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Cob Dome: possible?  RSS feed

 
Joseph Russell
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Not too sure if it's even possible, so I figured I would ask the opinions of the experts. I have seen videos of people building cob pizza ovens, using sand to form the dome. My thought is, could this be used on a large scale. I know the cob has to be right, and it may be a good idea to have an extra layer or two of the limestone plaster, since it will have full exposure. Maybe 12" wall thickness? But could it actually be done. Will it work. How long should it dry before digging out the inside of the sand dome, if the walls are a foot thick. Maybe I should start with a small shed size hut?
 
John Elliott
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How large? Do you mean a habitable space? I wouldn't want to inhabit one, especially if there was any seismic activity -- it's not going to be stable.

Remember that a dome is just a three-dimensional arch, and the principle of an arch is that it transmits its falling down stresses through solid block elements until they are loads directly down on the walls. With cob or adobe or any pour-and-harden material, you don't have those loads being transferred through integral blocks.

Now there's a whole lot of interesting building you can do with prestressed concrete, where the rebar is put into tension, the concrete is poured and it hardens, and then after when the forms are removed, the tension is removed from the rebar, putting the whole concrete piece into compression. In principle, the same thing can be done with adobe or cob, you just have to know your material and what you are doing with it.

Now you can take bricks or stones or other blocks, make your arch or dome, and then put a cob or adobe coating on it. That is going to have structural stability.
 
Jay C. White Cloud
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Not to be contrary John, but the "matrix" of a domes physical context has little to do with it's tectonic integrity as long as that matrix is of "sound" composition, whether in block or some other extruded and/or malleable material, such as cobb. Now I would stress that from a logistical perspective you probably can achieve a more functional shape with rectilinear design. Even Buckminster Fuller finally concluded that even though circular and parabolic space is efficient in design enclosures, it is not often sustainably functional, especially for durable habitation.

A PE is going to be warranted. They are going to want testing. They will require a consistent mix ration for every "lift" of Cobb applied, and/or a uniform application modality, most probable with an additional internal or exposed "wattling" or timber work- (while a pure Cobb form could work, but again is not warranted, compared to other methods of "inclosing the same volume of space.) You are probably familiar with earth bagging methods. This combine with a Cobb filler would most likely be one of your easier solutions if you really like this spherical shape esthetically, or a wattling armature in an upside down nest to Cobb into.

Keep us up to speed Joseph if you pursue this line of design further...

Good luck,

jay
 
Miles Flansburg
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Howdy Joseph, here is a thread that talks about cob domes.

http://www.permies.com/t/8822/cob/Adobe-Test-Dome-Diameter
 
John Elliott
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Jay C. White Cloud wrote:Not to be contrary John, but the "matrix" of a domes physical context has little to do with it's tectonic integrity as long as that matrix is of "sound" composition, whether in block or some other extruded and/or malleable material, such as cobb.
jay


Ah, but it does, Jay. That matrix needs to harden up to its ultimate strength, that's why bricks get fired to speed the process along (plus surface vitrification is a big help). Or if you have plenty of time, you can wait for concrete blocks to obtain their ultimate strangth, which takes 30-50 years or so. Since that takes so long, it's better to spend your time quarrying blocks (which have taken geological time to harden up), stack them up until you can put a keystone at the top, and hold the whole thing together with a cementitious mortar. Even then, when the ground starts shaking, cracks are going to show up at the weak spots, which are the mortar joints.

While I would be queasy in a newly finished cobb dome, I have nothing but admiration for cobb/adobe/rammed earth/"matrix" constructions that have completed their chemical reactions and stood the test of time:

 
leila hamaya
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i have heard of people making a huge armature (which is what you would call it in clay sculpture) basically an enormous lump of flamable materials and extra things that are sturdy enough to add support to hold the dome up from the inside while building....then in the end setting the middle on fire to bake the clay dome.
not sure how wise this is, and theres lots of other materials, but its a neat idea anyway.

why not earth bags instead? i think this would be way more stable

http://www.earthbagbuilding.com/articles/riceland.htm
 
Jay C. White Cloud
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Hey John E.,

Well, again, I am not sure that is completely true. The majoring of earth architecture in the world, present and past, is of an unvitrified earth. Adobe brick can be laid up in vault, arch, or dome format, depending on cultural application or individual skill set. The brick format, as I had mention earlier is probably the easiest modality approach to this effort though a large scale armature that you later remove would also work. I would agree that an internal permanent armature is probably best if you want the most seismic resistance, as "firing" a house sized dome, (a very interesting prospect) would not be cost effective fiscally or logistically, and is not necessary at all. Middle Easter, and some Asian faulting and dome methods out of earth have been around for millenia. As simple Google search for earth dome or vault will give a lot of information. If you know how to do research in Persian, Arabic, Turkish or Chinese, you are sure to find even more.
 
leila hamaya
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earth building with forms is much much easier, and much more forgiving for the beginner. it might be faster....

form building being bricks, earth bags, maybe even consider slip form, slip straw, with some kind of framework to hold up the top of a dome.
 
Joseph Russell
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I did originally think about the earth-bags. But the big setback is the cost. I do need a good size dwelling, due to the fact that I have 2 special needs step kids that will need to stay with us indefinitely. The idea of the sand dome came to me from watching a pizza dome being made, and I have a acquaintance that I can borrow ALOT of sand. I'm not even sure if I calculated it correctly, but just to purchase the earth-bags, would be around $5k. I'm thinking I may scrap the idea of a dome, and make forms for adobe blocks. I am working on getting the money up for a bunch of books, most of the research has been online. But thanks to everyone for the input.

Joe
 
leila hamaya
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i havent done that much with earth bags, but when i have done small projects i got all the bags for free. old feed bags, and big rice bags are some you can manifest for free sometimes. hard to find when youre looking for them, but i was given a huge amount once that an elder person had been stockpiling and saving for a long time.

i suppose people would want the best bags, but i actually dont think it matters much. because the place isnt held together by the bags really, they dont have to be that strong. the whole point is that whats inside the bags turns into a brick form by being there and getting tamped down and slightly moistened.
 
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