Abaza,
See www.rehau.com and/or www.hardingeotech.com
Both of these companies offer an enhancement to the traditional single, HDPE, U-bend / grout design for vertical geothermal bores that claim to reduce the number of bores required.
We haven't designed using these products yet, but are waiting for the right project to apply it to. I'm sure there are other products out there as well...
Jon Evans, PE, LEED AP
