Soil mechanics,Formation of soils, Classification of soils,Types of soils, Soil mechanics

 Soil 

The term soil has various meaning, depending upon the genral professional field in which it is being considered. To an agriculturist, soil is the substance existing on the Earth's surface, which grows and develop plant life. To the geologist also , soil is the material in the relatively thin surface zone within which roots occur, and all the rest of the crust is grouped under the term rock irrespective of its hardness.

Formation of soil 

Soils are formed by the disintegration and decomposition of rocks. The disintegration of rocks is caused by .

Mechanical weathering. 

Chemical decomposition.

Biological decomposition.

Mechanical weathering.

It is also known as physical weathering.It can be as follows.

(i)- Plant roots..

Plants and tress grow in rocks.The roots of these tree and plants enter into the cracks and fissures of the rocks .Over the period of time these roots become thicker and induce stresses on the rock causing disintegration.

(ii) Frost 

Water enters into the cracks and fissures of rocks during rain.In cold climate,water gets frozen and increase in volume.Due to increase in volume stresses are induced in the cracks causing disintegration of rocks.

(iii) Thermal expansion.

Due to temprature variation,the mineral present in the rocks expand and contract. Stresses are developed due to repeated expansion and contraction of rocks, resulting in disintegration and formation of soil.

(iv) Abrasion.

Abrasion of rocks occur under the action of flowing water ,moving ice , blowing wind . this abrasion results in the formation of soil. 

 

Chemical Decomposition 

Chemical decomposition depends on the pressure of water, temprature and dissolved materials in water. chemical weathering is dependent on available surface for reaction, temprature and presence of chemically active fluid. The  following processes are involved in chemical weathering.

(i)- Oxidation 

It is the process in which oxygen ion combines with ferrous oxide. Iron containing rocks are subjected to chemical decomposition by oxidation.

(ii) Hydration 

Hydration is the process in which rock minerals combine with water to form a new compound which will be different from the parent minerals. The decomposition of rock takes place due to change in volume which create stresses within the rock. 

(iii) Carbonation 

In this process Carbon di oxide present in the atmosphere combine with water to form carbonic acid .This carbonic acid reacts with rock minerals causing decomposition. 

(iv) Leaching 

Leaching is the removal of soluble materials by dissolving them away from solids. In this process some minerals dissolved out of the rocks and deposited separately causing decomposition. 

Biological decomposition 

The breakdown of organic matter in soils is done entirely by the microorganisms. Bacterias and other microorganisms induce chemical changes in their surroundings by producing organic acids, which helps in weathering of soils. 

Classification of soil  

 soil can be classified into two groups 

(1)- Residual soils.

(2)- Transported soils. 

Residual soils 

Soils which remains at the place of its formation are called residual soils. The thickness of residual soils depends upon the climate,time and type of source rock.These soils are found directly over the parent rock..

Transported soils .

Transported soils are soils which are carried away from their place of formation to some other place by the transporting agencies.Transporting agencies may be glaciers,water, wind or gravity. So the soils are classified as :

(1) Glaciers transported Soils.

(2) Water transported Soils 

(3) Wind transported Soils 

(4) Gravity Deposited Soils

(1)- Glaciers transported soils. 

In the geological past vast area of earth's crust were cover by glaciers. The glaciers tend to move slowly from the ploes towards the equator. It is estimated that almost one fourth of the total land area of the earth has been at same time or other subjects to glaciation. 

Glacier transported soils include matterials transported and redeposited by galcier or by ice waters following from glaciers.Glaciers bring with them huge quantities of mineral debris which mix with ice and carried away. These types of deposits are differentiated by using the following terminology.

(a)- Moraine .Moraine are glacial material deposited by ice and not by melt waters.

(b)-Drift: Drift are the materials transported by glaciers and deposited from the melt water.

(c)- Till: Till are unstratified , heterogeneous mixtures of clay or silt directly deposited by ice 

(2) Water transported soils 

.The soils which transported and deposited by action of water is known as water transported soils.

Soils carried by water are classified as:

(i)- Alluvial Soil.

(ii)- Lacustrine Soil.

(iii)- Marine Soil.

(i) Alluvial Soil: 

When the soil transported from its origin by the flowing water and deposited along the stream is known as alluvial soil.

These soils are very common and a very large number of engineering structure are built them. 

(ii) Lacustrine soils 

The soil transported by flowing water and deposited in lakes is called Lacustrine soil.Most 

Lacustrine soils are primarily silt and clay.their  suitability for foundation ranges from poor to average. 

(iii)- Marine soils 

The soil transported by flowing water and deposited in ocean is known as marine soil 

Marine soils are primarily silts and clays and are very soft.   

(3) Wind transported Soils (Aeolian soils)

Just as running water transports soil ,so does the wind . wind is air in motion. Soils which are transported by wind are called Aeolian soils. These are sub class into Sub classed into dune sand and loess.

Soils transported and deposited by wind are called aeoline soils .These soils are usually very loose and have fair engineering properties.Aeolian soils are of two types 

(a) Loess: 

loess is deep deposits of silts made by wind.such deposits are often found downwind of deserts. These deposits have a very high porosity .Loess is fairy strong when dry, but become weak when wetted.

(b) Sand dunes: 

A dune is a mound of sand formed by the wind, usually along the beach or in a desert. Dunes form when wind blows sand into a sheltered area behind an obstacle. Dunes grow as grains of sand accumulate.The low irregular hills formed by accumulations of sand along some beaches and in some desert areas are called sand dunes.

(4) Gravity Deposited Soils.

Gravity deposited soils are loose soil or rock fragments transported down slope under the action of gravity and deposited on or near sloping ground .These soils are also known as colluvial soils. 

The rock debris accumulated at the base of a hill, also known as talus ,is  termed colluvial soil. The cause of movement here is gravity. Initially , therefore ,the rock particles tend to assume the angle of repose of the broken mass. Gradually, this will flatten out due to wind and water. 

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