foundations on black cotton soil
Single storey building (clinic) to be constructed on black cotton soil. Guidance required on adequate foundations. Foundations will be required to carry 3.5m of 200mm brickwork and small roof load (galvanised roof). Water level not known. Assumed that black cotton soil is to a depth that can't be excavated. Building plan 5m x 8m. What should be provided in terms of foundations?
As we all know, the problem with black cotton soils is that their large volume changes with moisture content means that are a big problems with foundations. Tomlinson (1980) notes that seasonal swelling and shrinkage can extend down as far as 4-5m and says that piled foundations to below this level may be needed even for light buildings, or that raft foundations may be needed for important structures. Black cotton soils are a large problem in India. Khanna (1999) notes that in such soils: 1. Foundation loads are limited to 5 to 10t per m2 (way below your load). 2. Foundations are taken below the cracking zone 3. Trenches filled with sand or other material may be used alongside the foundation trench to prevent intimate contact of the black cotton soil with the foundation concrete and masonry. Sand piling (sand columns below the foundation) is not so useful in black cotton soils because of the risk of sand loss through cracks. He recommends that for ordinary buildings, foundations should be taken 15cm below the depth of cracking and complicated foundation arrangements with sand. Lime stabilisation is also used a lot in India with stabilisation in piles, trenches, or areas wide. Essentially this consists of mixing lime into the soil to change the way it behaves. Again this is fine for a low wage area like India, but probably not for more prosperous areas Another approach (and, in my view a far better one as it is more straightforward) is to use a foundation beam at plinth level with a ring beam at lintel level. A size of 20cm by 20 cm with at least 4 x 12mm bars would probably be ok for a small building like yours. Of course, a key issue is to avoid creating problems from the roof water by taking it away from the foundations. It is also a good idea to check what local practices are. If they normally use piles or raft foundations locally, then these are probably a good idea. Regards, John Cosgrave
Digging or piling through the soil is out of the question...it could be 10m + deep, so with a single storey building like this, use a reinforced raft foundation with 500mm (750mm better) deep downstand edge beams, so that scouring does not erode / wash away soil under raft. Do top and bottom mesh in slab, if you can get it, as the slab will have to resist heave and shrinkage. Tie mesh into the rebar used in the edge beams. OR....go for a simple more temporary construction with bush poles.....perhaps just a bit of improvement on indigenous methods?? Regards, Gordon Browne
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