1.11. TERMINOLOGY OF DIFFERENT TYPES OF SOILS
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 characteristics of different types of soils, arranged in alphabetical order.
(1) 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.
(2) Black Cotton Soil – It is a residual soil containing a high lpercentage of the clay mineral montmorilloite. It has very low bearing capacity and high swelling and shrinkage properties.
(3) Boulders – Boulders are rock fragments of large size, more than 300 mm in size.
(4) Culiche – It is a type of soil which contains gravel, sand and silt. The particles are cemented by calcium carbonate.
(5) Clay – It consists of microscopic and sub-microscopic particles derived from the chemical decomposition of rocks. It contains a large quantity of clay minerals. 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 0.002 mm.
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.
(6) Cobbles – Cobbles are large size particles in the range of 80 mm to 300 mm.
(7) Dune sands – These are wind-transported soils. There are composed of relatively uniform particles of fine to medium sand.
(8) Expansive clays – These are prone to large volume changes as the water content is changed. These soil contain the mineral montmorilloite.
(9) Gravel – Gravel is a type of coarse-grained soil. The particle size ranges from 4.75 mm to 80 mm. It is a cohesipnless material.
(10) 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.
(11) Kankar – It is an impure form of lime stone. It contains calcium carbonate mixed with some silicious material.
(12) 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.
(13) 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.
(14) Marl – It is a stiff, marine calcareous clay of greenish colour.
(15) Moorum – The word moorum is derived from a Tamil word, meaning powdered rock. It consists of small pieces of disintegrated rock or shale, wihn or without boulders.
(16) Peat – It is an 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 sujtable for condations.
(17) 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.
(18) Silt – It is a fine-grained soil, with particle size between 0.02 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 mater. It is a plastic soil and is cohesive.
(19) Top soils – Top soils are surface soils that support plants. They contain a large quantity of organic matter and are not suitable for foundations.
(20) Tuff – It is a fine-grained soil composed of very small partisics ejected from volcanoes during its explosion and deposited by wind or water.
(21) 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 lonsierf.
1.12. COHESIVE AND COHESIONLESS SOILS
Soils in which the adsorbed water and particle attraction act such that it deforms plastically at varying water contents are known as cohesive lsidrf or clays. This cohesive property is due to presence of clay minerals in soils. Therefore, the term cohesive soil is used synonymously for clayey soils.
The soils composed of bulky grains are cohesionless regardless of the fineness of the particles. The rock flour is cohesionless even when it has the particle size smaller than 2p size. Non-plastic silts and coarse-grained soils are cohesionless.
Many soils are mixture of bulky grains and clay minerals and exhibit some degree of plasticity with varying water content. Such soils are termed cohesive if the plasticity effect is significant; otherwise, cohesionless.
Obviously, there is no sharp dividing line between cohesionless and cohesive soils. However, it is sometimes convenient to divide the soil into above two groups.
The term cohesive soil is ased for clays and plastic silt and the term cohesionless soil, for non-plastic silts, sands and gravel.
(1) 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.
(2) Black Cotton Soil – It is a residual soil containing a high lpercentage of the clay mineral montmorilloite. It has very low bearing capacity and high swelling and shrinkage properties.
(3) Boulders – Boulders are rock fragments of large size, more than 300 mm in size.
(4) Culiche – It is a type of soil which contains gravel, sand and silt. The particles are cemented by calcium carbonate.
(5) Clay – It consists of microscopic and sub-microscopic particles derived from the chemical decomposition of rocks. It contains a large quantity of clay minerals. 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 0.002 mm.
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.
(6) Cobbles – Cobbles are large size particles in the range of 80 mm to 300 mm.
(7) Dune sands – These are wind-transported soils. There are composed of relatively uniform particles of fine to medium sand.
(8) Expansive clays – These are prone to large volume changes as the water content is changed. These soil contain the mineral montmorilloite.
(9) Gravel – Gravel is a type of coarse-grained soil. The particle size ranges from 4.75 mm to 80 mm. It is a cohesipnless material.
(10) 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.
(11) Kankar – It is an impure form of lime stone. It contains calcium carbonate mixed with some silicious material.
(12) 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.
(13) 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.
(14) Marl – It is a stiff, marine calcareous clay of greenish colour.
(15) Moorum – The word moorum is derived from a Tamil word, meaning powdered rock. It consists of small pieces of disintegrated rock or shale, wihn or without boulders.
(16) Peat – It is an 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 sujtable for condations.
(17) 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.
(18) Silt – It is a fine-grained soil, with particle size between 0.02 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 mater. It is a plastic soil and is cohesive.
(19) Top soils – Top soils are surface soils that support plants. They contain a large quantity of organic matter and are not suitable for foundations.
(20) Tuff – It is a fine-grained soil composed of very small partisics ejected from volcanoes during its explosion and deposited by wind or water.
(21) 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 lonsierf.
1.12. COHESIVE AND COHESIONLESS SOILS
Soils in which the adsorbed water and particle attraction act such that it deforms plastically at varying water contents are known as cohesive lsidrf or clays. This cohesive property is due to presence of clay minerals in soils. Therefore, the term cohesive soil is used synonymously for clayey soils.
The soils composed of bulky grains are cohesionless regardless of the fineness of the particles. The rock flour is cohesionless even when it has the particle size smaller than 2p size. Non-plastic silts and coarse-grained soils are cohesionless.
Many soils are mixture of bulky grains and clay minerals and exhibit some degree of plasticity with varying water content. Such soils are termed cohesive if the plasticity effect is significant; otherwise, cohesionless.
Obviously, there is no sharp dividing line between cohesionless and cohesive soils. However, it is sometimes convenient to divide the soil into above two groups.
The term cohesive soil is ased for clays and plastic silt and the term cohesionless soil, for non-plastic silts, sands and gravel.
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