Classification of Soil

The earth is covered with different variety of soils. It is desirable to systemize or classify the soils into broad groups of similar behavior. it is more convenient to study the behavior of groups than that of individual one. Soil may be classified on the basis of following parameters:

1) Classification based on mode of origin.

2) Classification based on composition.

3) Classification based on particle size.

4) Classification based on particle size and plasticity characteristics of soils.

5) Classification based on shear strength.

Classification based on composition

This classification is mainly based on chemical constituents present in the soil. It includes the following types:

a) Black soil

b) Lateritic soil

c) Pedzol

d) Peat soil

a) Black soil

The clayey soil having colored and formed from the basic igneous rock like. Basalts is a example of black soil. It mainly consists of iron, aluminum , magnesium, calcium oxides and also some organic matter. Its capacity to retain water is high. Also it swells considerably coming in contact with water. It is highly fertile and well known for growing cotton.

b) Lateritic Soil

It is brown colored soil derived from laterite and mainly consists of iron and aluminum hydroxide having a small percentage of manganese hydroxide, titanium oxide, silica, calcium, magnesium etc. it is not much suitable for agricultural works.

c) pedzol

it is a sandy soil of light grey color, with very small percentage of iron and organic content. Being deficient in nutritions components, this soil is not good for agricultural works.

d) Peat soil

This is a dark brownish colured soil formed by the decomposed vegetation matter. Thus, it is rich in organic content with very little sand and clay etc. it is highly porous with good water retaining capacity. For agricultural works it is not very fertile. Being soft and weak it readily subsides under loads.

Classification based on shear strength

On the basis of shear strength, soil can be classified as:

1) Cohesionless soils

2) Purely cohesive soils

3) Cohesive-frictional soils

1) Cohessionless soil

These soils derive the shear strength from the intergranular friction. It has no cohesion i.e. c′= 0. These soil are also called frictional soil. Example: sand and gravels.

2) Purely cohesive soils

These are the soils which exhibit cohesion but the angle of shearing resistance φ =0. These soils are also called φ_u =0 soil.

Example: saturated clays and silts under undrained conditions.

3) Cohesive-frictional soils

These are composite soils having both c′and φ′. These are also called C – φ soil.

Example: clayey sand, silty sand, sandy clay etc.

But, sometimes cohesive-frictional soils are also called cohesive soils. Thus any soils having a value of c′ are called a cohesive soil.