I have very rocky soil, so whenever I bury any type of line, water, electrical or whatever, I sleeve the line with PVC conduit. It comes in 10 foot sections and is easy to work with and not too expensive. I do not use any glue on the joints.
Damage from rocks is a real concern. Also damage from burrowing rodents, although my cats seem to do a pretty good job of keeping rodents away from the buildings.
I think if I was going to run an airline underground that far, I would sure look closely at PEX. PEX is not that expensive and seems to work very well for water pipe. I would want to research how well PEX stands up to oils, since most compressors put some oil in the air over time. You don"t want to have to redo the job in a few years. Good luck!