When I installed the solar pump system, I paid a backhoe operator to dig it down 4 feet, all 480 feet to the house. In a couple places he could not get deep enough because of solid rock, in these spots I laid 4" of blue foam board 4 feet out on both sides of the line, and buried the blue board. I actually laid THREE lines in the trench (in case one froze) since black poly is so cheap, but have never frozen the first line in 5 years. The backhoe also dug a "manhole" in the valley to place the pump and controller. I just filled the top of this with about 3 feet of fiberglas insulation and it has never froze either.

As you may have gathered, the cost of the excavation was about quadruple the cost of the solar panel, pump, controller and hose.