Instead of including the select fill as a layer in the Foundation Slab, try using a Building Pad or In-Place Family instead. Here are the general steps for the Building Pad methodology:
Create a Toposurface.
Create the Building Pad in the same shape as the Foundation Slab.
Create the Foundation Slab.
Add the Slab Edge thickened edge.
Join Geometry of Foundation Slab and Slab Edge if desired.
Use the Cut Profile tool on the Building Pad to graphically "cut" at the Slab Edge.
Please see the attached image.
Note that the Building Pad cuts the Toposurface to the depth you choose, thus giving a Cut/Fill volume. Set the thickness, depth, and material of the Building Pad to your preference.
The In-place Family methodology would be similar except that you would not explicitly need the Toposurface. In my opinion, the In-place method will take slightly longer to accomplish. However, the In-place method does allow for more flexibility to create complex shapes.
Brian E. Johnson, P.E.
AEC Technical Specialist
Autodesk, Inc.