In early 80's in Nigeria I was working on rural water supply programme where we had areas of black cotton soils. As far as I recall we rarely tried to put in anything where black cotton soils were thick. Indeed villages tended not to be based where bcs were thickest. When we did drill in these areas we often had problems - not least logistics in the wet season as roads became quagmires and many vehicles were stuck, damaged etc.
Yields in these areas were often low and poor quality as bedrock was a black shale formation.
I would check the area more extensively.
I was in Malakal twice this year once on route to Mabaan through a field office in Malakal and the second time doing infrastructure assessments for the judiciary of South Sudan.
In both cases the impression we had was of clay soil that tended to settle ... indeed many of the buildings including the circuit appeal court and judiciary buildings and hoses show cracking and other evidence of differential settlement.
The area is close to the Nile and we were repeated told of soft clay soils underlying most of Melaka which results in settlement of structures . I have just checked the assessment report and this is confirmed. Also most of the facilities and sites we checked had reticulated water which while not very clean (including our office / residence) was seen to be and told to be moderately reliable. I recall in the guest house we did not have water for about 25 percent of the time. Outside the centre of Melaka I have no information ... but all the evidence we saw and were told of suggested damp clays.
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.
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