Count me in also as being someone who is not in favor of a loopfield being close to a foundation. There is a possibility of freeze-cracking of a foundation.
A GSHP will monitor the evaporator HX to prevent it from freezing, so it should have a similar effect on the loopfield.
However, in the event of a severe prolonged cold snap, if a loop field is close to a foundation, and it has had heat withdrawn due to a heat pump... then there is an elevated danger of totally frozen ground, and ground heaving, and foundation cracking. All bad, all expensive to repair, not good.
I would say that it would be best to have the loopfield no closer than 16 feet from the foundation, for all the reasons that previous posts suggest.
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There is some great info here regarding how water content can increase thermal transfer. Very good info, very useful charts.
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An additional aspect that the document/powerpoint neglected is that, in the very same way that the heat pump condenser HX gets hot because of condensation, rain falling from the sky is carrying heat with it, because it is also a product of condensation, and it is releasing back to the earth, the same energy that was required to create water vapor from liquid water.
So, whenever possible, rain should definitely be captured and directed to the loopfield, because it aids heat transfer, and it is also, itself, a source of heat.
I know this to be true because I did a two month test of my little heat pump and I watched my heat pump's output increase after a heavy rain. Of course, there was a lag of about two days between rain event, and the heat output increase event.
-AC_Hacker