Building a foundation in black cotton soil requires careful consideration due to its expansive nature. To minimize potential issues like differential settlement, it’s advisable to use deep foundations, such as pile foundations, to reach stable strata below the expansive soil.
A foundation/footing is a lower portion of building structure or substructure that transfers its all coming loads of structure to the ground. The design of size and depth of foundation depends on the total load on the building and the type of sub soil and its bearing capacity.
Foundations are generally broken into two categories mainly, shallow foundations and deep foundations.
Foundation with depth less than 3 meter are shallow foundations. Such foundations are used for structures that do not carry much load. They are also used in case the soil has high bearing capacity. Examples of Shallow Foundation is isolated footing, combined footing, strap footing and raft footing.
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Foundations with depth greater than 3 meter are deep foundations. They are deep below the finished ground surface such that their base bearing capacity is not affected by surface conditions. Examples of deep Foundation is Pile and piers formation.
Black cotton soil (BC soil) is a highly clayey soil. The black colour in Black cotton soil (BC soil) is due to the presence of titanium oxide in small concentration. The Black cotton soil (BC soil) has a high percentage of clay, which is predominantly montmorillonite in structure and black or blackish grey in colour.
The black soil is majorly found in the parts of Gujarat, Maharashtra, Western parts of Madhya Pradesh, North- Western Andhra Pradesh, Karnataka, Tamil Nadu, Rajasthan, Chhattisgarh, Jharkhand up to Raj Mahal hills.
Black cotton soil absorb much water due to higher percentage of clay, it swells during rainy season and cracks in summer due to shrinkage. Hence foundation in black cotton soil need special care. These shrinkage cracks are 100 mm to 150 mm wide and 0.5m to 2m deep. Swelling creates upwards pressure on the structure and shrinkage creates downward pull. It results into cracks in foundations wall and roof.
Foundation depth in black cotton soil | Good foundation type in black soil
In India states of Gujarat and Maharashtra percentage of black cotton soil deposition is about 50% of mainland of India, also found in other countries of the world. most of Indian farmer grew cotton in black soil, black soil is very much favourable for producing plant of cotton hence it is known as Black cotton soil.
Black cotton soil have huge amount of fine grained clay material which obsorve massive amount of water and swell in rainy season and shrinkage during dry season. These shrinkage cracks are 100 mm to 150 mm wide and 0.5m to 2m deep. Swelling creates upwards pressure on the structure and shrinkage creates downward pull. It results into cracks in foundations wall and roof.
Building a foundation in black soil is risk and avoid to risk read this article and understanding the mystry of foundation in black soil.
Foundation depth in black cotton soil:- Generally, we provide depth of foundation in black cotton soil is more than 3m, deep foundation of spread footings/wall footings, under reamed piles or raft foundation (for special structure whose load is greater than 6 tone/sq.m) for the low-rise buildings.
Never rest the foundation on the black cotton soil. If the black cotton soil is found in the top layers, say 1 to 2m, then entire soil should be removed, and foundation should be rested on hard firm strata of good soil or Lay the foundation minimum 300mm to 500mm below the level where the cracks cease in the ground. This is very important to assess on the site.
Minimum depth of foundation in black cotton soil:– The minimum depth of foundation in black cotton soil should be at least 1.5m, Foundation of footing trench is made wider than required and the extra width is filled with granular materials of loam & sandy soil to prevent intimate contact of black cotton soil with the foundation of footing concrete structure.
Why black cotton soil is not suitable for foundation? Black cotton soil absorb much water due to higher percentage of clay, it swells during rainy season and cracks in summer due to shrinkage. These shrinkage cracks are 100 mm to 150 mm wide and 0.5m to 2m deep. Swelling creates upwards pressure on the structure and shrinkage creates downward pull. It results into cracks in foundations wall and roof. That’s why it is not suitable for foundation.
What are the problems of foundation on black cotton soil? Building a foundation in black cotton soil is risky . The swelling and shrinkage of expansive black cotton soil cause the settlement of building. When dry, it is very hard have large crack pull the structure downward and it loses strength during swelling in rainy season results upward pressure on structure, this leads development of the cracks in foundation and roof of building.
Which type of foundation is used in black cotton soil? The mat foundation or raft foundation of footing is used in black cotton/expansive soils whose bearing capacity is less, deep foundation of spread footings/wall footings, under reamed piles or raft foundation (for special structure whose load is greater than 6 tone/sq.m) for the low-rise buildings. If the black cotton soil is found in the top layers, say 1 to 2m, then entire soil should be removed, and mat or raft foundation should be rested on hard firm strata of good soil or Lay the mat foundation minimum 300mm to 500mm below the level where the cracks cease in the ground. This is very important to assess on the site.
Which Foundation is best for black cotton soil? The mat or raft foundation is suitable for black cotton/expansive soils whose bearing capacity is less for suitability of spread footings and wall footings. Make trench, soil should be excavated depth up to 1.5 to 1.6m deep where crack is seased. Width should be 450 mm wider, extra width should be filled of cement concrete, and depth should be filled with 150mm with sand, then layout mat footing making rcc.
Which type of concrete pile is most suitable for black cotton soil? safe,economical and most suitable pile foundations in expansive soils such as black cotton soil, filled up ground and other types of soils having poor bearing capacity is under-reamed piles, it is bored cast-in-situ concrete piles having bulk shaped enlargement near base. These piles are commonly recommended for providing foundation of footing in black cotton soil.