Foundations on black cotton soil

Problems of foundations on black cotton soils

Black cotton soil is a highly plastic clay containing montmorillonite.

1. Swelling in the wet season and shrinkage in the dry season, causing vertical movement that lifts and drops the foundation.
2. Deep shrinkage cracks (1 m or more) in summer, which let water in.
3. Differential movement, producing cracks in walls, floors and plinth.
4. Low bearing capacity and softness when wet; poor drainage.
5. Uplift pressure (swelling pressure) on footings, floors and pavements.
6. High compressibility and lateral movement.

Suggested foundation

An under-reamed pile foundation (as per IS 2911 Part 3) is best, with plinth beams (RCC tie beams) connecting the piles at the top.

Justification

1. The piles pass below the active zone of seasonal moisture change (3.5 m or more) and are anchored in stable soil.
2. The bulbs (under-reams) give additional bearing and a grip that resists uplift from swelling.
3. The plinth beam is kept clear of the ground with a gap (void) or compressible filler, so swelling soil cannot lift it.
4. Settlement is small, and the foundation is economical for light buildings.

Alternatives: a rigid RCC raft foundation over a sand cushion (about 0.9 m); removal of the black cotton soil and replacement with a non-swelling sand or murrum cushion; and lime or cement stabilisation. Shallow strip footings are not recommended.

   plinth beam  ===========  
  (gap)  |      |      |
  under- |      |      |
  ream  (O)    (O)    (O)  <- bulbs in stable soil