Oops - sorry. I failed to notice that you are a Mechanical engineer, and gave instructions for the Land Desktop software that Civ-E's use. I probably can't help much.
Open the dwg file with Rhinoceros, hack away at it, and save it as a dwg file. Rhino can do all that you would logically expect AutoCAD to do with surfaces, and more.
Better, open a _copy_ of just the parts you want to change, and edit that. Rhino may not preserve some AutoCAD entities.
It may be necessary to add one linear dimesion in Rhino to avoid a scaling error.
I am working on a surface which a 3D curve revolves along another straight line, and I want cut this surface with a plane to see the intersection.
But solid can not do the job.
If the curve is planar, you can add radials to the axis at its ends, turn it into a closed polyline, and revolve it to produce a solid of revolution, which you can slice at will.
Before you start, set the variable DELOBJ to 0 to retain primitives along the way.
Can you create that 3D surface with solid commands then use shell command to create that same 3D surface with a thin wall; then you probably can slice it with a plane.
