Some great methods here and have tied some of them... Including Dnkldorf's which does work well. But can say I have a modification to Obsaleet's that I figure is better, and have done often. I figure a diversabit might drift off somewhere... Pre-drill the hole with roto-hammer and a long masonary(in rocky soil), or you can use an old wood auger and several extentions on a regular hole hog. (Soft loose soil or clay)An old shop I worked for had and SDS bit welded to a long shaft extention for the purpose, about 6' long.
I have also seen on the internet (but never used personaly) an attachement that you can put on RMC up 2" I think. It has a carbid tipped water lubed bit that threads right on, and a water feeding coupling that attaches the drill to the conduit. Suposedly you can with this item drive up 2" pipe 20' or so with a hole hog. When you're done you pull the attachments off the counduit, allow to drain, and you're done.
