Classification of Soil Types.
A number of systems of
classification have been evolved for categorizing various types of soil. Some
of these have been developed specifically in connection with ascertaining the
suitability of soil for use in particular soil engineering projects. Some are
rather preliminary in character while a few are relatively more exhaustive,
although some degree of arbitrariness is necessarily inherent in each of the
systems.
Geological
Classification:-
Classification by
Structure:-
Unified soil
classification system:-
Preliminary
Classification by soil types:-
Terminology of different types of soil:-
The more common classification systems are enlisted below:
- Geological
Classification
- Classification by
Structure
- Classification based on
Grain-size
- Unified Soil
Classification System
- Preliminary
Classification by soil types
Geological
Classification:-
Soil types may be classified
on the basis of their geological origin. The origin of a soil may refer either
to its constituents or to the agencies responsible for its present status.
Based on constituents, soil
may be classified as:
- Inorganic soil
- Organic soil
Based on the agencies
responsible for their present state, soils may be classified under following
types:
- Residual Soils
- Transported Soils:
- Alluvial or sedimentary
soils
- Aeolian soils
- Glacial soils
- Lacustrine soils
- Marine soils
Over the geological cycle,
soils are formed by disintegration and weathering of rocks. These are again
formed by compaction and cementation by heat and pressure.
Classification by
Structure:-
Depending upon the average
grain-size and the conditions under which soils are formed and deposited in
their natural state, they may be categorized into following types on the basis
of their structure:
- Soils of single-grained
structure
- Soils of honey-comb
structure
- Soils of flocculent
structure
Classification based on grain-size
In the grain-size
classification, soils are designated according to the grain-size or
particle-size. Terms such as gravel, sand, silt and clay are used to indicate
certain ranges of grain-sizes. Since natural soils are mixtures of all
particle-sizes, it is preferable call these fractions as sand size, silt size,
etc. A number of gain-size classifications have been evolved, but the commonly
used ones are:
- S. Bureau of Soils and
Public Roads Administration (PRA) System of U.S.A.
- International Classification,
propsed at the International Soil Congress at Washington, D.C., in 1927
- Massachusetts Institute
of Technology (MIT) System of Classification of U.S.A.
- Indian Standard
Classification (IS: 1498-1970)
Unified soil
classification system:-
This system was originally
developed by A. Cassagrande and adopted by the U.S. Corps of Engineers in 1942
as ‘Airfield Classification’. It was later revised for universal use and
redesignated as the “Unified Soil Classification” in 1957.
In this system soils are
classified into three broad categories:
- Coarse-grained soils
with up to 50% passing No. 200 ASTM Sieve
- Fine-grained soils with
more than 50% pass No. passing No. 200 ASTM Sieve
- Organic soils
Preliminary
Classification by soil types:-
Familiarity with common soil
types is necessary for an understanding of the fundamentals of soil behaviour.
In this approach, soils are described by designation such as Boulders, Gravel,
Sand, Silt, Clay, Rockflour, Peat, China Clay, Fill, Bentonite, Black Cotton
soil, Boulder Clay, Caliche, Hardpan, Laterite, Loam, Loess, Marl, Moorum,
Topsoil and Varved Clay.
Terminology of different types of soil:-
A geotechnical engineer
should be well versed with the nomenclature and terminology of different types
of soils. The following list gives the names and salient features of different
types of soil, arranged in alphabetical order.
- Bentonite: It is a type of clay
with a very high percentage of clay mineral – montmorillonite. It is a
highly plastic clay, resulting from the decomposition of volcanic ash. It
is highly water absorbent and has high shrinkage and swelling
characteristics.
- Black cotton soil: It is a residual soil containing a high percentage of the clay
mineral – montmorillonite. It has very low bearing capacity and high swelling
and shrinkage properties.
- Boulders: Boulders are the rock
fragments of large size, more than 300mm in size.
- Calcareous soil: This type of soil contains a large quantity of calcium
carbonate. Such soils effervesce when tested with weak hydrochloric acid.
- Caliche: It is a type of soil
which contains gravel, sand and silt. The particles are cemented by
calcium carbonate.
- Clay: It consists of
microscopic and sub-microscopic particles derived from the chemical
decomposition of rocks. It contains a large quantity of clay mineral. It
can be made plastic by adjusting the water content. It exhibits
considerable strength when dry. Clay is a fine-grained soil. It is a
cohesive soil. The particle size is less than 0.002mm.
Organic clay contains finely divided organic matter and is usually dark grey or black in colour. It has a conspicuous odour. Organic clay is highly compressible and its strength is very high when dry. - Cobbles: Cobbles are large size
particles in the range of 80mm to 300mm.
- Diatomaceous earth: Diatoms are minute unicellular marine organisms. Diatomaceous
earth is a fine, light grey, soft sedimentary deposit of the silicious
remains of skeletons of diatoms.
- Dispersive clays: These are special type of clays which defloculate in still
water. Such soils erode if exposed to low-velocity water. Susceptibility
to dispersion depends upon the cations in the soil pore water.
- Dune sands: These are
wind-transported soils. These are composed of relatively uniform particles
of fine to medium sand.
- Expansive clays: These are prone to large volume changes as the water content is
changed. These soils contain the mineral montmorillonite.
- Fills: All manmade deposits of
soil and waste-materials are called fills. These are the soil embankments
raised above the ground surface. Engineering properties of fills depend
upon the type of soil, its water content and the degree of compaction.
- Gravel: It is a type of
coarse-grained soil. The particle size ranges from 4.75mm to 80mm. It is a
cohesion-less material.
- Hardpans: H8ardpans are the types
of soil that offer great resistance to the penetration of drilling tools
during soil exploration. The soils are designated hardpans regardless of
their particle size. These are generally dense, well-graded, cohesive
aggregates of mineral particles. Hardpans do not disintegrate when
submerged in water.
- Humus: It is a dark brown,
organic amorphous earth of the topsoil. It consists of partly decomposed
vegetal matter. It is not suitable for engineering works.
- Kankar: It is an impure form of
lime stone. It contains calcium carbonate mixed with some silicious
material.
- Laterites: Laterites are residual
soils formed in tropical regions. Laterites are very soft when freshly cut
but become hard after long exposure. Hardness is due to cementing action
of iron oxide and aluminium oxide. These soils are also called lateritic
soils.
- Loam: It is a mixture of sand,
silt and clay. The term is generally used in agronomy. The soil is well
suited to tilling operations.
- Loess: It is a windblown
deposit of silt. It is generally of uniform gradation, with the particle
size between 0.01 and 0.05 mm. It consists of quartz and feldspar
particles, cemented with calcium carbonate or iron oxide. When wet, it
becomes soft and compressible because cementing action is lost. A loess
deposit has a loose structure with numerous root holes which produce
vertical cleavage. The permeability in the vertical direction is generally
much greater than that in the horizontal direction.
- Marl: It is a stiff, marine
calcareous clay of greenish colour.
- Moorum: The word moorum is
derived from a Tamil word, meaning powdered rock. It consists of small
pieces ofdisintegrated rock or shale, with or without boulders.
- Muck: It denotes a mixture of
fine soil particles and highly decomposed organic matter. It is black in
colour and of extremely soft consistency. It cannot be used for
engineering works. The organic matter is in an advanced stage of
decomposition.
- Peat: It is a type of organic
soil having fibrous aggregates of macroscopic and microscopic particles.
It is formed from vegetal matter under conditions of excess moisture, such
as in swamps. It is highly compressible and not suitable for foundations.
- Sand: It is a coarse-grained
soil, having particle size between 0.075 mm to 4.75 mm. The particles are
visible to naked eye. The soil is cohesionless and pervious.
- Silt: It is a fine-grained
soil, having particle size between 0.002 mm and 0.075 mm. The particles
are not visible to naked eyes. Inorganic silt consists of bulky,
equidimensional grains of quartz. It has little or no plasticity, and is
cohesionless. Organic silt contains an admixture of organic matter. It is
a plastic soil and is cohesive.
- Till: It is a type of soil
formed by an unstratified deposit resulting from melting of a glacier. The
deposit consists of particles of different sizes, ranging from boulders to
clay. The soil is generally well-graded. It can be easily densified by
compaction. Till is known as boulder-clay.
- Top soils: Top soils are the
surface soils that support plants. They contain a large quantity of
organic matter and are not suitable for foundations.
- Tuff: It is a fine-grained
soil composed of very small particles ejected from volcanoes during its
explosion and deposited by wind or water.
- Tundra: It is a mat of peat and
shrubby vegetation that covers clayey subsoil in arctic regions. The
deeper layers are permanently frozen and are called permafrost. The
surface deposit is the active layer which alternately freezes and thaws.
- Varved clays: These are
sedimentary deposits consisting of alternate thin layers of silt and clay.
The thickness of each layer seldom exceeds 1 cm. These clays are the
results of deposition in lakes during periods of alternately high and low
waters.
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