IDENTIFICATION AND

CLASSIFICATION OF SOILS
 FIELD IDENTIFICATION OF SOILS
• Mainly to differentiate sand and gravel
• Problems in identifying silt and clay
• Organic Content and Colour
Fresh, wet organic soils usually have a distinctive
odour of decomposed organic matter, which can
be easily detected on heating. Another
distinctive feature of such soils is the dark
colour.
• 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.
• Dispersion Test. The procedure consists in
dispersing a small quantity of the soil in water
taken in aglass 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.
 SOIL CLASSIFICATION
• The more common classification systems :
1. Preliminary Classification by Soil types or
Descriptive Classification.
2. Geological Classification or Classification by
Origin.
3. Classification by Structure.
4. Grain-size Classification or Textural Classification.
5. Unified Soil Classification System.
6. Indian Standard Soil Classification System.
• Bentonite is a clay formed by the decomposition of volcanic ash
with a high content of montmorillonite. It exhibits the properties of
clay to an extreme degree.
• Varved Clays consist of thin alternating layers of silt and fat clays of
glacial origin. They possess the undesirable properties of both silt
and clay. The constituents of varved clays were transported into
fresh water lakes by the melted ice at the close of the ice age.
• Kaolin, China Clay are very pure forms of white clay used in the
ceramic industry.
• Boulder Clay is a mixture of an unstratified sedimented deposit of
glacial clay, containing unsorted rock fragments of all sizes ranging
from boulders, cobbles, and gravel to finely pulverized clay
material.
• Calcareous Soil is a soil containing calcium carbonate. Such soil
effervesces when tested with weak hydrochloric acid.
• Marl consists of a mixture of calcareous sands, clays, or loam.
• Caliche is an admixture of clay, sand, and gravel cemented by calcium
carbonate deposited from
• ground water.
• Peat is a fibrous aggregate of finer fragments of decayed vegetable matter.
Peat is very compressible and one should be cautious when using it for
supporting foundations of structures.
• Loam is a mixture of sand, silt and clay.
• Loess is a fine-grained, air-borne deposit characterized by a very uniform
grain size, and high void ratio.
• Shale is a material in the state of transition from clay to slate. Shale itself
is sometimes considered a rock but, when it is exposed to the air or has a
chance to take in water it may rapidly decompose.
 Indian Standard Soil Classification
System
• IS: 1498-1970 -Indian Standard on Classification and
Identification of Soils for general engineering purposes :
Soils shall be broadly divided into three divisions :
1. Coarse-grained Soils: More than 50% of the total material
by weight is larger than 75μ IS Sieve size.
2. Fine-grained Soils: More than 50% of the total material by
weight is smaller than 75μ IS Sieve size.
3. Highly Organic Soils and Other Miscellaneous Soil Materials:
These soils contain large percentages of fibrous organic
matter, such as peat, and particles of decomposed
vegetation. In addition, certain soils containing shells,
concretions, cinders and other non-soil materials in
sufficient quantities are also grouped in this division.
Coarse-grained soils shall be divided into two sub-divisions :
(a) Gravels: More than 50% of coarse fraction (+ 75 μ) is larger than
4.75 mm IS Sieve size.
(b) Sands: More than 50% of Coarse fraction (+ 75 μ) is smaller than
4.75 mm IS Sieve size.

Fine-grained soils shall be divided into three sub-divisions:

Organic soils
Soil may be oven-dried, remixed with water and retested for liquid limit. The plasticity of fine-grained
organic soils is considerably reduced on oven-drying. Oven-drying also affects the liquid limit of
inorganic soils, but only to a small extent. A reduction in liquid limit after ovendrying to a value less
than three-fourth of the liquid limit before oven-drying is positive identification of organic soils.
 STRUCTURE OF CLAY MINERALS
• Two fundamental building blocks
• are involved in the formation of clay mineral
structures. They are:
• 1. Tetrahedral unit.
• 2. Octahedral unit.
 SOIL MASS STRUCTURE
• The ‘structure’ of a soil may be defined as the
manner of arrangement and state of
aggregation of soil grains.
• The orientation of particles in a mass depends
on the size and shape of the grains as well as
upon the minerals of which the grains are
formed. The structure of soils that is formed
by natural deposition can be altered by
external forces.