Backfill material choice for climate battery/GAHT

Hi all:

I'm working on a 36' high tunnel with a GAHT system to help extend my season a bit in zone 5B.

I have most of it well sorted out but have some questions about backfilling around the tubes. The soil in this spot is nasty clay that doesn't drain worth $#@!, and I know that won't work as it needs to drain moisture that condenses out. So I am going to have to bring in alternate backfill.

As the GAHT pit is about 24x16', for each 6" of depth I am looking at 7 yards or ~10 tons of material which I'll have to buy and move 50-100 yards from my driveway to the building site as I can't get a large dump truck all the way there.

The gravel pits and quarries around here have a lot of different stuff available such as:

- Pond sediment, dead sand, or "silty fill" ~$8/yd
- "Dry screened state road sand" $17/ton (assuming this is just mixed size sand/grit of the kind they put on roads in winter)
- Washed native stone in a variety of sizes $19/ton
- Screened topsoil $30/yard
- River rock $80/ton

I have two questions:

1. Any comment on which of these would be good/better/best or a bad idea?

2. Does the fill above the tubes matter, or just what is below and immediately around them?

In other words, if I had a single layer of 4" tubes, could I go with a 6" layer of sand/gravel fill that encases the tubes, and then backfill with the existing junk on top of that? I could put a layer of filter fabric or something on top if infiltration was a concern.

I am also planning to build a slight slope into the pit with a drain tube at the bottom edge so that any condensate or ground water can easily exit.

I do have a small tractor and mini-ex so I've been doing the excavation myself. Depending on the amount of material I need to bring in I might look at renting a mini dumper or small loader for a day just because my little 1025R is good for about 1/3 yard per trip and I'd like to avoid having to make 100+ round trips with a little machine.

Hi Colin,

the way I understand it you want high thermal mass and high conductivity around the tubes. The air flows through the tubes, exchanges heat with the tube and surrounding "dirt" and gets warmer (at night, in winter) or cooler (summer, midday). So you need material that absorbs and hold on to heat.
The condensation that you're talking about will happen when you cool down hot (moist) air and will happen inside the tubes, not in contact with the backfill. So I don't see why it shouldn't work with heavy clay. It might actually be your best material.

But maybe I'm misunderstanding the whole setup. GAHT didn't get me many results googleing.

My understanding of earth tubes is that they only work if constructed at deeper depth and level.

Dumping a load of washed rock or even river rock could disrupt the level.

I like Benjamin`s  suggestion of using clay though the silty fill might work and it is the cheapest.

Do you not have an area where you would like a pond and can use the dirt or fill from there?

Benjamin Dinkel wrote:
The condensation that you're talking about will happen when you cool down hot (moist) air and will happen inside the tubes, not in contact with the backfill. So I don't see why it shouldn't work with heavy clay. It might actually be your best material.

Every single book/article I've read about these systems states 'heavy clay soils will not work because the condensation needs to be drained.' You use perforated drain pipe for that reason.

Anne Miller wrote:
My understanding of earth tubes is that they only work if constructed at deeper depth and level.

"Earth tubes" refers to a different type of system that's (mainly, IIRC) used for cooling, at least in greenhouse applications. They work on the (relatively) constant temperature of the ground at say 6+ feet down.

GAHT is a totally different architecture and works by taking excess heat generated during sunny days and heating earth below the greenhouse, and then pulling that heat back out at night.