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I have seen oil-drum linings used (fairly successfully) in household pit latrines in northern India, but must admit that I don't know how the collapsible black cotton soil is likely to affect functionality.

Investing in a good pit lining (like fired bricks) is only worthwhile if you plan to re-use the (decomposed) contents; if you have very collapsible soils (obviously the case in Sudan); or if there is no space available for new pits. A lot of latrine designs don't cater for what happens when the latrine pit fills - when you need either to divert (or move) the latrine to a second pit, or to dig out the single pit (which now contains fresh excreta, thus presents a significant health hazard both for those digging it out, and for those that live near the site where the pit contents are dumped). In many cases, the investment in the pit lining is lost (as people often opt to build a new latrine rather than dig out the old one) ... which suggests that it is best to minimize the cost of the pit lining. Assuming that some form of pit lining is required, then oil drums are a reasonable alternative (if readily available). Obviously, there is a strong likelihood that the drums will corrode and have a fairly short life, but even a heavily corroded oil drum may provide enough support if the pits are not too deep. And if the latrine pits are going to be dug out at some stage, then there is always the option of adding some internal strengthening (timber, woven matting, other local materials) while the pit is empty? In South Asia, one of the most effective approaches for low-cost household latrines has been to dig a small, shallow pit (1.2m diameter, 1.2-1.5m deep); leave the pit unlined if possible - but use a disposable (woven bamboo or similar) lining if needed; then cover with some form of latrine slab (usually precast concrete). When the pit is full (usually in 6-18 months), the household dig another pit nearby, move and re-use the slab (plus rebuild a simple enclosure), and plant a fruit tree over the old pit. I guess this approach works less well in highly collapsible soils, unless the latrine pits are kept shallow and some local building material can be fashioned into a reasonably strong pit lining - what do people use to build their houses (there is a huge sustainability gain in using local building materials); is there any way that these local building materials can be used to weave/build some form of cheap lining? And I'm not sure why you plan to have such a deep latrine pit - presumably to lengthen the time taken for the pit to fill (or because this is the local practice). I always advise people to keep latrine pits shallow - deep pits are more difficult to dig; must resist higher forces (at the bottom of the pit); are more difficult to empty; and (perhaps most importantly) pose a higher risk of groundwater contamination. OK - sorry that I couldn't really answer your questions directly. My only experience of black cotton soils was in trying to drive through them, which was memorable enough to make me realise that you face a big challenge.