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Sounds like a good challenge to me. So, looking around, there seem to be several ways to do this but all of them are very time consuming, until I stumble upon this little gem. Someone out there in the great wide internet wrote a lisp routine that will create a solid out of 3d faces. So, in Civil 3D, you can display your surface as 3D faces, explode the surface (or better yet use the Extract Objects from Surface routine in 2008), and run this neat little routine and you have a solid. It took about 5 minutes to process 1,400 faces in 2008.
Now, I know I focus primarily on Civil 3D (all of my previous three posts so far) but this will work for Land Desktop as well. Just import your 3d Faces into your drawing and away you go!
If you are interested, you can find the lisp routine HERE