My projects have used only wells or lakes for their water-to-air heat pumps.  I am not up on horizontal applications.

What is the practical depth for burying the polyethylene water lines for horizontal applications in the Southeast?

What is the maximum depth that can be achieved if no rock is found?

What type of equipment is used to dig the trenches?  That is, do most contractors use track hoes instead of back hoes to dig the trenches.  I know some of the larger track hoes can dig 15' to 20' deep, at least they did when they built roads for me in my subdivision.

Can the water lines be placed in the trench without anyone having to be in the trench?  Thinking about OSHA rules regarding safety while working in deep trenches.

there is a video on using slinkys for horizontal and no one has to be in the trench . look on ebay for the video. scroll down the page of some sellers and you will see the video of them doing it.

I suspect most contractors use backhoes vs trackhoes owing to speed and expense. As trenches get deeper, productivity slows, and the relationship is nonlinear owing to the time to lift and set aside each scoop of dirt.

I haven't designed a closed loop horizontal system, but I suspect diminishing returns kick in at depths much below 6-8'

Usually the practical depth is around 6 feet, some do less, some do more, and the total size is dependent on ground conductivity, the amount of heat extraction or rejection, etc. We do not go below 8 ft, we rather add another loop. The first 6 ft take as much as the last 2 ft down to 8 ft. We only do it if we run out of space.

Excavators are what we use. Much quicker than a back hoe. However, the best tool for the job is the one most available, so many do use back-hoes as they are cheaper.
6ish feet is where so many of the osha regs weigh-in so we stop there.
With slinkies much less time need be spent in the trench.
j

We have installed slinkies in 8' deep trenches before. We don't get down in the trench. We place the pipe with long poles.

What is the opinion on chain trenchers and vertical slinkies?

Posted By jonr on 14 Apr 2011 09:28 AM
What is the opinion on chain trenchers and vertical slinkies?

Developed by an equipment manufacturer with IGSHPA help, I don't know of anyone who uses them.
In MI, unfortunately if the soil is stable enough to hold the trench, it makes backfill around a vertical slinky very difficult.
j

Posted By jonr on 14 Apr 2011 09:28 AM
What is the opinion on chain trenchers and vertical slinkies?

We use vertical slinkies when we do not have enough room, and space then 4 ft apart, works as well as horizontal slinkies. You have to put them deeper, standing at 8 ft, since their upper edge is now at 5 ft. I agree that the backfilling is problematic with our great, dense, moist clay. It only works if you stand up the vertical slinky on each side of the 4' trench, and backfill in between them.

could you backfill with sand for say two feet instead of screened top soil.

Posted By rikmeister on 15 Apr 2011 04:09 AM
could you backfill with sand for say two feet instead of screened top soil.

Sure, but there generally is no economic advantage to trucking in different fills.
j

Posted By rikmeister on 15 Apr 2011 04:09 AM
could you backfill with sand for say two feet instead of screened top soil.

We only do it if the pipe wold be exposed to sharp rocks or stones, in order to protect it to get damaged. Otherwise not worth it. Rain and gravity is a great compactor.....

sand is much much cheaper here than screen top soil.

We do not screen backfill. As doc pointed out the only time we would change from the earth we dug up was if it was injurious to the pipe.
j

could you backfill with sand for say two feet instead of screened top soil.

The economic benefit depends on how much that costs you and the conductivity of the materials. Underground transmission lines sometimes find it beneficial (they can overheat). Utilities are more scientific about it and measure the soils with something like this. http://www.thermalresistivity.com/