What is Soil? Formation and types of Soil?

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What is soil?

The term "Soil" has different meanings depending on the field of general professional consideration.
To an agriculturist, Soil is the material on the surface of the earth which grows and produces plant life.
To the Geologist, Soil is the substance in the relatively thin surface area within which there are roots.
To an engineer, Soil is an unconsolidated agglomerate of minerals found on or near the earth's surface with or without organic matter through which and on which engineers build structures.
It covers the entire thickness of the earth's crust that is accessible and feasible for practical use as foundation support or building material.
The behavior of soil as a foundation or building material is heavily influenced by the following:

• Moisture content present in soil pores
• The fluctuation of the groundwater table
• Freezing and thawing phenomena
• Presence of organic matter
• History of the formation of soil
• Seismicity of area

Mechanical or attractive forces bind the particles of the soil together.
The binding strength of the soil is very low compared to the rocks.
The type of soil will differ between clay and gravel and even between cobble and boulders.
The topsoil, normally two feet deep, contains organic matter and is generally considered unacceptable for use in civil engineering.

Formation of soil:

Soil is generally formed by rock decomposition or disintegration (rock weathering) on or near the earth's surface through the action of many natural, physical, and chemical agents that often break them into smaller and smaller particles.

The weathering of rocks may be the following: 

1. Physical/mechanical weathering
2. Chemical weathering 

1.Physical/mechanical weathering:

It is the disintegration of rocks caused by changes in temperature, freezing and thawing, swelling, flowing water erosion, natural disasters (earthquake, land sliding, etc.) and animal activities including people. 

Soils formed by physical weathering retain parent rock minerals.

Coarse-grained soils (gravels, sands and their mixtures) are the physical weathering products

2. Chemical weathering:

Weathering caused by oxidation, hydration, carbonation, desilication, and leaching of rock minerals is known as chemical weathering.

The common soils formed by chemical weathering are different types of clay and organic soils (peat, muck, hummus, etc.).

Types of Soil Based on Particle Size:

Types of soil based on engineering considerations depend on particle size. As the engineering properties of soil change with particle size change, different names are given for different particle size ranges. The range of particle size defined for each soil type varies among agencies.

clay:

Composed of fine particles with a size of less than 0.002 mm. Flaky in the form with a large area of the surface. Have a strong attraction of inter-particles and therefore have sufficient cohesion. Poor permeability is vulnerable to swelling and shrinking. Typically the color is brown.

silt:

Composed of particles between 0.002 and 0.06 mm in size. Have high capillarity and dry strength very low. The intermediate particle size between clay and sand, therefore, has sand and clay properties, i.e. it shows slight friction and friction as well. The silty soil color is mostly brown.

Sand:

Particle size from 0.06 to 2 mm, in shape, can be rounded to angular in color brown. No plasticity, high resistance in a confined environment and significant resistance to friction. angular particles have a high resistance to friction compared to rounded particles. It is extremely permeable and has low capillarity.

gravel:

The particle size ranges between 2 and 60 mm. Form good material for the foundation. Show high resistance to friction. Angular particles have a high resistance to friction compared to rounded particles. The gravel produced by rock crushing is angular in shape, while those taken from the riverbeds are sub-rounded to rounded.

Cobbles and Boulders:

Particles larger than gravel are commonly referred to as cobbles or boulders. Cobbles vary between 60 and 200 mm in length. Boulders are the material greater than 200 mm.

Soil types According to ASTM and AASHTO:

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