I agree, bury below local ...

I agree, bury below local frost depth.  If this cannot be achieved there is an option of adding insulation above the pipe.   In a recent project I specified Owns Corning closed cell extruded polysytrene insulation board, 2-inch thick, across the width of the trench just above the pipe.  It was difficult to find exact design guidance but I came to the conclusion that this works better in soil not rock and one 2-inch layer could reduce depth by 1 foot.