This interesting structure is an earthbag dome. Earthbags, sometimes called sandbags, were originally used by the military as protective shelters, or for flood control. The concept is also applied for creating housing. The strong walls are able to resist harsh weather, they’re fully fireproof, and they can withstand natural calamities like earthquakes, cyclones and even tornadoes.
The cost is minimal as the bags are usually filled from local and natural materials. This lowers the cost involved in manufacturing and logistics. With the wide variety of materials that can be used as filler, earthbags can provide both thermal mass and insulation.
It’s well suited for many purposes. You can use it as a cool getaway space in summer. A warm escape for the winter. Earthbag buildings are also great as storage sheds, disaster shelters, and are commonly used as rootcellars. In most cases building one won’t require a permit as it’s below the minimum size required by building codes, unless it’s inhabited or attached to a residence.
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Project Details:
- Exterior walls: 18″
- Interior diameter: 8’
- Exterior diameter: 11’
- Interior floor space: 50 sq. ft.
- Materials cost: $300, $6/square foot
Tools:
- Tampers
- Round nose shovels
- Grub Hoe and trowel
- 13” x 16” sheetmetal slider
- Knife, hammer
- 2’ level
- 2” x 4” x 10’ leveling board
- Tape measure
- Fencing pliers
- Handsaw
- Stepladder
Steps:
Let’s briefly take a look at the plans so you’ll better understand the building process. The first drawing is the floorplan that shows the dimensions for the dome base and the angled buttresses in front. The buttresses provide reinforcing on both sides of the door and also act as retaining walls to hold back the earth. Make a rubble trench under the buttresses the same as the dome, and interlock (overlap) each course where the dome and buttresses intersect.
The second drawing is the dome section that shows key vertical dimensions, earthbag foundation, door and arch. Note the wood stake in front of the door. The string line on this stake guides the curvature of the dome (112” radius in this case). This dimension remains constant. Put the stake off to one side of the doorway so you’re not tripping on it during construction.
Use the same earthbag foundation building technique described in previous Instructables. This includes poly bags, double-bagged for strength, filled with gravel to prevent wicking moisture into upper courses. The bags sit directly on the rubble trench. Build the buttresses at the same time, in the same way. After the two gravel bag courses are finished, add soil around the sides of the dome and tamp solid to prevent the bags from moving.
Use the string line on the center stake to measure the radius. Move the string to make sure each bag is the same distance from the center point. This creates a perfect circle. Repeat this step for every course except the very highest ones. By that point, the circles become so small that you can just eyeball them. Note: the string line on the center stake is lengthened to measure each course. Just make sure it remains a constant length for each entire course. (You could use temporary knots or pinch the string between your fingers.)
Check each course for level after tamping. For small domes like this one, a straight 2×4 works great. Put the level on the 2×4 and gradually move it around over the center stake to check each bag. We fill each bag with the same number of buckets so the walls are almost self-leveling. Also, we’re careful to use the same technique for each bag.
You’re almost ready to start laying soil-filled earthbags. The soil in the earthbags needs to be slightly moist – just enough to pack densely into the bags. Use a garden hose to mist the soil. Test the moisture content by making a ball in your hand. If it looks wet or if water oozes out of the bags, then there’s too much water. You’ll soon recognize the proper moisture content. Also, note how the soil in the photo below has been distributed around the dome to minimize labor. Each bucket load is only moved a few steps. The reddish soil is called road base. It’s the same material used to build roads in many parts of the world, and is ideal for earthbag.
Now that you’re above the level where moisture can cause problems, you can start filling bags with soil. We use 2-gallon buckets to fill bags. The easiest way to fill buckets is with a sturdy hoe that’s made for digging. We use a grape hoe to pull the soil into the buckets. And with just a flick of the tool, the bucket is pulled upright.
Tamp each course solid, starting with the high spots. Keep the tamper moving so you don’t create low spots. The first pass doesn’t require much force. After you’ve gone around once, you can start tamping more vigorously. Final tamping is usually done with more force. You’ll hear a change in pitch as the earthbags become solid.
There are various ways of attaching door and window bucks. On this dome we made six anchors with small pieces of steel welded together in a T-shape. Drill holes in the anchors and pin to the earthbags as shown with ¼” steel rod. Distribute the anchors, three per side, so there are two near the bottom, two in the center and two near the top of the arched door opening. A steel door buck was welded to these anchors after the dome was built.
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Remember to always use 4-point barbed wire between courses. We used one strand on this dome due to its small size. Use two strands of barbed wire between every course for domes larger than this one.
On straight earthbag walls, you can just butt the bags against each other. But for roundhouses and domes, you need to align the bag ends to the center point so the bags sit tightly against each other. Hit the bottom of each bag with a board until it aligns with the center of the dome. The second photo shows how the bag joints appear looking from above. (They look like mitered joints.) Repeat this process throughout the dome. At the top, the process becomes more pronounced as the radius decreases.
You need a way to support the bags as you build the arch over the door. You could build an elaborate wood and plywood form. This might be worthwhile if you’re building lots of arches the same size. But this dome has just one arch, so we lashed two used tires together for the form. Build some supports on each side of the door opening to hold the tires at the correct height. Add some horizontal blocking to hold everything together, but leave enough space in the middle to squeeze through and for the string line that will determine your dome curvature. Note: you could temporarily fill the space with adobes, bricks, etc. and remove them later, but then you lose access to the dome and can’t use the string line.
It looks really complicated to build an arch, but it’s not. Make tapered earthbags about 12” high. Make sure you use the right soil mix that will withstand high pressure. Compact the bags from the sides and top as much as possible. The taper aligns with the center point of your form (the center of the tires in this case). You could use a string line for this or just eyeball it like I did.
It’s a good idea to include a protected overhang above entry doors to protect the door and shelter people going in and out. This eyebrow or overhang is made with hardwood table legs embedded between bags. Then short pieces of insect resistant eucalyptus wood were nailed to the table legs and covered with 6 mil plastic sheeting. The wood was varnished with polyurethane for greater durability. (The table legs were varnished in advance.)
Continue building the buttress at the same time as the dome. The bags and barbed wire interlock at every course. We added a few ½” pieces of rebar for added strength where it seemed it would do the most good: down through the top of the bags to help preventing overturning, and where the buttresses connect to the dome. The buttresses are stepped to match the slope of the earth. The steps make it easy to climb on the dome. The lower part makes a nice bench.
Continue the same process of laying and tamping bags. The corbel (overhang between courses) will increase the higher you go. Use the string lines at every course to arrive at the desired shape. Make sure each bag is angled toward the center point and tightly butted to the next bag. (This is what makes the dome really strong.)
The photo shows the last bag being put in place with a great feeling of accomplishment. The last bag looks like a mushroom cap.
The easiest method of finishing the dome is plaster. We live in a rainy climate where a plastered dome might leak. Plus, we wanted a beautiful plant covered dome in our backyard. Living roofs require a lot of regular maintenance just like gardening, so think long and hard before going this route. Without regular watering, weeding and fertilizer the living roof will turn into a giant weed patch, die or maybe even erode away. We’ve worked hard to establish really healthy grass, as you can see in the photo.
The main steps for the living roof include adding two or three layers of 6 mil plastic sheeting (“black poly”). I put one layer directly against the dome and back side of the buttresses. Then I backfilled with earth (more road base) about half way up and added another layer of plastic. I added a third layer of plastic on the top of the dome where the risk of water penetration is greatest. Work carefully and try not to puncture the plastic. Gradually add soil on top of the plastic starting at the bottom and working up. Compact it slightly as you go. I used ‘field dirt’ – soil from rice fields for this step. It has enough clay to stay in place, but still enable grass to grow. Add drought resistant sod once the dome is covered in a layer of soil. Some pieces of sod require bamboo pins to keep from slipping. Time the project so the sod gets started during the rainy season. We have a lawn sprinkler on top of the dome and water it whenever it gets dry.
The outside (front area only) is plastered with cement plaster with yellow iron oxide pigment. All edges are rounded slightly to soften the look. Plaster mesh isn’t typically needed on earthbags, but we used plastic plaster mesh in this application due to the buttresses being exposed to the elements. Plastic mesh won’t rust. We never plastered the inside since our dome is used as a tool shed. This allows people to come and see how the dome was built.
Domes have lots of other advantages. They create the most floor space for a given length of wall. There are no wasted corners. The feeling inside is magical. Those who live in domes (and roundhouses) most likely never live in boxes again. Wind flows around domes and does not build up pressure against them.
You can build domes without wood. You can build domes with minimal tools and materials – no nails, no wood, no plywood, no shingles. This makes domes a good candidate for those who lack carpentry skills and for emergency shelters for disaster areas and war refugees. Give people some rice or grain bags and a little training, and soon they can build their own sturdy, safe shelters.
Thanks to Owen Geiger for this great tutorial!
Original article and pictures take diyprojects.ideas2live4.com site
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