Sharing information and promoting earthbag building
|Using Barbed Wire and Twine
Questions answered by Kelly Hart
Q: You used poly twine to anchor the bags as well as barbed wire - is it vital? I was also a little confused about how many levels were lashed together?
A: The polypropylene twine is not essential, but it does help with the stability while building, and more importantly, it gives the eventual plaster material something to grab onto and lock into place. The twine circled three bags, so each bag was lashed both upward and downward.
Q: Does securing the bags with courses of barbed wire really give them enough strength to not need load-bearing beams?
A: The barbed wire is used for two reasons: to help stick the bags together and form a matrix within the wall system, and also to resist any tendency for the wall to expand from weight from above (in domes), so in this instance the wire would help with load-bearing capacity.
Q: I have available barbed wire that is two point. Will this work if I use two strands? And can I wind the wire over and under each bag rather than keeping the wire in a single line between courses for added integrity?
A: Two-point barbed wire will work...it just isn't as good in gripping the bags as four-point would be. In either case I recommend using two parallel strands, at least 6" apart. As for weaving the wire, doing this would not be a particularly good idea, since it could actually diminish the integrity; the wire needs to be able to act under tension to resist outward expansion of the wall from the pressures placed on it from above. Also interweaving the wire between courses would make it difficult to get adjacent bags to make good contact.
Q: Is it better to not make courses so perfectly level; add in small sine wave and more like seashells, which would make structures considerably stronger as then sides come up with strong ridges.
A: This might be true, although if you use the barbed wire between the courses they need to be relatively level and uniform in order for the wire to be effective as a bond beam. I have found that the wire adds considerably to the strength of the construction.
C: The thought of rust attacking barbed wire (in those paces where humidity is often between 90 and 100%) kept me from looking into earthbag construction at first. But when I realized that the barbed wire is most needed to hold bags in place until the earth has cured and dried, the lights went on. It seems irresponsible in very damp climates to rely for strength on a material that may eventually rust. But once the walls are cured, the longevity of the barbed wire is not as critical. I think of the bags and wire as a sort of formwork. The earth itself is strong enough once it has cured.
R: (Owen Geiger) As far as the barbed wire, it's embedded inside a dry earth wall and is coated with zinc. Old adobe buildings of 100+ years still have their nails intact, probably because the earth wicks moisture away. I think the barbed wire would still be there in 100 years.
R: (Kelly) I agree about the barbed wire; it will last a very long time, and even it doesn't most buildings would do fine without. I think there are some earthbag domes that rely on the wire to hold their shape under tension.
Q: Is it a must to use the barbed wire as binder? Since the cost of steel wire is very much on the higher side, and since I am going to build a small house, 10 feet high, can I leave the barbed wire? I intend to build a mud brick dome over the walls.
Q: With a double earthbag wall (two columns adjacent to each other), how can you connect the two columns?
A: Short chunks of barbed wire put in occasionally at right angles to the straight wire will work. I basically did this on the double stem wall of my Carriage House, and extended the short pieces out far enough to then use them to attach the stucco mesh later.
Q: I may build a simple, small home out of earthbags. One thing I'm wondering about is if I use 2 point barbed wire, am I jeopardizing the strength of the structure? I have an unlimited free supply of 2 point barbed wire. I was wondering if I use 4 or more strands between each row if that would be comparable to using 2 strands of 4 point barbed wire.
A: Four-point barbed wire is recommended because it tends to grab the bag material better than two-point wire, so that the bags don't slide relative to each other. In terms of providing tensile strength to keep the building from expanding from any outward pressure (especially with domes) two-point barbed wire would work just as well. I suggest that you go ahead and use the two-point wire just as you would the other, since it does work nearly as well.
Q: My question is about the barbed wire. This is a little bit counter-intuitive for me; I would expect the wire to tear the bags. Does this happen?
A: While the barbed wire does occasionally snag the polypropylene material, is rarely tears it very much. The barbs help keep the wire in place and also help the bags from slipping relative to each other.
Q: I saw this in a video called "Making the Soweto Hospice with Volunteers" where they said that it was not necessary to use the barbed wire because of these rebar stakes. I decided to do it like this because it´s cheaper. I could not afford the barbed wire, but if you tell me thatIi definitely need the barbed wire I will try to get some money to buy it.
Q: What is the best place to pick up 4-point barbed wire?
A: I would look for the barbed wire at your local farm and ranch supply companies. They might have to special order it for you.
Q: What are your thoughts on eliminating the 4-point barbed wire between bag layers? I have read that vertical earthbag walls do not need barbed wire? Barbed wire is madly expensive down here in Grenada. We had planned on using lengths of rebar driven vertically through each 4/5 rows of bags in corners and alongside door/window openings. If we used temporary vertical wood supports until the bags hardened up would that be sufficient? Our bags will be filled with a red clay soil. (we have used the soil test and determined the clay content is not too high and will have gravel on site to mix with if necessary).
A: While barbed wire is essential in a dome, it also plays a role in vertical wall structures. It does keep bags from slipping relative to each other, and provides a cohesive element should any seismic event occur. I would use barbed wire on the entire structure if it were my house.
Q: I've been looking at eco bag building and I was wondering if instead of using barbed wire to help hold bags together could we use brambles? They are strong, should last and are eco friendly. I have fields of them I need to clear.
A: You could use the brambles and they would definitely help keep the bags from slipping around on each other and also lend some tensile strength to the wall. If you are in an area where there is significant concern about earthquakes, then barbed wire would be a better choice because it is much stronger and would keep a wall from fragmenting better.
Q: I'm looking to find the 4-pt barbed wire necessary for linking the earthbags together; do you know of anywhere that would sells barbed wire by the foot, or of any rancher or earthbuilder that has surplus barbed wire? I'm willing to drive far to find it!
A: When I bought 4-point barbed wire a decade ago I had to special order it at my local farm supply store since it isn't so common any more. I've never heard of anybody selling it by the foot; it is always sold by the spool (about 1/4 mile I think). In a pinch you can use standard 2-point wire...it just doesn't grab quite as well.
Q: Is single strand barbed wire used for cattle fencing suitable. The double strand barbed wire available here is three times the price which makes a big difference in the cost.
A: Single stand wire is fine; if it has 4 barbs rather than 2, that is better.
Q: If it is not bad enough that we have to fight the damp constantly we also have to worry about acid rain from the volcano! It rusts everything! Would barbed wire eventually rust even with it being encased? If so is there an alternative to using it?
A: I think it would take a very long time for galvanized barbed wire to rust when embedded in an earthbag wall, even in your acid rainy climate. Another approach with your project might be to use the mesh tubing for earthbag material and periodically reinforce it with rebar or bamboo stakes if you didn't want to rely on the barbed wire. This method is called hyperadobe.
Q: Since the frictional coefficient between like materials is the lowest (from my hi-school physics), what keeps the bag layers from sliding on one another (polypropylene is very slippery! on itself), esp. in an inclined state (like for a dome or a sloping wall)? In a masonry walled system both rebar and and joint reinforcement are used to prevent toppling and bulging of the wall. Of course the primary binding/bonding material is mortar.
A: The "mortar" with earthbags is mainly the two strands of barbed wire that is placed between every course of bags. Also, under the immense weight of the entire wall, there is quite a bit of friction between the bags. Add to this the plaster on both sides that generally has a mesh embedded, and the wall becomes rather stiff and unlikely for individual bags to slide around. Furthermore, typical earthbag wall construction does used embedded rebar in strategic places, such as around opens and in the corners, and this is ideally also embedded in a solid bond beam at the top of the wall, which tends to make the whole structure more monolithic. With domes, the bags are tamped horizontally, even though they corbel inward, and this helps with any tendency to slide.
Q: How do I determine the barbed wire?
A: If you want a very accurate measure of the length of barbed wire needed, you'll need to run some calculations that estimate the circumference of each horizontal slice of a dome at roughly 6" intervals, minus any openings, and double this if you use two strands per course.
Q: I will be building a 20'x36' earthbag home in southern Missouri. I plan to use buttresses on every intersecting wall. My question is, how absolutely necessary is the barbed wire between layers? Needs to be every layer? My budget is minimal and I already have the sandbags.
A: Barbed wire is always a good idea because it helps keep all of the courses of bags into a more monolithic whole. It is essential with domes to keep them from expanding outwards. With vertical walls it is not as crucial, so you could probably get by with just one strand instead of two, especially if you periodically pound vertical rebar stakes through the walls. Also, in earthquake country the barbed wire is good insurance against collapses.
Q: If I' am doing the mesh bags, would it be overkill to still put in barbwire, And also I will be mixing cement and soil together as my mix, and rebar throughout. If so, out of all that, what will be overkill?
A: I doubt that barbed wire would be needed, given everything else.
Q: How important is stainless steel metal in the wall? For example I've seen some guides recommend only stainless steal barbed wire, others don't specify.
A: I have never used stainless steel in earthbag construction, although I'm sure it is extremely durable. I think that ordinary galvanized barbed wire is just fine, considering that once the bags are cured and dry the wire should not corrode.
Q: Overlapping rebar is generally stronger. Why not crisscross the wire?
A: The wire needs to be fairly taut to create the needed tensile strength.
Q: Even if I use a bond beam do I still have to use barbed wire between each earthbag layer? I won't be using individual earthbags, I was able to find uncut polypropylene bags which I think will make it more stable and easier to work.
A: The barbed wire is essential for earthquake preparedness, even with the long tubes of bag material. I agree that the long tubes are better than individual bags for stability.
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