Soil & Soil Mechanics | Definition, Origin of Soil, Types of Soil

 Soil & Soil Mechanics | Definition, Origin of Soil, Types of Soil 

   Definition of Soil:-

• Soil has different meaning depending upon various professions.
• According to Geologist, Soil is upper thin layer of earth crust in which roots of plants occur.
• According to Agriculturist, Soil is a part of earth crust which provides minerals to plants to grow up.
• According to Engineer, Soil is an unconsolidated/uncemented/unaggregated material which is composed of solid particles (Inorganic and/or Organic), produced by disintegration of rocks.
• The void between the soil particle may fill with air, water, or both.
• The size of soil particle may range from a fraction of a micron (10-3 mm) to large boulders.

Definition of Soil Mechanics:-

• According to Karl Terzaghi, Soil mechanics is the application of the laws of mechanics and hydraulics to engineering problems dealing with sediments and other unconsolidated accumulations of solid particles produced by the mechanical and chemical disintegration of rock.
• Dr. Karl Terzaghi is known as father of Soil Mechanics.

Origin of Soil:-

The Geological cycle

• Soil is formed by Geological cycle which continuously works in nature.
• For completing one cycle it takes millions of years.
• Geological cycle consist of Erosion, Transportation, Deposition and Earth Movement/Upheaval.

Erosion phase:-

• Geological cycle starts with erosion phase.
• In this phase exposed rock get disintegrated by weathering process.
• There are two weathering process-

1.      Physical Weathering

2.      Chemical Weathering

Physical Weathering:-

• It is also known as Mechanical weathering.
• Physical weathering occurs by-

Ø  Disintegration due to Temperature change, Spreading of plant roots and alternate freezing and thawing in cracks in rock.

Ø  Erosion of exposed rocks by direct erosive action of wind, Running Water, Glaciers.

• The soil formed by physical weathering has same composition as parent rock.
• The soil formed by physical weathering falls in group of Sand and Gravel.

Chemical Weathering:-

• Chemical weathering occurs by chemical process like- Hydration, oxidation, Carbonation and Hydrolysis.
• Soil formed has different composition as parent rock.
•  Soil formed is Clay.
• Most of clay mineral has plate like structure having high specific gravity.

Transportation & Deposition:-

• It is second phase of geological cycle.
•  In this phase Soil particle formed by disintegration is transported by agencies like- Water, Wind and Glaciers or by combined action.
• If the soil is still located at the place where it is formed is called Residual Soil.
• If the soil transported by any agencies from the their origin and has been deposited to other place is called Transported Soil.
• The characteristics of transported soil are influence by agency of transportation.
• Transported soil has generally small in size and large amount of pores.
• Soil transported by different agencies are-

Ø  Soil deposited by River water/ Running water -- Alluvial soil

Ø  Soil deposited by Still Water/ Lakes Water      -- Lacustrine Soil

Ø  Soil deposited by Sea Water                               ­­­-- Marine Soil

Ø  Soil deposited by Wind                                      -- Aeolian Soil

             Ø Soil deposited by Ice                                          -- Glacial Soil

Different Types of Soil:-

Bentonite: - It is clay with very high montmorillonite. It is high plastic clay, resulting from disintegration of volcanic ash. It absorbed high water and has high swelling and shrinkage.

Black Cotton Soil:- It is residual soil containing high percentage of montmorillonite.  It has very low bearing capacity and high swelling and shrinkage.

Boulder:- It is rock fragment of large size greater than 300 mm.

Clay:- It formed by chemical disintegration of rocks. It is fine grained soil. It is cohesive in nature. It can be made plastic by adjusting water content. It has Considerable strength when dry. It has particle size less than 0.002mm.

Cobble:- It is large size particle range from 80mm to 300mm.

Gravel:- It is coarse grained soil having particle size 4.75mm to 80mm.

Loam:- It is mixture of sand, silt and clay.

Loess:- It is wind deposited silt. It is generally uniformly graded soil.

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

Moorum:- it consist of small piece of disintegrated rock or shale, with or without boulders.

Peat:- It is organic soil formed from vegetal matter under excess moisture condition. It is highly compressible and not suitable for foundations.

Sand:- It is a coarse grained soil, having particle size between 0.075mm to 4.75mm. It’s particles are visible by naked eye. It is cohesionless and pervious soil.

Silt:- It is a fine-grained soil, having particle size between 0.002mm to 0.075mm. It is not visible by naked eye. It is cohesionless and has very little plasticity.

Till:- It is an unstratified deposit formed by melting glacier. It is generally well graded.

Cohesive Soil:- Soils in which particles and absorbed water attract each other such that it shows plasticity at varying water content. this cohesive properties is due to presence of clay mineral. The term cohesive soil is used for Clays and Plastic Silt.

Cohesionless Soil:- It is composed of bulky grain are cohesionless in nature. The term cohesionless soil is used for Non-plastic Silt, Sand and Gravel.