Precautions to be Taken for Choosing Shallow Foundation on Black Cotton Soil?

Shallow foundations on black cotton soil are very susceptible as severe cracking occur in soil in relation to atmospheric conditions. Adopting some precautions and providing some limitation in choosing foundation type, shallow foundation can be used:   

1. To provide reinforced concrete ties or bands all around 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 roof, R.C.C slab it self acts as a tie and as such no extra band needed to be provide near the roof in such cases.

2. If the depth of the 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 may be laid on the hard bed below.

3. To limit the load on the soil to 5.5 tonnes/ sqr. m. if water is liable to to find an access to the foundations, the limit of loading should be restricted to 4,900 kg/ sqr. m.

4. To take the foundation to such depths where the cracks cease to extend. The minimum depth of foundation should be at least 1.5 m.

5. The swelling of soil in direct contact with the foundation material causes maximum damage. Hence it is necessary to prevent the direct contact of black cotton soil with masonry work below ground level. These can be achieved by making wider trenches for foundation and filling spaces on the either side of the foundation masonry with sand or moorum.

6. 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 desire height to place the foundation concrete bed block upon it.

7. In case of ordinary buildings, the foundation should be taken at least 30 cm deeper than the depth where the crack stop.

8. In important structures raft foundation should be provided so as to float the building on the bed below the depth, quite independent of the surrounding soil.

9. 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.

10. Construction in black cotton soil should be undertaken during dry season.

11. The masonry for the walls should start at least 15 cm below the general ground level.

12. The width 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.