Sharing information and promoting earthbag building
|Landscaping with Earthbags
Questions answered by Kelly Hart
Q: I have been looking for an alternative way to build a terraced/retaining wall system for a slope adjacent to my home that has "slumped" over the years. All the traditional methods, such as timbers, block, Keystone systems are much too expensive. I stumbled across your site this morning looking for alternative building techniques and noticed your article on earthbags. Have you used earthbags for this application or might you suggest other methods?
A: Earthbags are an excellent choice for retaining walls. I once built a dam with earthbags that held a large body of water. For such a wall you should be able fill the bags with your native soil. It is important to keep the polypropylene bags shaded from sunlight exposure, so they must be eventually plastered with stucco or something.
Q: I am looking for information on building a retaining wall out of stacked bags of concrete. Any how-to information you could provide would be great.
A: You can certainly make a retaining wall with bags of cement stacked up, but I might suggest a simpler, cheaper, and more sustainable approach to making such a wall: just use earthbags (polypropylene bags) filled with your local soil, stacked up like you would with bags of cement (laid in a staggered, brick-like pattern). If the wall is curved against the soil being retained it will be much stronger. Two strands of barbed wire placed between each course also makes it stronger, but this may not be necessary. The earthbags will need to be plastered with stabilized earthen plaster or stucco to keep the sunlight off of them and make it look nice, but so would the sacks of cement approach, and this would use way less Portland cement, which has an environmental toll.
Q: Can one build a retaining wall with earthbags similar to a retaining wall constructed out of tires?
A: This is done much easier with earthbags than with tires.
Q: We are planning to build four small and cheap houses using strawbales. The north wall of the houses will touch the wall of a ground. We are thinking to build a retaining wall against this land using superadobe technology. This wall would be covered with something not permeable as is usually done. We have seen in your website that you advise using earthbags for retaining walls. What we are trying to calculate now is the exact measure of the curvature to make the retaining wall resistant enough. In our original plans we already considered a curvature in the plan of the wall (overhead view), but we don't know how much curvature it may have. Another question is the bag size. Our wall is 16 m long and 2 m high and our local soil is basically clay.
A: From what I can see, your plan looks good. The curvature of your north wall is probably adequate, but if it were me, I would make the arc a little more (use a shorter radius), just to be sure. As for bag size, the standard size that we recommend for earthbag construction should be fine: 18" x 30" (46 cm X 76 cm) measured when empty.
Q: I need to build a 20 'L X 3-4' H retaining wall. I have access to volcanic rock and would be planning on using that as the filler. Also I plan on using a PVC pipe as drains to allow water to drain. Do you have any other recommendations of what to fill the bags with. (I'm located in southern Utah) Also once constructed I planned to cover it with a stucco. Would you suggest stuccoing or covering both sides of the wall or just the side that does not have soil against it?
A: For a retaining wall, there is no particular advantage of using volcanic stone in the bags, although it should still work; the on-site soil should also work, and perhaps better, since it is likely heavier. Plastering the exposed bags with stucco is good, and if this is carried down the back enough to assure that the bags will never be exposed to the sun, then this should suffice.
Q: I want to make a swimming pool in Portugal, utilizing the natural rock formation. I will have to build retaining wall for half of the pool. Is this feasible with a curved earth bag wall, wire and buttresses?
A: I think that you should be able to build a retaining wall with earthbags for your project. A curved wall is naturally much stronger than a straight one, especially if the curve is convex toward the direction that most of the pressure will be coming from. If it is reinforced with wire and buttresses, so much the better.
Q: I want to build into the side of sloping terrain and erect two curved retaining walls, each approximately 10 feet high, 30 feet across and 60 linear feet. Any suggestions?
A: For a retaining wall that big, there are various issues to address, obviously. Moisture won't negatively affect the bag material, so you don't need a barrier for this reason, but moisture can affect the stability of the wall in other ways. If the fill material is not stabilized, then it can become saturated and potentially deform, causing some failure. And, as with any retaining wall, some means of dealing with the enormous hydraulic pressure that can build up in the soil being retained. Usually this is dealt with by providing "weep holes" or vents to allow the any accumulating water some way to spill through the wall. Or possibly a French drain can be created to carry water away from the retaining wall. All of this depends on the specific types of soil and geology of the site.
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 am wanting to build a retaining wall using earth bags and was wondering if there is a recommended set back or offset needed. The wall I want to build is about 6 feet tall and 65 feet long. I have read about inward curves to increase strength but cannot find anything about the offset. Could you be of some to help me or point me in the right direction?
A: If you mean by offset the distance between the middle of an arc and the straight line between the two ends, this would vary depending on the length of the wall. With a 65 foot wall this might need to be perhaps 6 ft. to be effective. I think an even more effective way to make a retaining wall is to incline the wall significantly into the berm and backfill it with each course of bags,so that it has no tendency to topple. With this approach the wall could be straight.
So on the incline, I am using poly bags that are 18" wide X 30" long. Should the incline be 3" or 4" out of the 18"? If I figured the incline right and I go more than 3-4" I will gain very little usable space.
I recently built a recessed pantry about 6 feet underground and offset each bag by about 1.5 inches, providing an incline of about 16 degrees, and this worked out well. I didn't even use barbed wire between the courses and the soil is pure sand.
Q: My wife and I are building a passive house in Massachusetts. It's taking forever because aside from the shell, the whole thing is freecycle, craigslist, and dumpster-built. Probably 80% or more, anyway. But the excavator over dug the foundation by a lot and left us with a big 6 foot retaining wall need. We have tons of stone in the area. But no money to do it. Quotes for a wall using onsite and local stone range from $30,000-$46,000 for 110 foot long 6-foot 2 tiered wall. Do you know of ANY alternatives to stone that are truly green in nature (not greenwashing but actually sustainable with low carbon footprint)? I am looking into concrete rubble but my hunch is the wall guys will all say, "more expensive than stone to build. Gotta jackhammer it, etc. Even if it's free.
A: This sounds like a perfect situation to use earthbags to build the retaining wall. They are very effective, especially if the wall is curved and/or inclined against the embankment, and it is very inexpensive. The bags do need to be plastered with a durable plaster to protect them, but this is not hard to do.
Q: I understand that plastering with a cement stucco mix preserves the bags from sunlight, but is it necessary to plaster or seal the backfill side of the wall? How long will the bags last with earth up against them? Should I place pvc pipe as weeps in between bags?
A: Earthbags make great retaining walls. The back sides do not need plaster, but you might go up the front, over the top and down the back a little ways to protect that edge. Weep pipes are a good idea. I suggest leaning the bags into the berm for greater stability. About the only thing that gets to the polypropylene is UV in sunlight. Moisture and dirt won't hurt it.
Q: Do I need to use gravel filled bags on all rows due to water?
A: I don't think that gravel fill is needed with retaining walls, but the addition of periodic pipes placed through the walls as "weep holes" is good. As an extra precaution you can back-fill the wall with gravel to help with draining water all the way to the rubble trench'/French drain.
Q: I am building a retaining wall, about 3 feet high at the tallest. I’m making a double bag row with 14x26 bags. Is it necessary to use better, heavy-duty bags? The ones we have available locally are light-duty, and I have been doubling them up. Is this necessary? They will be covered so not exposed to light.
A: I would think that a 3' retaining wall could be easily built with bags that size, and double bagging them is probably a good idea. If you lean the wall a bit into the bank it will hold better. Keep them our of the sun as much as possible while building and cover the exposed parts of the bags with a durable plaster.
Q: We are about to build a retaining wall to the rear of the house. Our question is about the covering - to protect from sunlight. Would coating the poly bags in two layers of UV resistant landscape fabric suffice? (Run a layer of landscape fabric between each layer and fold over - tucking the bags in) then top plant with evergreen climbers to hang down and eventually completely cover the area?
A: Earthbags make great retaining walls, and you are right that protecting the material is critical. If it were me, I wouldn't rely on UV resistant landscaping material to do this job over time. I would prefer a more durable plaster with a synthetic mesh embedded, such as lime or cement stabilized earthen plaster, or cement-based stucco.
Q: I'm ready to do an earthbag-style retaining wall project at the end slope of my front yard. I'm looking at an earthbag-style product called Flex MSE, which is basically UV-safe PPE material that can be sliced open to insert seeds or native plants. Flex MSE apparently strengthens as a retaining wall as vegetation becomes established. But the bags are specially formulated and have aforementioned UV-blocking properties, which is opposite of the much more affordable PPE earthbag that has to be shielded from the sun. The result of putting plant matter in a small slit in the Flex MSE bag is that the vegetation spreads and covers the bag, which makes UV protection unnecessary so long as it is fully developed in cover. I assume that plant coverage could protect a plain PPE bag from the sun, just as long as it is a vine or perennial that doesn't recede during winter months.
Any thoughts? Plain earthbags would save us hundreds of dollars here on our retaining wall, as the Flex MSE product is just about $4 a bag; we need 128 for the project, plus a proprietary interlocking plate product that would act as the barbed wire does in a traditional earthbag setup to hold the bags tight to one another, working in tandem with gravity. The plates are about the same price per unit and are used in a 1:1 ratio with the build style we're using for our retaining wall, although I feel that using barbed wire instead would be just as effective and so much cheaper.
A: Todd, I don't believe that there is a completely UV-proof Polypropylene material. The coatings that are used do have a lifetime, and they are usually measured in a matter of weeks, or months at the most. I would not trust the coating to protect the bags for very long. On the other hand relying on plantings to ultimately protect the bag material from sunlight is equally uncertain; as you know, nature never provides complete and uniform cover.
As for slitting the bag material in order to directly seed the soil sounds counter productive to me. Such cutting of the bags can only weaken the integrity of the matrix that creates the monolithic nature of the earthbag retaining wall. And, as you point out, replacing the barbed wire with specially fabricated connectors just adds extra cost to the project.
If I wanted a green retaining wall, I would use standard earthbag methods and protect them with stabilized plaster, then add a secondary commercial green living wall system, similar to what is shown in this article. By doing this you would create a more durable and likely less costly retaining wall.
On the subject of planters/raised garden beds - would you recommend plastering the inner vertical surfaces that will be in constant contact with moist soil?
A: With planter beds, it doesn't really matter if the bags or their fill material gets damp. But in terms of protecting the bags from wear and tear and abrasion over time, a good solid plaster is a good idea...and the bags need to be protected from the UV in the sun. I would recommend a cement-based stucco below grade.
Q: Just wondering if the rice bags are biodegradable enough to use as a base for garden beds? I use cardboard often, but have quite a few bags and thought that might be a good use for them.
A: The only thing that I know that degrades polypropylene is the UV in the sunlight, but underground they do not generally degrade.
Q: I'm looking into building a simple fence/wall using your technique, but can only find small garden beds or home plans. Is there a plan for simple fencing?
A: Making an earthbag fence or wall is a fairly simple project. You can usually use the soil that is on-site, and use the same technique for stacking the bags that you would with a larger home or a small garden bed. It is better if the path of the wall is curved to give it more stability from toppling over. If the wall is straight for over about 8 feet, you might need to buttress it for support, depending on the height of the wall. Standard 50 pound bags can probably be stacked up to 2 or 3 feet without the need for buttresses, but above that it would be a good idea.
Q: I am in the process of building a replacement fence around my yard with sandbags and was wondering how high I could build a wall/fence without the need of rebar reinforcement?
A: (Owen) It all depends on how it's designed. For example, curves, jogs, buttresses and built-in benches will make it stronger. Wider bags will be more stable than narrower bags. Avoid long straight sections. Tamp each course level and make sure walls are plumb as you're building. You'll easily be able to tell when the walls start to feel unsteady. But just for starters, I'd guess around 4' high for 18" wide bags (measured when empty) for a typical wall. You can go higher as you add curves, buttresses, etc.
Q: I live in Arkansas, in a flood prone area, with a wetter climate than most desert earthbag homes. I want to build a fence in my back yard of earthbags. I am considering Type S lime, but am uncertain about how to mix it, or if I should just fill the bags with Type S lime only, instead of mixing it with soil. Also, I have a row of pine trees going along the south side of my property line where I want to build the fence. I am considering building the fence in a squiggly line, since I don't have room for buttresses, and since there are many roots in between the rows of trees, would it be feasible for me to build the fence on top of the ground instead of trying to dig a trench down into all those roots?
A: Squiggly fence lines are great for stability. I don't think that a fence requires a foundation as substantial as a house would, because even if the soil heaves a bit with frost, there is not likely to be much damage. I would advise using gravel in the bags for the first course or two so that water doesn't wick upward into the wall.
Yes, but if I'm using hydrated lime, wouldn't water reaching the bags only help to strenghten the wall? Also, is using gravel for the first few courses of bags relying on the integrity of the bag only, instead of relying on the bag, and the strength of the mix ? Is it wise to use fence staples secure the barbed wire onto the top of the bags?
If you're using commercial hydrated lime to fill the bags, it will certainly set up and be quite rigid in the bags once it gets damp. This could cost quite a bit of money though. You can also mix the lime with sand or soil to reduce the cost. Yes, you do rely on the integrity of the bags with gravel, but we recommend double bagging it (one inside the other) and covering with a good plaster so it remains intact. Fence staples would not likely hold the barbed wire in place; I think they would be constantly popping out. It is much easier to just place bricks over the barbed wire until the bags are placed on the wire, then nothing will move.
Q: I'm planning on building an earthbag garden wall at my house in southern Oregon, maybe fifty feet long, enclosing a small hillside area on three sides. The bags I want to use are 12 inches wide when filled. Is that wide enough if I stack them carefully and bind them well with the barbed wire for a 5 foot wall? (I'd like to make it relatively narrow.) And I suppose I'll punch in some rebar through the top courses to stabilize it.
A: As for bag size, I would think that a 12" thick, 5' high wall would be plenty stable. You might want to either make it curved some or provide some buttresses to make to more stable. Also, in areas where it might be bermed on one side, you can lean it into the berm some. Normally rebar stakes are not necessary, but they will do no harm.
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: I would like to build a perimeter wall around my land. The land in total is about 1.5 acres. I'm going to do a blend of high tensile electric wire/wooden posts and earthbags. The earthbag wall would be about 2-3 feet high (thoughts on strength of such a short wall?) and every 30-50 feet would be a wooden post about 5 ft high. The wall would be keeping in dogs and the electric wire is for horses. Would it be OK to put the wall between the fence posts or should I just make a continuous wall either directly in front or behind the fence posts?
A: A wall that short should be plenty strong; you might even get by with smaller bags than standard (more like standard sand bags for flood control). Having the wall between the posts sounds best to me.
Q: Do you have any thoughts on how you could integrate a roof rainwater catchment system with an earthbag dome structure? Have people done that before?
A: I did have some discussion with another person about the possibility of rainwater catchment on a dome, and had some specific suggestions. I was imagining a way of creating gutters that circled the dome at some level above the doors, possibly using large diameter black plastic pipe that was cut in half lengthwise and then attached to the dome and plastered into place with the exterior plaster, which would probably have to be a cement-based stucco to shed the water well enough. Some experimentation is certainly in order.
C: (Jeff Bousquet) I have often thought that Earthbag domes could make a great cistern. Make a smaller say 8 foot dome with no openings. Have buttresses radiate out from the structure on 3 sides, or better yet berm it. Leave a manhole at the top of the dome. Climb in and ferrocement the inside. Use a rain barrel as a slow sand filter at the top of the dome. This could be make a really low cost water storage system minimizing cement and creating a large water store for irrigation and household needs.
See Your Ad