Posted: 26/08/2013 at 14:08
I would dig a trial hole and see what's underneath.
I live in a converted school and our raised beds, for mainly vegetables, are on the Tarmac because having dug a trial hole we found nasty hardcore laid straight on clay and we reasoned the expense of digging it up and disposing of the spoil was just too much.
The raised beds are a sucess but they do dry out quite quickly. But the best thing is having a really good surface to walk on to work from, between the beds.