Soil
The word 'soil' has different meanings for different professions.
To the agriculturist, soil is the top thin layer of earth within which organic forces are predominant and which is responsible for the support of plant life.
To the geologist, soil is the material in the top thin zone within which roots occur.
From the point of view of an engineer, soil includes all earth materials, organic and inorganic, occurring in the zone overlying the rock crust.
Soil is defined as a natural aggregate of mineral grains, with or without organic constituents, that can be separated by gentle mechanical means such as agitation in water.
The behavior of a structure depends upon the properties of the soil materials on which the structure rests.
Origin Of Soils
Soils are formed by the process of weathering of the parent rock. The weathering of the rocks might be
by mechanical disintegration, and/or chemical decomposition.
The process of weathering of the rock decreases the cohesive forces binding the mineral grains and leads to the disintegration of bigger masses to smaller and smaller particles.
The properties of the soil materials depend upon the properties of the rocks from which they are derived.
Rock is defined as a natural aggregate of mineral grains connected by strong and permanent cohesive forces.
Mechanical Weathering
Mechanical weathering of rocks to smaller particles is due to the action of such agents as the expansive forces of freezing water in fissures, due to sudden changes of temperature or due to the abrasion of rock by moving water or glaciers.
Temperature changes of sufficient amplitude and frequency bring about changes in the volume of the rocks in the superficial layers of the earth's crust in terms of expansion and contraction.
Such a volume change sets up tensile and shear stresses in the rock ultimately leading to the fracture of even large rocks.
This type of rock weathering takes place in a very significant manner in arid climates where free, extreme atmospheric radiation brings about considerable variation in temperature at sunrise and sunset.
Erosion by wind and rain is a very important factor and a continuing event.
Cracking forces by growing plants and roots in voids and crevasses of rock can force fragments apart.
Chemical Weathering
Chemical weathering (decomposition) can transform hard rock minerals into soft, easily erodible matter.
The principal types of decomposition are hydration, oxidation, carbonation, desalination and leaching.
Oxygen and carbon dioxide which are always present in the air readily combine with the elements of rock in the presence of water.
Phase Diagram Of Soil
Chemical weathering (decomposition) can transform hard rock minerals into soft, easily erodible matter.
The principal types of decomposition are hydration, oxidation, carbonation, desalination and leaching.
Oxygen and carbon dioxide which are always three phase diagram is an artifice used for easy under standing and convenience.
A soil mass consists of solid particles, water and air. The three constituents are blended together to form a complex material
However, for convenience, all the solid particles are segregated and placed in the lower layer of the three-phase diagram
Likewise, water and air particles are placed separately; as shown in three-phase diagram.
It may be noted that the three constituents practically cannot be segregated.
ays present in the air readily combine with the elements of rock in the presence of water.
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