Now on subscription centre for download, the 12d import tool.
Very similar user interface as the GENIO tool which is also on subscription.
Using this tool you can import geometry and surfaces into Civil 3D objects.
Using data-shortcuts, place all your 12d design into one drawing and share out the geometry and surfaces to other drawings and users.
Make a change in 12d, delete the content of that original design in the drawing and reimport and just hit save. This will then update all Civil 3D objects in your project.
Jack Strongitharm