Underground irrigation fittings

I am in the process of setting up an irrigation system and was planning to do the following coming out of the backflow preventer (Febco 765-1" PVB): 1" vertical copper pipe going into the ground ~12 inches deep + soldered copper elbow + ~6" of 1" copper pipe away from the house + female copper thread adapter + PVC male adapter to connect to PVC mainline. (note irrigation will be blown out each year)

Are there any issues with having the copper elbow fitting be a soldered connection and buried in the dirt? I am planning to have the copper to PVC transition accessible from a valve box since I will also have a tee there for the mainline along with a few PVC valves.

 

1 foot deep in Colorado. don't you need a foot below local frost line? I sleeve copper or metallic pipe underground

 

This part of the line would be winterized and drained so it doesn’t need to be all the way down below the frost line

 

30+ years on many copper standpipes with copper unions on them so that the backflow valve can be removed for winter in my neighborhood. No failures underground that I know of, but many unions damaged over time from putting the backflow valve back on in the spring. The unions are getting dinged by folks just dropping the backflow valve into place. Soil is clay where I live.