Surface cannot be simplified in Civil 3D.
The surface was created by adding drawing objects – 3D faces – and the option “Maintain edges from objects selected” was activated.
With that option activated it is not possible to change the triangulation of the surface at a later time.
- Export the surface as IMX or LandXML file
- Import the IMX/LandXML file again into Civil 3D
- The surface can be simplified after the export/import process