workflow for existing pavement terrain

does anyone have suggestions for best workflow to create an existing pavement terrain?  I want to reference it to my earthwork files in order to calculate unsuitable.  I tried extracting the same 3D graphics that I thought were used to create the overall existing topo, but I get different triangulation.  Perhaps terrain was modified or cleaned up from these original graphics?  I also thought about clipping a terrain from the original, but does that mean I have to create a closed shape outlining all of my existing pavement?  How can I easily extract a boundary?  

Thanks for any help.