Never Xref a drawing just to get a Surface. Use a Data Shortcut. It keeps your file size small and allows the surface to update dynamically.
Don't just drag and drop. Use the (type XREF in the command line) to maintain control. civil 3d xref
A critical distinction in Civil 3D is the difference between an and a Data Shortcut (DREF). data Shortcuts and Xrefs Never Xref a drawing just to get a Surface