I don't believe a direct conversion is possible, however see http://forums.autodesk.com/t5/AutoCAD-Civil-3D/Riv
Does anybody know if it is possible to convert an autocad surface to civil 3d surface?
The easiest way is to explode the Acad surface, getting a bunch of 3Dfaces.
Finally you add those 3Dfaces to a new C3D surface definition.
When you add 3D Faces, make sure to check off "Maintain edges"
Unless you really don't want to.
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