Black Cotton Soil

Black cotton soil has a tendency to shrink and swell excessively. 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 foundation which in turn causes cracks in 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.

Following precaution should be employed during construction on black cotton soil.

Precautions to be Taken

  1. 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 foundation, then the load should be limited to 4.9 tonnes/m2.
  2. Foundation should be placed at a depth where the cracks cease to extend. The minimum depth of foundation should be at least 1.5 m.
  3. The main wall of the building must be provided with all round reinforced concrete ties or bands.
  4. Reinforced concrete ties or bands having 10 to 15 cm deep should be placed at plinth level, lintel level and eaves level.
  5. 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.
  6. Try to avoid direct contact of black cotton soil with 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.
  7. 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 highest possible density. On this compacted layer of morroum, place either sand or stone upto desired height where concrete foundation bed has to be made.
  8. In case of important structures, raft foundation should be provided.
  9. For less important structures (such as boundary wall construction), the foundation should preferably taken at least 15 cm below the depth at which cracks in soil cease to occur.
  10. Construction should be done in dry season.
  11. For main walls or for load bearing walls, the width of the trench should be dug 40 cm wider than width of 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 case of compound wall this width of sand filling can be reduced to 15 cm on each side.
  12. Under reamed pile foundation is also a good choice of foundation in black cotton soil.

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