F.E.Modelling of Deep Basements

Hi!
You'd better consult a geotechnical expert, but as much as I remember, overburden removal won't affect soil load bearing in granular soils like sand as they are in quick drained conditions. I suggest a test on subgrade reaction of the soil in a depth on which the foundation is going to be laied, and then use a compression spring model for the soil, instead of brick element model. It would be easier and more realistic, if the soil layer is more or less uniform on a depth ,say, 1.5 times the foundation dimension. If you persist on using a brick model, a media of soil with dimensions around 3x3x3 times of foundation dimension should be discretized. If the soil is layered, with extremely varying characteristics, an infinite element method may be the best approach. Also try a boundary element approach if possible.
Good luck.