The 300% rain that we received here in Tennessee this summer finally dried out enough where I was able to finish burying the cooltubes out into the forest. It turned out to be fairly labor intensive the way I did it as I was unable to use a machine to bury the tubes directly. The clearing I had made in the forest for the ditch was fairly well surrounded by trees making it impossible to get my backhoe in there. Also, the ditch was only 3-4' deep which required me to be very particular about getting the fill directly in contact with the tubes just right to maximize heat the transfer from the tubes to the earth.
There is no "industry standard" as yet for the installation of this system. I had to take into consideration the soil type, topography, available equipment, and budget to design my earth air heat exchange system based on the knowledge available to me at the time. I put the tubes 3-4' underground and had them exit the ground horizontally to maximize drainage. The fall in elevation from where they exit the house is 15' which made me decide use a solid, non perforated pipe as any water due to condensation would drain out of the downhill end eliminating the need for a perforated pipe. I did not want to use perforated pipe as the holes are an entry point for underground moisture. With the solid pipe making any condensation drain out only at the end of the pipe, there is no need for gravel in the ditch around the tubes. The dry-ish crumbly clay I packed around the tubes has much more surface area than gravel maximizing the contact with and heat exchange from the air through the wall of the tubes and into the earth.
Dirt has .1R value per 1 inch. I buried 2" of white beadboard insulation (the cheap stuff) on top of the 6" of dirt on top of the tubes. So the math: 2" of insulation at 5R value per inch= 10 Rvalue/.1 Rvalue per inch of dirt = 100"/12 = 8.3'+3'deep burial = 11.3' deep