Unfortunately, the answer is that you should really take the tarmac up first if you want to replace it with a gravel driveway. Gravel, being a ‘soft’ driveway medium, needs a soft bedding.
Health and Safety
The problem with laying gravel on any hard surface, such as Tarmac or Concrete, is that it will roll around. This is unsafe and also means that you’ll lose the gravel quite quickly. As cars drive over it the gravel will be squeezed out to the sides and into whatever borders the driveway.
Obviously, we’re not sure of the exact situation with your driveway, but if you are able to build border walls about 3-4in (8-10cm) high that retain the gravel, then it might stay in place. But car tyres would probably dig furrows quite quickly, so you’d have to regularly rake the gravel back in place.
You would also have to put down three or four times the amount of gravel than you would normally, so it would cost a lot more than an ordinary gravel drive. It would still have the potential to be unsafe for people walking on it too, so we must advise strongly against this course of action.
Taking up the tarmac drive shouldn’t be that big a deal anyway. Unless you have a very long drive all you need to do is spend a Saturday with a strong pick, a shovel and a skip. Break the top layer up, being careful not to go down more than a few inches and, assuming the tarmac had proper foundations, you should be left with a reasonably good surface to lay gravel on.
Don’t try to smooth the revealed foundation too much, as any undulations will help to keep the gravel in place. Depending on the surface that you reveal, you many even want to put down a layer of sharp sand to give something for the gravel to bed into.
Pick the Right Gravel
Of course, there are many different types of gravel. If you want to go for something that’s a bit sharper, this will help the gravel bind together. Smooth, small, round stones can be a bit too keen to roll away to where they aren’t wanted.
Gravel that’s less ‘finished’ also has the benefit of generally being cheaper than smooth, round products. But if you go too far down that road you can end up with gravel that is painful to walk on. The best advice is try to get to see a lot of samples before you decide what to buy.