[Vlad]

AC_Hacker you forgot to add header 1 1/4 pipe connecting 3/4 vertical loops. When you have 5 holes you only have 60 feet of double header. When you have 12 holes you have 165 feet of double header. In our climate official frost line is above 18"(the deepest frost I saw was only 2-3" bellow ). The headers are 4 feet down. I have nice yellow wet clay at that depth. I would consider headers as part of the loop.
Another thing I was thinking about is we do not have real winter. Cold is only at night. During day time the temperature is often 48-55. You can use loop as large thermo accumulator (even low temp). During day time when heat demand is low and temperature outside is "high" you can gain some heat from air and transfer it to underground loop, increasing loop capacity. In my case I can add couple sun collectors on my roof, which has south exposure and during day time return line fluid before going back underground can go through sun collectors, gain heat and transfer it back to ground. At night when heat demand is high, you can extract this heat.
Convenient heat pump(air to air or air to water) do not work properly in our climate because when you need heat you have cold air and a lot of moisture in it which freezes the coil. You have to cycle heat pump to melt ice. And during day time when you do not need much heat you have plenty of it in air (moisture creates the same problem).
I think in our particular climate you can have much smaller loop combined with air coil or sun collector which will heat loop back (not directly refrigerant coil) in this case loop is not going to be only source of heat but the accumulator as well.