Compacted fill within an existing structure

[rikmayall]

Hopefully the attachment will explain this better than I can in writing.

I using Staad Pro to model a substructure on poor ground (subgrade modulus 2800kN/m2/m), with trenches throughout. To avoid attracting high stresses at the wall/slab intersection I have made a lower level slab throughout with the supporting trench walls built from it, with our high floor level slab spanning between walls (max 8.5m).

We are laying the low level slab, building up the walls, putting compacted fill between the trench walls and then pouring our floor slab on the compacted fill.

I am trying to find a solution on how to model the higher level floor slab supports - which although will be supported between the walls, will also be supported on the compacted fill. This fill will be relative to the rest of the substructure and can't really be modelled as an independant support as the floor slab loads will still be travelling through the structure (and not to a support).

It's almost like modelling a concrete slab sandwich with fill filling if that makes sense