Terrain Model Exterior Boundary

I have created a terrain model from a corridor top mesh.  By default there is no exterior boundary created.  I have two questions:

1. Why does the terrain model not simply match the top mesh?  In the terrain model there are several areas of sliver triangles (I have the edge method set to remove slivers).  It appears to me that the top mesh is some kind of triangulated surface itself.
2. What is the preferred method for creating an exterior boundary for a terrain model of a corridor.

In the SS2 version this was not even a thought as the roadway designer created an exterior when a dtm was made.  It seems to be MUCH more difficult in OpenRoads to create a terrain model and exterior boundary.

Marc