My property is really close to the lava flow, and lava rock is everywhere. My friend wants me to use his silt/cement mix, which means buying the silt and having to unload it by truck. I'm thinking on somehow using that lava rock in the mix, but it might take too much physical energy to crush that lava with simple tools. And I'm not sure on how good the lava powder is against humidity.

A: As for the need to use cement in your mix, I am of the opinion that it is generally unnecessary. It certainly makes for a solid wall that doesn't rely on the bag material to hold it together, but the same could be said about ordinary adobe soil. As far as I know Nader Khalili didn't use cement in his buildings. I used crushed volcanic rock in the house I built in Colorado, and it worked out fine, and I expect it to last for centuries. The polypropylene is not affected by moisture; it is only effected by sunlight, so as long as there is a good plaster protecting it, there should be no problems. Besides, once you have a solid plaster on both sides, the wall becomes nearly monolithic. I actually think that using cement can make the building more vulnerable to earthquakes, because the material cannot give or sway in any direction...it can only break apart, whereas looser material is more malleable. In some instances, very fine, slippery sand can be a problem in building domes because it doesn't pack well and deforms too easily. In this case a bit of cement might make it usable. I don't know what the qualities of your silt is, whether the cement might useful or not. I would think that you could somehow incorporate at least some of your lava rock...I built a whole house with it. Mine was very well insulated...much better than a cement building would be.

I'm not sure we are talking about the same thing. You mentioned that you used crushed volcanic rock in your Colorado site. I am talking about lava rocks abundant in my property. Can I crush my lava rocks with hand tools, or maybe a bulldozer to get some fine powder for the mix? Would the resultant powder be of any quality for the earthbags?

From what I can see in the pictures the lava rock you have there is similar to what I used. The main useful quality about what I used (which was red) is that it is very lightweight, full of air pockets, so that it acts as a natural insulating material. If it is crushed to a powder, then this quality is diminished considerably, so I made sure that I eliminated as much powder as I could before using it. I used a pre-crushed gravel that averaged about 3/4 inch, but somewhat larger or smaller is also possible. If you were able to use this, then you wouldn't necessarily need to use any cement at all...I didn't.

Q: My wife and myself are considering building a small house in a desert Oasis, in Egypt. As you may know, the main material available over there, besides Palm trees and salt blocks, is ...sand, sand, and sand! I do not know what are the limitations, drawbacks or advantages of using sand instead of another non-available material. Do you think the result (stability, thermal isolation, humidity, long lasting, etc..) could be positive ?

A: I have had some experience building an earthbag structure with sand. My first experimental building was done this way, and unfortunately it didn't turn out too well. The problem was that the sand where I was building (near the Great Sand Dunes National Park in Colorado, USA) was a very fine, rather spherical sand that didn't want to pack into a solid form in the bags. You might say that it was a "shape shifter," so that when I was up to about two meters high with this little dome, the wall simply collapsed. I rebuilt the dome using some local crushed volcanic stone. This doesn't mean that you can't use the sand that is available to you; it just means that you need to be careful about using it. Some sorts of sand, like a sharper river sand, does pack well. Even if your sand is more like what I tried to use, you can stabilize it in various ways, through either adding some cement and wetting it before you fill the bags, or possibly adding some clay to the mix. Lime is another possible stabilizer. In any case, I suggest that you do some experiments to see what works best, before committing to a large project like a house.

The real drawback we find with sandbags is the necessity of having at least 3, 4 or 5 persons around working on the project. Regardless of budget (and bearing in mind that red bricks are quite cheap in Egypt), one positive point about using bricks (clay, mix, cob etc..) is that one can still work alone, or two people can work on different parts/areas.

I built most of my earthbag house in Colorado all by myself, so you don't necessarily need a crew to do this.

Q: I'm trying to find urbanite, but so far all I've been able to find is broken up asphalt from a parking lot. Is this an ok substitute for cement? Will it off-gas? It would be used in a stem wall for a straw bale building on top of a rubble trench. Both sides would be coated in a lime/perlite stucco for better insulation.

A: I would not use asphalt chunks, not only because they might off-gas, but because they are not really all that solid over time and can easily "melt" and therefore settle. You are much better off using concrete chunks or gravel.

Q: I've found a statement that "you would only want about 30% clay to 70% sandier soil" to avoid the dangers of shrink-swell that many clays exhibit. I've also run across the comment that "soil stabilization is rarely needed in earthbag construction". It seems to me, however, that the troubles of expansive clays could be eliminated by completely stabilizing the clay with hydrated lime. My site is rich nearly pure red and gray clay that is known to be "severely expansive", predominantly montmorillonite. It has no sand to speak of. Having done my own experiments, I found I can eliminate the clay's shrink-swell property with hydrated lime (8% w/w dry); with some water it turns it into a highly workable moist dirt that, when dry, shows more than 100 psi unconstrained compressive strength. I would like to know if anyone has used only clay, completely stabilized with lime, as a fill material. If so, where might I find guidance beyond my own experiments? If not, does my description of the properties make this clay seem viable as a sole-ingredient for use in earthbag construction? Would I be recommended, nevertheless, to include sand in the fill material and, if so, how much and why?

A: I have not heard of people stabilizing pure clay with lime, although from your experiments this sounds like a promising approach. If doing this eliminates that expansion/contraction cycle of clay, then this material should make an excellent fill material for earthbag construction. I suggest that you do some further experimentation with a small project and see how it fares. In my experience pure lime plaster also will shrink quite a bit when it dries, which is one reason that it also needs some sand added to keep this from happening. It is a mystery to me how both clay and lime mixed together will not shrink!

C: Isn't lime a wonderful substance? The chemistry of its many uses seem straightforward, but it is chemistry. Actually, the pozzolanic reactions of hydraulic cements are more complex.

First off, the pH does need to be correct for "full" stabilization. The normal protocol calls for attaining pH 12.5 after a 30 minute period in a standard 5:1 water mix. One paper I read said that full stabilization may be achieved at pH as low as 10, depending on some soil characteristics, but that by following the guidance of pH 12.3 any expansive clay-bearing soil can be counted on to become fully stabilized. The soil at our "ranch" is basically 3 to 8 inches of silt over solid, sticky, plastic clay. Using a wide range of lime concentrations, I tested a 30% clay/70% silt mixture which yielded a 1.5% by weight, dry, of hydrated lime required to reach the pH 12.3. All well and good, except we'd have to use all the top soil, and only a little of the clay, scraped off of a large area to supply fill material. Our test holes have shown that our clay extends to feet in depth, a veritable bounty, and we would rather not use the seemingly meager topsoil in the bags - that belongs to the biosphere.

My tests with liming a large amount of raw, heavy, sticky clay showed a rather magical transformation to a non-sticky dirt. I chopped up the clay into pieces 1 to 1-1/2 inches in size, wet it and distributed the hydrated lime over it. After roughly mixing it up, I let it sit - within minutes the physical characteristics were clearly changing. After 24 hours, and a couple more quick mixes, it was "done" - completely transformed. Here's where I made cylindrical samples - including one with 15% of silt topsoil added. After 4 days of drying in ambient conditions, the stabilized-clay-only easily bucked 100 psi (14,400 psf) while the sample with silt was crushed at half that pressure. Clearly, any silt is not desired. The clay-only actually failed on it's edge only as the varying weight shifted and rocked the sample during loading.

As I understand the mechanism of lime stabilization of clay, an ionic exchange happens in the highly alkaline environment to replace aluminum and sodium with calcium supplied by the lime. Doing so, the clay flocculates and the ability of the clay plates to hold water in between them is decreased. Plasticity index and water content drop while plasticity limit and "optimum water content" increase. These results create a material that then doesn't shrink or swell with varying water content. Being less plastic, of course, means it doesn't ooze. Over time, the components can even engage in some pozzolanic (cementation) reactions too. It's all very cool, but my ignorance makes me anxious now that it seems I'm on the "bleeding edge" of research! Ha!

Oh, before I forget: Texas Department of Transportation is big on using lime to stabilize our expansive clays for use as roadway subgrades. Also, our Dallas/Fort Worth International Airport was built upon stabilized clay. Without the use of lime to eliminate the "shrink-swell" of our clays, our roads and the airport runways would have cracked up into un-usability. It's kind of weird that foundation repair is big business here ... it's apparently not worthy to stabilize the soil before setting the concrete slab foundation! There is one caveat to lime-stabilization: unknown sulfates in the soil can destroy subgrades. Texas is keen on remembering this as a nearby highway was destroyed before learning the threat of sulfate-caused formation of ettringite made the roadway heave by feet after constructing it. If the sulfate concentration is stable, and not monstrously high, it can be countered with appropriate lime and/or pozzolanic silica additions. I've not been able to find any evidence of sulfates at our "ranch", thank goodness.

And now for something completely different ... Class C fly ash! It's really a blast experimenting with a power-plant waste product that acts like portland cement. I got a 100-pound sample of it from a local company that specializes in oil-field supplies; it's apparently used to strengthen drill hole casings and what not. Straight up, it's useful for some applications. Better, though, is adding 9% to 11% hydrated lime and using 40% water-to-cementatious ratio. It's wonderfully workable (but quick setting). The stuff begins setting up seconds after adding water, so my next experiment is to see if I can lengthen the working time to something closer to portland cement. Perhaps you already know that the last ingredient added to portland cement, while the clinkers are ground up, is gypsum. Powdered drywall, I mean gypsum, stalls the earliest pozzolanic reactions ... I suspect I can achieve a similar effect with fly ash concrete as well. One thing I've been concerned about is the relative concentration of bad stuff in the fly ash, heavy metals in particular. I've read a peer-reviewed paper, though, that determined that the heavy metal leachants from cast fly ash were no more than from any other building material even though the content is there. I think it's better to use it instead of Portland cement, especially since casting it "sequesters" it in a fixed locality for a useful purpose.

Q: I do like the scoria approach, but I don't think we have much in the way of volcanic rock in my neck of the woods.  I'm going to check into perlite to see if that's available hereabouts and if so, if it would be an affordable alternative.  But if that doesn't work, I'll probably have to scrap the bagging idea and go with some other approach. 

A: (Owen) Perlite, scoria, vermiculite and rice hulls are all good options.  Rice hulls are still experimental and it would be helpful to get more info on how they've worked for others.  Although rice hulls are resistant to decay, they will rot if they get wet, so I would use them only in dry areas with a good roof (no domes).  Price each one and see what's the most affordable.  For scoria, check with landscape suppliers.  You can get perlite and vermiculite from gardening supply outlets.  Try to buy things direct by the truck load.

Q: What is the best bag filling to deal with high moisture content and heavy rains?

A: The most secure fill material for earthbag buildings would be Portland cement stabilized soil, as this could actually survive some immersion in water for a period and still retain its shape.

Q: I recently acquired a .50 acre parcel of land in southwest Florida. I've been wanting to build an earthbag home for the longest time. Iis half an acre enough to build a 3 br/1.5 bath earthbag home, taking bag fill from the actual land?

A: The best fill material for earthbags in Florida would be some that is a combination of both clay and sand, in a ratio of about 1 to 2. If your property has these ingredients, I'm sure that your half-acre would suffice; it doesn't take much land area, especially if you can dig down vertically to access it.

Q: I'm soon starting a project in Mexico, but I'm having a problem regarding the soil mixtures I should use for the wall. Accordingly to what I've read,  silty soils should be avoided. The problem I'm facing is that almost all the soil in this region is made of something near to 85 % silt and 15% clay resulting in very unstable walls. Trying to import good soil will be expensive due to the transportation fees. Could I still use this soil mixture or is it simply out of question?

A: I suspect that your local soil would work just fine, especially if you make sure that it does have that clay content. I suggest that you run some tests with packing some of your moist soil in a few bags and see if it hardens into a reasonable solid block. This is more important if you are building domes, and less important if the walls are vertical.

A: (Owen) It shouldn't take many truckloads of clay to give you the necessary proportions.  I would experiment to find a stable mix by adding increasing amounts of clay.  Keep track of the percentage of clay in each sample.  Once you get a stable sample, calculate the approximate volume of clay you will need.  On a small house, I'm guessing two truckloads may be enough.  This is just a guess, so do the math and calculate how much it will cost.

The other option is to use other fill materials such as scoria (lava rock), pumice, perlite, etc.  Maybe you have something like this in your area.  Scoria and pumice are volcanic in origin, so look for volcanic areas nearby.  Rice hulls may work also, but are less proven.  (I would only use rice hulls on a small round or curved building with vertical walls.)

Q: I'm testing bags for my construction and my soil is nearly 100% clay. I trucked in some sand and mixed an experimental bag with 1/3 clay soil and 2/3 sand. I mixed it dry in a wheelbarrow and broke down the clay as small as I could but most of it still stayed in small clumps. I mixed with water and filled a test bag, tamped it down very well, and it's been sitting there for 3 days. The bag weeped some water but not very much. When I press on it, it still feels more like sand than adobe. Should I mix a different ratio of sand to clay, pulverize my clay into finer dust, or just keep waiting and hope this bag eventually sets up correctly?

A: I suspect that your test bag will eventually harden up just fine, but it might take awhile. If water was weeping out of your mix, it may have been too wet in the first place.

A: (Quentin Wilson) Just use the soil you have. The problem with too much clay is that sun cured adobes crack. Since you have the soil in bags, cracking is not a problem so you don't need to make the job any harder than it is. Any clay that oozes through the mesh will just help stick the bags together. Assuming that you are using the dratted woven polyolefin or polysomethingelse bags, they have such a low coefficient of friction that keeping them aligned as wall go up is troublesome. The oozing clay goop will help. Being old fashioned, I prefer hemp, jute or cotton bags.

Don't worry about clumps. Moisture will suffuse throughout the material over several hours or days as it slowly dries. If that seems sleazy, let the soil sit in a pile for a week or two that you keep damp. Nature will do the work for you. If you come back in 10 years and for some reason (such as solar degradation when plaster is gone) you will find that the bag contents will hold up on their own even if it does not seem to be a perfect blend in the beginning.

A: (Michael G. Smith) Sounds to me like you are either using too much water or not tamping hard enough. No matter what's in the bag, it should feel very solid once it's been compacted.

Q: What parameters make for good earthbag fill? Here at the office we often talk about, 'select fill' and 'general fill'. I wonder if either would be suitable bag fill material. Are there any specific recommendations for good fill?

A: The best fill for earthbags is similar to soil that can be used for making adobe blocks, cob, or rammed earth walls, although for earthbag construction there is more leeway for the ratio since the bag will help consolidate the material. In general soil that is between about 10% to 35% clay, with the remainder primarily sandy aggregate works well. When filling the bags you want to moisten the soil just enough that it will form a solid block after being compacted, but not so much that it will weep water. Once the compacted soil dries it will be quite hard and brittle.

Q: My husband and I have just purchased land in the hills of Montana and will start a test dome shortly in preparation for building a home next year. I have been searching locally for scoria for the bag filling. All I can find is 3/8" or 1.5". I'm a bit worried by that - the 3/8" may be on the small side and so may not have adequate insulating properties, but the 1.5" may be too bulky and untampable (if that's a word!). Perhaps we could mix them.

A: I think that your suggestion to mix the two sizes would be the best, but try to order the stone with as little fines as possible (or even screen it out yourself) since the fines will also lessen the insulative value.

Q: My concern with road base is based on what I hear about some having clay and other types of road base not having clay.  The people at the recycle yard could not really tell me much about the contents.  I have been adding clay to the recycled fines and that works well, but the nail split the sample in a few pieces.

A: Driving a nail into most earthen blocks will crack them. I think the deck screw idea tends work better (at least according to our adobe expert). "Road base" is a generic term that applies to a large range of specific soil compositions, depending on the locality, so your addition of clay may make the difference with how well your particular material consolidates into a solid block. The thing about earthbags is that the fill material doesn't necessarily need to form a solid block, since the bag material itself will hold it all together.

Q: I am going to build an addition to my main home. I am using earthbags to build the walls. I am going to build a rubble trench. I am going to fill the bags with crusher fines from a local rock quarry. I guess the fines are a bit larger than ag-lime. Would ag-lime work as filler or should I stick with the fines? Should I mix some sand in with the fines or can I use straight fines or straight ag-lime? The walls will be load bearing. The addition will be round with a masonry stove in the center that will be load bearing as well with timbers running off masonry stove to the earthbag wall.

A: As for fill material, the ideal is something that will solidify after compaction, like a moistened adobe soil would. Adobe has a combination of about 1 part clay to two parts sand. Your crusher fines might do this just fine, or possibly also the ag-lime, or even a combination of the two. I suggest that you run some tests to see what works best. Make several mixes, moisten them slightly, place them in small bags, tamp them well, and wait for a week or two and see which ones remain solid when dropped or handled. I doubt that you would need to add sand to the fines, but they may need some added clay to bind them better.

Q: I am not quite sure that the numbers for hydrated lime add up as Kaki suggested. I was close to buying a couple of hydrated lime bags to do some testing and pay for some PSI certification from a local lab. But on a cost basis, I don't see how hydrated lime is going to be cheaper than cement. On the Big Island, a 94 pound bag of cement costs close to $15. On the other hand, a 35 pound bag of hydrated lime costs close to $12 In the cindercrete mix we use around 8 parts of cinder, 1 part of blue rock, 1 part cement and 1 part water. If we are going to use 1 solid part of hydrated lime, it is just too expensive. Unless Kaki was talking about 1 part of very liquid hydrated lime paste.

A: I doubt that Kaki was suggesting that you use the lime only. From my experience, pure lime will shrink a great deal as it cures (and it takes a very long time to cure). You need to add sand to the lime to make it stronger and keep it from shrinking too much...sort of like adding sand to clay to make adobe. I suggest that you try replacing the cement in your formula with the lime and see how well this performs. It doesn't need to set up to the specs of concrete, only become firm enough that it will stay in place without shifting over time. Then if you leave the bag material on before plastering, your wall will be extremely durable. Lime is considerably lighter than cement per volume, so that 35 pound bag will go almost as far as the 94 pound bag of cement.

Q: We have land with at least 169 different soil types.  But the area for building is a heavy mix of sand, with a small amount of clay.  We planned on using some portland cement mixed in with our soil.  So if we cover them with a tarp for a week or so after installing them that would give us a sufficient cure time?

A: I suggest that you keep one test bag separate from your wall and use it a guide for how long it may need to be cured; there are so many variables in all this that it is hard to say in advance.

Q: I am having a bit of trouble discerning the clay to sand proportion of my soil and have attached a photo in the hopes you can give me your opinion. Do you think it is possible to use this soil for earthbag building without mixing any other soil or sand with it? How about for Adobe or cob? I realize it is difficult to tell by looking at photos but perhaps you can give a hint before I begin experimenting. I am really hoping I will not need to truck in other material.

A: It appears from the photo of the jar test that your clay content runs about one third, with the rest courser material. I would think that this soil would do fine packed in earthbags, especially since it doesn't really matter if it eventually cracked a bit. For cob or adobe there may be too much clay, in which case some sand can be added to it.

Q: Are scoria and/or perlite filled earthbags as sturdy and even bullet proof as are those filled with sand or dirt?

A: Scoria or perlite filled bags are not quite as solid as an adobe soil would be, but they are plenty solid for this sort of building and the insulation value makes them well worth the extra expense...especially in that climate. They may not be as bullet-proof however.

Q: I went to the Lava Rock facility to get a price list and to see the scoria.  Their 2009 price list showed one ton of the 3/8 inch or less material for $16 a ton.  I have a feeling you used something larger than that.  Am I correct?

A: Yes, we used a 3/4" minus aggregate and it worked fine. The 3/8" would also work, but might be slightly less effective as insulation. The problem with any of this is the "fines" or dust that often comes with it, which tends to clog up the air pores and add to the weight. If you can ask them to screen out these fines, then either of these sizes should work.

Q: The building people are concerned with what the material is made of. Since I have the same type of climate that you do in CO I want to go with the lava rock.

A: While lava rock worked well for me, since it provided excellent insulation, I suspect that the building officials will recommend the adobe-like soil.

Q: I know a missionary in Niger that is interest in earthbag building, but he is concerned that the local sand is too fine, thin, and dry for that system.

A: It might be that the local sand does not pack well enough into a firm block to be good fill material, but this can be amended with some clay or even cement or lime and dampened to help it solidify some.

Q: I thought the mixture was 10% concrete / 90% sand, (more or less and I know this is varied depending on location) moist but not dripping. If you use a machine to fill the bags you'll have lots of dry bags to stack. Does that make the finished structure weaker?

A: There have been many different mixes of materials used successfully for filling bags, and the concrete/sand mix is just one option...one that is not particularly green because of all that cement. A better, and equally durable, mix would be something approaching standard adobe (20-30% clay and the rest sand). Even just 10% clay can make a decent mix; all that is needed is that material solidify sufficiently after having been dampened and tamped to form a block that will not shape-shift over time. Also, lime can be used to stabilize a loose soil, and this is more environmentally benign since it reabsorbs the CO2 released during its manufacture. The house I made in Colorado used scoria (a type of volcanic rock) that was crushed to fill the bags, and this worked our great because it is light-weight and insulating.

Q: I am looking into building semi-subterranean sandbag spaces in a dry desert environment in Africa. The rainfall is less than 3 mm per year, but the area receives daily moisture through fog. The available soil is fairly granular river sand (from dry riverbeds), and we are thinking of either importing lime or cement as stabilizer, as clay is virtually non-existent within reasonable distances. We want to sink the units at least 1.5m deep, and cover the domes with at least 600 mm sand, to provide a thermally buffered interior space. Any suggestions or examples of previous experience will be extremely helpful.

A: Stabilizing your sand with cement or lime will likely be essential to build domes; otherwise it might collapse because of the sand shifting inside the bags, especially with pressure from any berming. I am not aware of any earthbag domes having been completely buried like you propose, so this would clearly be experimental. If you want to proceed with this, I suggest that you stabilize your sand with quite a bit of cement (at least 10%) and start with a small test dome to become confident that it will work. Please let us know how this works out.

Q: Do you think one layer of scoria / perlite / pumice / vermiculite would be strong enough to support an earthbag (compressed earth only fill) structure? as compressed earth is much heavier (12' high).

A: I know that scoria would do fine under such compression, and I am pretty sure that both pumice and perlite would also; vermiculite or rice hulls might compress too much.

Q: We are in the process of plastering a 15' dome and a 15' vertical cylindrical building, side by side. After constructing both, I found out that the dome requires only slightly extra effort but delivers more economy for space and strength. However, you are right, it is more difficult to waterproof, esp. in the tropics like Belize. Can you say if it is better to stabilize the material with cement?

A: Nader Khalili tended to use cement to stabilize the soil in his dome projects, especially toward the end of his career. I am not convinced it is necessary, but it adds a degree of security if a dome did develop a significant leak and saturated the material in the bags.

Nice sketch of a cross section of your dome project. It all looks very well thought out to me, and should create a very thermally efficient small building.

Q: What about using sawdust as fill material?

A: Sawdust might work, but I have never heard of anybody using it. Possible problems are: rot if it ever gets wet long enough, compaction over time, insect infestation (termites?). I know that sawdust has been used historically for insulation, and sometimes it is mixed with lime to inhibit mold or insect infestation.

Q: Do you know from where I can get rice hulls?

A: Rice hulls are generally available from the plants where they process the rice, which is usually near where it is grown. The hulls have been recently baled and sold for agricultural use in the US, so they might also be available internationally, but I don't know for sure.

Q: Is there a problem with making an entire part of the wall filled with dense gravel and not just the first three courses?

A: I would say that this is definitely possible. I built a substantial house with bags filled primarily with crushed volcanic stone. This was over a decade ago, and the house is as sound today as when it was built. This crushed stone did pack very well in the bags, so that it didn't tend to change shape, which a slipperier gravel might do, so you might make a test wall before committing to doing this. Filling the bags with loose material does mean that the bag material is being relied on to help hold the shape, so it is very important that the poly material is not compromised by UV...keep the bags covered when possible and plaster them soon!

Q: I've got a stumper for the experts. I am building a small Earthbag dome as a test project in the tropics. I've jar tested my local soil and found it to be 30% clay/silt and 70% small fines and sand. I'm using 10% Portland cement by volume. My first mixes of came out of the mixer as cement coated earthen balls. So I started fine screening (1/4") the soil to break it up and reject any large clay clods. This has worked well until last night. We received a large amount of rain. Now the soil is very wet and clumps/rolls back into cement coated balls. Is it possible to mix stabilized cement soils using wet soil?

A: That is a bit of a stumper. I know that clay and cement don't always want to combine, and they tend to serve the same function in binding the larger sandy particles together. You might need to increase the sand portion of your soil to get good results.

Q: I tested 2 bags of soil, one with dark top soil and another with the more yellowish, rocky sub soil. Both were damp and tamped and left in a shaded place to set. Both are now rock hard except if I push on the edges it crumbles a bit inside of course; with bags on top and plaster it wouldn't do this. Any way I can't decide which of these soils is going to be better?

A: I would say that either of your soils would do fine. Often, the darker soils are richer in nutrients and better for gardening, so I would lean toward the other.

Q: Anyone been building earthbag in heavier soils? I just wish we had a way to test and predict more accurately... To tell one person, you better add some sand, another it'll be ok. The CEB info I have says 1/32 inch shrinkage over a foot length is acceptable. That's about 0.2%. I am certain that earthbags can tolerate much more. But how much more? And how will lime plaster perform on a heavy clay base? Would it crack? At least if one uses the same clay for plaster it should expand uniformly with the base?

A: I'm afraid that we just don't have the database to answer your questions with authority. I'm sure that people have used all sorts of soil in earthbags by now, but how successfully it is hard to say. All we really can go by is from our own experience and knowledge. The needs of CEB, adobe, cob and rammed earth are well documented...but only pertain to earthbags in a rather general way. I believe that unless heavy clay used in earthbag building is very expansive (and this varies a lot with different clays) and it gets soaked through after it has cured initially, that problems are not likely to appear. An earthbag wall that is properly made, with a reinforced plaster, is not going to fail even if the clay in the bags is riddled with cracks. The exterior plaster is more likely to expand or contact with moisture content because of its exposure to the elements. For this reason, I would not recommend using a heavy clay earthen plaster. Lime or standard earthen plaster is more likely to hold up well.

Q: We are currently planning on stabilizing the earth inside of the sacks with lime. We like lime as an option because it is more environmentally friendly and there is a local place that mines and prepares lime (hydraulic type s). What percentage of lime do you recommend that we add to the mix to improve the impermeability and increase the strength?

A: I suggest that you run some tests with your particular soil to see what works best. Try several batches with different ratios. I have always heard that working with lime can be tricky, partly because it does behave differently with different soils.

I've read that lime works well as a stabilizer with soil mixes that have some clay content, and that they shouldn't be too sandy. We've got a really wide variety of soils available to choose from. Approximately what percentage of clay is ideal to have in a soil that will be stabilized with lime?

I have used lime with straight sand, and it actually has hardened just fine.

Q: What would be the best material to put into these earthbags to make them harden up solid?

A: Soil that contains at least some clay (between 10 and 40%) and the rest courser aggregate and sand will harden quite firmly when damp and then tamped. I should mention, however, that this material will provide a lot of thermal mass and virtually no insulation. So be careful where you place such material so that it assists with your overall design intentions for thermal efficiency.

Q: I'm trying to build an earth bag dome here in Merida, Yucatan. The soil is pretty bare and mostly composed of limestone mother rock. There is an abundance of limestone dust that is not very expensive here and that I would like to use for bag filling, instead of the cement, lime, red earth, stone dust mix that my neighbours are using to build their earth dome house. My plan is for a 5 meter diameter dome. simple. Is their anything I could add to the limestone dust/gravel to stabilize it?

A: My guess is that if you just add enough water to dampen the limestone dust and then tamp it well in the bags that it will pack into a rather solid block. You might try this and see. If it doesn't "set up" after it dries, then you will need to add some cement or perhaps clay to it. I think you are going to have to experiment with a variety of mixes to see what works.

Q: I am very interested in the use scoria as fill material. However, when I referenced this in Kaki Hunter's book, their take was that, while the scoria has excellent insulative qualities, it does not pack down well from a 3/4" size material, resulting in lumpy bags that do not fit well in an earthbag wall. Did you have any such problems in your building, and if so how did you get around it.

A: This is one point that I disagree with Kaki and Doni about in their book. I know that they suggest mixing the scoria with soil so that it will pack well. The problem with this is that it severely diminishes the insulative value of the scoria.

As you know I used straight scoria in most of the bags in the large domed house that I built over a decade ago. (see http://earthbagbuilding.com/projects/hart.htm) The scoria does not pack quite like adobe soil does, but it packs quite firmly none-the-less, certainly firmly enough to hold its shape in the dome configuration. All those sharp angles of the 3/4" minus aggregate interconnect in such a way that once they are tamped they tend to hold that shape, especially after being plastered on both sides. That house has not shifted or sagged in any way since the day I finished it, and I expect it to still be here a century from now.

Q: There's grass growing out of the earthbags. Is this a problem?

A: Adobe and cob often sprout little seedlings, but as they cure these die off and actually help integrate the soil to some extent I have heard. Obviously you don't want very much organic material that will decompose and cause voids though.

Q: What about using straw or rice hulls in a dome earthbag home that is reinforced with curved beams and wire net?

A: I would not advise using bags of straw or rice hulls for a dome structure. The possibility of the straw or hulls rotting from moisture is too great. If the dome is protected on the outside with an adequate moisture barrier, you still have the danger of condensation forming from within.

Q: Could you tell me what the ratio of sand and cement is needed to stabilize the sand?

A: You only need about 4 - 6% cement by volume to stabilize the sand. It needs to be mixed thoroughly and moistened to work. Run some tests with your sand to make sure what works.

C: Unfortunately the clay content in our soil is about 2%--so we will be supplementing our fill with about 10-15% Portland cement.

R: Have you actually tried using the native soil to see how well it compacts after being dampened, tamped into a bag, and allowed to cure until dry? If not, you might be surprised at how solid it is. If this natural soil is still too loose, then you might try to find a source for additional clay, rather than Portland cement (which is not only quite expensive, but also terrible for the environment, with all of the CO2 emitted during it's manufacture.) If you can't make the natural materials work, then you should at least consider the addition of hydrated lime instead of the cement, as it is much more environmentally benign (and also cheaper). If you are certain that the only way forward is to use cement, then you should know that normally only about 5-6% cement is used for soil stabilization, not 10-15%.

Q: If I wanted to build a 1500 square foot barn/house using the lava rock, how much scoria would I need to buy?

A: Of course there are many variables, but given standard 50# earthbags and a single story structure, you might need about 1800 bags (minus doors and windows), which at about 35 lbs/bag of scoria would weigh about 28 tons.

Q: We recently purchased a lot in Southern Baja, Mexico and are very interested in building a small vertical wall building with earthbags. Around 200 square feet. The soil in the area seems to be mostly a sandy dirt, pretty fine in texture, it is basically desert, very dry fine soil. Do you think this would be acceptable for this type of structure?

A: If you make the building circular, or at least with curved walls, then I would say that you could likely use your fine sandy soil. I did this for a circular structure once, about a decade ago, and the building is still standing. See earthbagbuilding.com. Or, if you stabilize the sand with some added clay, Portland cement, or lime, then you can build most any shape.

Do you recommend doing a soil sample first?

Yes, if you run a soil sample test, you might discover that there is enough clay to bind the soil together. Or you can moisten some of the soil and see if it will form a cohesive ball, or just fragments once it is dry.

Q: We are being put through the pains in Tucson, AZ as to building our Owen Geiger Zero Two Energy Home. They are requiring us to have the earthbags tested; we have tried all and they won't move. They want cement to get it to pass the test. Do you have any recommendations for what amount of cement? The only cement usage I found said 10% cement 30% gravel with 30% sand and soil. Any help you can provide for a correct mixture is appreciated.

A: A variation of poured earth that has been around since cement was first formulated is soil cement. This is a dry pack (moist) earth/cement mixture which works especially well with rather sandy soil, but will also work with other soil types. The heavier soils with more clay content will probably require more Portland cement. Soil cement has been used to form walls, make floors, pave roads, stabilize river banks, etc.

Here is some information about formulas: Make it by mixing earth with Portland cement to the desired depth, add water and mix again. Tamp, and cover with plastic to let it cure properly. Use 6 to 16 percent cement by volume according to the density of the soil. The denser the soil (clay, for instance), the higher percentage of cement to use. Six percent translates to 1 part cement to 15 parts soil; 16 percent translates to 1 part cement to 6 parts soil. Hopefully you can get by with less than 10%.

A: (Patti Stouter) Strongest soil has all the particle sizes- is called 'well graded'. Adding extra strong clay and/or sand to your soil could put it up there for less expense than cement. When I stabilized some rather silty soil for a single shed course here in NY, I was appalled that it took about 12% to make it somewhat strong (my goal was to make it water resistant not stronger). If you must stabilize, you should add some lime with cement if the soil has much clay, and tweak to get least stabilizer, strongest mix. You should do your own tests for flexure by stacking weights on a block that's spanning 18" or so, to test mixes before you give it to the lab again.