Examples show an option with no variable thickness components in the slab, and an option with variable thickness for the earth component (if you want a flat underside of slab)."
David Light, Excitech says: ."This really breaks the Revit rules as my example is a very large floor slab which has been edited, but this does allow you to cut a void to show a tunnel.
At the moment this cannot be achieved using the toposurface tool. The only other options to consider are an inplace family or maybe a combination of an inplace family and a toposurface."
In summary, we are fighting to use Revit for planning and landscape design although I guess a half mile long tunnel constitutes a major Civil engineering project (we could use Autocad Civil 3D without much connection to Revit). In early design we would like these kind of tools in Revit Architecture. We need better Revit tools for site modelling to include a tunnel and roads features. Autodesk? As workarounds these suggestions are great, thanks!