Arches are a lot like Amal Clooney: Smart, well-put together, and began life in the Levant. Some of the first arches (as far as we know) were constructed around 2000 BC in Mesopotamia, and as anyone who has wandered round an old Roman ruin will tell you, an arched doorway is often the only thing remaining after an earthquake.
Arches work because any external pressure pushing inwards or downwards on the arch stones only serves to compress them tighter. But...before you leap up in arch-building excitement, there are a couple of points to etch onto your brain.
Before starting an earthbag arch remember...
1. Earthbag arch wedges need to be a bit wedge shaped, and must be jammed in tightly, otherwise the arch principle doesn’t work.
2. As the forces of the arch push down to the ground, the arch will push outward at the base. So, you need some decent ‘abutment’ (aka a good fat earthbag wall on either side of the arch to stop it pushing outwards).
Here’s how to make an earthbag arch, step by step
1. You need a mould or strong arch frame to lay your bags around. In our small arch, we were incorporating part of an organic tree trunk, so the arch wasn’t perfectly round. On a small arch this won’t matter, but a larger one needs more precision.
2. Attach a piece of string in the centre of the bottom of the arch. You use this to check the angle of your arch wedges.
3. Make a wedge mould for your earthbag 'slices'. Stick your empty earthbag in the mould, fill it with clayey dirt, and tamp it firm until you have an earthbag the shape of a slice of brie.
4. Arrange your bag wedges around the arch. Use the string to check they're lined up correctly. Remember, if your bags are not wedge-shaped but straight-edged, they’re not going to compress together. As you lay the bags, weave the barbed wire in and out of the wedges (see top photo). Make sure the wedges are tightly jammed together.
5. Once all the earthbag wedge pieces have been jammed in, time to drop the keystone in the middle. It may end up being more of a squeeze and shove than a drop, but as long as you've jammed the keystone piece in nice and tight, it will hold.
6. Run a course or two of barbed wire over the top of the arch, and lay another layer of flat bags over the top of the arch to lock it all in.
Extra note from the Costa Rican Earthbag Team
Our Costa Rican team in The Mud Home FB group made the wide and beautiful earthbag arch in the top photo. Because of the span, I asked Murat if their arch technique had differed from mine in any way. “At the core, our method was the same, but we interwound/integrated a lot more barbed wire and aimed for super symmetry just because of structural demands,” Murat said. He also drove in a bunch of rebar just to make sure the whole thing was pinned together invincibly.
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