First, determine the required flow, (approx. 3 GPM/ton), and desired type of antifreeze -- then choose the piping to yield a satisfactorily low friction loss and high Reynolds number. WF Pressure Drop Calculation Instructions Also run the numbers for a pair of smaller diameter U-tubes in parallel (in the same bore). Depending on GPM requirements, that might allow you to get away with a lower-wattage circ pump.
No danger of bursting a water-filled pipe if it's in a standing column of water or water-saturated soil. I don't know whether there are (realistic) situations where crushing might be a concern.