Thank you for the clarification.
The question about fluid-like behaviour was due to the possible consideration of using a pressure, i.e. a load always directed orthogonally to the container walls.
If the vertical load you used corresponds to the weight of the material, I think it may be a good first approximation, although perhaps out of professional bias I still think the uniform distribution you are implicitly using may not correspond to reality: after all, intuitively, I think at the bottom there may be higher loads per unit area due to the ‘column’ of ore above.
Finally, if the material moves, there is a frictional force parallel to the wall, to be considered, which (I think) is not taken into account in your model.
Please bear in mind that these suggestions come from someone who is completely outside this industry.