Posted: 27/08/2013 at 22:59
My main path and patio is tarmac. If the tarmac has been laid properly there will be hard core underneath and checking whats under the hard core could prove difficult, so you may need to take your chances with your soil type but the good news is most soil types can be improved by feeding it with a good compost.
Double digging in well rotted hourse muck or mushroom compost will improve the soil whether clay or stony. Then putting on a surface mulch regularly will feed the soil.
I took up a concrete garage base before putting down turf for a lawn and then several years later turned part over for my first veg bed. There was very little life under the concrete, no worms but fortunately although stony ground the soil was fairly good stuff. I double dug in well rotted horse muck for the veg beds in the Autumn before planting in Spring.
In another part of the garden I discovered clay under the top soil after taking up slate which had laid there for years on a weed membrane. After a double digging of mushroom compost the flowers have thrived and because of the clay that part of the garden retains water well when we get the sun.
Regarding electric cables I would have thought they would be near the surface and in some sort of protective covering. Are you expecting to find cables?