Black Cotton Soil: Exploring Properties and Foundation Types for Black Cotton Soil

Black Cotton Soil

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 Definition

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:-

1. Shallow Black Cotton Soil
2. Medium Black Cotton Soil
3. 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.

Type Of Foundation In Black Cotton Soil

There are five main types of foundations utilized in black cotton soil foundation construction,

1. Strip Footing or Wall Footing
2. Inverted Arch Footing
3. Raft or Mat Foundation
4. Under reamed pile
5. Pier Foundation
6. Drilled Shafts Piles
7. Pile Foundation

Read More: Water Content of Soil Test – Procedure, Result & Report

Designing The Footing On Black Cotton Soil

Black cotton soils and different expansive soils are dangerous due to their shrinkage and swelling aspects.

In addition to this, these soils possess a very low bearing capacity, ranging from 49.05 KN/m² to 98.10 KN/m². In designing the footings on these types of soils.

The following points should be remembered in the mind

The safe bearing capacity should be appropriately determined, carrying into account the impact of sustained loading.

The long-term outcome of loading effects in slow consolidation. In lack of the tests, the bearing capacity of these soils may be particular to 49.05 KN/m² to 98.10 KN/m².

The foundation should be carried at Least 50 cm deeper than the depth of moisture movement. This depth should also be extensively more than the depth of tension cracks.

Where this soil exists exclusively in the top layer, and where the thickness of this layer does fol exceed 1 to 1.5 m, the whole layer of Black Cotton Soil (or different expansive soil) should be removed, and the foundation should be established on non-shrinkable non-expansive soil.

Where the depth of the clay layer is massive, the foundation or footing should be prevented from entering in contact with the soil.

This can be executed by excavating a wider and deeper foundation trench and interposing a layer of sand/monogram around and beneath the footing.

Where the soil is extremely expansive, it is relatively important to have the lowest contact between the soil and the footing.

This can be nicely achieved by transferring the loads through deep piles or piers and by supporting wall weights on capping beams that are kept some distance (generally 5 to 15 cm) above the ground surface, to allow unrestricted expansion of the soil.

Where the bearing capacity of soil is low, or soil is extremely soft, the layer of the foundation trench should be created firm or hard by ramming mom and ballast mixed.

The foundations should be established during the dry season. Similarly, desirable plinth protection around the outer wall should be created on the ground surface, with its slope out from the wall, so that water does not penetrate the foundation during the rainy season.

Precautions While Laying The Foundation On Black Cotton Soil

Following are some precautions which are similar for both the RCC framed structure and load-bearing structure. Here we distinguish the precautions for low-rise structures and high-rise structures.

Precautions For Low-Rise Buildings (RCC Framed Structure And Load Bearing Structure) On Black Cotton Soil:

Provide spread footings/wall footings, under-reamed piles, or raft foundation (for a particular structure whose load is higher than 59 KN/m²) for the low-rise buildings.

Never lay the foundation on the Black Cotton Soil. Rest the foundation minimum 300 mm to 500 mm below the level where the cracks cease in the ground. This is very essential to analyze on the site.

Provide minimum depth of foundation as per Soil Bearing Capacity and stabilize the soil if required. Stabilization is the procedure in which 5  to 7 % of materials like lime, cement plant dust, etc. are added to the soil which enhances the bearing capacity of the black cotton soil/expansive soil.

Provide the RCC ties on the boundary of the building at plinth level, lintel level, and roof level, mainly in the case of the load-bearing structure.

In the case of the RCC framed structure, the floor beams and roof slabs with beams themselves act as ties.

Prevent the direct contact of the black cotton soil with the column, masonry work, foundations, etc. as it will injure it.

Thus it is necessary to avoid direct contact of black cotton soil with masonry work below ground level. Greenery, vegetation, and the planting of trees and shrubs near the building should be avoided.

Particular procedures like horizontal and vertical moisture barriers should be given to prevent moisture movement within the depth of the foundation.

The bottom of the foundation trench should be created firm tough by ramming it well. On this layer either sand bed or stone should be allotted to the desired height to place the foundation concrete bed block upon it.

In Black Cotton Soil, it is advisable to stop the backfilling of excavated plinth refilling. In the case of a load-bearing structure, the wall footing should be dug 40 cm broader than the width of the foundation.

Allocate at least a minimum 20 cm broad layer of coarse sand on either side of the foundation to segregate the substructure from maintaining direct contact with black cotton soil and foundation concrete.

Provide the raft foundation for the significant structures as it will enable to float of the building on the bottom below. It is advisable that the raft foundation should be relatively independent of the surrounding black cotton soil.

In the case of a limited significant structure like the compound wall, the width of a sand layer on either side of foundation masonry should be 15 cm.

The masonry for walls should be begun at least 15 cm below the normal ground level. Foundation construction in black cotton soil should be conducted in the dry season.

It is advisable to rest the foundation at such a deep where the impacts of seasonal differences are not significant. Allot a flexible type of construction or rigid foundations to exclude the unpleasant impacts due to the relative movement.

Precautions For High-Rise Building (RCC Framed Structure And Steel Framed Structure) On Black Cotton Soil

Precautions for High-Rise Building are given below;

Provide pile foundation, raft or mat foundation, or under-reamed pile for the high-rise structures. It is advisable that the foundation should be relatively independent of the surrounding black cotton soil as it will injure it.

To avoid such develop a trench around the foundation filling space of trench with murum or sand or backfilling with yellow soil.

Replacement Of Black Cotton Soil

This is the easiest technique in which Black Cotton Soil is replaced with some different suitable soil like a combination of moored and sand with cohesive soil to prevent shrinkage characteristics or water-bound macadam mix and compacted at Optimum Moisture Content. 

It is always asked up to what depth, black cotton soil should be replaced? The answer to this question is to replace all the black cotton soil up to whatever depth it exists. 

However, there are two problems with this answer, one, whether it is economically possible and two, whether it is technically needed.

Black cotton soil possesses swelling and shrinkage characteristics up to a specific depth called an active zone. The soil of this region just swells and shrinks.

In case, this soil is replaced with different types of convenient soil and the replaced soil is properly compacted, the soil would not have swelling and shrinking characteristics.

Read More: Specific Gravity of Soil Test with Sample Report

Preventing Water From Entering In Contact With Black Cotton Soil

It is not often economically achievable to replace all the Black Cotton Soil. Thus, damages require to be avoided by preventing moisture from coming into contact with foundation soil or soil below floors

It is known that the soil which is 1.5 to 2 times the width of the foundation below the footing level carries the load.

Therefore, if the width of the foundation is 1.2 m and the depth is 1.2 m, the impact of pressure would be between 3 m to 3.6 m. 

The zone up to which the soil gets affected for swelling and drying is known as the active zone.

The water table in the regions of black cotton soils is deep hence it is the rainwater and stormwater that is to be avoided from appearing in contact with black cotton soil of the active zone.

Rainwater can come in contact with the black cotton soil of the foundation from the following details:-

• Water from rainwater pipes.
• Water from open courtyards.
• Water from cracked drains was constructed near the foundation of the buildings.
• Water from broken roadside drains.
• Water arrived through the joint between the plinth conservation and the walls.
• Water appears into contact from the outside unpaved area.
• From the roots of plants/trees planted near the foundation.

Thus, it is important that the water arriving on the surface and seeping into the soil below the foundation or floors from the above points is acquired care of in such a manner that either water does not arrive into the black cotton soil or flow path of the water is lengthened in a way that water does not arrive into the active zone.

The water flow and length of flow way depending on several parameters like the intensity of rain, the amount of water, permeability of the soil above footing level, length of the horizontal flow way of water, length of the vertical direction of flow, and the vegetation in the vicinity, etc.

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