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Black cotton soil is notoriously difficult to drill unless approached with caution, but it can be done. The use of a down the hole hammer is out of the questions as is rotary drilling. Compressed air will blow it away and rotary flushing systems can turn it to mush. A simple cable percussion system driving casing at the same time can be the answer, a system which has been used (and recorded) for four thousand years and mentioned in my book (in REDR's library) "Drilling for Water". The casing can get tight and even stuck so a quantity of drilling polymer mixed with water is poured inside the casing whilst driving which stabilises the bcs and eases the casing down. If the bcs starts to rise in the casing, fill the casing with the polymer mix and drill through it. Should you need more information please do not hesitate to contact me.