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buried cistern inside a hugleculture mound...

 
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I'm looking at buying a manufactured, plastic cistern tank, designed to be buried underground.  I'm planning on putting it on a plot of property, with a small catchment system (for now) in advance of a cabin or wofati living structure to be built later.  I'm considering making this cistern as the 'center' of a huglecultre mound instead of a berm.  I'd like to know how wide/thick the "slope" pf this mound should be, as well as how thick I should make the top.
 
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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.

For the record, I think it's a good idea once you figure out the details. It bothers me to see tanks like that exposed to the sun as I think they'd last *much* long protected from UV, and water that's kept cool, will have better quality for longer also.  However, a bit more info about what you're thinking would be helpful. For example, if your could partially dig into a sloped bank, that would give you dirt to work with and decrease the overall amount of dirt that needs to move.


 
Creighton Samuels
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Jay Angler wrote:So you're planning to "earth berm" it above ground?


Sort of; but mostly because while the soil is quality in this area, it's not very thick.  It's about 4 feet to the limestone bedrock.  So even if I scratch all the way down to the bedrock with a "low profile" type cistern of about 5' 6", the top would be sticking up above the natural grade by a bit.  I'm also considering this mound to be the elevated base for a yurt, but that part still needs thinking through.



But you're calling it a hugel mound because you want to bury wood in the dirt you're berming with?


Correct.



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.


This is the type that I'm looking at...
https://www.tank-depot.com/productdetails.aspx?part=A-AST-0325-1W
...although I'm just as likely to get this one's bigger brother...
https://www.tank-depot.com/productdetails.aspx?part=A-AST-0525-1W



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.


Is that the "riser" thing?  How much of a riser should I plan for?  16  inches seems to be the tallest that is offered with these particular cisterns.



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.



The frost line in Kentucky is about 18", but even then I'm not very worried about that because I expect the slopes to be thicker than that anyway, and the top might be covered by an occupied yurt.  Eventually, anyway.  As far as the steep slope goes, I've heard Paul talk about the need for a steep slope, but I've never heard the explanation as to why it's preferred over a more gradual slope.
 
Jay Angler
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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. I also don't see the point. The purpose of the hugel mound is to grow plants without irrigation. Plants aren't going to grow under a yurt. Is your land sloped? If so, I could see putting the hugel mound at higher elevation than your yurt so that you can gravity feed the water to the yurt? If it were me, I'd sink posts down to rest on that bedrock and build a yurt platform attached to them and have the yurt firmly attached to the platform so that no matter how bad a storm blows through, I'd feel safe. I admit I'm getting a little anxious about how much bigger the storms have been getting and I believe in planning for what we think will be the worst to the best of our ability.

 
Creighton Samuels
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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.


Is that one rated for underground? It looks like an aboveground tank to me.



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.


I like the box access with a removable fill bag idea.


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.


This won't matter, because I will be using a small collection surface until I move there, then changing it to a larger surface but remote.  In the meantime, I won't need much of a collection surface because there won't be a demand upon the water.  And there's plenty of rain in Kentucky for the hugel to soak up besides.


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.



Yeah, building on a shifting mound probably wouldn't be wise, and it would shade too much of the slopes for reasonable growth.
 
Jay Angler
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Creighton Samuels wrote:

Is that one rated for underground? It looks like an aboveground tank to me.  

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.
 
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