Origin of Soils, Type Of Soils, Regional soil deposits of India

Origin of Soils

1. Soil: It is defined as an unconsolidated material, composed of solid particles, produced by disintegration of rocks and decomposition of organic matter. The void spaces between the particles may contain air, water or both. 

2. Soil Mechanics: It is a branch of mechanics which deals with the action of forces on soil and with the fl ow of water in soil. The term ‘Soil Mechanics’ was coined by late Dr. Karl Terzaghi, who is recognized as the father of soil mechanics.

 
3. Foundation Engineering or Soil Engineering: It is an applied science dealing with the application of principles of Soil Mechanics to practical problems. It includes site investigation, design and construction of foundations, earth retaining structures and earth structures.

4. Geotechnical Engineering: It is a broader term which includes soil mechanics, foundation engineering, rock mechanics and geology.

5. Origin of soils: Soils are formed by weathering of rocks and decomposition of organic matter. Therefore, soils may be organic or inorganic (depends on source of origin). Organic soils are called ‘Cumulose soils’. Examples for organic soils are Peat, Humus, muck etc.

6. Types of weathering: (a) Physical (b) Chemical

7. Physical weathering: 
• It is due to physical eff ects like temperature, abrasion ,wedging action of ice, penetration of plant roots etc. 
• Physical weathering results in no change in chemical composition of particles. 
• It produces coarse grained and non cohesive soils. Examples: Gravel, Sands. 

8. Chemical weathering:
• It is due to chemical actions (oxidation, hydration, carbonation, solution, leaching, hydrolysis, etc.,)
• Original rock minerals are transformed into clay minerals, due to chemical weathering.
• It results in fi ne grained and cohesive soils. Examples: Clays. 

9. Geological Cycle: Weathering of rocks – Transportation – Deposition – Upheaval. 

10. Sedimentary soils: These soil particles created at one location, transported and fi nally deposited in another location. 

11. Residual soils (Sedentary soils): These are soils which remain on the parent rock without getting transported. 
 

Examples: Black cotton soils, Laterite soils. 

Source of transportatione	Type of soil
River	Alluvial soils
Lake	Lacustrine soils
Sea	Marine soils
Wind	Aeolian soils (GATE’95), Example: Sand dunes, Loess
Gravitation	Colluvial soils, Example: Talus.
Glacier	Glacier deposited soils Example: Drift, till, outwash

12. Gravitational and surface forces: The Gravitational force in a soil particle is proportional to its mass. Therefore, the larger the particle size, the greater would be the gravitational force. The gravitational forces are predominant in the case of coarse grained soils viz. Gravel and sand.

Bonding or surface forces between the particles depend upon the surface area of the particles and the surrounding conditions. As the particle size decreases, the surface forces increase. The surface forces like chemical forces, electrical forces are predominant in clays. 

In the case of silts, both gravitational and surface forces are important.

13. Important field names of soils: 

Loam: It is a mixture of sand, silt and clay. Approximately in equal proportions. 

Varved Clay: It contains alternate thin layers of clay and silt. It is generally a lacustrine deposit. 

Loess: It is a loose deposit of wind–blown deposit (Aeolian soil). It has silt sized particles. It is weakly cemented by calcium carbonate. It is formed in arid and semiarid regions in nearly vertical banks. The permeability is generally more in the vertical direction than that in the horizontal direction for the loess soils. 

Bentonite: It is chemically weathered volcanic ash. It is highly water absorbent and has high shrinkage and swelling. Contains high percentage of clay mineral, montmorillonite. 

Marl: It is a stiff marine calcareous clay of greenish colour. 

Caliche: A soil which contains gravel, sand and silt and the particles are cemented by calcium carbonate. 

Dune sand: It is a wind transported soil, containing relatively uniform size of particles of fi ne to medium sands. 

Fill: It is a man made deposit of soil. 

Moorum: It consists of small pieces of disintegrated rock. Drift: It refers to soil deposit made by glaciers directly or indirectly. 

Till: It is an unstratifi ed deposit formed by melting of a glacier. It consists of particles of different sizes ranging from boulders to clay. It is also called boulderclay. 

Talus: It is a colluvial deposit of broken rock pieces. It is generally found at the foot of cliff s or steep slopes. 

Peat: It is a highly organic soil, fi brous nature and entirely of vegetative matter in varying states of decomposition. It posses an organic odour. Brown to black in colour. Humus: It is an organic amorphous earth of top soil, consisting partly decomposed vegetative matter. It is dark brown. Muck: It is a mixture of fi ne particles of inorganic soil and black decomposed organic matter. It is usually found in conditions of imperfect drainage as in swamps. Peat and muck are called cumulose soils. 

14. Regional soil deposits of India: The soil deposits of India may be broadly divided into the following groups based on the climatic conditions, topography and geology of their formation. 

1. Alluvial deposits 

 2. Marine deposits 

 3. Black cotton soils 

 4. Laterites and lateritic soils 

 5. Desert soils 

 6. Boulder deposits 

Alluvial deposits: Large part of north India is concerned with alluvial deposits. These deposits have alternating layers of sand, silt and clay. The fine silty sand deposits loose in nature are prone to liquefaction during earthquakes.

Marine deposits: These are confined along a narrow belt near the coast. They contain a large amount of organic matter and the marine clays are soft and highly plastic.

Black cotton soils: These are expansive soils or (clays of high plasticity) present in large part of central India and a portion of south India. Formed due to chemical weathering. Parent rock is basalt or trap. Exhibits large swelling and shrinkage due to presence of montmorillonite mineral. 

Lateritic soils: Formed from basalt rock. These exist in the central, southern and eastern India. These soils are soft when wet and harden with time. Formed by disintegration of rock, removal of the bases and silica and accumulation of iron oxide and aluminium oxide. Laterite soils possess red or pink colour due to iron oxide. 

Desert soils: These are wind blown deposits of sand. Sand dunes are desert soils. 

Boulder deposits: These are large quantities of boulders deposited by rivers flowing in highly terrain near the foot of hills.

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