Michael
I don't proffess to be an expert on the subject but a bore hole can be expensive if you don't have the room and I'm not sure of the ratio of piping because you may enter the water table somewhere in your depth and this may reduce the amount of piping (depth)required.
For our installation in my area I did ask around. A reliable installation for a trench here is 6 ft below ground in a 4 ft wide trench two loops not getting any closer than 2 ft apart each tube. The tubing was 3/4" HDPE designed for that application and welded connections. The installers had cautioned about the slinky installations, although the amount of tubing that goes into the trench gets used up the fact that for many feet of tubing are essentually overlapping and its not as good as having a minimum of 16" of dedicated zone of earth for heat collection. Now again this may be a specific detail for our harsh climate. We are subject to some wind swept areas that the ground will freeze down to 4 ft below grade. You may be able to source enough heat 1 ft below grade because your milder climate has injected enough heat into it.
I would check around locally. its a lot of time and money to not have your loop installed correctly.
Randen
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