The figure  below shows a comparison of thermal conductivity data.
   
In the pink area the estimated thermal conductivity values have been too high, TRT was required in order to adjust the BHE (borehole heat exchanger) design to avoid under-designing.
  
In the green area the TRT allowed for cost savings, where the underground conditions were better than expected.
 
   

  
   
 
The next figure shows how many total metres of BHE would be necessary for a 15kW heat pump. For a 2.5 heat conductivity this means if you drill just one BHE it has to be 205m deep. If you decide to drill 2 BHEs (because of restrictions or geology etc.) the total length (of both) needs to be around 234m (2 @ 117m). With  3 BHEs the total length is 240m (3 @ 80m).