Conduits are supposed to be surrounded by sand before they are backfilled with the local material. Probably only a few electricians actually do this, I'm sure nobody else does.
Our power company has pretty good instructions on burying conduits:
http://www.cmpco.com/MediaLibrary/3/...ted_080707.pdf
I have seen a strong metal conduit pipe crushed by small rocks that was buried 5-6 feet down. Probably the rocks moved around and damaged the pipe (and the wiring in it, and caused water to flow from the pipe into the building).
At our office, there was a small stream flowing through the basement. The water main out front (probably 6") was leaking according to the water analysis. The water company dug things up, flooded our street and they had to shut off our street. A rock had worked its way up (over many years) and punctured the pipe underneath. This was probably a 6-8" fieldstone.
Gallery 2006- :: 2005/03/31 - AquaMaine fixes our leaking sidewalk :: DSC_0071
If you don't have much to bury and don't want to go all out to protect things, perhaps liquidtight conduit might be flexible enough to bend around moving rocks instead of break. It's not cheap though. Otherwise, don't count on things lasting forever underground.