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I think the best discussion of the design of leach pits is to be found in The design of pour flush latrines published by the World Bank. It is a bit dated but still sound in my opinion.
https://www-wds.worldbank.org/external/default/WDSContentServer/WDSP/IB/2003/03/14/000178830_98101903445990/Rendered/PDF/multi0page.pdf