Don't know who mentioned here to use a pressure washer, but boy did it help me out. I needed to get under a 4 foot wide concrete sidewalk. After a few hours of digging out dirt/large/small rocks I tried the pressure washer. Presto. A three inch hole right through.
I used a 1" Sched 40 natural gas pipe. Started it with a 16oz hammer and then started hitting it with a 16lb sledge. No exaggeration it only took a few minutes to get it under my 4ft sidewalk. Once I pulled the pipe out the 3/4" poly slipped right underneath. I also didn't wash out the dirt from under the sidewalk.
My fiancee's brother recommended using a hose attachment but I didn't want to risk a sidewalk that cracks after a few years.
I plan on using this same technique to get underneath my walkway from the driveway to my front door.
Gas pipe/sledge - good idea. Actually in my case I needed a three inch hole for a 3 inch drainage pipe. I planned to insert four ploy pipes just in case I need them for irrigation in the future....but I forgot. I'll keep the gas pipe in mind, thanks