I am asked to design a pile cap which also serves as a collision barrier. What special design considerations need to be addressed in order to raise pile caps above ground level?
A pilecap is something to distribute loads into piles. There are no particular "rules" for pilecaps that mean it needs to be treated differently above or below ground. It just needs to be designed like any other structural element: for all loads that could be applied on it, with all resistances that may occur.
So, you'll need to design it for collision loads, any vertical loads coming onto it, with resistance from the piles and from soil that is against any below-ground parts.