Convert a Civil 3D Tin Surface to AutoCAD Solid

Issue

In some instances, usually for interoperability reasons with other software, you may want to convert your Civil 3D surfaces to AutoCAD solids.
There is no direct way to do this but the method described below will help you do it efficiently.

Solution

1. Have your Surface style display Triangles
2. Use the _AeccSurfaceExtractObjects command in order to extract the Triangles and obtain 3D FACEs
3. Set your view to Isometric and draw a vertical line (the length of the extrusion you would like to obtain from one triangle vertex)
4. Use the EXTRUDE command with the Direction option. Select the 3D faces, then select the start and end point of your line for the direction (All 3D faces will get extruded vertically)
5. UNION all those new 3D Solids... 
6. In case you get a message "the maximum number of history mode would be exceeded...  then run the BREP command and select all the solids to remove their history.
7. Run again the UNION command to union all those solids.
   

Now you should have one big solid.
The bottom of the ground is not flat but reflect the top surface
SUBSTRACT a big box if you need to make the bottom flat...