Creating a Tunnel in Revit

Aungwara N, ATL says: We have a project that requires us to carve out areas that are larger than the pad openings and connect with each other below street level, like tunnels. Has anyone came up with a way to create a pad and able to control its height? The whole site will end up being about 0.5 mile long and the void will be filled with utilities and rail system.

Adam S, ATL says: "If you have enough spot elevation data, one option is to establish fixed elevation points at the corners / perimeter of the pad, and then create a new floor type of earth / structure to match the pad dimensions. Click on new floor and use the ‘modify points and edges’ tool and select each corner point and match its elevation to the spot elevation of the toposurface. Given the scale of the site, this will probably have to be done with a number of slabs in order to make the model reasonably accurate. I don’t think it is possible to join geometry with the toposurface, so the joint between slab and topo may be apparent in shaded views (or get creative with visibility/graphic override). It should fade away when rendered, and can be hidden in section views by using the linework tool.

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!