared to a uniform or poorly
graded material.
Fine-Grained Soil Materials
Inorganic Soils: The constituent parts of fine-grained materials are the silt and clay fractions. Since
both these materials are microscopic in size, physical properties other than grain size must be used
as criteria for field identification. The classification tests used in the field for preliminary
identification are
1. Dry strength test
2. Shaking test
3. Plasticity test
4. Dispersion test
Dry strength: The strength of a soil in a dry state is an indication of its cohesion and hence of its
nature. It can be estimated by crushing a 3 mm size dried fragment between thumb and forefinger. A
clay fragment can be broken only with great effort, whereas a silt fragment crushes easily.
Shaking test: The shaking test is also called as dilatancy test. It helps to distinguish silt from clay
since silt is more permeable than clay. In this test a part of soil mixed with water to a very soft
consistency is placed in the palm of the hand. The surface of the soil is smoothed out with a knife
and the soil pat is shaken by tapping the back of the hand. If the soil is silt, water will rise quickly
to the surface and give it a shiny glistening appearance. If the pat is deformed either by squeezing
or by stretching, the water will flow back into the soil and leave the surface with a dull appearance.
Since clay soils contain much smaller voids than silts and are much less permeable, the appearance
of the surface of the pat does not change during the shaking test. An estimate of the relative
proportions of silt and clay in an unknown soil mixture can be made by noting whether the reaction
is rapid, slow or nonexistent.
Plasticity test: If a sample of moist soil can be manipulated between the palms of the hands and
fingers and rolled into a long thread of about 3 mm diameter, the soil then contains a significant
amount of clay. Silt cannot be rolled into a thread of 3 mm diameter without severe cracking.

2. Soil Classification
Dispersion test: This test is useful for making a rough estimate of sand, silt and clay present in a
material. The procedure consists in dispersing a small quantity of the soil in water taken in a Soil
Phase Relationships, Index Properties and Soil Classification 69 glass cylinder and allowing the
particles to settle. The coarser particles settle first followed by finer ones. Ordinarily sand particles
settle within 30 seconds if the depth of water is about 10 cm. Silt particles settle in about 1/2 to 240
minutes, whereas particles of clay size remain in suspension for at least several hours and
sometimes several days.
Organic soils: Organic material in soil is usually derived from plant or root growth and consists of
almost completely disintegrated matter, such as muck or more fibrous material, such as peat. The
soils with organic matter are weaker and more compressible than soils having the same mineral
composition but lacking in organic matter. The presence of an appreciable quantity of organic
material can usually be recognized by the dark-grey to black color and the odor of decaying
vegetation which it lends to the soil.
Table 3-1 Field Identification Test
Test ML CL OL MI CI OI MH CH OH
a. Dilatancy Quick None to
slow
Slow Quick
to slow
None Slow Slow to
none
None None to
Very
Slow
b. Toughness None Medium Low None Medium Low Low to
medium
High Low to
medium
c. Dry strength None
of low
Medium Low Low Medium
to high
Low to
medium
Low to
medium
High
to very
Hign
Medium
to high
3.2 Soil Classification Systems
Descriptive classification of soils
Under this classification system, soils are designated as
a. Boulders: Most of the systems recognise any material larger than 7.62 cm (3
inches) size as a boulder.
b. Gravel: gravel is the intermediate size between boulder and sand
c. Sand: particle size between 2mm-0.06mm is sand.
d. Silt: organic silt (0.002-0.06mm) which is usually dark to brown colour, highly
compressible. Similarly, non plastic silt ( < 0.002mm) are plate like bulk grains and
incompressible.
e. Clay: soil particles smaller than 200mm size and capable of showing plasticity
when wet.
f. Peat: peat is partly carbonized organic matter and is fibrous in nature. It is very
light compared with other soils.
g. Black-cotton soil: it contains high percentage of montmorinollite clay and it has
very high value of swelling and shrinkage. When dry this soil is very hard but
become very soft in wet. It is black in colour and not good for foundation.
Textural classification

3. Soil Classification
The visual appearance of the soil is called texture. The texture depends upon particle size,
shape of the particles and gradation of particles. The textural classification incorporates
only the particle size, as it is difficult to incorporate the other two parameters.
The triangular classification system suggested by U.S. Bureau of Public Roads in
commonly known as the textural classification system.
Unified Soil Classification System (USCS)
The Unified Soil Classification System was first developed by Casagrande in 1948. It is based on
the recognition of the type and predominance of the constituents considering grain-size, gradation,
plasticity and compressibility. It divides soil into three major divisions: coarse-grained soils, fine
grained soils, and highly organic (peaty) soils.
Table 3-2. Symbols used in USCS
Division Symbols Description
Primary G
S
M
C
O
Pt
Gravel
Sand
Silt
Clay
Organic
Peat
Division Symbols Description
Secondary W
P
M
C
L
H
Well-graded
Poorly graded
Non-plastic fines
Plastic fines
Low plasticity
High Plasticity

4. Soil Classification
Table 3-3. Unified Soil Classification System
Major Division Group
Symbols
Typical Names Classification Criteria
Coarse-
Grained
Soils [More
than 50%
retained on
No 200
(0.075mm)]
Gravel
[50% or
more of
coarse
fraction
retained
on No. 4
sieve (
4.75mm)]
Clean
Gravels
GW Well graded
gravels
Cu > 4
Cc = 1 to 3
GP Poorly graded
gravels
Not meeting both criteria for GW
Gravels
with fines
GM Silty gravels Atterberg Limits below
A-line or plasticity
index less than 4
Atterberg
Limits in
hatched
GM-GC
GC Clayey gravels Atterberg Limits above
A-line or plasticity
index greater than 7
Sand [50%
or more of
coarse
fraction
passing
through
No. 4
sieve (
4.75mm)]
Clean
Sands
SW Well graded
Sands
Cu > 6
Cc = 1 to 3
SP Poorly graded
Sands
Not meeting both criteria for GW
Sands
with fines
SM Silty Sands Atterberg Limits below
A-line or plasticity
index less than 4
Atterberg
Limits in
hatched
SM-SC
SC Clayey Sands Atterberg Limits above
A-line or plasticity
index greater than 7
Fine-
Grained
Soils [50%
or more
passing No
200 Sieve
(0.075mm)]
Silts and clays Liquid
limit 50% or less
ML Inorganic silts of
low plasticity
See Plasticity chart
CL Inorganic clays of
low plasticity
OL Organic silts of
low plasticity
Silts and clays Liquid
greater than 50%
MH Inorganic silts of
high plasticity
CH Inorganic clays of
high plasticity
OH Organic silts of
high plasticity
Highly organic Soils Pt Peat, muck and
other highly
organic soils
Visual-manual identification

5. Soil Classification
Coarse-grained soils: The coarse-grained soils are designated as gravel (G) if 50% or more of coarse
fraction is retained on No.4 (4.75 mm) sieve otherwise it is termed as sand (S).
If the coarse-grained soils contains less than 5% fines and are well-graded(W), they are given symbols
GW and SW, and if poorly graded (P), symbols GP and SP. If the coarse-grained soils contain more tan
12% fines, these are designated as GM, GC, SM or SC, as per criteria given. If the percentage of fines
is between 5-12% dual symbols such as GW-GM, SP-SM, are used.
Fine grained Soils: Fine grained soils are classified on the basis of plasticity index (PI) and liquid limit
(LL). If liquid limit is less or equal to 50% then soil of low compressibility (L), these are given the
symbols ML, CL and OL. If liquid limit is more than 50% then denoted by symbols MH, CH and OH
(Fig. 3.1 )
Fig. 3.1 Plasticity chart (USCS)
3.3 Practical Implications of the Soil Classification System
The individual constituents of a soil mixture can be separated and identified as gravel, sand, silt
and clay on the basis of mechanical analysis. The clay mineral that is present in a clay soil is
sometimes a matter of engineering importance. According to the mineral present, the clay soil can
be classified as kaolinite, montmorillonite or illite. The minerals present in a clay can be identified
by either X-ray diffraction or differential thermal analysis. A description of these methods is
beyond the scope of this book.
Buildings, bridges, dams etc. are built on natural soils (undisturbed soils), whereas earthen dams
for reservoirs, embankments for roads and railway lines, foundation bases for pavements of roads
and airports are made out of remolded soils. Sites for structures on natural soils for embankments,
etc, will have to be chosen first on the basis of preliminary examinations of the soil that can be
carried out in the field. An engineer should therefore be conversant with the field tests that would
identify the various constituents of a soil mixture.
The behavior of a soil mass under load depends upon many factors such as the properties of the various
constituents present in the mass, the density, the degree of saturation, the environmental conditions etc.

6. Soil Classification
If soils are grouped on the basis of certain definite principles and rated according to their performance,
the properties of a given soil can be understood to a certain extent, on the basis of some simple tests.