Plumbing Questions? Ask a Plumber and Get Answers ASAP.
Welcome to Just Answer, my name is ***** ***** I will do my best to help you with your issue. If my initial response doesn’t answer your question then let me know and we can continue our conversation.
Your description leads me to think that this is a drain line. There is no minimum depth for drain lines to prevent them from freezing since it should never have any water (except moving waste water) in it. Only water lines need to be protected from freezing since they are always under pressure and full of water. Minimum depth to bury a water line is 1' below deepest expected frost depth which varies depending on where you live.
Ok, so my original answer applies.
Do you have any more questions?
I'd appreciate a favorable rating