The site needs 25 feet of fill over a large area. Could the piping for the geothermal system be added as the loads and loads of dirt are added to the site or would the the site be build up first and then horizontal installed later? I would think the concern on compacting the fill would protentially damage the piping.
It might be a good strategy to add loop before fill, if the fill is relatively clean. Rocks and sharps in fill would be a primary concern. I'd think as far as compaction, the HDPE should be up to the chore. One concern with settling is how much stretch that might add on a loop. A straight out and in run may create a bit of tension through compaction and settling. If you had slinky loops, there would be some margin of slack to reduce that tension.
