Jay Angler wrote:So you're planning to "earth berm" it above ground?
But you're calling it a hugel mound because you want to bury wood in the dirt you're berming with?
It's just your description wasn't quite clear to me, so I figured clarify first, but if the answers are "yes" and "yes", I would start with deciding the size and shape of the tank.
If it's a round tank, usually they have some sort of a clean-out entry large enough for a human on the top. You'll have to plan for that by either adding some sort of extra tunnel above the entry, or arrange things so that you can dig out the entry when you need to access it. From dealing with our septic tank, I recommend the first idea over the second.
Next I'd consider what your low temps are like. In my area it's rare to get serious freezing for long enough that a large tank with even minimal protection would freeze. "Frost Line" is an easy bit of data to find, and if it's say 3 ft, I'd be aiming to make sure all parts of the tank were covered by at least that much to start, and then you'd have to consider how much extra to add near the bottom to have a stable slope. Base Camp's done a bunch of work on how to make tall hugels with fairly steep sides, so maybe someone with experience with those will speak up on techniques they've used.
Jay Angler wrote:Have you considered tanks more like this shape?
It would allow you to make an even sand base, and then build your hugel around it. This shape of tank is popular in my area where we get a lot of rain in the winter and a drought for months in the summer.
The shape you've linked to seems to me as if it would be harder to build a hugel mound around - would wood go under the round part of the tank, or only soil there? Water is *really* heavy (1 kilogram/liter) so even a small tank will put a lot of pressure on the ground under it and you don't want anything puncturing it.
So far as the cleanout -I was thinking a wooden box about 2 feet high with a hinged lid would be fine. If it was me, I'd fill a cloth bag full of some sort of insulation that I could set in the box over top the cleanout hatch so it was easy to lift the lid and lift out the bag to access beneath it. That way you can have 18 to 20 inches of soil on top of the tank without worrying about the dirt getting into the cleanout area, but still having enough soil depth that things will grow without needing too much supplemental water.
Remember, the point of a tall hugel is that it stays wet enough for plants to grow without extra irrigation. I think if you have a tank filling up the center of the mound, you will need to make the mound fairly large to get the same effect, or you will have to compromise and irrigate to some extent.
You keep mentioning that you might build a yurt on top of this mound. Hugel mounds contain wood. My understanding is that the wood decomposes gradually over years, but that process results in the hugel gradually reducing in height. I would be uncomfortable using it as a "foundation" for even a temporary house.
Quite likely - I was more showing the "shape". That said, a tank buried in the ground will have more force on it than one buried in a hugel mound, but either way, I was able to find vertically and horizontally cylindrical water tanks rated for burying - much heavier ribbing on them, but that would help the hugel dirt stay in place better I expect. Alternatively, if you're using a cheaper tank, you could give it extra support either internally or externally. You probably don't want huge amounts of dirt on top unless you're sure the tank's rated for that.
Is that one rated for underground? It looks like an aboveground tank to me.