I'm in the process of installing a pressure treated fence and have some post questions. My soil conditions are terrible ( 18" of soil and then the rock hits). I was going to do the usual cement the posts in gig, but I'm concerned about the possibilty of the frost shifting them. I've been told to just pack the posts in with sand (no cement) and they won't shift at all. Not too hot on the sand idea! Any comments or ideas out there guys? Much appreciated.