Black cotton soil is a very good soil from an agricultural point of view, but very bad from the structural point of view. This soil when dry is quite strong but its bearing capacity reduces to less than 5 t/m2 when wet. This soil undergoes considerable change in volume due to a change in its moisture content. The variation in the volume of this soil is as much as 20% to 30% of its original volume.
This soil develops very wide and deep cracks due to excessive shrinkage when it is made to dry from the wet condition. It becomes too soft to carry any load and swells enormously when dry soil is saturated with water. The cracks developed in drying black cotton soil may be as wide as 20 cm and may extend to 2 to 4 m depth. Due to the peculiar behaviour of this soil, the construction of a foundation in them has always been a major headache for engineers.
Principle for Foundation in Black Cotton Soil
In such soils following three main principles should be adopted for the construction of any foundation:
- The foundation should lead to such a depth that cracks in the soil cannot reach the bottom of the foundation.
- Measures should be adopted that water cannot reach the bottom o of the foundation.
- Foundation of the structure is prevented from coming in direct contact with the black cotton soil by introduction of a layer of sand or of some other granular material, at bottom and faces of the foundation.
Methods of Foundation in Black Cotton Soil
If the depth of the black cotton soil is 1 to 2m, the soil should be completely removed and the foundation should be laid on the hard soil below it. If depth of black cotton soil is considerable and it is not feasible to remove it completely, any one of the following two measures can be adopted.
(a) R.C.C. cap on pile-heads, then boulders and then masonry wall.
(b) Sand layers on moorum layer and then cement concrete layer below brick masonry.
(c) Moorum layer on boulders, then cement concrete and lastly masonry.
This method is suitable when depth of black cotton soil is considerable and rainfall is not large and it is sure that water cannot penetrate the soil for a depth of say 1 to 1.5 m. Procedure of foundation construction is as follows:
Excavate the foundation 300 mm in excess of the possible depth of the cracks.
Width of the trench should be such that the intensity of pressure on the soil does not exceed the safe bearing capacity which is taken as 7.5 t/m2 (75 kN/m2) in case water is not likely to reach the foundation. But if it is likely that water may reach it, its safe bearing capacity is taken 4.9 t/ m2 (49 kN/ m2). In this case since water cannot reach the foundation, safe bearing capacity of 7.5 t/m? (75 kN/ m2)may be adopted.
The trench of the foundation is rammed well and gravel layer is spread in the trench and it is also rammed into the foundation.
A layer of concrete about 300 to 500 mm deep is spread on the rammed gravel base. .
Now masonry work of the foundation is started on top of the concrete layer and carried up to the plinth level.
Open space left in the faces of the foundation masonry walls and foundation trenches are filled with sand. Arrangement of foundation is shown in Fig-2 (b) and Fig-3.
This method is suitable where depth of black cotton soil is quite large and rainfall conditions are such that rain water can teach the foundation soil and may affect it.
In this case foundation trench is excavated about 2 m deep and side portions of the trench filled with concrete, each having a section of about 250 mm x 250 mm. The hollow space left between the concrete blocks is filled with sand.
On the top of cement concrete blocks and sand filled in them, a R.C.C. slab 120 mm to 150 mm thick is constructed, covering the full width of the trench. Foundation masonry work can be started directly on the top of R.C.C. slab. Open space left between trench faces and foundation masonry is filled with sand as is shown in Fig-4.
To drain off the water which has reached the foundation, 80 mm diameter inclined pipes are embedded in the foundation at an interval of 1 m to 1.5 m. These pipes are led to the plinth level and filled with sand. They are also plugged from the top. These pipes should be checked from time to time.
The two methods discussed above are not the only methods of construction of foundation in black cotton soil. There may be many more arrangements. Some of the arrangements are shown in Figs. 1.33 and 1.34. In case black cotton soil is for a very large depth and load to be carried by the structure is very heavy and also water table is likely to rise right up to the surface of the ground, the foundation may be constructed on piles.
In this case, the number and depth of piles depend upon the load to be transmitted and the location of the hard bed below the ground level. Piles are driven deep to rest upon the hard bed. All the piles are then grouped together into a single bearing base by providing a pile cap connecting all the pile heads. See Fig-2 (a). The usual foundation may be constructed over the pile cap while the space around the foundation block is filled with sand.
All the structures resting on black cotton soil are always likely to suffer from the difficulty of differential settlement. To prevent the cracking of the structure, R.C.C. tiles or bands 100 mm to 150 mm deep should be provided at plinth level, lintel level and eaves level. If R.C.C. roofs are provided over the structure, ties at eaves level may be eliminated.
In case of less important structures, width of the foundation trench should be at least 1.3 m, out of which 1 m is the width of foundation bed and 150 mm width is left on both the sides to be filled with sand. In the case of important structures, the width of the trench should be 0.9 m to 1.2 m larger than the width of the foundation block.
Under-reamed pile foundation is the most successful foundation in regions of black cotton soil. It not only bears the heavy load successfully but also keeps structures safe against the heaving effect of swelling black cotton soil during heavy rains. It keeps structures anchored to the foundation because of concrete bulbs developed in them.
I hope this article will help you to understand the Problem of Foundations in Black Cotton Soil and its Solutions. You may also want to see my other post from my Blog. If I have missed anything here, please let me know about that in the comment below this post.
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