Is the cross slope the same for all the layers?
You could of course create one single subassembly and it would be very easy to do so. But unless you know that nobody else will be needing to open your file, I wouldn't do so. If others will need your file, it becomes a major hassle to distribute the SA and maintain the correct version everywhere in case there are changes (such as if you notice a problem).
But if you do still want to create the SA, do a search through the AU classes for anything from Kati Mercier. She's done a number of AU classes on SAC (Sub Assembly Composer).
In addition, I wrote up a short SAC how-to sheet that you might find helpful. It was originally intended for my co-workers. But since that time, I've posted it here too.
http://forums.autodesk.com/t5/autocad-civil-3d-general/sub-assembly-composer-getting-started/m-p/530...
Don Ireland
Engineering Design Technician
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