Site work and Foundations for Earthbag Buildings
Questions answered
by Kelly Hart
Q: I have decided to build an earthbag structure similar to Riceland. The land I am contemplating building on is in a hilly area. Is it easier to excavate a flat area or can I integrate the slope of the land in my foundation & structure?
A: I suggest that you excavate a flat area so that you will have a level floor inside. You can probably use the excavated soil to fill some of the bags.
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 might be 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: Does anyone install any kind of land drains from the foundation trench?
A: Depending the soil type and the contours of the grade at the site, this might be recommended. The soil at our site is pure sand and drains quite readily, so we didn't bother with drains.
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: You built a house with a sand foundation. How is this done? Do you have some details on foundations?
A: Actually I built an earthbag house that happens to sit on a huge bank of sand, since that is the native soil at this site. Earthbag structures do not really need a traditional concrete foundation, since the bags themselves become a sort of foundation. Earthbags can also be used as a foundation for other sorts of buildings, such as strawbale buildings. The sand is nice because it drains readily and therefore does not present a problem of frost upheaval, as would more expansive soils. One way to deal with these kinds of soils is to utilize a rubble trench foundation.
Q: Would there be a water run off problem building in England where there's lots of rain?
A: Our experience is in the fairly arid Southwestern U.S. As with any building, adequate drainage around the house would be essential.
Q: Is that a case against having the floor inside the building lower than the ground outside - wouldn't that encourage dampness to seep in from the outside?
A: I would usually advise keeping the interior floor level above the outside grade where the doors are. It is possible to create covered entrances and special drains to keep the water from entering the house when it is dug in lower than the grade, but this must be carefully done.
Q: I want to get your thoughts on drainage issues. Berming a home into a hill obviously raises some concerns here, perhaps particularly where our sandy soil means VERY RAPID water movement over and through the soil. I think if you can channel that water effectively, it very quickly moves on, but if you don't, I could imagine some problems.
A: Obviously, drainage is very important. First of all you would want to create a circular swale around the back of the house to direct surface water around to either side and not into the house. Then the water that percolates into the soil near your house needs to be kept from entering the house with a moisture barrier. I used two layers of 6 mil polyethylene draped over the earthbags before they were backfilled to accomplish this, and it has worked out well. In very wet climates, you might also want to place a French drain at the base of the plastic to direct water around the house, with "daylight" exits for the water to run out.
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: Would gravel be best for bag fill in the lower rows to prevent wicking in a wet climate?
A: This would likely be an excellent idea. Such bags with gravel have been used successfully for foundations for strawbale building.
Q: I am planning on building an earthship and am researching the possibility of building it using earthbags. I would be doing a multiple U and hut design so most of the walls would be curved for strength. The soil at the site is a heavy red clay that gets rock hard and is very rocky. Most of this area (central NC) seems to have similar soil. I would like to do a partial berm on the north side for cooling. My first question is about foundations. What kind of foundation do earthbags require? Could I place a couple of layers of tires down and use those as a foundation?
A: I think your idea of building an earthship using earthbags instead of tires is an excellent one. Earthbags are much more versatile, easier to use, just as durable as tires, much less work to "pound", require much less plastering to make a smooth wall, and are less likely to off-gas over time. Earthbags themselves provide their own foundation. In fact earthbags filled with gravel are often used as a foundation for other types of construction, such as strawbale walls. Starting out with tires is unnecessary and would create much more work.
Q: Do we need to dig out a perimeter trench? I read that you did not do this. Is this because you are in a more desert-type climate?
A: You're right that I didn't use any particular foundation other than the sand that is on our lot, because the sand drains so readily and isn't really subject to frost upheaval. In other types of soil it may be advisable to prepare a kind of rubble trench as a foundation. Another possibility is to fill the first few courses of bags with gravel to created a kind of raised foundation that won't wick moisture. Recessing the first row of bags below grade would be a good idea if there is any concern about the soil washing away.
Q: My question is about earthbag foundations and strawbale buildings in a seismic area. Any thoughts on this subject? The structure is going to be a small, pole built shed with a bale wrap, so the bales won't be "structural" in the sense of holding up the roof. I'm guessing that the concern in a seismic area would be the multiple forces on the foundation....up down and side to side, effectively taking the foundation apart at the seams. I really appreciate your help on this as down here in Costa Rica.....I'm kinda on my own!
A: I know of several strawbale projects built upon earthbag foundations quite effectively. In Costa Rica you wouldn't have frost upheaval issues to deal with, so it should be pretty straight forward. I would suggest filling the first course or two of bags with gravel, so that water will not wick upward towards the bales. As far as seismic concerns, I think bag foundations would likely hold up way better than concrete, which has no give whatsoever. At least the bags can flex some without failure. You might be surprised how strong a bag wall is, once the two strands of barbed wire are placed between each course, the whole thing is tamped into place, and some sort of plaster is applied to the exterior to protect the bags. Seismic testing that was done at Nader Khalili's CalEarth site demonstrated considerable resistance to failure. If the bales are simply infill material in a post and beam structure, I don't see why you should be concerned, even if the walls shook a bit. You might somehow attach the barbed wire from the bags to the poles, to help tie the foundation to this rigid structure.
Q: My son has just returned from an earthbag building course. We live on a Caribbean island. Our plan is to build a house onto an already existing concrete foundation and we have been advised to drill and insert rebar vertically into the concrete with a layer of gravel between the concrete and the first row of bags to anchor them.
A: Pinning like this is a good insurance that the bags will stay put, but I see no need for the layer of gravel; in fact this gravel might actually act in the opposite way of causing the bags to slip.
Q: I am trying to find a formula for materials ratio in bags for an earthbag foundation for a strawbale home. Specifically what materials and what ratio.
A: I recommend the use of washed 3/4 minus gravel to fill earthbags for strawbale foundations. This will keep any water from wicking upward and will provide a bit of insulation at the same time. Of course the bags need to be well protected from the sun and physical damage with a good plaster...and rather soon.
Q: How much sun is too much? We put the bags up in October and covered the site with a tarp. At some point the weather exposed some of the bags, I would say the bags have seen as much as 4 months of a Vermont winter sun. We'll stucco the bags in the next 2 weeks.
A: You can tell if the bags have deteriorated too much by poking them with your finger. If you can tear the fabric at all this way, then it is usually too far gone to be very dependable. Obviously, it depends on how extensive the damage might be as to how serious it is. A spot or two of this softness won't affect things much, but a large area is not good. I have seen bags rot from the sun within a month of very intense Southwest sunlight. Even with a larger area of damage, you might be able to save the situation by wrapping them with chicken wire or other mesh reinforcement material before you apply the plaster. This is not a bad idea regardless of the state of the bags, since it will make your stucco more permanent.
Q: There is a high spot on the property that we have chosen as the home building site. It just so happens, that it is a high spot due to being on top of solid lava rock. Here is where my question starts. How important is the trench for the sandbag foundation? Is the trench mainly for drainage or also for stability of the structure? I guess I could get a jack hammer, but it will be quite a job to cut through. Also to level the site, do you think we can just bring in some gravel or cinder?
A: I built my earthbag/papercrete house in the natural sand of the site without any trench foundation and it has been fine, showing no tendency to shift whatsoever. If you were to build on solid rock, I think you would be fine. Rubble trench foundations are mainly needed where the soil doesn't drain well, or frost upheaval is a problem, but this would not be likely where you are.
You could level the site with gravel fill, as long as it is sufficiently compacted and contained so that it doesn't shift over time.
Q: My site has hard packed soil and ledge and rocks below surface. This is in Jordan and it is quite arid, getting rain only in winter. Would it be possible to build in such a site using only a foundation of gravel filled bags for the first few courses?
A: I think that a gravel-filled bag foundation would do fine in the climate of Jordan.
Q: I live in Mozambique and am about to start building, and over the last few months have been introduced into sandbag building. Firstly, my land is on a sandy small slope. Can I build a retaining walls with sand bags? Do they need foundations with vertical support beams? Do I need to lay down a solid stone foundation/slab before starting work on the sandbag walls?
A: Earthbags, or sandbags, make an excellent retaining wall. I would suggest leaning them at a bit of an angle against the hill, so that they want to fall into the hill, and then they shouldn't need any other support. You can backfill the wall as you go up with it to keep it from falling. You do not need any other foundation for this wall...it should serve as its own foundation. You might dig the first course down into the ground to make sure that it doesn't slide on the surface...and fill this first course with gravel to help with drainage. Be sure to plaster the bags to keep the sun off!
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.
Q: We get deep frost in the winter time. What kind of footing do I need under the wall? Need it extend below the the frost line?
A: I would imagine that a rubble trench foundation would be adequate (for a potato storage building). If the soil generally drains well, this need not go below the frost line, especially for such an agricultural building. If the soil does not drain well, then a French drain can be installed in the rubble trench. Grade the surface soil away from the building. Of course the safest approach would be to place the rubble trench down to below frost level...
Q: The next thing I am looking at is foundations and how to do them on a budget...it's probably going to be a gravel trench with maybe some stone walling to get the bags off the ground. Any tips?
A: Rubble trench foundations are one of the best ways to go; they are inexpensive, and they work. If the first few rows of earthbags are filled with gravel, they can go right on the rubble.
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 foundation would be needed to support a two-story structure? Would we have to assume that the standard rebar cement foundations found in modern building techniques would be necessary?
A: I expect that a two-story structure could be designed without the need for a conventional concrete foundation. This is one of the beauties of earthbags: they can create their own foundation.
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: One portion of the wall will go straight up a hill. When laying bags on that portion (about 10 feet long), do you cut steps into the trench in order to keep the bags laying flat? It's in the middle of a sort of C-shaped garden wall.
A: In a situation like this I think that is mainly a matter of aesthetics, whether you want the top of the wall to be staggered or to have a straight incline that matches the slope of the hill.
Q: Our ground is of course rocky and in places very irregular - can we make the first layer less full to accommodate for the difference in terrain or should we try to excavate a "footing"
A: I would suggest digging down far enough to create a level trench to start laying your bags, and then stepping this as necessary to accommodate the terrain.
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: How to prepare a building site, which is on solid, lava rock with a steep slope?
A: To build on solid lava rock that is inclined you might have to build up a perimeter stemwall with earthbags to create a level base for your house, and then fill this in with soil to make the floor level.
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: I like the idea of gravel bags (double-bagged) for the stem/foundation wall on top of a rubble trench, seems very simple and straightforward. But how do you finish them? Since they would be in contact with the ground (and water), I'm guessing not with an earthen plaster. So would you use lime - or cement - plaster then? Would this stem wall need wire for such a plaster to hold onto? And if you wanted to use an earthen plaster for the rest of the building, would you need some sort of edging or lip for it to rest on and separate it from the stem wall below, so moisture couldn't migrate up the outside of the wall?
A: You are right that a stabilized plaster with either cement or lime is a good idea for a stem wall foundation, and using a stucco mesh is also a good idea. I don't think it is customary to place an edging between stabilized plaster and earthen plaster. I might suggest that you bevel the top of the stem wall plaster, so when you apply the earthen plaster it overlaps just a bit, like a shingle does. This should at least discourage moisture from draining behind the lower plaster.
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 intend to build a earthbag house of approx 800-900 sq. ft. on land which is overlooking a beautiful lake. But there are lots of wild bush/trees which obstruct the view. Can I build earthbag house on an elevated platform / terrace with pillars made of concrete to hold the weight? An elevated house will give a superb view and also the remaining space can be used as terrace or elevated patio.
A: I suppose that with a sufficiently substantial and well-engineered platform that it would be possible to do this, but it sounds like a lot of trouble and expense to me. A simpler approach might be to elevate the home by creating an earthbag stemwall that is filled with soil on the inside to provide the platform for your house at the level you want. This would provide a solid foundation without so much expense and trouble. You could still build an elevated deck outside that wouldn't have to be nearly as heavy duty.
Q: I intend to build a earthbag house of approx 800-900 sq. ft. on land which is overlooking a beautiful lake. But there are lots of wild bush/trees which obstruct the view. Can I build earthbag house on an elevated platform / terrace with pillars made of concrete to hold the weight? An elevated house will give a superb view and also the remaining space can be used as terrace or elevated patio.
A: I suppose that with a sufficiently substantial and well-engineered platform that it would be possible to do this, but it sounds like a lot of trouble and expense to me. A simpler approach might be to elevate the home by creating an earthbag stemwall that is filled with soil on the inside to provide the platform for your house at the level you want. This would provide a solid foundation without so much expense and trouble. You could still build an elevated deck outside that wouldn't have to be nearly as heavy duty.
Q: I'm getting ready to break ground on this earthbag compost toilet. It is going to be a small circular structure with a foot print of 10' rising to 3' where a wooden floor will create the chamber and from there up another 7' or so. First I am wondering if I need to dig down the full frost depth here for the rubble trench foundation (nearly 3' feet here) or can get I away with less because of the size and use of the structure?
A: Yes, in anything other than very well-draining soil, it is best to dig down below the frost depth for a rubble trench foundation; otherwise you risk upheaval that could disturb the structure above. Admittedly, this might be very slight, and not much of a disturbance for a small building like you are planning.
Q: I would like to show you the bags that were destroyed by the sun. As you will see they are still under the trench, so I was wondering if maybe there won´t be a problem...I hope so.
A: I can see from the photo what you mean about the earthbags in your trench. You are probably right that the fact that they are down in that trench will keep the material from being lost and undermining your foundation. I'm a little concerned about the amount of water in that trench; it appears to not drain very well. How deep in the trench? Is it filled with more heavy gravel or stones beneath the bags? Ideally, you want the rubble trench to drain water away from the foundation, and if this doesn't happen naturally, you might want what they call a "French drain" put at the base of the trench. This is large plastic pipe with holes drilled in it that will collect the water and direct it away from the building.
Because it´s pure clay, it retains the water a lot. I dug a one meter and a half deep water pond, and the rain water stays in pond like seven months or more, that´s why I am thinking in digging another pond 2 meters away from the house, so I suppose all the water would go into the pond. Would that be a good idea?
Your pond idea might work, or it might just fill up with water and your trenches would still be full also. Unless you can somehow drain the water to a lower location on your land, you might just be stuck with having wet soil conditions there. Here is another idea. Suppose you leave the rubble trench the way it is, and build up the foundation with several more rows of bags filled with gravel. Then, bring in more gravel to fill in the area inside where your floor would be. Perhaps, if you build up the interior floor with gravel this way before you actually make your floor (with concrete or whatever) then the fact that the soil is saturated with water won't matter, since the water won't be getting up to your floor level.
Yes, it sounds like a good idea. How many centimeters should I leave for the earth floor? I mean how high should it be, including the gravel and the floor? Should the volcanic rock (under the floor) inside the house be in bags?
I would think that something on the order of 30 cm of gravel plus the floor would be enough. I don't think the gravel under the floor needs to be in bags. But you might use larger stones at the bottom and the smaller gravel on top, so that the earthen floor material doesn't tend to sink downward. It doesn't have to be volcanic rock under the floor either; ordinary gravel should do fine.
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.
Comment from Owen Geiger: So far very little has been written about earthbag basements. Earthbags are suitable for basements and other underground applications such as cisterns and rootcellars. Round or curved designs are inherently stronger than long, straight walls, which require reinforcing columns or buttresses. I have one plan with a basement: Habitat Earthbag House http://earthbagplans.wordpress.com/2009/03/16/habitat-earthbag-house/
General guidelines for earthbag basements:
- 24" wide poly bags (measured when empty)
- lime stabilized soil tamped solid
- 10%-20% type-S lime hydrate to dry soil by volume
- mix lime and soil thoroughly before adding water
- two strands 4-point barbed wire
- rubble trench with French drain
- taper walls slightly outward
- use 6 mil polyethylene as a moisture barrier
Build on high ground, and grade the site away from the building. Dry climates are obviously more suitable than wet climates. Use caution -- working below grade is dangerous. Temporary shoring may be required. Avoid problem clay soils. The final design should be based on soil tests and calculations by a licensed engineer.
Response from Kelly: I am a bit uncomfortable with the idea of inclining the wall as one might do with a retaining wall. I have several concerns. Basement walls are also foundations for what might be above, so obviously they have to be able to bear a tremendous weight, which well-compacted earthbags are capable of doing, but if they are not vertical, then some portion of the force of the vertical load will be borne by the surrounding soil. This could be problematic because the backfill is rarely compacted, but instead is just tossed into place, so it cannot be expected to be as firm at the bag wall itself...and thus might compromise the stability somewhat. Also, inclining the wall complicates the whole design and construction, in that the footprint at the floor of the basement is smaller than the ground-level floor. This can lead to confusion or errors in layout. A third objection is that walls that are not vertical pose various problems functionally, in terms of hanging cabinets, placing shelving, etc. I think all in all, I would recommned vertical-walled basements to avoid these problems.
Response from Owen: There isn't much slope (about 10" in 8' high wall). What engineers do is draw force vectors to show lines of force. So you'd have 1. forces straight down representing the weight of the house (this is covered due to the earthbags being nearly vertical); 2. horizontal forces from the surrounding earth pushing in (this is covered due to the slight outward angle to counteract the force). So this design accounts for and balances both main forces.
Yes, vertical walls would be more functional in terms of storage, etc. as you say, but they wouldn't be as strong. 8' of earth pushing in exerts enormous forces, and since earthbags are not as strong as concrete and steel I think a little outward slope is called for. And note: the surrounding soil is not supporting the earthbags. The earthbags are self-supporting and actually resisting the horizontal forces of the surrounding earth.
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'm planning on having the first layer of bags full of earth instead of gravel as I'm not sure how the gravel bags will hold up over time, as the weight is relying on the poly bags instead of the fill. As I have closed cell foam and liquid rubber on the exterior of the first row, a gravel trench underneath and a french drain for water collection it seems the bags should be ok, what do you think ?
A: You are probably right that what you have in mind will be OK. We usually recommend using double bags (one inside the other) for the first course or two. Doing this, plus a good stabilized plaster should keep even gravel in place since nothing can really move. And if you use a sharp gravel that packs well rather than rounded gravel, that would also help. If the bag material deteriorates along with your plaster, the dirt is not going to stay in place either.
I know you must have considered this already but I have to ask, are you sure the sharp gravel wouldn't puncture the bags ? Especially with all the weight.
I think this is unlikely, especially with two bags. That poly material is surprisingly strong and even if a few little pointy pieces poke through the whole mass of gravel is not going to spill.
I really want every thing that I build to last ideally hundreds of years, so I hope the bags will hold up that long underground. Does 1/2" sharp gravel wick moisture? If it compresses better there would be less air gaps. Gravel in tubes or bags or either?
The best information that I have is that polypropylene does not degrade in the presence of water (or even most acids); the only thing that gets to it is the UV in sunlight and physical abrasion. But of course with only a few decades of existence in the world, who can say this for sure?
Well washed gravel should not wick water, whether sharp or not. Sharp gravel packs better because it has more angular facets to lock together, but there is still plenty of air trapped in there.
It shouldn't really matter whether you use the gravel in individual bags or tubes. I did once have the experience of tubes filled with crushed scoria tending to roll, but this is not likely to happen once they are compress as a foundation...I was using the tubing high up on a dome.
Q: I have a question for you about earth bag building on a slope. We live on the small caribbean island of Grenada and are building on the site of my husband's grandmother's house. We have cleared the site of the rubble from the original house that was almost completely destroyed during Hurricane Ivan. Though the land is almost level where we propose to build, we have had to dig into the slope to open the area where the back of the house will be situated. This means that the floor level will be below ground in the back, but above ground in the front. We are concerned about drainage and are planning a rock & gravel foundation including a French drain, will this be sufficient? Our soil is clay base but drains quickly. Is it a problem to butt the back wall of the house up against the embankment? Would it be better/stronger to bring the back wall 2/3 feet away from the embankment? We are planning to construct several small earth bag retaining walls up the mountainside behind the house site, but would like to do that after the house is complete.
A: A rubble trench foundation with a French drain should keep most moisture out of the building. I think that you could berm up against the earthbags in that back wall safely, as long as the wall is sufficiently buttressed to withstand the pressure. I also suggest that you place a plastic curtain on the outside of the bags that will have the dirt up against, as insurance that moisture will not penetrate. This is basically what I did with the house that I built, and it worked out just fine.
Q: I'd like to build an earthbag home (20' diameter) on a parcel near Capitol Reef that is subject to flash flooding. I'd like to sink the home for the thermal advantages. Options I'm considering: 1) Building on a platform or hill of bulldozed earth. 2) Elevating the door and windows 4' above the interior floor. (ugh.) 3) Encircling the home with a "garden wall" that has wide steps over it. What other adaptations might you suggest?
A: Obviously you do need to mitigate any construction against the possibility of flooding. This pretty much eliminates the possibility of digging into the ground directly. You could build onto a hill of well-compacted earth, as you suggest. Your second option of simply raising the doors and windows high enough to avoid flooding sounds a bit risky to me, since you would be relying on the walls and foundation to keep the interior dry and this is hard to guarantee. Creating a garden wall to surround the building also sounds a bit risky to me, as breaching this wall is always a possibility.
If I were to build under these circumstances, I think that I would look at designing a building with a raised floor on the inside. If the walls and foundation were well engineered, you could start at the existing ground level and then go up with the walls until you reach a safe level to avoid flooding. At this point you could import enough soil to dump inside the house to build up the floor. I would use gravel for the last half a foot before the actual flooring. Then continue on with the rest of the house.
In addition to this, you could increase the thermal performance and the flood protection by berming more soil up around the exterior of the house as high as reasonably works with your design.
C: (Owen) I'm wary of setting bags right on grade; I put two courses of gravel-filled bags below grade and one above.
A: I think that toeing the bags into the ground, as we have been advising is a good idea, but I am not convinced that it is essential in every case. The house that I built was right on grade, and there has been no shifting over a decade. Of course this is not a seismic area and the sand drains very well. In fact I didn't even use a rubble trench there, but the bags were first placed on a bed of scoria.
Without two continuous courses locking the bottom of the wall together, the structure is almost certainly less strong. What will hold the walls together at exterior doorways? You don't want to rely on mass/weight alone. Not everyone builds buttresses on each side of doors. For seismic areas, I still think two continuous courses is best.
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.
Q: I'm planning to build a house in Paraguay, South America. I got a piece of land and thought of earthbags as building material, as the climate is very humid and extremely hot. Now, the land has several springs. Moisture, if not water, just comes out of the ground. I wonder if earthbags are really the right decision and, in that case, how should I insulate the floor and walls.Can I go for earthbags on an extremely humid place, where I can actually have water running under my floor?
A: If your floor level is raise above the surrounding grade and you have a layer of about 16 cm of gravel below your actual floor, I think you would be safe from water coming up into your house.
Q: I am trying to find the simplest, low-cost solution for building on my site, near the mountains in Romania on a well compacted clay and sand soil with very little slope. I have drain channels build on the entire area, because the water does not drain in the soil, and so there is no water in my soil (as the geologist said). We have rain in the summer, and we have a long winter, big snow and cold winds. The temperature in winter are about -5F, and the frost level is at 3.5 feet. We are on a very low budget, and I know that sandbags are not the first choice for cold climates, but because that is what we can afford, I would like try to take a chance and try to make the best of it. I have some ideas, like: 1. Buried sandbag structure, just below the frost level, for better thermal comfort, but: How do I avoid flooding ?
A: You would need to provide a swale around the house to drain surface water away. And I would advise installing a moisture barrier on the outside of any bermed walls to keep moisture from entering.
Do I need thermal insulation for the part that is underground?
For greater comfort and energy conservation, yes, some insulation would be a good idea. I looked up the underground temperature in your area, and it appears to be around 54 degrees F. year round. That's a little chilly for my blood...so unless you insulate the structure, the ground will always be sucking away your heat. Even 1 inch of rigid foam insulation would help.
Do I need a French drain? (A French drain below the frost level would need an extra trench 4 feet deep, to the very next valley, which is not too close)
If, as you say, the soil does not absorb water and is dry year round, then there would be little need for a French drain. What you might do is fill the first few layers of bags with gravel, and elevate the actual floor some above these.
2. 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: A roofer told me that my earthbag house is not sound, because there is no foundation (it was built on a pad that was raised two feet higher than the natural land and we do not get very much rainfall). He told me that I should sell my house because he feels the whole thing will collapse or sink in time. I can say the house has been in a storm with winds of approximately 90 mph with no signs of instability. The walls seems solid as rock. I plan on digging a trench approx 3 feet from the exterior walls, filling with about 8 to 10 inches of cement, then building a 4 inch form above ground and extending the cement to the base of the walls, with hopes that it will prevent water seepage. Any words of advice?
A: A proper foundation really depends on the soil conditions of the site and how the foundation was done. Earthbags can provide their own foundation to some extent, so they are different from most other methods of building. Frost upheaval and excessive moisture are the real enemies, and I doubt that either of these are much of an issue where you built. If the soil drains well (like sand would) you can actually build right on the ground. I did this in Colorado over a decade ago, and the house has not move an iota. In most areas, a rubble trench foundation that goes below the frost level, along with a French drain to carry away excessive water, is all that is needed.
If you have not experienced any subsidence with your house yet, I doubt that you will. Grading the soil around the house so water flows away from the walls is a good idea. You say that the house was built on a pad two feet higher than the surrounding land, so that should be easy. Now, if that pad of raised soil was not sufficiently compacted before you started building, then there is the possibility that it could settle over time, and maybe this is what was concerning your roofer, as this would be an issue with any house. If this were to happen, it would not be catastrophic; the most damage would likely be some cracked plaster and such...there would be no collapsed walls.
The concrete cap that you describe that would surround the base of your walls, would certainly keep that raise pad in place over time, so you could rest assured that the house would not be undermined, and this would probably help with any moisture. You could also just bring in a load of large drain rock and spread it around the same area to stabilize the soil there.