Those that have been trying out the new Labs tool for exporting your Civil 3D corridors to solids have noticed that what we export are in the form of a solid body rather than an actual 3D solid object in this release.
However I was talking to one of our top experts in the Benelux area, Cristian Otter of Breijn who mentioned to me if you export to a .SAT file and then import that file back into AutoCAD it will convert those 3D bodies to 3D solids.
I am sure there are other ways also, but this was quick and easy.
Why you want to convert? We have users wanting to merge solids together and do other creative ideas.
Give it a try