What is black cotton floor

The Foundation in Black Cotton Soil / Expansive Soil Secret Revealed

In India, black cotton soil accounts for almost 15% of total soil deposition. This type of land is found in many other countries around the world. Previously, most Indian farmers grew cotton in black soil. Black soil is very favorable (good fertility) for the production of the plant of cotton and therefore this soil is especially known as black cotton soil in India and South Asia.

Essentially, the black cotton soil has fine-grained clay particles that cause a massive change in volume with changing humidity conditions; H. It swells excessively when wet and shrinks during the dry season. This is why black cotton soil is also known as expansive soil. Please note that all of the black cotton flooring is non-expandable and all of the flooring is not black. It is the expansive nature of soil that is challenging and creating problems, not color.

A foundation in black cotton soil is a risk and to avoid the risk, read here what will reveal the secret of foundation in black cotton soil.

Base in black cotton soil
Courtesy - Flickr

Why Foundation in Black Cotton Soil is a Risk?

The shrinkage and swelling of black cotton soil has become a serious problem for the entire construction industry. The swelling and shrinking of expansive soil cause the building's differential billing. It is very hard when dry, but completely loses its strength when wet. From this, the development of the cracks in the house emerges. If the building settles more, then it can become the reason for the failure of a building or a house, especially in the case of load-bearing structures. Therefore, in such soils it is necessary to either place the foundation deep enough that the effects of seasonal changes are not important, or to enable the foundation to eliminate the undesirable effects due to relative movement, either by flexible construction or by a rigid construction is created foundations. Sufficient pressure equalization swelling pressures also provide a satisfactory basis. Resting the foundation of the building on the black cotton floor is therefore a very demanding task.

What precautions should be taken while the foundation rests on black cotton soil?

In order to read the following precautions for better understanding, be sure to read the following 3 terms before reading the precautions.

Soil investigation: soil test

  • Before starting work on a site, it is good to evaluate the properties such as swelling pressure, shrinkage index, compaction (compressibility of the soil) and strength of the black cotton soil. It helps to determine the suitability of the floor or say black cotton floor for the construction of the building.
  • Usually according to IS 1893: 2016, if the N-value of the soil at a certain height is less than 10 in Zone III, IV or V and less than 15 in Zone II, then high buildings on such floors should be avoided or pile foundations or adequate soil stabilization should be carried out.

Source pressure

  • It is the pressure that the soil exerts on the soil when the soil is not allowed to swell or when the change in volume is stopped.

Shrinkage index

  • It is the measure of the degree of expansion of the soil.

Cracks appear due to the movement of the soil due to alternating swelling and shrinking. The cracks thus formed are sometimes 15 to 20 cm wide and 2.5 to 4 m deep. To avoid this, the following precautions are typically taken while the structure is being built on black cotton soil.

Precautions When Resting Foundation On Black Cotton Floor:

Below are the precautionary measures that are the same for both RCC frame structure and load-bearing structure. Here we differentiate between the precautions for low-rise and high-rise construction.

(A) Precautions for low-rise structure (RCC framed structure / load-bearing structure) on Black Cotton Soil:

01. Provide rounded piles or raft foundations (for special structures with a load-bearing capacity of more than 6 tons / m2) for the low buildings. Never lay the foundation on the black cotton floor. Place the foundation at least 300 to 500 mm below the level where the cracks in the ground stop. This is very important to evaluate on the website.

02. Provide minimum foundation depth such as SBC and stabilize the ground if necessary. Stabilization are the processes in which 5 to 7% materials such as lime, cement plant dust, etc. are added to the soil, which improve the load-bearing capacity of the black cotton / expansive soil.

Also read:

Dry process for the production of cement
Wet process for the production of cement

03. The ____ is provided by RCC connections on the building periphery on the base level, lintel and roof level, especially in the case of load-bearing structures. In the case of the RCC frame structure, the floor joists and the roof slab with joists themselves act as connections.

04. It is advisable to limit the total structure on the black cotton / expansive floor to 5.5 t / m². When the groundwater has access to the foundations, the exposure limit should be restricted based on its SBC.

Also read: Classification of loads on structure

05. If the black cotton soil is found in the upper layers, say 1 to 1.5 m, then all of the soil should be removed and the foundation should rest on hard solid layers of good soil.

06. Vegetation and planting of trees and shrubs next to the building should be avoided.

07. Special treatments such as horizontal and vertical moisture barriers should be provided to control moisture movement in the depth of the foundation. Some examples of the horizontal barriers are the surface pavement, the paved or buried impermeable membrane on the outer wall periphery. Commonly known as "plinth protection" and vertical barriers, include vertical trenches approximately 15 cm wide and 1.5 m deep that are filled with lean gravel concrete. or the mixture of granulated rubber, lime and fly ash. All moisture barriers should be provided with an adequate drainage system.

Strip footing with special treatments

08. Avoid direct contact of the black cotton floor with the foundation, pillar, masonry, etc. as it will damage it. It is therefore necessary to prevent the black cotton floor from coming into direct contact with brickwork below floor level. This can be achieved by digging a trench around the foundation and filling the space with sand or moorum. Remember that the layer of sand or moorum should be well watered and compacted.

09. The bed of the foundation trench should be made firmly in place by ramming. On the rammed bed, a 30 cm layer of good hard moorum should be spread in a 15 cm layer, with each layer well watered and rammed before the next layer is laid. On top of this layer, either a stone or sand bed should be provided at the desired height to place the foundation concrete base on top.

10. It is also advisable to avoid backfilling excavated black cotton soil in the plinth fill.

11. In the case of a load-bearing structure, the foot of the wall should be excavated 40 cm wider than the foundation width. Provide a layer of coarse sand at least 8 inches wide on each side of the foundation to support the subgrade by making direct contact with black cotton soil and foundation concrete.

12. Provide the raft foundation for the important structures as it will help the building float on the bed below. It is advisable that the raft foundation be fairly independent of the surrounding black cotton soil.

13. For less important structures such as composite wall, the width of a layer of sand on both sides of the foundation masonry should be 15 cm.

14. The masonry for walls should start at least 15 cm below the natural ground.

15. The construction in black cotton soil should be carried out in the dry season.

16. It is advisable to lay the foundation so deep that seasonal changes do not play a role. Provide the flexible type of construction or rigid foundations to eliminate the unwanted effects due to relative movement. A sufficient pressure equalization source pressure also provides a satisfactory basis.

17. Sometimes excavation for foundations in the water-bearing black cotton soil becomes a big problem for the civil engineers. It is advisable to use the "sand drain system" and the "electro-osmosis system" to drain the water. This system helps to control the water table and also helps to improve soil bearing capacity.

Sand drainage system and electro-osmosis system

(B) Precautions for the high-rise structure (RCC Framed Structure / Steel Framed Structure) on black cotton floor:

Provide raft foundation, under rubbed pile, or pile foundation for the high-rise structures. It is advisable that the foundation be fairly independent from the surrounding black cotton soil as it will damage the soil. This can be achieved by laying a trench around the foundation and filling the space of the trench with sand or moorum, or backfilling with yellow soil.

What Kinds of Foundation are Recommended in Black Cotton Soil / Expansive Soil?

As for the black cotton floor for one-story or two-story buildings, soil stabilization will come in handy. But for multi-story or high-rise buildings or super-high-rise buildings, the mat foundation or pile foundation will be useful.

What to do if good floor is not available say up to 5m?

Usually, foundations on black cotton soil are not advisable at all, for both load-bearing and RCC structures. But what to do if there is no good soil to lay the foundations at a reasonable depth of 2.0 or 2.5 m, or 4.0 m for one or two story buildings. If the layer of black cotton soil in the upper layers is 3 to 5 feet or more, how deep should you go for small buildings, or what type of foundations should you construct and at what depth? Because as you go deeper and deeper, the start-up costs will go up.

In such cases, the solutions need to be split based on

  • Low-rise building (G + 2 floors)
  • skyscraper

Do not go too deep in low buildings. One can either do soil stabilization with different techniques or use different types of foundations. In high-rise buildings, you have to choose special types of foundation systems.

The foundation transfers and distributes the loads to the ground in such a way that the load-bearing capacity of the foundation bed is not exceeded. There are many types of foundations that transfer and distribute the loads of the building to the ground. The following types of foundations are mostly used to build a building / house in the black cotton soil, depending on the soil conditions and the load on the structure.

01. Spread floor and wall coverings on black cotton flooring:

Spread floor and wall coverings on black cotton flooring

Propagation foundations and wall foundations are used where a layer of good soil (soil that can bear the load on the building or soil that has sufficient bearing capacity) is within 2 to 3 m (6 to 10 feet) of the natural surface. Propagation foundations and wall foundations should not be used if the high groundwater flow is in the depth of the foundation, as this can lead to abrasion. However, the load-bearing capacity of the floor must be sufficient to support the weight of the structure.

02. Under reamed stakes on black cotton soil:

Under framed stacks in black cotton bottom

An undermined heap is a pile of shallow depth (1 to 6 m) that has one or more tubers at its lower end. If this onion is used in a deeper layer than the critical depth of moisture movement in black cotton / expansive soil, the foundation will be anchored to the ground and will not move with the movement (shrinkage and swelling) of the soil. So if the soil is up to 1 to 6 m long black cotton / expansive soil, you can use the piles driven down (in situ concrete piles with a massive extension near the base) for the foundation of a low building. It is the safest and most economical way to lay the foundation in black cotton soil.

With this type of foundation, the building or house is anchored to the ground at depth, where ground movement due to changes in moisture content is negligible. When the water table is high, bentonite slurry is used to hold the sides of the borehole against collapse.

Based on research carried out at the Central Building Research Institute in Roorkee, India, the piles driven below are the ideal solution for the foundation in black cotton / expansive soil, especially for low structures.

One or more than one under driven piles with a pile cap can be used for a pillar, depending on the load on the pillar.

03. Mat foundation on black cotton floor:

Mat foundation on black cotton floor
Courtesy - carrasquillo staff

The mat foundation is suitable for black cotton / expansive floors, the load-bearing capacity of which is lower for the suitability of expansion foundations and wall foundations. It is also recommended and or economical when the area of ​​the floors becomes greater than 50 percent of the area of ​​the building. This is generally recommended for high-rise buildings. It would be very expensive for low buildings. Mat foundation should not be used if a high water table is found deep in the foundation, as this can cause it to wash out and liquefy.

04. pile of foundation in black cotton bottom:

Pile foundations are used to transfer the heavy loads from the structure to hard rock layers that are much deeper below the ground level and where shallow or matte foundations are impossible due to weak or extensive soils in the top layer. The pile foundation also prevents buildings from lifting from side loads such as earthquakes and wind. In general, the depth of strata of hard rock can be 5 to 50 m (15 feet to 150 feet) deep from the natural soil surface, depending on local topography and geology. The pile foundations withstand the stress caused by structural friction from skin friction and end bearings. The use of pile foundations also prevents the foundation of foundations.

05.Bored piles in black cotton soil:

You can use drilled shafts for the deep foundation of high-rise buildings where the depth of hard solid layers below the ground is 10 m or up to 100 m (25 feet to 300 feet). Drilled shaft pins are not suitable if there are deep deposits of soft clays and loose water-bearing granular soils. It is also not suitable where carved formations are difficult to stabilize, i.e. soil made up of boulders and artesian aquifer exit.

Also read:

Need for the Pile Foundation
Types and classification of pile foundation

In summary, since the capacity (SBC) of black cotton soil is very low, e.g. B. 4 to 6 ton / m², the foundation on such a floor is very dangerous for your building / house. Therefore, never create a foundation on the black cotton / extensive floor. Propagation foundations and wall foundations should be provided if good soils are found within 3 m of depth. In the case of heavily loaded structures, mat foundations should be preferred. You can also use under corrugated stakes if the black cotton soil is up to 4 to 5 m high or if the soil depth is more than 5 m. It is always advisable to see the structural engineer before building your home on black cotton flooring.

Must read:

Importance of building physics for professionals: Architects - Civil engineers - MEP Consultants
Permission to repair a house & when it is necessary!
Posted in Build a House