Updated: Sep 12, 2021
By: Lochan Yadav
BLACK COTTON SOIL
Black cotton soil has a tendency to shrink and swell excessively. It is basically due to the presence of a compound named montmorillonite. When these type of soil come in contact with water, they swell and when becomes dry, it shrinks. This alternate process of swelling and shrinking results in the differential settlement of the foundation which in turn causes cracks in the building.
The cracks thus formed are sometimes 15 to 20 cm wide and 2.5 to 4.5 m deep. Therefore necessary precautions need to be taken during construction to avoid any damage to building foundation.
PRECAUTIONS TO BE TAKEN
Construction should be done in the dry season.
Under reamed pile foundation is also a good choice of foundation in black cotton soil.
In the case of important structures, raft foundation should be provided.
For less important structures (such as boundary wall construction), the foundation should preferably be taken at least 15 cm below the depth at which cracks in soil cease to occur.
The maximum load on black cotton soil should be limited to 5 tonnes/m2. If there is a chance for water to come in contact with the foundation, then the load should be limited to 4.9 tonnes/m2.
Foundation should be placed at a depth where the cracks cease to extend. The minimum depth of the foundation should be at least 1.5 m.
The main wall of the building must be provided with all-round reinforced concrete ties or bands.
Reinforced concrete ties or bands having 10 to 15 cm deep should be placed at plinth level, lintel level and eaves level.
In case the depth of black cotton soil is only 1 m to 1.5 m, then completely remove the entire black cotton soil and place the foundation below that depth.
10. Try to avoid direct contact of black cotton soil with a foundation material. This can be achieved by making wider trenches for foundation and filling spaces on either side of the foundation masonry with sand or morroum.
11. Ram the bed of the foundation trench to make it farm and hard. On this rammed bed, spread a thick layer of morroum (i.e. 30 cm) in two layers, each layer being 15 cm. each layer should be water and rammed properly to get the highest possible density. On this compacted layer of morroum, place either sand or stone up to the desired height where concrete foundation bed has to be made.
12. For main walls or for load-bearing walls, the width of the trench should be dug 40 cm wider than the width of the foundation. Then fill the space on either side of the trench (i.e. 20 cm in each side) with coarse sand. This is done to separate the foundation masonry from direct contact with black cotton soil. In the case of a compound wall, this width of sand filling can be reduced to 15 cm on each side.