Black cotton soil – Properties, Chemical composition, and Stabilization

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What is black cotton soil?

Black cotton soil is very favorable for the cultivation of cotton. It is called black cotton soil because it is black in color formed by the presence of titaniferous magnetite.  

Black cotton soil is the clay-rich soil i.e. it contains calcium, carbonate, potash, and hold moisture and is mainly formed in the tropics and subtropics region.

Black cotton soil is also rich in lime, iron, magnesium but contains a low amount of phosphorous, nitrogen, and organic matter. So, it is more fertile in low lands than on the uplands.

We can see the cracks in many lands having black soil, this is because during the dry season they form the crack for the circulation of the air.

Though it is very good soil for cultivation but is problematic soil for civil engineering work due to its swelling and shrinkage property.

Black cotton soil - Properties, Chemical composition, Stabilization for Construction

Properties of black cotton soil

  1. Black soil has a texture like clayey and is highly fertile.
  2. Black soil structure is cloddish or sometimes friable.
  3. Black soil when dry gets a contract and develops deep wide cracks.
  4. Black soil expands when they are wet and they are hard to plow.
  5. Black soil contains almost 50% of clay and can hold water for a long time.

Engineering properties

Black cotton soil - Properties, Chemical composition, Stabilization for Construction

Some of its property are:

PropertyValue
Dry density1300-1800 kg/m³
Liquid limit(40-120) %
Plastic limit(20-60) %
Specific gravity2.60 – 2.75
Maximum dry density(20-35) %
Compression index0.2 – 0.5

Chemical composition of black cotton soil

MineralValue
Alumina10%
Iron oxide(9-10) %
Lime and magnesium carbonates(6-8) %
Potash< 0.5%
Phosphate, nitrogen, humuslow

Types of black cotton soil

On the basis of thickness black soil is divided into:

  1. Shallow black soil.
  2. Medium black soil.
  3. Deep black soil.
  • Shallow black soil:

This type of soil is suitable for the production of the gram, wheat, cotton, and rice. They are mainly found with a thickness of less than 30cm. 

  • Medium black soil:

Its thickness ranges from 30cm to 100cm.

  • Deep black soil:

Its thickness is measured more than 1m and is found in low lands.

Deep black soil is very fertile and in the field of crops, it is suitable for rice, cotton, sugarcane, citrus fruits, etc. It contains 40% -60% of clay.

Formation of black soil

Black soil is a sedimentary type of soil that is found in place of its origin, i.e. it is not transmitted from its original place. It is formed by the wear and tear of the specific rock.

The moderate climatic condition and volcanic erupted igneous or basalt rock as a parent rock is required for the formation of black soil.

Then, due to the weathering or breaking of the igneous rock and cooling and solidification of the lava, the black soil is formed. Since, it is formed from lava it is also known as lava soil.

Identification of black soil in the field

  • It is black in color.
  • It is a very fine particle that can’t be seen by the naked eye.
  • Cracks of high width are formed in the summer season.
  • In the rainy season, it looks muddy.
  • It is sticky when wet.

Advantages of black soil:

  • As they are very fertile in nature, they are very helpful in cultivation.
  • They are resistant to wind and water as they have an iron-rich granular structure.
  • Black soil is high moisture-retentive, so it can help in rainfed agriculture.
  • The cracks formed by black soil can be used as a passageway for rainwater.
  • It has a high bearing capacity in a dry state.

Disadvantages of black soil:

  • it loses its bearing capacity when it gets moist.
  • Due to its swelling and drying nature it is very hard to manage the soil.
  • Building made on this soil is not safe as increase in its volume extends from 20% to 30% of original volume and this causes cracks in foundation.
  • High stabilization should be done for any construction as it has unusual characteristics.

Stabilization of black cotton soil

Black soil with lime:

Over the past few decades, lime is being used for stabilization of the black soil. The high plasticity of the black soil is reduced gradually after the use of lime which can make it workable.  And the reaction between soil and lime forms more strength resistance.

Black soil with brick powder:

When the brick powder is mixed with black cotton soil, the strength of the soil increases significantly. Brick powder with bagasse ash when mixed with black cotton soil improves the unconfined compressive strength of soil gradually.

Foundation in black cotton soil

Black cotton soil - Properties, Chemical composition, Stabilization for Construction

The following precaution should be taken if construction is done in black soil:

  1. Foundation loads should be limited to 5 tonnes/sqm.
  2. RCC raft foundation should be done.
  3. Foundation excavation should extend beyond the depth of cracks.
  4. For preventing intimate contact with black soil, the foundation trench width is made bigger than the requirement and the extra width is filled with granular materials.
  5. It is good to remove black soil if strata of black soil are up to 1200mm.

Construction of foundation on black soil:

  • Black cotton soil removal and granular material filling:

The depth and the thickness of black cotton soil can be determined by the cracks in the soil. If the depth is up to 1200mm the soil is removed. Foundation trench is made 150mm wider and depth of 1500mm below from where cracks cease.

The remaining depth and width of the foundation are filled with other materials like sand, bricks, fly ash, etc.

The foundation is rammed and the foundation bed is prepared over which normal foundation can be made.

  • Excavation of extra depth:

After the removal of black soil, the excavation is done up to 750mm extra depth. 

 Each side of the foundation is made 75 mm wider and single bricks with mortar is constructed in extra width.

Now, extra depth is filled with 450mm moorum and 300mm sand in the layer of 150mm and well rammed by iron rammer.

  •   Extra depth filled with boulder:

After the removal of black soil, the extra depth is filled with boulders and over which the foundation can be laid.

For the compound wall foundation, sides are filled with sand for 1500mm, and for the main wall of the building, extra sides are filled with 450 to 600 mm with sand.

I hope this article remains helpful for you.

Happy Learning – Civil Concept

Contributed By,

Civil Engineer – Shreya Parajuli

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