How to Tamp Earthbags
Questions answered by Kelly Hart

Q: Do earthbags settle? If I support the roof partially on earthbags and partially with poles will that cause problems due to the bags settling?

A: It really depends on the type of material the bags are filled with, and how well they are tamped. Some of our bags are filled with sand (which basically does not settle) and some are filled with scoria (crushed volcanic rock), which has settled slightly. Had I tamped the bags more thoroughly as I was building they would likely have settled even less.

Q: To speed up the earth bag tamping process do you know of anyone who has successfully used a small powered plate tamper (as opposed to the Pogo stick variety)? Apart from the safe physical handling of the machine would there be any disadvantages?

A: I don't see why you couldn't successfully use a power tamper, although for the sort of earthbag work that I have done this would be overkill, since the loose volcanic gravel and the sand that I used to build my house don't need that much compaction. For adobe soil compaction, then I think such a power tamper could actually save quite a bit of work.

Q: I wonder - after reading about your designs - how compactable the volcanic 'crush' filled earthbags are once installed. Do they 'set-up' like the ones Kaki Hunter and Doni Kifmeyer advocate?

A: The volcanic stone does compact some, but never "sets up" like Kaki and Doni advocate. I disagree with them about the need for this degree of solidification, and have proven this through the construction and durability of the house I built.

Q: I have started with my first wall! We are using 100lb. Tubing, so it is quite a mission as the volumes are quite daunting. How do I keep the walls even? As we lay the tube, it does not have an even side - this is the side I need to plaster at the end. Is there a way of getting it a little more straightened out - do I just to a bit of tampering? If so, I guess first the side and then later the top?

A: Yes, tamping the soil in the bags is the way to get it more even. I would tamp the top first, and then the sides if you want them more even. Often people just fill in the unevenness between the bags later with plaster, but it can be tamped fairly smoothly while the soil is still damp.

Q: We started with our dome in Brasilia and laid a row of bags filled with gravel over a rubble trench. Then we laid the first bags filled with earth but we didn't tamp them immediately (only two bags were tamped). Then a big big rain came. The two tamped bags are still in a good conditions, but the rest of the row is all soaked up with water. If you step on the bags, water floods out of any side. Right now, we are not sure, how to continue. Do we have to tear the row away and re-do it? Could we lay the next row on top of the soaked, wet row? If we decide to wait, how long might it take for the bags to dry to a state where they can be tamped?

A: I suspect that if you were willing to wait long enough, the soil in the bags would dry sufficiently that you could them tamp as normal and proceed with the wall. How long this might take would depend on the weather, exact soil composition, etc...so it is hard to tell. If you wanted to continue without waiting, the safest thing to do might be to remove those saturated bags and start over.

Q: I am going to get a large pneumatic tamper/rammer for earthbag/tire ramming/ and rammed earth. Otherwise its too labor intensive. Do you know of anyone using the tampers on earthbags?

A: I suppose that pneumatic tampers could save some labor with tamping earthbags, and I'm sure it has been tried, although I don't know of a specific instance of this. I'm not sure you could get one to work with ramming tires though, because of the cramped space and angles involved. Speaking of labor intensity, I think that ramming tires has got to be the worst of all "natural" building methods; I would much rather build with earthbags.

Q: What worries me is the weight of the house. The earth seems soft, if I jump I can make an indentation under foot. I'm thinking a conventional slab foundation.

A: With such a high water table, you might want to raise the pad for building several feet with gravel or something that drains well. I wouldn't worry about the soil being too soft. Once it gets compacted in a bag it will hold a great deal of weight. You might do some sample bags to prove this to yourself.

Q: For quality control how does one make sure that bags are sufficiently compacted?  Have a penetrometer or other compacting testing tool been used to test compaction?

A: Hand compaction with a tamper is typically used with earthbags, and you can feel the degree of compaction pretty easily with a bit of experience. There is not only a sense of hardness when you hit the bags, but there is also a corresponding change of sound from a moosh to a ping. I have never known of scientific equipment being used to assess compaction, but I suppose it could be done.

Q: Would there be any benefit to putting the earthbag in a mold and squeezing it with a hydraulic press? 

A: I have seen bags formed in boxes to create rectangular blocks which makes for flatter walls that may not need as much plaster to make smooth. You can also just tamp the bags on their sides once they are laid to accomplish the same thing. Actually hydraulically squeezing the bags would probably leave loose fabric that could be a nuisance. If you wanted to do that you might just go with compressed earth blocks and not  have the bag material.

Q: In a few of your responses you indicated that, understandably, the size of the filled, finished earth bags were a different size than when they are flat and unfilled. How much does the size change and how much 'shrinkage' can I expect? I am preparing to attempt to build The 'Solar Pit House' designed by Owen Geiger. I read on your site that for walls to hold back earth and building into the side of hills/mountains (which we are) the walls need to be 24" think. I'm eyeing bags that are 27". I am concerned that once filled the bag will no longer be 24" thick. Should I buy the 30" instead? The bags will be filled primarily with clay.

A: The standard 18"X30" bags will create a wall that is maybe 14" thick, so that is about the "shrinkage" you might expect. 20" walls with your larger bags should be plenty adequate for your project.


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