Black Cotton Soil is a cohesive soil. It is considered difficult or problematic soil for civil engineers. It possesses the characteristics of swelling during the rainy and shrinking during summer. In both situations, it poses difficulties.
Swelling is caused in Black Cotton Soil during the rainy season, the structure has uplift pressure and generates heave in the foundations, plinth beams, ground floors of the buildings and canals, roads surfaces, etc, and on shrinkage in the summer season, cracks are created in walls, slabs, plinth protection, floors, etc.
In the rainy season, Black cotton soil swells due to a higher percentage of clay. It swells during the rainy season and cracks in summer due to shrinkage. The cracks are generally in the range of 100 mm to 150 mm wide and 0.5m to 2m deep.
Black Cotton Soil
The Black Cotton soils are highly compressible and have extremely low bearing capacity. The soils have great shrinkage and swelling characteristics.
The shear strength of the soils is incredibly low. Black Cotton soil is also known as Expansive soil. Black cotton soil is the Indian name given to the expansive soil deposits.
A large part of central India and a part of South India covering Madhya Pradesh, Maharashtra, Karnataka, Tamil Nadu, South Gujarat, and Utter Pradesh is covered with Black Cotton Soils.
The covering is approximately 3,00,000 sq. km. These soils have been created from basalt or trap-rocks. These soils are very favorable for growing cotton.
The soils are highly compressible and have extremely low bearing capacity. It is greatly tough to work with such soils.
Lightly loaded structures are greatly susceptible to defects as a result of the volume changes in the soil. Under-reamed piles are considered largely suitable as foundations for buildings or houses and different light structures. These piles are carried to depths below the zone of seasonal deviation in water content.
Particular construction technology has been acquired traditionally in most of the places where such type of soil is found, yet, cracks are identified in several structures as sufficient precautions are not carried out during the construction and maintenance period of the buildings.
Read More: 11 Soil Stabilization Methods & Their Use
Black Cotton Soil Properties
The following are the properties of black cotton soil,
Black Cotton soils are black in the colour and also recognized as regur soil. These are prepared up of lava discharges. It is suitable for the cultivation of cotton and is also known as black soil.
This soil is formed of incredibly fine clayey material and is well-known for its moisture-holding capacity. They formulate deep cracks during the hot climate which enables the adequate aeration of the soil.
These soils are sticky when moist and tough to work on. They enclose the plateau of Maharashtra, Saurashtra, Malwa, MP, and Chhattisgarh and extended in the southeast direction along with the Godavari and the Krishna valley.
It is supposed that climatic conditions along with the parent rock substance are the significant factors for the formation of black cotton soil. Black Cotton Soil is fine textured and clay in nature. It has great quantities of lime, iron, magnesium, and mainly low amounts of phosphorus, nitrogen, and organic matter.
It has up to 50% clay content and accordingly is highly retentive of moisture. Because of the huge clay content, the soil expands when wet and becomes tough to plow through. During the arid season, this soil shrinks and formulate a huge crack which enables the circulation of air. The soil is incredibly fertile in most of the locations where it is found.
Black Cotton Soil Engineering Properties
Some of engineering black cotton soil properties are as follows,
| Property | Value |
| Dry Density | 1300 – 1800 kg/m3 |
| Liquid Limit | ( 40 – 120 %) |
| Plastic Limit | ( 20 – 60 %) |
| Specific Gravity | 2.60 – 2.75 |
| Maximum Dry Density | ( 20 – 35 %) |
| Compression Index | 0.2 – 0.5 |
Chemical Composition Of Black Cotton Soil
Some chemical compositions of black cotton soil is,
| Mineral | Value |
| Alumina | 10 % |
| Iron Oxide | ( 9 – 10 %) |
| Lime And Magnesium Carbonates | ( 6 –8 %) |
| Potash | < 0.5 % |
| Phosphate, Nitrogen, Humus | Low |
Types Of Black Cotton Soil
Based on thickness black cotton soil is divided into three types:-
- Shallow Black Cotton Soil
- Medium Black Cotton Soil
- Deep Black Cotton Soil
1. Shallow Black Cotton Soil
This type of soil forms with a thickness of less than 30 cm. It stands in Satpura hills (Madhya Pradesh), Bhandara, Nagpur and Satara (Maharashtra), Bijapur, and Gulbarga areas (Karnataka). This type of soil is desirable for the cultivation of jowar, rice, wheat, gram, and cotton.
2. Medium Black Cotton Soil
Its thickness ranges between 30 cm and 100 cm. It encloses a bigger area in Maharashtra, Gujarat, Madhya Pradesh, Tamil Nadu, and Andhra Pradesh.
3. Deep Black Cotton Soil
Thickness is greater than 1 meter. It encloses huge areas in the lowland zones of Peninsular India. The clay volume ranges between 40 % to 60 %.
Its reaction is alkaline. The soil is fertile and desirable for crops of cotton, sugarcane, rice, citrus fruits, vegetables, etc.
Read More: Water Content of Soil Test – Procedure, Result & Report
Advantages Of Black Cotton Soil
Advantages of black cotton soil are listed below;
- As Black Cotton Soil is relatively fertile in nature, they are extremely useful in cultivation.
- They are resistant to wind and moisture as they possess an iron-rich granular structure.
- Black soil is great moisture-retentive, so it can help in rainfed agriculture.
- The cracks created by black soil can be utilized as a passage for rainwater.
- It has a great bearing capacity in an arid state.
Disadvantages Of Black Cotton Soil
Disadvantages of black cotton soil are as follows;
- Black Cotton Soil loses its bearing capacity when it receives moisture.
- Expected to its swelling and drying nature, it is extremely difficult to manage the soil.
- The building is created on this soil is not safe as the rise in its volume expands from 20% to 30% of the actual volume and this affects cracks in the foundation.
- Increased stabilization should be achieved for any constructions it has different characteristics.
Why Black Cotton Soil Is Not Convenient For Foundation?
As per the description, the Black Cotton Soil contains swell and shrink properties. Due to shrinkage and swelling, a difference in the volume of soil occurs which formulates cracks in the foundation or the building structural system.
The cracks are reducing the durability and strength of the structure. And greatly the cases of failure of structure occur due to cracks.
Similarly, the load-bearing capacity of black cotton soil is extremely less which is affecting several types of foundation failure like punching failure, general shear failure, etc.
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