fence post questions

[20027700F150]

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.

[pushkins]

When you say "the rock hits" is it solid rock or "rocks" ?

Either way sand will not stop the posts heaving.
The only way to put the posts in correctly without future problems is to sink them in 32" (depends on your frost line) and concrete them in.
Sorry for the extra hard work but it's the only way to do it right.