Autodesk CIVIL3D - CIVILDESIGNCOMP 2007 SLM CD Tutorials Manual page 738

20 Change the name of the new extrusion modifier to Box Outside. Repeat
these steps for the inner rectangle using a distance of 44 and a name of
Box Inside.
The inner extrusion stops 4 inches shy of the outer extrusion, creating a
4-inch thick floor.
726 | Chapter 18 Part Builder Tutorials