I have been watching a Geo Thermal install in a nearby town. After the pipe was layed in the horizontal loops the installer dumped truckloads of sand on top of each pipe loop. I saw two huge piles of sand and he was back filling sand over the pipes with an excavator. I was always told that sand does not conduct heat, hence why you put sand around insulated pipes to and from an outside boiler. Does this sound strange to anyone else. The ground seemed to have clay and shale.