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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