No stone.. You have to ram it down 8' and sever a clay or PVC sewer line.. in a very short period of time..
It's never a good idea to do this sort of thing in a short period of time. If you hit a gas line or underground electrical line, your life will suck. Aside from getting hurt, you're responsible for the damages if you didn't call Blue Stakes first (or whatever it is in your state; they're the people who run TV ads that say, "Call before you dig!").
I know of a company that dug without calling Blue Stakes and cut a fiber-optic phone line. They were fined $80,000 per day that it was down and had to pay all costs for the phone company to come out and fix it. They also got sued by various companies that used the fiber-optic comm line for their downtime costs. The company wound up going bankrupt.
What I'm telling you is: be really careful. You could wind up making a very dangerous and/or expensive mistake.