What Can I Put in my Earthbags?
The answer is: It depends what you want to do with them.
If you want superadobe, which is a specific technique to create solid clay/mud bricks by allowing the mixture to cure on the wall, then you need an damp earth-clay mix. About 20-30% clay is ideal. The mud needs to stick together in a ball when you roll it in your hand. It shouldn’t be breaking up like oatmeal.
What’s the big deal about superadobe? With superadobe, the earthbags cure on the wall, and once dry are rock solid WITH OR WITHOUT THE BAGS. A friend of mine forgot a stove pipe hole in his superadobe wall. In the end he had to dig out a hole with a teaspoon because the walls were so darn solid. He said it was like escape from Colditz:) You could burn the bags off, and your building would stay standing.
Experiments have shown you can add things into the damp clay-earth mix without too much impact, especially if the bags are left intact. Stones less than an inch (2-3 cm) in width can be left in. Some people have added things like pumice for insulation.
Stabilized bags for foundations.
You can mix your mud with lime to stabilize it for foundations, which obviously see a lot more water and pressure than the rest of your walls. You can also fill them with limecrete, gravel, or a mixture of both for the foundations.
Can’t you just fill the bags with sand? Or stones? Or any loose aggregate?
In theory, if your earthbags are tough and hold, you could put anything in them. The army have been building sandbag bunkers for years. They’re strong, bullet proof and safe. But here’s the thing. Rip those bags open and you have...you got it. A mess. There are a number of ways the bags can lose their integrity: Polypropylene bags deteriorate fast in the sun. Jute bags tend to rot if used in damp climates. And if you happen to have mice attempt to move in – which take it from me if you live in the wild you will – then every rustle and scamper is a harbinger of doom. Mice love to shred things like sacks, polypropylene or otherwise. The only thing they can’t comfortably destroy seems to be metal.
In truth these events are unlikely, especially once you've plastered the house (though mice can and do gnaw through earthplaster). But the bottom line is, if you want your earthbag house to remain solid even if someone slashes the bags open, superadobe is the way to go.
So there you have it. In an earthbag.