Thanks for this very informative video. To add a bit of technical info on the type of soil that is being used, I believe this is laterite soil (containing iron oxides that give red color) not very common in Sri Lanka. A sieve analysis and parameters like Liquid limit, Plastic limit and Plasticity Index of fine particles are indicators of a suitable soil. Perhaps NERD which may have done research on this should come out with details for identifying such soils. All the best for the father and son!.