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Foundations for Earthbag Buildings
Questions answered by Kelly Hart

Q: I was wondering when you start laying the bags is there any footer under them or do you start laying them directly on the ground? Also, how deep do you dig to start laying them?

A: Earthbags don't really need a conventional foundation; you can fill the first course or two with gravel to keep water from wicking upward. And you can dig the first course into the ground a few inches to make sure that it has a good "toe-hold". If your soil drains poorly and there is a danger of frost upheaval, then it is a good idea to dig a deeper rubble trench, and possibly install a French drain, to guard against this.

Q: Can you give me any information on the type of footer I would need for an earthbag building? What about a stemwall if you are building in a wet climate?

A: There are several options for making a foundation for an earthbag building, depending on the type of soil and climate in your locality. One of the simplest is to make a "rubble trench foundation", which is described here and here. A frost-protected foundation is described here.

The need for a stem wall depends more on the building design than on the climate; the foundations described above would work fine in wet climates.

Q: I am interested in building an earthbag home for my mother and stepfather however we are kind of stumped by the foundation portion. We can get up to 2 weeks of rain at once and then go 2 months without any rain, the soil is kind of unpredictable.  I assume we would need to build up the foundation so it will not gather water but should we dig below ground level and use small rocks in the bags for the first 3 rows? 

A: I would recommend digging below frost level with a trench that is as wide as the bags and filling this with enough rubble or drain rock so that the first course of bags will be embedded several inches below grade level. And yes, filling the first few courses of bags with gravel is a good idea to assure that moisture does not wick upward. If it seems likely that this rubble trench will fill with water during heavy rains, then a French drain arrangement, where a perforated pipe embedded at the bottom of the foundation can collect the water and run it off to some "daylight" location away from the building, might be a good idea.

Q: As I thought about your recommendations concerning the rubble footing/sand bag stem wall, something occurred to me that I wanted to check with you. You mentioned it was best to place a layer of large (4 inch) rocks at the bottom of the trench and then layer(s) of progressively smaller gravel until just above grade - then start the bags. At first I thought this was just for drainage, but then it occurred to me that tamping these layers would wedge progressively smaller stones between larger stones, thus stabilizing and distributing the load in a horizontal as well as vertical plane. Does that make sense or am I inferring something that wasn't intended?

A: I think you are absolutely right in your thinking about the distribution of aggregate for a foundation...this is what Mother Nature would like.

Q: If the foundation bags are laid on a rough stone foundation, doesn't the weight of the walls puncture the bags?

A: Perhaps rounded drain rock would be best as a foundation material in the trench. A smoother gravel would be easier on the bags. Also I used two bags (one inside the other) on the first row. Once the bags are packed into place there is little stress on the bag material.

Q: I have decided that earthbag/superadobe is for me. I have found a retired building inspector who will draw my plans for a building permit. I need information on the foundation.

A: Earthbag buildings do not really need the sort of continuous concrete foundation that most other building forms require, but, unfortunately, the building codes are not likely to agree with this. There is an example of a simple rubble trench foundation here that might help visualize one method. Your retired building inspector should be able to help you determine what the authorities will allow.

Q: I was wondering if you might be able to make any comments on a foundation. I plan to use rocks found on or near the site.

A: A good stone foundation should work well for an earthbag structure; just make sure that the top of this foundation is relatively smooth so that the bags are not punctured.

C: I am nervous about the longevity of the gravel filled bags though...they have a lot of weight on top of them.

R: The key to longevity with polypropylene bags is to keep the UV from the sun off of them, otherwise they are virtually indestructible. You can read about the results of loading tests at Testing/prismtest And actually, once you have the bags sandwiched between two good layers of plaster, they become similar to structural insulated panels, which have a soft inner core and rigid skins...and these are structural components. Tests have shown that even strawbale walls that are rotting within will often remain standing because of the strength of the plaster!

Q: What kind of a foundation would be needed for an underground structure?

A: The main issue is dealing with possible water entry into the structure; the bags create their own foundation. It may be necessary to create a French drain around the exterior at the foot of the wall, depending on soil type, water table, grading, and rainfall.

Q: I noticed in your riceland how to, you didn't put a silt barrier between your footer and the outside earth. Is that because you don't have that much silt? We do. In order to keep the rocks from filling up with silt between them, should be put a silt barrier?

A: Right...we were building on pure sand. I dug out the rubble trench foundation for Riceland, thinking it was necessary, but then realized that with this sand that drains so well there was really no need for it, and on my larger house I built right on the surface of the sand. With your soil a silt barrier of some sort might be a good idea.

Q: I'm building a compost toilet and I'd like to do it with earth bags. There is plenty of large stone near and I'm wondering about using it down to the frost line and on up a good three feet for a stem wall. Now, I'm not sure it will be a benefit to use the stone or just do a gravel foundation and start with bags at ground level...just trying to use what's here. If I use the large stone I wonder if I can do the foundation part without cement or rebar?

A: Either stone or earthbags could be used for such a foundation. I would expect the earthbags to be easier and go faster. If you use the stone, it is customary to mortar a stone foundation into place, so there is no danger of slippage or settling later...but rebar is usually not necessary, especially for a small toilet room.

Q: I am building a stick-framed house (30' X 40') using salvaged lumber. I have built a rubble trench foundation and would like to use earthbags to form a grade beam for my structure. I read on earthbagbuilding.com about a similar project where "flat wooden sill plates are spiked to the bags to provide a base for the walls". Can you tell me how the sill plate is pinned to the earthbags?

A: A simple way to pin the sill plate is with long sections of  half inch rebar that are pounded down through the sill (with pre-drilled holes) at intervals of about every 3 or 4 feet.  To make them even more secure, you can bend the top few inches 90 degrees, and pound them in at various angles, so they are not all vertical. To further keep the sill from potentially lifting off the bags, you can wrap heavy duty strapping tape around the bags and sill and cinch it into place, as is commonly done with strawbale walls. To do this, you need to lay the tape under the bags before they are put in place.

Q: We would like to fill the below ground earthbags with rocks to avoid freezing and thawing and frost heave. Would you agree?

The conventional wisdom is to actually dig a "rubble trench" that goes below frost level to avoid any possibility of frost heaving. The need for this depends of the soil type and climatic conditions at the site. If you have clay soils, or other soils that don't drain well or are susceptible to frost heave, then this is a good idea. You don't need to use the bags for this foundation; just fill the trench with cobbles, and then start with the bags near the ground level.  The Riceland series shows how we did build a rubble trench on this project. 

Q: I am curious about the effects of expansive soils on the foundation systems used in earthbag construction. I would think that there would be ample opportunity for earthbag domes to expand/contract/rise and fall along with the soils they bear on, but I shudder to think what will happen if an earthbag home moves enough to crack the surface coating.  Again, I don't know what your experience is, but we commonly find native soils around here that expand and rise 3-4" or more going from a dry state to a saturated state. Those expansive clays can and do horrible things to foundations!

A: The best assurance that foundations are stable in expansive clays is to create a rubble trench with a French drain that goes below frost level and the expansive clays.

If expansive clays get wet and intrude horizontally into the French drain, possibly compressing the sandbags, they might heave upward due to lateral pressure. I think that placing the bags on a concrete footing may be best; in this case is, would the sandbags tend to slide outwards off the footing without some kind of lip to catch them?

While expansive clays will certainly exert lateral pressure, I doubt that this would adversely impact earthbags resting on a true rubble trench, because of ample space between and around the drain rock. I have certainly never heard of this happening. If expansive clays actually did heave, as you suggest, a concrete foundation will not be solution, since it will heave as well. If you do place earthbags on a concrete foundation, probably the best way to connect them to that foundation would be to leave rebar pins raised above the concrete so that the bags can be pierced by the pins when placed.

Q: My question is about a patio slab that we want to enclose to expand our living space.  Code is not an issue, so I was wondering if I could just drill some holes on the slab sink some rebar into them and lay the first course on top of the rebar?  I am only using the earthbags as a stem wall and straw bales for the rest. 

A: I think that what you propose with tying the earthbags to the existing patio slab should work just fine.

Q: I am planning to make a earthbag cylinder with scoria. It will be around 5 meters diameter. It will be on a stone foundation laid on the ledge/ground. The height of the earthbag section will be around 2 meters. Do I need any special treatment between the stone and the first course of bags?

A: If your stone foundation brings the bags above the ground level, then you should need nothing between it and the bags.

Q: Is it possible to lay tires on the ground in lieu of footings, fill them with gravel and begin laying earthbags on top?  I saw an earthbag round building being constructed on YouTube flick that appeared to be done that way.

A: Yes, I have seen that video on YouTube, and you could do the same, but I would not recommend it because it creates way more work than is necessary. A much easier foundation can be made by digging and filling a "rubble trench," and then filling the first few courses of bags with gravel. This way you have guaranteed drainage (you can also place a French drain at the base of the trench if necessary) and don't have to deal with the contours of those large tires at the base of the house.

Q: My architect has doubts about the foundation with gravel bags. He thinks that in due course gravel will spill from the bags if it is not stabilized. And there I would be without the foundation. Can you refute this?

A: The stone in the rubble trench is confined by the trench itself. In the bags the gravel does rely to some extent on the poly bag material to hold it in place, and with the bags kept out of the sunlight, they will not deteriorate. We recommend using two bags (one inside the other) for these courses of gravel. Also with good cement-stabilized plaster at the base that has a mesh embedded in it, there is a secondary protection against losing gravel. As an added precaution you can add some cement as a slurry with the gravel packed into the bags, so that even with the loss of these coverings it will not fall apart.

Q: I don't know much about drain systems (the french drain) and was wondering how would I go about to do this? I know I have to go below the frost level so do I fill the trench with just loose rock for the 4 ft and then start my rows?

A: Yes, basically that's what you would do, although the size of stones used can be graduated so that the smaller aggregate is near the top to make a nice solid and relatively smooth base for the bag foundation. And the first few courses of bags (double bagged) can also be filled with gravel to avoid wicking further up into the wall.

Where do I run the drain to?

A French drain should ideally be run to daylight somewhere on the property where it will feed via gravity. Or it could be taken to a large sump area filled with cobbles.

Q: What is the BEST SIZE of gravel to use for the few layers? I want to use a size that will compact well like the smaller gravel, but will drain better like the large gravel, so I am uncertain.

A: Practically any gravel that is relatively free of fines should drain well enough. I would suggest something under 1" minus.

Would it work if I just dig about 1.5, 2 feet rubble trench and start just below grade instead of 4 feet (frost grade) with first 3, 4 rows of stabilized earth hydro-isolated on the outside, and an isolation skirt around the perimeter of the house to prevent freezing?

Yes, frost protected shallow foundations have been proven to be quite effective in eliminating the need for deep foundations and providing better thermal protection for the house.

Does my concept drawing have all the necessary elements for our earthbag building frost protected shallow foundation, with 1.5 or 2 feet deep trench?

Your drawing looks perfect to me!

Q: Do you think that a reinforced grade beam on grade is necessary for a high-seismic zone?

A: No, I am not convinced that a reinforced grade beam is essential for seismic stability. I think that there is natural resiliency to earthbag walls that is built with the standard two strands of barbed wire running between each course. Owen Geiger and I generally advocate the use of rubble trenches with the first several courses of bags filled with gravel and the very first one toed into the ground below grade. We do not claim that grade beams are necessary, even in high risk seismic zones. But we are not licensed engineers.

Q: In respect to keeping a structure floating, I don't know that I see the reasoning for starting one course into the ground/trench with the barbed wire between this in-ground course and the later above-ground course. I would think this would counter balance the ideology of having a "floating system" as it's keying itself into the ground, right?

A: Well, I didn't key the house that I built into the ground, and I am not sure it is absolutely necessary; this idea is one that Owen came up with and I have gone along with because I didn't think that it would do any harm. It is an interesting question. In some circumstances I think the keying might help keep a wall intact since it might be less likely to shift in such a way as to disrupt the integrity of the wall. On the other hand, locking the structure to the ground might cause parts to give way that wouldn't if the whole structure could move more as a monolithic whole. I bet that either of these sceneries could happen. This is an area where we could really use some scientific testing to have a better understanding.

Q: How deep do I need to go with the foundation?

A: Going below your local frost line is a good idea, but the foundation itself can be bags filled with gravel, so if your soil drains well, you might be able to get with less. In any case I would suggest at least a 1 foot deep foundation.

Q: Is there a way around the gravel-filled trench system for the earthbag homes? My home site at the land purchased is of a very rocky terrain.

A: It all depends on the climate and how well your soil drains. Foundations need to both keep water away from the wall structure and avoid any heaving from freezing soil. So if your frost depth only goes a few inches and your soil drains very well, then you might get by without the rubble trench foundation.

Q: I have a question about the foundation for my earthbag house, currently in the first stages of construction. Your proposed method of double bagging the first few rows and filling them with gravel (set down on top of a gravel trench) is very simple and elegant. I share some of the concerns that I have seen on this site about the gravel filled bags relying completely on the structural integrity of the bags. I have no reason to believe the double bags will ever fail, but it seems a bit unfortunate that while the rest of the house will not rely on the bags holding together because of the fact that the earth will cure into bricks, the most important rows will not have the same properties.

A: In defense of the gravel bag foundations, I might add that it would take a severely neglected house that was allowed to deteriorate to the point where gravel started leaking out of the foundation bags. Normally, a well stabilized plaster with embedded mesh covers both sides of the foundations, and this layer would also have to fall away before the gravel could release. Any house that is allowed to get to this point would be vulnerable to failure no matter what type of structure it is.

Q: When constructing an underground/earth sheltered/earth bag structure, (all walls are earthbag) is trenching necessary as foundation for all walls including interior walls? If it is underground does trenching need to be done at all?

A: It depends on your climate, soil and depth whether you need trenching or not. Rubble trench foundations serve two functions: to keep water from wicking up into the walls and to keep frost from heaving those walls in the winter. So you need to dig down below the frost depth in your region to avoid the heaving. Some soils drain very well (like sand) so water doesn't tend to accumulate anyway, and in this case you might be able to get away without a rubble trench. Or in in some very arid climates, water intrusion is not an issue. Interior walls are less crucial usually because they are protected by the outer walls, but if you have a high water table, then trenching the inner walls is also a good idea.

Q: We are looking at building in north Texas on red clay soil. Is there anything special needed for this due to the heavy heaving and contraction that happens here? Also this soil stays wet during the rainy season and extremely dry during the summer months. Is there anything extra we should do for the footings?

A: The standard foundation that we suggest for this is a rubble trench dug down below your frost level, with a French drain installed that directs accumulated water to some daylight area away from the house. Then the first few courses of bags can be filled with gravel to assure that moisture doesn't migrate into the wall. In your case, with such expansive clay, you might dig down far enough to avoid the clay if this is possible; if not, make the trench deep enough that the soil doesn't cycle so much with wet and dry seasons.

Q: My wife and I have been considering adding a small storage/shop building in our yard, and we think earthbags would be a fun way to go. We are next to a "pond" (small cities storm drainage retainage area) that overflows it banks for a couple of days each year. The area we want the shed would be in this flood plain. The "flood is only a few inches of standing water. My question is, would it still be possible to do earthbag on this site? Could my foundation bags be filled with sand and this keep water out? If not would a sandbag perimeter be sufficient to keep the few inches of water at bay?

A: Yes, I think that earthbags would be a good choice for building in that situation. I would first build the site up some to gain a little bit of elevation, then make a rubble trench foundation to support the first row of bags. I would fill the first several courses of bags with gravel (double bag these) so water intrusion into the upper part of the wall is unlikely.

Q: If I am going to put a concrete slab under our earthbags to build a storage building approximately 21 ft by 15 feet, how thick would the concrete need to be to be able to support the weight?

A: Earthbags don't necessarily need such a foundation as a rubble trench is usually sufficient and provides better drainage, but if you want to pour concrete it probably should be about 6 inches.

Q: In the spirit of building at least partially underground, I was thinking about digging the entire area of the house 2-3 feet down and starting the bags there (using 6-mil as a vapor barrier). Would that then preclude the need for a rubble trench foundation? I'm thinking that would be the foundation, except that we could put in the floor at a slightly sub-ground level to increase its insulation.

A: Certainly, digging down several feet into the ground would help to keep the house cooler. It depends on the type of soil, how well it drains, and how much moisture that portion of the wall might receive whether you need the rubble trench foundation or not. The bags in the underground portion could be filled with gravel, and that would help keep water from wicking up into the rest of the wall, but moisture could still seep into the floor under the wall. The safest thing to do is make the rubble trench foundation with a French drain if necessary.

Q: With an Earthbag structure (a square building vs a dome building), could you build an underground basement below a single story house that is going to be built?

A: Yes, you could build a basement foundation for a single story house with earthbags, but it would have to be well engineered to avoid any problems with the pressure of the surrounding soil from deforming it. This can be done with proper buttressing in the basement, or by inclining the basement wall outward to counteract that pressure.

Q: I have some questions regarding building an earthbag home in earthquake/mudslide zones in costa rica.  What kid of base is best?  Are the homes typically build on a concrete slab, or on solid level ground, or something else? I am guessing that keeping the home to one level would minimize damage/threat of death in earthquake, as there would be less rubble.

A: I have heard different opinions about this. Nader Khalili, one of the pioneers of earthbag building, claimed that a free-floating, non-anchored foundation was best to avoid stresses that would tear apart an earthbag building, especially a dome. Nabil Taha, a structural engineer has outlined a different approach here. I agree that one level would be safer.

Q: We are getting close to building our earthbag house in Hawaii but we still have a few remaining questions. First, we will be building our earthbag house on solid lava rock but are not sure what kind of foundation to use. Should we place the bags directly on the ground? Or should we dig a trench for the foundation? Should the walls be attached to the ground with rebar? Earthquakes are a concern of course but I am not sure if anchoring the walls to the ground is a good or bad thing during an earthquake. Also, do you think there will be issues with building on top of solid rock when it comes to living in a wet climate?

A: A solid rock foundation is as good  as it gets, so I would build right on top of it. If you want to level it somehow that might be preferable, but the bags can go right on the ground. I would suggest filling the first course or two with gravel to avoid moisture from wicking further up into the wall. These do not need to be attached to the ground with rebar. Nader Khalili, one of the originators of earthbag systems, suggests that floating foundations might be better in an earthquake anyway. To avoid moisture problems, make sure that drainage is such that water naturally moves away from the house, that the exterior plaster is stabilized somehow, and that the walls are protected by wide roof eaves.

Q: There seems to be some debate about using straight gravel in some or all of the courses, or a mix of clay and gravel. In my mind, a bag filled with gravel offers better moisture resistance in the event of wicking, but little in the way of binding strength -- if the bag fails, it seems like the foundation goes with it. Gravel will seemingly just pour forth. In the gravel/clay scenario, the clay should "set up" and provide a much more rigid building block, but the chance to wick moisture is much greater. However, it's my guess that with proper grading and trench design (and proper protection of the bags themselves from direct moisture), there shouldn't be much moisture to travel up... isn't that the whole point of the rubble trench anyway? What do you think about this? Have you head any failure stories in either scenario?

A: It sounds like you have thought this through rather well, and I agree with most your analysis. We do usually recommend using straight gravel in the first couple of courses (with one of them being slightly below grade as a toe into the earth), mainly to assure that no wicking of moisture will enter the wall above it. The integrity of the bags is the main concern with this, and to help with this concern we recommend using double bags (one inside the other) for these gravel bags, and of course a very stable mesh reinforced plaster on both sides. Even with clay and gravel filled bags at that level there is still the possibility of failure, because if the clay gets saturated it can flatten or ooze as well. If you do go with this option, I suggest the use of a moisture barrier above it to avoid possible wicking problems.

Q: We are looking for a suitable design which includes an elevated platform as front veranda, looking out over the water hole. Elephants are regular visitors so it will have to be sturdy, but we like the idea of building the platform on stilts as opposed to solid walls made out of sand bags. Do you know of any sand bag house plans that involve stilts? And do stilts give enough support to build part of the sand bag house on it?

A: As for building an earthbag house on stilts, I'm sure it could be done with a very sturdy platform system, but it is not a very good idea. It would be much better to simply build the an earthbag foundation for the deck and house.

Q: We want to build some yurts (we want to create a sort of spiritual retreat to bring people closer to earth and off the grid) in Belize. We want to build small yurts - probably 16' to 20' - maybe three or four all told, plus our own. We will be putting the yurts on decks. This is where my questions come in: With heavy rainfall and being located in an area that has the Caribbean ocean on one side and jungle on the other, is it feasible to use earthbags for a basic foundation to lay a deck on? The yurts obviously won't weigh much (except for ours, which will be our permanent home) but we'll want to build the decks large enough to hold the yurts plus a small front deck. My second question is: What about the soil there? If the soil is sandy/fast-draining, then it should be okay, right? But won't that make the bags shift/sink?

A: Earthbags do serve well as foundations for many different wall systems, and traditional yurts would be no exception. Circular walls or foundations are the strongest of all, so that is also a plus for yurts. In fact, you could build the entire yurt wall with earthbags as round houses are if you want. As for well-draining or sandy soil, it is generally surprisingly stable and will not compress much with weight on it. But if there is any question, then using a rubble trench foundation might be advisable. When I built the dome complex at Peaceful Valley it was on pure sand and has not shifted at all in nearly two decades.

Q: I am finding it very difficult to work out whether we should have a grade beam. We are in the Caribbean which is seismic and obviously has a lot of hurricanes etc. I am getting conflicting advice about whether a grade beam would increase or decrease suitability in these regions. What do you think?

A: All of the engineered specs for code approved earthbag construction do show a grade beam, so building officials may be more comfortable with this. It partly depends on other aspects of how the building might be stabilized or reinforced. Some systems show rebar wrapping the entire wall, encased in concrete both top and bottom. I am not convinced that it is necessary, given that many earthbag buildings have survived earthquakes without having grade beams. We really need my controlled tests to find out what is best.

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