The Foundation of Building in Black Cotton Soil

Here are the precautions that are generally adopted while building structures on black cotton soil:

  • To limit the loads on the soil to 55 tonnes/m². If water is liable to find an access to the foundations, the limit of loading should be restricted to 4900 kg/ m².
  • To take the foundation to such depths where the cracks cease to extend. the minimum depth of foundation should be at least 1.5m.
  • To provide reinforced concrete ties or bands all round the main walls of the building. The R.C.C. ties or bands which may be 10 cm to 15 cm deep should be placed at plinth level, lintel level and eaves level. In case of flat room, the R.C.C. slab itself acts as a tie and as such no extra band need be provided near the roof in such cases.
  • If the depth of black cotton soil at a given site is only 1 to 1.5 m the entire black cotton soil above the hard bed may be completely removed and the foundation laid on the hard bed below.
  • The swelling of soil in direct contact with the foundation material causes maximum damage. Hence it is necessary to prevent direct contact of black cotton soil with masonry work below the ground level.This can be achieved by making wider trenches for foundation and filling spaces on the either side of foundation masonry with sand or moorum.
  • The bed of foundation trench should be made firm or hard by ramming it well. On the rammed bed a 30 cm layer of good hard moorum should be spread in layers of 15 cm. Each layer being well watered and rammed before laying the next layer. On this layer either stone or sand bed should be provided to the desired height place the foundation concrete bed block upon it.
  • In case of ordinary buildings, the foundation should be taken at least 30 cm deeper than the depth where the cracks stop.
  • In important structures raft foundation should be provided so as to float the building on the bed below, quite independent of the surrounding soil.
  • For less important structures like compound walls etc., the foundation should preferably be taken at least 15 cm below the depth at which cracks in soil cease to occur.
  • Construction in black cotton soil should be undertaken during dry season.
  • The masonry of the walls should start at least 15 cm below the general ground level.
  • The wall of trench for main walls or load bearing walls of a building should be dug 40 cm wider than the width of foundation. This is necessary to ensure provision of at least 20 cm wide layer of coarse sand on either side of foundation masonry thereby separating the sub structure from having direct contact with black cotton soil.
  • In case of compound wall, width of sand layer on either side of foundation masonry could be 15 cm.

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