A surface in Civil 3D is built on the basis of mathematical principles of planar geometry. Each face of a surface is based on three points defining a plane. Each of these triangular planes shares an edge with another, and a continuous surface is made. This methodology is typically referred to as a triangulated irregular network (TIN). On the basis of Delaunay triangulation, this means that for any given (x,y) point, there can be only one unique z value within the surface. What does this mean to you? It means surfaces in Civil 3D have two major limitations:

No thickness Operations on the basis of solid modeling are not possible. You cannot add or subtract surfaces or look for their unions as you can with a solid that has thickness in the vertical direction.

No vertical faces Vertical faces cannot exist in a TIN object since two points on the surface cannot have the same (x,y) coordinate pair. At a theoretical level, this limits the ability of Civil 3D to handle true vertical surfaces, such as walls or curb structures. In reality, the micro-miniscule amount of pitch required to keep a surface from going vertical is impossible to discern; it just has to be recognized and input to the system.

Beyond these basic limitations, surfaces are flexible and can describe any object's face in astonishing detail. The surfaces can range in size from a few square feet to square miles and generally process quickly.

There are...