My system put in last year is vertical with drilled holes through rock and clay then backfilled around tubes with a Bentonite slurry pumped in. Seems to be working very well. There is no way to put in a horizontal system in my rocky hills.

Has anyone ever considered backfilling a trench system with a layer of Bentonite over the tubes for contact and thermal conductivity?

The key to a successful system seems to be having enough tube length/hole length to get the heat transfer your heat pump unit needs for your house and conditions.

Jim at Dawn