I'm not sure if I understand you correctly but I assume that you want to model soil with solid elements. In Robot there are no special elements for modeling soil behavior and personally I'd rather use elastic supports instead (as in the sample model I sent you) but of course you can use solids if you think it is a better solution.
Please mind that the attached model is intended only as the example of use of solids.
As I mentioned this is just the sample solid model and you have to decide what is the best way of supporting the structure. In my sample model I defined supports in the directions perpendicular to each side of the solid (UZ for XY bottom side/ UX for YZ sides/ UY for XZ sides).
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