Shape and Support of Earthbag Buildings
Questions answered by Kelly Hart
Shape
Domes
Elliptical Domes
Vaults
Vertical Walls
Buttresses
Bond Beams
Attaching a Roof
Multiple Stories
Building Underground
Ponds, Pools and Cisterns
Q: Can you use the earthbag technique to build a house of any shape other than domes?
A: Earthbags can be used to build a house of almost any shape, but care must be taken in the engineering design to allow for sufficient reinforcement and buttressing of the walls where needed, such as with buildings that are more rectilinear, with more or less conventional roofs. There are, of course, advantages to domes that are worth considering: they are generally stronger under stresses such as wind or earthquake, they use less material in creating the same amount of living space, and they do not require other tensile materials, such as wood, to enclose a roof area. Even vertical walls if they are curved are inherently stronger, especially with earthbag construction.
Q: I am going to build a lighthouse cottage on the ocean! The main structure will be round but preferably a little higher than a normal earth bag dome and I am looking at using a different type of roof rather than the dome. I am not really sure... I am also planning to attach a smaller "pod" or two to it. I want to see if you have any recommendations with respect to the best type of roof to use for my design.
A: A circular cylinder of this sort should be quite strong and an excellent shape for using earthbags. This is a fun design. There are many possible roof concepts that would work with this. One would be a dome, which could be either somewhat conical and be made entirely with earthbags, or it could be hemispherical if a rigid framework is provided for it. Another possibility would be a simple shed type roof that truncates the cylinder at an angle. One consideration is whether you want to have a look-out on the roof, in which case the shed roof could be raised enough to provide this. This type of roof would need to be made of other materials than earthbags, obviously.
I was confused at first in trying to figure out how much I would have to bring the bags in with each layer by the cm or inch and was looking high and low for formulas on truncated cylinders and cones. In fact, it turns out that I am building a frustum rather than a cone or cylinder! I am concerned about making sure that I get the exact angle on the walls without losing any stability. Do you think that it will be OK to go from 20 feet to a 10 or 12 foot second level (supported only by cross beams to support the loft). Also, is there an easy method to bring the bags in on the right angle?
I have a vague memory of being introduced to the concept of frustums in solid geometry in school, but I have not heard the term for quite awhile. As far the angle of leaning or decrease of the walls of such an earthbag frustum, I would suggest that you can safely incline the wall by as much as 60 degrees (with respect to the horizontal) without compromising safety. The lighthouse design you showed earlier doesn't even come close to this...so I wouldn't worry about it.
The most accurate way to measure exactly where to place the bags as you go up would be to create a central column of pipe that is well braced into place, and use this with an adjustable compass that is attached to it. Then, for each course you figure out from a scaled drawing exactly what the radius should be at that point and adjust the swing arm accordingly.
What formula would one use to calculate the way to set the compass?
The simplest way to do this is to make an accurate scale drawing of the shape you want to achieve, looking from the side, and then divide this into vertical intervals, of perhaps every 6 inches, and draw horizontal lines across at these intervals. Where they intersect the shape line can be measured, either to the central axis line, or as a diameter across the entire distance and then halved for the radius.
Q: I'm looking for ideas to improve the basic process of making walls of very lightweight material. One potential glitch is getting the bags to align in straight, vertical walls. Lightweight bags will shift around more than bags of tamped earth. Maybe you can think of something better, but I thought of building a light gauge metal frame of rebar that's covered with plastic mesh. Rebar is cheap and easy to weld. It would take extra time and money to build, but it should speed up the bag stacking process and get walls better aligned.
A: This is an interesting idea that would be fun to try out some day. I think it depends on the design and shape of what is being constructed how well it would work. When I was doing some design work for a couple who wanted a more hemispherical earthbag dome, I came up with a concept that fits in with your ideas here. I never got to making any drawings for it, but basically it was to make a welded truss with rebar curved to the exact shape that you want. Only instead of making triangulated braces between the two parallel arcs, the cross pieces would be welded so that they would be horizontal when in place. This would allow the earthbags to be laid directly in between the two arcs, and also between the cross braces. I think that this system could be quite strong, and would eliminate the need to have any other forming or placement system while building. Each arched truss would be attached to a central hub, and perhaps also embedded or attached to a foundation bond beam of concrete. I also think that a system like this might allow the construction of larger diameter earthbag domes.
Q: Is a round dome (or corbeled dome) the ideal shape to withstand high hurricane winds?
A: I believe that circular domes have been proven to be the most stable building shape (other than underground structures) under severe wind conditions.
Q: How would a dome shape vs a non-domular roof effect the basic price?
A: A dome most certainly can be built for less than a standard roof structure, since all of that material to frame and sheath a roof is eliminated.
Q: Would building a round structure be a bit easier to erect than connecting several connecting domes?
A: It is always complicated to connect domes together because you have to figure out how make the connection between them, whether it is a vaulted passage, or they intersect in some way, allowing the passage. Vertical-walled structures are easier to build in this regard.
Q: According to the size of the structure, how do you know how much to reduce the size of each row of bags as you proceed up in order to come together at the top to form a dome?
A: Well, there are various shapes that you can attempt to build, and the shape that you want determines how you place the bags. A hemispherical shape is difficult to create because the higher you go up, the more the bags must come in, and it may eventually become unstable, unless there is a framework built up to support it.
The domes that I have made are generally measured with a piece of string or rope, or a rigid piece of pipe. I would first lay out the circular perimeter the way I wanted it, then use the rope or pipe to measure for the placement of the subsequent layers. I basically created an arc, with the fixed length describing it from the perimeter at the base, through the center of the circle to the opposite side of the wall, and rising upward to about the height of one story. At this point I set up a simple tripod of poles to use as a guide for placing the rest of the bags. In other words, the top of the dome is actually a cone, going up at an angle that would allow head space in the loft area, and not come in too abruptly for the bags to need support. I would suggest not coming in any more than a cone that would be described by an equilateral triangle, if that makes sense.
Q: Has anyone come up with a height /diameter ratio that makes bag building unstable as far as the cone slope is concerned? I plan a circular structure 20 feet diameter (outside measurement) with a 13 feet height to the apex.
A: Not that I know of. Our pantry has a slope of about 45 degrees, which I think would not be stable without the pole framework to help support it. A safer angle would be about 60 degrees, which would make your height about 20 feet. This would be a 1:1 ratio of diameter to height. I would say it partly depends on what shape you want the dome to take. Also, if you want a roomy feeling at floor level, you don't want the walls to come in very sharply.
Q: I'm still not sure how to increase the corbel size radically at the top of the dome. As is, we increased the corbel each row, usually 2" and sometimes as much as 3". It's a little tricky going beyond that, but others have done it so I know it's possible.
A: You can corbel the bags quite a bit more than 2 or 3 inches, especially when you get that high and the radius is getting small. It is a little surprising, but the tight circles actually don't want to fall in as much as the larger ones.
Q: Also, Kelly, in your experience, what would be the biggest diameter you would build without an elaborate support structure?
A: Our bedroom dome is 16 ft. interior diameter and is totally self-supporting. I might try one that is say, 20 ft. It helps having a loft at about the 8 ft. level to tie the structure together at that point and make it more rigid. This also provides more usable space and gives a platform for working at the higher levels.
Q: I know that the size of domes are all a matter of personal taste and budget, but I wonder if domes for bedrooms will be sufficient at 11 ft. in diameter. I plotted the sizes of the various domes and they all seemed too small with the admittedly modest diameters. (5 for the pantry, 15 for the living room, 13 for the master bedroom, 11 for the other bedrooms.) Will increasing each of the diameters by half have a severe change on heating and cooling? (I am going to use on-site soil and each room minus the pantry will have lofting to provide some extra storage space.)
A: The bedroom that we happily occupied was 16' diameter, which allowed space for a curved earthbag staircase up to a small loft. A 14' diameter room would allow space for a smaller ladder to access a loft. Anything much smaller than this will not have much loft space. The difference in size should not affect the thermal attributes much.
Q: What is the largest dome structure we can build and use just scoria in poly rice bags (as opposed to cob and scoria in poly tubes)? We would like to build it about 18-20 feet in diameter but we're worried about structural integrity. But we will be building a loft, too. Does that add greatly to the structural stability? Then we would have to build it as high as it is in diameter at the base, right? So 18' x 18'?
A: The largest earthbag dome that I know of is one that I build, which is an elliptical one, roughly 20'X30' on the inside. I don't recommend trying ellipses after this experience, since I had to rebuild it once with a rigid pole framework on the inside to help support the shape. With a perfectly circular dome, you should be able to go up to a 20' diameter (interior) without too much concern. I built my dome with rice bags filled with the scoria. Yes, a loft within the dome does help support and rigidify the structure considerably. Depending on your design, it will likely be at least as high as it is wide at the base.
Q: I'm looking at building a multi-dome home within the next few years. How much weight can a larger (20-24 foot diameter) support? The reason being I live in the Upper Peninsula of Michigan and we get quite a bit of snow in the winter.
A: You don't say what the dome would be made of. This can make a big difference. Monolithic concrete domes can withstand enormous pressures; geodesics do pretty well, depending on what materials go into their construction; earthbags are perhaps less certain about their strength in this regard. My earthbag/papercrete dome withstood quite a bit of snow in Colorado, but I built it with a rigid pole framework on the inside.
Q: The dome may be easier, but as I would be trying to avoid shear stresses to maximize span, the coil of soil has to be shaped uniquely with each lift. Something has to be in place to carry the horizontal loads until the dome is complete, and there almost has to be an 'eye', which would be a rigid frame that can carry all of the compressive loads. All of this becomes very complex. I like the simplicity of a corbeled dome, but it can not be nearly as strong (thus, wide) as a property designed and built catenary (or nearly) version. The math says I can get a 20' radius, 13' high in the center, and still carry the hoop stresses around the perimeter with a 1" steel cable. From your experience, how would it be best to construct a catenary dome?
A: The trick with building a stable earthbag dome is to keep it perfectly symmetrical (circular) and use the two strands of barbed wire between each course. It is the wire that constrains the horizontal forces. The bags of soil can handle all of the vertical compressive loads just fine. The other trick is to not corbel the bags at a rate beyond about a 30 degree angle, making the dome somewhat more conical than hemispherical. This can change for the last few courses at the very top if you want.
C: I think catenary allows a much larger dome. The corbeled dome requires fairly thick walls, and relies on the shear strength transferred by the barbwire. You are missing a great deal of the available strength that is within the bag itself. On paper, a catenary dome 12m radius, 8m high has an maximum internal stress of 336 kPa, (assuming self weight=24 kN/m3, applied distributed vertical load of 6 kN/m2) Using the test results from Colin McDonald at Queens University, one can get 750 kPa from rejected feed bags partially filled with soil. With stronger bags, uniform filling techniques, and the best aspect ratio of each bag, much higher loads are achievable. Thus, the size of the dome can be huge.
The minimum thickness of the wall is estimated by comparing the difference between the catenary and the parabola of that size. The parabola is the right shape when self weight is much less than the vertically applied loads, and the catenary is the right shape when the self weight is much greater than the vertically applied loads. The line of force always acts between these two shapes, regardless of the loads applied. The line of force must always pass within the middle third of the wall. Thus, the difference between these two curves must be less than 1/3 of thickness of the wall. To be safe, we can assume the building is built 'self-loaded only', and the catenary governs, and the line of force acts through the middle of the wall. Only half of the middle third is then available. Thus, the wall should be at least 6x the difference between the two curves, without any further analysis. I get the dome thickness should be at least 8.2% of radius. A more detailed analysis may be able to reduce this substantially, as the applied loads may be significantly less than the self weight.
I was thinking I could use plywood forms during construction. As the height is 4m, I need poles 4m long. When the forms are placed near the bottom of the dome, the poles need to be anchored 2m from the center - use steel rings set at specific radii. Each 4'x8' wide sheet would be anchored against 2 rings, with some overlap in the top corners. 15 sheets would do the first meter of height, all the way around. 12 more would be needed for the next meter, and would sit on the lower tier. A total of 5 tiers would bring it up to less than 5' from the center. One the 2nd tier is complete, the first tier would be removed. It will shift slightly, as the bag's shape change with the change in loading, but I'm pretty sure it won't go far.
A: I hope that you document all of this well when you get around to building this. I would love to feature it at this site! I am not trained as an engineer, but what you say sounds reasonable to me.
C: Beware of any unreinforced construction method, especially in earthquake country. You will recall from news stories about earthquakes in many Asian countries that they have high death tolls. This is often because in many impoverished countries metal reinforcement ("re-bar") is omitted, to reduce construction costs. In an earthquake, such buildings just collapse. Sandbag construction and Earthships must be reinforced. This is best accomplished by placing re-bar vertically on two-foot (or narrower) centers through the wall stacks, making sure that the top of each piece of re-bar passes through a wood top sill, or that it is at least firmly wired in place.
A: (Owen Geiger) Vertical rebar through earthbag (sandbag) walls is not necessary, as shown by tests at Cal-Earth and the US Military Academy. (Please refer to the Earthbag Testing webpage at for more information.)
Earthbag building is exceptionally strong and ideal for survival shelters. Tests have shown all that is typically needed is two strands of 4-point barbed wire between courses. Reinforced plaster (plaster with stucco mesh) on both sides and a reinforced concrete bond beam add further strength and is all most people need. In earthquake regions, Gripple tensioners (a tie-down system) or the equivalent is recommended.
Also note, earthbag building is much faster and easier than building with tires and rammed earth (‘Earthships'). Not convinced? Time yourself ramming earth into one tire and compare it to filling and tamping the equivalent length of earthbags. Multiply this times the number of bags or tires needed in your design.
Q: If one were to use a removable form system, would it work to construct a semispherical dome by tamping each course of bags at the appropriate angle in the way one does when building an arch?
A: This approach might work to form an earthbag hemisphere, but I would never feel totally secure living in it without some sort of rigid interior framework. I did once design and build such a framework for a small earthbag dome that was created with a wooden geodesic framework that was actually quite handsome when finished. I think if you didn't want a framework, then an catenary arch would be inherently stronger and more durable.
Q: I'm building my dome shelter by following your instructions in "Building Riceland." Its up to a little over 3 feet in height. I deviated from design by installing another dummy door for later expansion to another dome. Hhow do I interface another dome at the dummy door opening? Pictures at www.autobestbuysweekly.com/Dome
A: As for interfacing the domes later, there are several ways, depending on how you want the space to be arranged. One is with a small vault between the two domes, and another would be to actually overlap the domes slightly, so that the second one rests partially on the first.
Q: How to corbel the roof in earthbag construction? What is the formula to keep the inclination from falling? Will it be possible to construct a Nubian vault over rectangular walls of earth bags? (Nubian vaults by adobe bricks / smaller earth bags).
A: Earthbags can easily be corbeled in a dome as long as you don't exceed about a 60 degree angle (from the horizontal) as you go in. Anything over this is a bit risky, and may require formwork. Vaults with earthbags are much more difficult, and certainly require forms to accomplish. I made a vault spanning about 8 feet (2.5 m), and it required a very thick base to be stable. I have never seen an earthbag vault spanning more than this, without a permanent form under it.
Q: Here is a sketch of the home / restaurant I want to build in my wife's country of Kyrgyzstan. Any Comments?
A: Your design concept is certainly fanciful and quite lovely, really. Could be built with earthbags? Not easily. Domes that are more of a catenary arch form are more easily accommodated with earthbags; these spherical shapes, or convex shapes would likely require some internal rigid framework to help support them. This is possible, but it definitely adds to the complexity of the project.
Q: I intended to make a 19' dome. Your FAQ says 20' is about the max. But I realized after everything was finished that the outside DIA is 22'.
A: I came up with that 20' max suggestion just as a conservative figure that I knew would be fine, based on my experience building the large dome in Colorado. It was ellipse that measured 20' X 30' on the inside...so my thinking was actually inside dimensions. And it was off the top of my head. Your 19' dome is well within this rough suggestion.
Q: I love the concept of dome homes but am interested in a larger diameter than the limited 20 foot model. I'd like your input on the possibility of building the cylindrical floorplans you have with say about 32 foot diameter (two bedroom round house), but instead of putting a traditional roof on it, buying one of those prefabricated 32 foot fiberglass domes that are out there. Examples of what I mean can be found at domesintl.com or the same thing at bestdomes.com. The question is, do you think this very light structure could be tied down to the earthbag walls with some sense of security that it won't blow off? My hope is that I can build a first floor sunken into the earth (basement) and a second floor at regular earth level, and put the dome on top. This way I don't have to build supports to hold up the dome and I'm still free to design the interior however I like.
A: Yes, I think what you propose is entirely feasible. Obviously the floor between the two stories would need to designed in such a way to be well supported, but that shouldn't be too hard. There are various ways to attach such a dome to the earthbag wall that should be sufficiently secure; a dome doesn't offer much resistance in a wind anyway.
Q: Do dome walls need to begin "stepping in" immediately from the ground level, or could they be vertical to a given height and then begin the corbelling? If the latter, would you need buttressing on the outside to keep the lower walls from bowing out from the pressure of the corbelling above them, or would the curve of the wall be strong enough in and of itself?
A: I think that it is quite safe to raise a circular wall vertically before corbelling in for the dome, without the need for buttressing (except for doorways). This is predicated on the use of two strands of barbed wire running between all of the courses of bags. The wire creates mini tension rings (or bond beams), similar to a yurt structure, which resist outward expansion. By the time you get to the last row of bags on the dome the forces become vertical, which the lower bags can easily handle.
Q: Any way to give it a more circular arch instead pointed?
A: I did once design a completely hemispherical roof for someone and this was accomplished by first building a geodesic wood-framed structure that was left permanently in place and supported the bags toward the top.
Q: We're on the Sunshine Coast of BC (Canada), where we have lovely autumns and warm but rainy winters. So my question is: Is there any reason not to build with earthbags in the winter up here? We're considering a small dome (14' inside diameter) with loft and attached post-and-beam porch. We are also debating whether to go with a dome/cone-on-loft shape, or just cylindrical walls with loft and a stick-framed/cedar shake roof to shed all that wet-coast rain.
A: My sense is that a cylindrical building with a more traditional roof would suit your climate better than domes. The issue with domes is always how to keep the moisture out over the years, and this is easier done with a roof with eaves.
Q: You built a wooden skeletal tipi type framework above the first floor level of your large central section. Would that be necessary with my 20' diameter cone?
A: I had to build this framework because I was working with an ellipse. With your circular project, you might get by without the framework.
Q: I am wondering if earthbag construction can be an oval instead of a circle. Would this too ambitious for a first-time project? What design or structural challenges would you encounter?
A: The large living room kitchen of the earthbag home that I built was an ellipse, and this turned out to be one of the most difficult challenges of the entire project. Because of the uneven forces inherent in such a shape, I eventually had to create a rigid pole structure inside the dome to force it to stay in the shape I wanted...otherwise it would have been unstable. I generally advise people to stay with circular domes for this reason. However, if I did it, someone else could too...it just requires more careful engineering.
Q: My idea: Dig a trench as long as you want and build an earthbag vault. About 2/3 would be below grade, depending on the water table. Add plastic and backfill on top and then add more plastic and more backfill, plus plants. What do you think?
A: It turns out that earthbag vaults are not so easy to construct. The entryway to my house in Crestone is a vault, and it was one of the more difficult parts of the plan to accomplish. It only spans about 6 feet, and this is about the most that I would recommend anyone trying. Even Khalili doesn't use earthbags in his vaults...he switches to stabilized adobes. The problem is the same as trying to make very large arched doorways: it is too easy for the bags to deform and you lose the overall shape. Also, vaults put tremendous outward pressure on the walls they sit on, so they have to be quite thick (my entryway had 30" walls) or be buttressed. Of course this wouldn't be a problem in a dug-out pit arrangement like you describe. A carefully designed dome could be completely buried on the other hand. Domes are inherently much stronger in every way.
Q: I know it is possible to build one dome, then add another at a later date. Is this done through vaults and arches?
A: Yes, you have to prepare the place where you might want to join another dome or other structure in advance of actually doing it, i.e. as you are building the first wall. You basically create an arched doorway, and then just fill it back in with more earthbags, so that when you go in later to create the doorway, the opening just needs to be cleared of the infilled bags. Then a small connecting vault or passageway can be constructed.
Q: I'm thinking of using earthbags to make a set of vaults, much like Nader Khalili has done at Cal-Earth. More particularly, I was attracted to the idea of using earthbags for the end walls and load-bearing walls, but instead of using pure earthbag construction to form the roof of the vaults, I'd use curved rebar to form the roof of the vault and overlay that with insulation and a clay-earth mixture. The reason I would like to do it this way instead of constructing the each vault wholly from earthbags has to do with the width of the vaults, which (as I'm planning them) is too wide to permit this (15' x 25' is my working number for each vault).
A: I don't think that even Nader Khalili used earthbags exclusively for his larger vaults. There is just too much danger of collapse. I think he built up walls with earthbags to the spring line, and then did the vaults with stabilized adobe blocks.
Q: The trouble I've gotten to is how best to build vaults and domes. I could use falsework to support the vault as it's being built, and then lower the falsework slowly, allowing the vault to shift to carry it's own weight. But that falsework must be pretty damn strong, and therefore not cheap. I don't want to have to add $10,000 for a reusable steel frame, if I don't have to.
A: Earthbag domes are way easier to build than vaults, as they are generally self-supporting while being constructed. I did build an earthbag vaulted entrance to my house in Colorado. This was only about 8 feet wide at the base, tapering up to 6 feet where the vault began, and was two bags wide so that it required no further buttressing. I used a movable wooden form, about four feet long, and released this to go on to the next section. From this experience, I would venture to say that attempting an earthbag vault any wider than this would be very difficult, even as a catenary shape. As far as I know Nader Khalili never built any vaults with earthbags; he switched to adobe blocks or bricks or ferrocement for the vaulted portions of his designs.
Q: How to corbel the roof in earthbag construction? What is the formula to keep the inclination from falling? Will it be possible to construct a Nubian vault over rectangular walls of earth bags? (Nubian vaults by adobe bricks / smaller earth bags).
A: Earthbags can easily be corbeled in a dome as long as you don't exceed about a 60 degree angle (from the horizontal) as you go in. Anything over this is a bit risky, and may require formwork. Vaults with earthbags are much more difficult, and certainly require forms to accomplish. I made a vault spanning about 8 feet (2.5 m), and it required a very thick base to be stable. I have never seen an earthbag vault spanning more than this, without a permanent form under it.
Q: In order to build stable earthbag walls 12 foot tall I need a base at least 1.2 feet wide?
A: It is a rule of thumb that all earthen walls need to have a base that is at least 1/10 of the height, so yes, 1.2 ft. should work...which is easy since most earthbags are at least this wide.
Q: I like the idea of vertical perpendicular interior walls. Actually the floor plan I have in mind is roughly a 48' long by 24' deep, with a largely open interior layout but with two interior walls spaced about 16' apart to break up the space and add a little extra internal thermal mass. I assume these would lend some additional stability to that rear wall if they were tied into the foundation, etc.
A: I'm sure such walls would help with stability. Periodic rebar rods (maybe 3/4") pounded all the way through the bags and into soil and tied to the top plate for your roof, would probably assure that the wall remains intact.
Q: My husband and I live in Oregon and are building a conventional pole barn with a hefty 8' retaining wall/foundation on one end. We're wondering if an earthbag structure, with reinforcements, is possible, instead of going with a conventional concrete design?
A: It is quite possible that earthbags would work just fine for your retaining wall/foundation, but the design would need to be carefully considered to make sure. 8' is pretty high for a retaining wall, and there can be a great deal of pressure upon this, which is why reinforced concrete is a more sure approach. Two things that can help stabilize retaining walls is to curve them and also to slant them into the hill or berm. Unfortunately neither of these may work for your foundation. If the wall has to be both straight and vertical, then buttressing is an option, but this would take up space within the structure and might be problematic, unless you planned to have stalls or divisions within the barn that could double as buttresses for the earthbag wall.
Q: I have been trying to get more information about sandbag construction using ecobeams. I have emailed Mike Tremeer, the engineer that developed the system in South Africa twice. No responce either time. Is there some one in the US that uses this system that you know of? Do you know of people other than Mr.Tremeer in South Africa that I can contact? I'm not interested in the dome sandbag technique.
A: I don't know of anyone in the US using that system for building. In fact I have only heard of it being used in South Africa. I have put up this article about it: articles/eco-beam. Earthbags can be used to create a wide range of housing styles, not just domes, as can be seen by browsing some of the projects shown at that site. It is not necessary to use the Eco-beam system for most applications...
Q: I'd really like to build a cylindrical tower shaped building with, in essence, two loft levels (so three floors). The third floor would essentially be an enclosed balcony, with walls pierced with frequent Roman arched windows. To do something this tall, I'm thinking we may need to use double walls - perhaps just on the first floor, perhaps on both the first and second floors. My question is this - what is a good way to tie the two concentric walls together into a monolithic structure? Given the dimensions of the bags, which seem to have an aspect ratio or around 1.3-1.5 to 1 as opposed to the usual masonry 2 to 1, I'm having trouble envisioning something like a rat trap bond that would work.
A: I think that a cylindrical building makes a lot of sense with earthbags because of its inherent stability as a form. I do think that you could go three stories with this. Each diaphragm will contribute more stability. I would suggest perhaps the double wall on the first floor only.
You can tie these two walls together with pieces of barbed wire running perpendicular to the direction of the courses of bags. I did this when building a stem wall for my carriage house: http://earthbagbuilding.com/plans/carriagehouse.htm . If you fill the outer bags with insulating material and the inner bags with soil, this is the best thermally. The insulation on the outside will insulate the thermal mass on the inside from the outside atmosphere and make the home much more comfortable, with a stable temperature.
Q: I understand the beauty of the dome structure in earth bag construction, because it can be done without precious lumber for framing and roofs. If lumber were available on site ,would a round structure support a viga type or other wooden roof structure? Would a truss roof work without pushing the walls outward?
A: Yes indeed, vertical walled structures are possible and many have been built. For some examples see these projects. With a circular building the barbed wire that typically runs between all of the courses of bags will effectively withstand any outward pressures, and with any shape a bond beam or top plate will provide adequate bearing on the bag wall and make the attachment of trusses or vigas a simple matter. In some instances with straight vertical walls it is necessary to provide buttresses or interior walls for stabilization.
Q: If I go above ground, how high can I stack bags with no worry of them collapsing in on themselves.
A: This all depends on the design. Curved walls are inherently more stable. It is sometimes necessary to incorporate buttresses, or interior walls periodically to stabilize a wall. A cylindrical wall could probably go a couple of stories high if it is well designed...
Q: I figure to satisfy the building officials, as well as myself, I will go two and a half feet at the base and go no higher than 12 feet, getting to about 20 inches at the very top.
A: The rule of thumb with earthen construction is that the wall should be at least 1/10th as wide at the base as it is high. For a 12 foot wall, this would be about 15 inches, which is about how thick my walls were using the standard 50# rice sacks. Wider bags would be fine, especially at the base, but 30 inches seems a bit excessive.
Q: What would it happen if a wall starts leaning a little to the left, or to the right? The other day I saw this in the house. I invited a house builder, and he told me the same. He said that I should have used a "plomo." It is a piece of lead, and it hangs from a cord; they use it so the walls won´t lean. I tried to correct the leaning, and I was able to move the bags a little bit, but not enough. So, I am kind of worried.
A: It is best to try to keep your walls vertical, and using a plomo might help do this. Some slight variation is not likely to be a problem; earthbag building is not an exact science. If you are nervous about the leaning wall, I suggest that you prop it up with some braces temporarily so that it can't fall. Also, it might be possible to use a car jack to force the wall to be vertical if it is off very much. It is hard for me to say without actually seeing it. Once you have the wall up as high as you want to go and put in a bond beam up there, and get a roof on there, chances are it won't be able to move any more. The higher it is, the more danger of falling, especially if you have some questionable parts of walls. The curved walls will be sturdier than the straighter sections.
Q: Can buttress walls can be as far apart as 10-14 feet and be as little as 2 feet deep and 2 feet wide?
A: I wouldn't go much more than about 10' between buttresses on vertical walls, especially if they are over 8 feet high. I would say that 2' is about the least depth to use, but they can be thinner, say the width of a bag should work. I like tapered or stepped buttresses that are deeper at the base.
Q: What exactly is involved in building vertical earthbag walls without buttresses? Is there a safe way to build vertical walls without buttresses?
A: The very best design for vertical walls without buttressing is to curve them. Otherwise putting rebar vertically through the bags can help rigidify the plane of the wall, but this will not necessarily keep the entire wall from buckling or toppling. Also, the thicker the wall the more stable it will be in general. Then reinforced concrete bond beams, or wooden box beams integrally connected to the wall, especially at the top will add quite a lot of stability. Such a bonding layer could also be introduced lower down in the wall, especially with rather tall walls.
I noticed that the black South African couple seemed to build without buttresses. How are they doing it?
They are using a unique system of double bags with a void between them, and then using this void to create lintels and bond beams with reinforced concrete. You can see in one of the pictures that the lintel over the windows actually runs the entire length of the wall, so they are basically creating a bond beam at this level.
I also was intrigued by the white South African woman's home that seemed to cleverly hide the buttresses in the design (am I right?) but the black couple seemed to not use them at all.
Using interior room divisions to naturally buttress vertical walls is an excellent idea. She used the "eco-beam" construction described here to build her house, and this system basically uses earthbags just as infill within a rigid framework.
They also weren't used in the Haiti home.
This is a small home with an L-shaped footprint, so there were only two longer walls that might really need buttressing, and it appears that they used interior wall partitions to accomplish this. Also, they did use rather large bags to begin with.
Is it possible to build without buttresses if you dig a deeper trench for your foundation?
I don't think that in general deeper trenches will help much. One approach might be to create periodic vertical posts of reinforced concrete that are buried into the ground, and this could substantially stiffen a wall.
OR If the bags were stacked side to side instead of end to end, would this MUCH thicker wall make vertical walls without buttresses possible? I realize this would use a lot more bags, but do you know if it would work?
Yes, thicker walls help, but doing as you suggest makes it hard to create a running bond by overlapping the bags, and it also means that each bag has to be more securely fastened, which can take time. Double columns of bags are also a possible way of making thicker walls and provide the opportunity to place insulation on the outside and denser, thermal mass material on the inside. Another way of making thicker walls is to taper them somewhat by starting with very large bags at the base, and then graduating to smaller ones as the wall rises.
Q: Many earthbag house plans seem built around curved or circular walls with most square walled plans showing external buttresses for lateral stabilization. Is all this buttressing necessary for non-circular walls and will internal walls suffice?
A: Straight vertical walled earthbag structures do often need buttressing, but this can be done with internal walls, as you suggest. Each design is different, and needs to be evaluated independently to determine where buttressing might be needed. Also, it is possible to stabilize earthbag walls in other ways, such as with periodic columns and horizontal bond beams.
Q: I am planning an oval with about 30 foot of vertical wall on both sides. I would think three buttresses on each side should be enough right? As to the buttresses, you only need them on the outside right?
A: Buttresses can be either on the inside or the outside (or both), as long as they are well integrated with the wall. Interior walls can effectively act as buttresses. Conventionally, buttresses seem to be spaced at least every 12 feet, so 30 feet of vertical wall may need 4 of them, dividing the space into 3 sections. A continuous reinforced concrete bond beam at the top of the wall will greatly strengthen it. There is some good discussion of building design here.
Q: Do you use any kind of bond beam if you are only building walls with the earthbags? How would a roof be attached so that it is not lifted in high winds? (We do get tornados and hurricanes here fairly often)
A: I didn't use any bond beams in my house, but then I built domes. There are several different methods of attaching a standard framed roof to an earthbag wall. This can be accomplished with a reinforced concrete bond beam and tie bolts, or simply using a wooden top plate that is strapped to the earthbags several courses down. I think this is an excellent way to rigidify and stabilize the wall and provide a good, level base for the roof structure.
Q: I've decided to build a small earth bermed dome (14-16ft diameter) as an experiment prior to starting on our full size home here in Texas. Other than experimentation with the technique itself, the dome will provide shelter while we're working on the other house (we might even decide it's adequate as is and not build any bigger.) I've been looking at the pictures of the interior of your pantry, thinking of using a similar manner of construction. Did you use any form of bond beam or cable to secure the bottom ends of the beams before stacking the bags on the "roof"?
A: No, actually I did not; those beams were just "toenailed" into the bags with some long rebar spikes. For a larger building, or for more weight put on top than I did, a bond beam and/or cable might well be a good idea.
Q: How does the wood roof attach to the bag wall?
A: I assume that you are referring to a conventional wood framed roof over vertical earthbag walls. The specific details of such an attachment would depend on the exact circumstances, but there are several ways that I can imagine. A top plate of either wood or a reinforced concrete bond beam can be attached to the bag wall with embedded rebar pins or poly straps or bands that loop around several courses of bags and the top plate and are cinched down to anchor the plate to the bag wall. Then the roof rafters or trusses can be attached to this in the normal manner.
Also, I attached large log vigas that support a loft in an earthbag dome by drilling half-inch holes through them where they intersect the bag wall and driving long (~ 24") half-inch sections of rebar through the poles and into the center of the bag wall. These vigas were simply resting on the bag wall, which was bearing their weight.
Q: Can you build double or triple story?
A: I would be cautious in building three stories with earthbags. I know of some two story buildings that seem to be fine, but going much higher than that would require some very careful engineering. Actually, any earthbag house needs to be carefully designed to consider the special requirements of this form of building.
Q: What brings me to contact you is curiosity if anybody that you are aware of has constructed, or has considered, a three-story silo-shaped (round) building. Perhaps 25' diameter with poured concrete floors/ceilings at 10' intervals, and a poured concrete rooftop as well.
A: I am not aware of anyone having constructed such an earthbag structure, but it does seem feasible to me. Ordinarily I would caution someone building over two stories with earthbags, but a cylinder-shaped building might be an exception to this, since it is inherently such a stable shape.
Poured concrete floors and ceilings can be very strong when properly reinforced, and these would provide excellent stabilizing diaphragms in your design. With this method, each story would basically be independent from the one under it in terms of calculating wall thickness/height ratios (see below).
I have been pondering and drawing up different wall configurations and am curious as to the effects that changing some variables would make. For example, using oversized bags (24” x 24” etc); double-thickness walls for lower levels;
Thicker walls at the base would lend greater stability. A rule of thumb for earthen walls is that the ratio of thickness/height needs to be at least 1/10; so in your case with a height of 30', the base should be at least 3'.
Vertical concrete pillars (or recycled telephone poles) at 10' intervals - especially with straight walls;
As long as these pillars don't interfere with the running bond of the earthbag walls (i.e., either inside or outside the wall), then they could lend additional support. I actually don't think they would be necessary. In a true cylinder shape there would be no straight sections.
Q: I am planning to build an earthbag structure and I was wondering if you could tell me how high is "safe" without narrowing to a dome? Supposing I was to put some other sort of roof on it...
A: I can't give you a definite number for a safe height, because there are too many variables in design that affect this. It depends on the thickness of the wall, the placement of any buttresses or interior walls, whether the walls are curved or not, the way that lofts, second stories and roofs are attached, etc. So you really need to carefully assess all of these things, and you may need the assistance of a knowledgeable engineer. In general, with earthen architecture, you need to maintain ratio of 1:10 in wall thickness to height.
Q: I was thinking of making a two story house with earthbags. Basically it would just be a cylinder extending straight up. Around the cylinder I would have posts extending up to roof height. On the first floor I would use these posts to make a balcony and extend the posts right across the roof to make the floor. I would use the top of the posts to repeat this to make a roof. My question is would a single thickness of bags in conjunction with the posts be strong enough to build more than one level?
A: I would think that you could do this successfully, especially if you tied the earthbag wall to the posts at intervals to lock the structure into a monolithic whole.
Q: Can one design a earthbag home with two stories?
A: Yes, this is distinctly possible. My dome home actually had a large loft area, so it was essentially two stories. Obviously, the engineering needs to be carefully analyzed for such an undertaking. Placing diaphragms (or floors) across an earthbag structure greatly strengthens the form.
Q: How about underground construction? As in a storm or fallout shelter. I am looking for a construction method that can be done individually over time without having to hire any big earth moving equipment or hired help.
A: Earthbags certainly fit this bill, since they can be used below grade and they are easy for one person to handle. You will need to carefully engineer whatever you design so that it will withstand any weight that is placed upon it.
Q: How about waterproofing considerations?
A: Yes, it would have to be waterproofed, and could be done with a few layers of heavy polyethylene plastic.
Q: Has anyone done this already?
A: I buried my earthbag pantry dome, which was covered with plastic and backfilled with maybe 6 inches of soil and gravel.
Q: This is a sincere request for your input on using earthbag techniques to build an underground shelter.
A: I feel that earthbags are an excellent choice for building shelters that are earth-bermed or even earth-sheltered...but these must be carefully designed and engineered to withstand the incumbent pressures. One major advantage of earthbags is that they are generally not adversely affected by moisture, unlike many other building materials. And, of course, earthbags are a good choice because they are environmentally benign, representing very little embodied energy.
In general I would suggest considering designs that have curved walls, as these tend to be stronger and can withstands the pressures of earth better. As for a roof on an earth-sheltered home or a totally underground structure, this would be a most important element. Domes are a possibility, but burying earthbag domes is largely untested, so there might be some risk here.
Q: Have earthbags been used for shoring an underground tunnel or shelter?
A: Earthbag vaults are a bit tricky, but possible if they don't span any more than about 6 feet max.
Q: Or to ask the question another way, if you wanted 2 or 3 feet of earth surrounding you on all sides and overhead, how would you go about building it?
A: I actually would consider burying a prefabricated steel quonset structure, which could be insulated on the outside with bags of crushed volcanic rock, then covered with plastic before backfilled. This sort of thing has been done successfully, as long as the backfilling is done carefully, and evenly from side to side, so that there is not too much stress from any one direction.
Q: I've decided to build with earthbags on our nearly 3 acre lot in Chattanooga, TN. I want to do a series of domes and kivas. Do you think that I could get away with a 25' kiva that is two story with a bond beam between levels and is bermed into a south facing slope up to ten feet? I know that the round earthbag walls give it better structural integrity, but do you think it is enough to push that much dirt up against it?
A: A circular, vertical-wall earthbag structure of 25' diameter should be very strong indeed; I would say it could easily withstand the pressure of a 10' berm. You would want the bags to be well compacted into place and the use of barbed wire to help maintain integrity...to make it as solid a wall as possible. Also, obviously you would want a moisture barrier between the earthbags and the berm, and possibly some further drainage provided, depending on soil, climate, water table, etc.
Q: I'm an architecture student, and the more I learn about conventional building methods, the less sense they make to me. I'm looking at building my house and I have a question. Can an earthbag dome 25' diameter and 20' high, supported by an internal framework of whole logs every 4' OC, be covered by 2' or more of dirt? What would be the load bearing capacity of such a structure?
A: I wish that I could give you a definitive answer to your question, but unfortunately there are too many variables to do this. Also, I am not a qualified engineer, so it would not be a good idea for me to offer such critical advice.
If the structure were not to be earth-sheltered, I would think that what you describe would be fine, but with 2 or more feet of earth over it, this is a tremendous amount of weight. Some of the factors that need to be considered are: exact design of the structure, size and quality of the logs, and how they are attached, weight of the soil to be placed on it, whether the upper part of the dome is reinforced with steel or wire mesh of any sort...
From my experience it is difficult to keep soil up on an earthbag dome because it naturally wants to slide off, so this is another factor.
Q: Is there anyone combining an underground house with earthbags AND passive solar.
A: I haven't heard of any actual underground earthbag homes, although I do advocate this if the engineering is done carefully. The earthbag house I built in Colorado was substantially bermed on the north side, and the pantry was completely covered with earth and stone.
Q: Is it possible to build a subterranean structure buried in earth with earthbags? The idea is to dig a great big hole, build a series of interconnecting earth bag domes as a storage facility and then to bury it, leaving only the entrance and exit leading to the ground level.
A: I think that this may be possible, but the concept has not been fully explored. Some experimentation is called for.
Q: I and am currently building a shelter that will be primarily underground. It will be 7 foot high to the bottom of the joists and the floor is appx 6 feet underground. My question is this, I am using standard sandbags, 14" by 26" as I had a supply of these available, the design is a 14 ft inside diameter circle. I will be using a center pole to support a framework of double 2x6 to create and support a living roof. It is my intention to lock these joists in between the top course of sandbags and attach them to the wall. Does this sound like it will be strong enough considering how much of the structure will be underground? My thought is that any lean I might have due to the thinner wall will be negated by the fact that the joists will provide integrity to the top of the wall. I am currently on the fifth course of bags. Any concerns you might have or considerations I should look at? I live in the Florida Panhandle and the ground composition is called Lakeland Sand, primarily sand with some clay content.
A: I would expect that a cylinder of earthbags like you suggest would be quite strong enough to withstand the pressures of berming, especially if you compact the soil well so that it locks into solid rings that resist any lateral forces. A sandy soil with some clay is usually ideal for earthbag work. If you moisten is slightly before compacting, and then let it dry thoroughly before backfilling it will be even stronger. I presume you will be placing a plastic moisture barrier over the exterior of the bags before berming...and include a French drain at the base to avoid water working its way inside.
Q: We are going to be building a 16' underground hobbit house. The basic design of the hobbit house follows principals in the Mike Ohler $50 and up Underground house book; the problem I see is that dirt is packed against the side walls and we want the interior dry. The hobbit house will have an earth roof and unless you knew it was there will be invisible to my neighbors. What you would do to protect the underground earthbag wall?
A: To keep your Hobbit House dry I suggest placing a curtain of two layers of 6-mil polyethylene plastic over the outside of the bags before backfilling. Again, a carefully designed French drain at the base of this should be installed, and the drainage at grade kept away from the house. I did this on my bermed earthbag house in Colorado, and never had a drop of moisture come in.
Q: I am interested in building an underground earthbag home as a home for me and my family, I intend on digging the hole and then building the earthbag dome in it. Would domes attached to the sides (as in main center dome is main house with a possible pantry or bedroom on the side) built in dome form effect the structural strength of a dome shape. Are there any sorts of engineering tables for earthbags? I understand this could be hard as the bags contents vary from place to place.
A: I wish that I could tell you that what you describe is entirely possible and safe to do, but unfortunately very little experimentation has been done with burying earthbag domes. I believe that it is possible, given careful design and backfilling procedures, but I know of no actual examples. I have been communicating with someone who plans to do this very thing for a large project in Mexico, but he is still in the planning stages of this. So, if you want to try this, it could be extremely valuable for furthering the knowledge of earthbag building...but the outcome cannot be assured.
Q: I've just gone through your website and I like the idea of an earthbag home. However I would like to build a home that is mostly underground, probably with a living roof (I kind of like hobbit houses...). Can this be done with an earthbag home? Do you have to do anything to support the walls so the backfilled earth doesn't gradually force them in? Do you have any plans for such a home. The county we are moving to said they are willing to let us build an unconventional home IF we have plans certified by an architect. I appreciate any help you can give me with this!
A: With proper design you can go underground with an earthbag home. It is generally better if the walls are curved against the berm so that any pressure against them will tend to compress the bag wall rather than expand it. There are several plans for earthsheltered designs at http://dreamgreenhomes.com/styles/earthsheltered/livingroofs.htm and earth bermed plans at http://dreamgreenhomes.com/styles/earthsheltered/earthbermed.htm It may be necessary to have the plans certified by a local architect or engineer, depending on the requirements of your local jurisdiction.
Q: I have read that being round is strong, so I am assuming the (vertical) walls should be ok after berming?
A: I agree that a round earthbag structure of this sort can withstand the pressures of berming.
Do the inside and outside surfaces need to be plastered (using multiple layers of plastic outside for waterproofing) or could I have the walls sprayed with foam insulation 3-4 inches thick plastic draped and then bermed?
Yes, I would advise using layers of plastic as a moisture barrier below grade, or in bermed situations, but a plaster is not necessary. The walls could be spray foamed, but this may not be necessary either, since the structure is intended to be a root cellar/storm cellar, and doesn't need the insulation...in fact a true root cellar does better if it is earth-coupled.
Would bags filled with rice hulls be ok to use for a bermed structure?
I would rather see rice hulls used above grade, since moisture and stability could be issues...
Q: Is it possible to build water cisterns using earthbags? The idea being that one would construct a large cylindrical earthbag structure with a dome cap atop. If it is possible to build something like this how would one seal the interior so that the water would not seep or the walls would not need constant maintenance i.e. re-application of lime plaster. Is it possible to build this structure both above ground and below?
A: Yes, this has been successfully done, either above or below grade. The easiest way to seal such a cistern is with plastic sheeting, either heavy duty polyethylene or commercial pond liners. It is also possible to use a cement-rich stucco that is virtually waterproof...and there are various coatings that can also be applied.
Q: I am interested in constructing an earthbag cistern. You mention this has been done with success. Can you point me in the direction of any plans or offer any advice?
A: I don't know of any plans for earthbag cisterns, but the concept is really pretty simple. Basically you want to build an earthbag cylinder, making sure to use two rounds of barbed wire between each course. This can be either dug into the ground, bermed, or not...it doesn't matter. Then the best way to seal it would be with a pond liner coming up over the top. The cistern could then be covered in various ways if desired...with an earthbag dome, with ferrocement, or some other roof.
Q: I am wondering if anyone has used the earth bags to build a swimming pool? A friend of ours has a pool built with concrete blocks with a liner covering them. I was thinking that the earthbags would be a great alternative with a much better price. Now their pool is completely in-ground. I would be only able to dig down two feet (really big boulders after that) and then the other two and a half feet would have to be above ground. Do you think this is possible?
A: I didn't build a swimming pool, but I did make a pond by damming a small arroyo with earthbags filled with the local soil and then draping heavy polyethylene over the whole thing. This has lasted some 4 years now. I don't see why this approach wouldn't work for building a pool as well, especially if the pool were circular and several strands of barbed wire were placed between each course of bags.
Q: I am planning to build a number of large (35m3/9246 gal) outdoor water cisterns using earthbags with maybe some rammed earth-tires (foundation only). I would like to use a natural lining both internally and externally. We live in South America where we have intense rainy seasons, followed by a relatively dry summer. I was wondering if you could recommend a natural waterproof lining solution that doesn't involve plastic liners or cement? I was wondering if lime/sand stucco with a number of boiled linseed oil coats would do the trick? These cisterns will likely not go dry completely, and I'm not sure if we should completely put them below ground to help with any moisture escaping through the breathing that would happen with a lime/sand stucco mix (with or without linseed oil). Thoughts please?
A: First of all, I doubt that you really need a rammed tire foundation for an earthbag cistern; the bags themselves serve that function quite nicely. Expecting a lime/sand plaster to hold water over time, even if coated with linseed oil, is asking quite a bit. It might work to some extent, but I would not expect a lime plaster to hold up well over time when immersed in water. If you were willing to experiment with this, and allow some seepage to occur, it might function for awhile...and then if you needed to line the cistern later with plastic you could do this.