Making design choices on a 10' diameter earth bag root cellar project. We have very poor soil percolation and have already prepped a compacted raised pad to build on and will build a hill over the structure. We completed an earthbag root cellar at my neighbors property and I hope to improve this one based on the experience. The existing structure has an interior diameter of 8' and we used the catenary curve for wall structure. The inherent stability of that style is a proven technology, yet has some drawbacks that I would like to avoid. It yields a sloping wall with excessive height in the center. The compound-curve wall makes storage more difficult and the added height requires more fill for insulation and an overall larger profile to the resulting mound. I would like to do a circular structure, but with vertical walls. If we use earthbags to the level of the door lintel and cap the bags with a concrete bond beam and build a robust insulated roof attached to the bond beam, I'm hoping to reduce the overall height of the mound while retaining sufficient insulation and earth cover. The structure will be used as a storm shelter as well, and the interior space should be more accommodating with the vertical walls. A local expert on root cellars suggested that I tent the entire structure with a layer of foam board sandwiched in the soil covering. I am considering that option along with internal earthbag buttressing to resist the thrust on the walls. The interior buttresses would take up interior space, but I could use them for shelf supports or as dividers for different zones. I am in an area where 5' frost depth is not unusual.
Cool you already got some experience and a raised tamped pad for your slow draining soils. As far as thermal mass performance on an above ground EB root cellar (sandwich foam, berm, or insulated berm, etc.) depends on your location: climate, shade, wind...
As far as roof choice, for a round house: shed, conical, octagonal, lower pitched dome with permanent formwork. Personally with that safe easy diameter, I'd suggest berming a hexagon/octagon with 6/8 interior posts for easy attachment to rafters, door, shelves. This will most effectively utilize the storage space for that size and shape. Interior Posts and shelves will help retain berm Oehler style.
Thanks, Christopher. I hadn't thought of using interior posts for both roof support and shelving. I have been thinking of yurt type roof construction with a compression ring at the peak and a cable tension element towards the heel of the rafter. I would also tie them to the bond beam. Besides insulating the roof and covering with earth, a used grain bin roof could simplify and reinforce it as well. I have used epdm roofing for a vapor barrier. There are occasionally silo roofs on Craigslist.
That isn't a commen bin size. You would have overhang.
You might like a Hogan roof. Google image search that. Don't know your resouces.
Gonna tamp some berm retaining wings beside your door like Owen's bermed shed?
Please pardon my ignorance but I was under the impression that you don't need buttresses on a circular wall. Maybe that changes with roof choice but I'd think you still wouldn't need one if you have a bond beam on top. Or would buttressing just be needed near the door which would be a break in the circle? With 5' of frost, most of the root cellar will be in the frost zone so I'd be leaning towards insulation myself. The "warm" floor will be fighting the frozen ceiling and upper walls and you want the endpoint to be above 32F. As for the roof, a 10' span isn't that big, can you just use poles or beams and have a flat roof?
I think it's a matter of degree. The catenary curve is stronger than a vertical curved wall. Buttresses are another tried and true way to increase strength. Given our frost depth, the amount of fill needed for insulation is a significant load. If I integrate another type of insulation (i.e. foam board), I can reduce the amount of soil covering. Because this structure will be its own independent hill, I'm not as concerned about the hydraulic pressure as I would be if it was part of a larger drainage area. It would be great to see examples that are comparable and how they perform. Given the variables with siting, materials, skill, and regional climate, it's challenging to make comparisons. When I sent questions directly to some of the pioneers in EB construction, they weren't ready to make recommendations for underground construction parameters. It's great that we have forums like this one to share our efforts and hopefully accumulate pertinent knowledge.
A flat roof would certainly be easier. I think my preference for a pitched roof is from the inherent ability to channel water away...and experience with failure of flat roofs on above ground structures. I'm not ruling a flat roof out yet.
I think I'm catching on now. I was imagining a curved above ground wall without tons of dirt pushing on the sides of it. If I imagine the root cellar as a soda can, the pressure from the sides may crush it halfway up. The "underground" element of your project is interesting.
How much dirt do you plan to have up against the sides? And is it primarily for insulation and to make the whole root cellar look like a hill? If it's going to be thinner than 5', it may still freeze solid. Have you considered doing a cylindrical double earth bag wall with insulation between the walls? That would give you a decent thickness of wall (3'?) plus some insulation. You could even cantenate the outer wall to look sexy or support a thinner covering of dirt. Of course then you're doing twice the earthbag labor to build the cellar. But you'd have straight walls.
Regarding flat roof and drainage, how about if you do a flat sloping roof? Just build up one side with an extra layer or two of bags to generate a small pitch. The dirt pile above it could still be a symmetrical hump but the underground drainage would still exist.
A circular structure will not need internal buttresses to resist earth pressure from outside, except as noted at the door. The curved walls will transfer the load sideways. You would need a strong doorframe for this; a stout frame would eliminate any need for buttresses. A bond beam at the top would stiffen the whole thing against any unevenness in stresses. The slightly sloped flat roof would be considerably more reliable against leakage than a dead flat roof, and still easy to build.
With a 5' frost depth, you would definitely need some insulation near the door, and around the upper walls and roof would help.
I'm not addressing the possible issues with underground earth bags, as I don't have any experience with them. What sort of precipitation do you get? Unless you are in a normally dry area, I would have concern for the earth bags getting waterlogged and softening, possibly letting the fill slump in and collapse the whole thing. The perimeter posts would help protect against that.
Thanks for the feedback. The earthbags will be protected from outside precipitation with a covering of epdm and/or heavy plastic vapor barrier. The EB root cellar that we built at my neighbors has more issue with interior moisture. Improvements are being made with that ventilation system that should help.
One other issue that I hope to address is the potential for radon exposure. I understand that I can test for it and that there are ways to mitigate should it prove to be unsafe levels. The structure will be used as a storm shelter, as well as for food. I doubt that the time it's used for personal shelter would provide sufficient exposure to be an issue, but I don't know if it would affect the food that is stored. I would appreciate hearing from folks that are knowledgable about this.
Moisture is a good thing in a root cellar as long as you have air circulation. The keys are temperature near 35F, humidity near 95% and some air circulation (not sure exactly how much). The air circulation would likely take care of the radon issue but I'm not sure.
95% humidity, especially where the walls will be colder than the interior for maybe six months of the year, would make me concerned about moisture traveling into the earthbag walls and softening them. Maybe it will not be an issue, but I would keep an eye on the condition of the upper walls in winter.
Diego Footer on Permaculture Based Homesteads - from the Eat Your Dirt Summit