the rule of thumb roosie is to dig out any soil that is capable of growing vegetation ,and as a builder i would 150% put hardcore underneath it to support the concrete .make sure you roll or whack in the harcore as well.
i would dig down minimum 14 inches and put 8 inches of hardcore and then blind it off with 1 inch of sand ,lay a 1200g visqueen/plastic membrane on top and put a layer of 383 mesh in then lay 5/6 inches of concrete on top of that.
one of the problems with concrete cracking is when the soil heaves /expands in wet conditions.thats why you should take the soil down to the harder subsoil underneath.

all the measurements are what i would use as a minimum to be safe