The word SOIL is derived from latin word Solium which according to Webster’s dictionary, refers to a loose upper layer of the earth where plants can grow. The soil is a mixture of unconsolidated material, composed of solid particles produced by disintegration of rocks. The voids in soil are filled with air or water or both. Soil may contain organic matter.


Soil Mechanics –

A German Scientist Dr. Karl Terzaghi defined soil Mechanics in his book Erdbaumechanic. He quotes,

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, regardless of whether or not they contain an admixture of organic constituents.


Hence, it is a branch of mechanics dealing with the action forces on soil and water flow in soil.

Process of Soil Formation –

The process of soil formation is very simple. A parent rock is disintegrated into soil particles due to chemical or physical action.


Physical Disintegration –




  • Temperature changes – Different minerals in rock have a different coefficient of thermal expansion. With changes in temperature, expansion and contraction of minerals can cause detachment of particles from parent rock. The soil is formed.
  • Abrasion – Movement of water ice and wind occurring over the surface of rock causes abrasion and scouring. It results in the formation of soil.
  • Wedging action of ice – Water flows into the minor cracks of rock and due to fall in temperature it freezes. The ice in the cracks expands and the cracks are widened.
  • Plant Action – Plant roots are also a reason of splitting of rock into pieces. Roots grow into the cracks where they get moisture and tend to break the parent rock into smaller grains.




Chemical Decomposition –





  • When chemical decomposition of rock takes place it changes the composition of rock. The original mineral structure changes to the different structure due to chemical action. Thus the soil formed do not have same properties of the parent rock.
  • Chemical processes include Hydration, Carbonation, Oxidation, Solution etc
  • Chemical processes transform rocks into clay which imparts plastic properties into the soil.
  • Clayey soils are formed due to chemical decomposition.



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