As you might guess from the name, choosing the earth and the bags are important steps in earthbag construction. Essentially, any type of soil can be used, but knowing the makeup of soil can help you to achieve the best mix.
Soil is made up of clay, silt, sand and gravel. Silt is extremely fine-grained, and using too much in an earthbag structure will weaken it. Gravel, or jagged pieces of rock, is sometimes used in earthbags, mostly at a foundational level, but builders primarily use a mix of clay and sand. Clay serves as the glue to hold sand together, while the loose, gritty particles of sand form the bulk of an earth wall's stability. Coarse, jagged sands are best because there are lots of sides for the other grains to adhere to. Earthbag builders Kaki Hunter and Donald Kiffmeyer recommend a ratio of 70 percent sand to 30 percent clay.
No matter the mixture, topsoil should not be used. Grasses, twigs and the miscellaneous debris found in topsoil will eventually decompose, leaving cavities in the bag and undermining the structural integrity of the building. Once these are cleared from the soil, most clean soil can be used. It's also possible to import soil from a local gravel yard, where reject materials from gravel usually have an appropriate sand-to-clay ratio.
Playing around with moisture content will also affect the composition of the soil, with moistened material creating a more stable structure because it presses everything together. A measure of about 10 percent moisture will work well [source: Hunter and Kiffmeyer]. How can you tell the right ratio? Soil with about 10 percent moisture will form a ball in your hand, but it will shatter if dropped.
The bags are typically 50-pound (23-kg) or 100-pound (45-kg) sacks of polypropylene or burlap. Burlap isn't as durable as polypropylene, but it is a more natural material. Generally, the weaker the fill material, the stronger the bag should be. It's also possible to use long sandbag tubing, such as the superadobe tubes developed by Nader Khalili. The lack of seams in the wall could possibly lead to greater wall stability, but some builders find these harder to work with, as they are cumbersome and sometimes roll after they've been placed on the wall.
Several other materials are necessary to build an earthbag home:
- Barbed wire is used between levels to hold earthbags together.
- Plasters are applied to the inside and outside of the home. Commonly used plasters include mud, a combination of clay and sand, and lime.
- Wooden forms are used to create windows and doors.
- A tamper is used to compress the soil; they are usually made of a wooden pole with a heavy metal plate attached. You can buy a tamper at garden stores, or you can make your own with pieces of concrete.
- Wheelbarrows, shovels and various other tools will help you move the dirt. Some builders also use tools such as bag stands or funnels to fill the bags. We'll take a look at these in the next section.
Enough talk about earthbag homes. Let's build one.