This is the very reason for the ground loop to be insulated whenever it's withing 2m of foundations(and as it passes through them).
Whilst it's not desirable to run the ground loop that cold it does happen.
A new install we did around 3 years ago which had 800m of 40mm HDPE in 4 loops, within hours of switching it on the return to the ground loop was at -1.5C.
This was an 11kw GSHP as below.
https://www.google.co.uk/url?sa=t&rc...76247554,d.d2s
Steve