Can I Use my Soil to Build with Earthbags?
Earthbag buildings are very simple to build. If you have never built one before, you may wonder about your soil.
Many soils become very strong in earthbags. Soil usually needs to have a little clay in it. Soil used for roads (called road base) usually works fine. It will harden into a very hard block after it is tamped and left to dry out of the sun. Most soils do not need any cement or lime added.
1. Getting Soil
If you are digging soil up to use, don't use the top layer.
Pick out any debris like sticks or leaves and stones.
If your soil sticks together it's too wet to test.
Dry a little out in the sun or over a flame.
You might need to cut a heavy clay soil into pieces to dry it, like the soil on the left.
2. A Test Bag
The best way to be sure about soil is to fill a bag. Tamp it by pounding with something heavy. (When you build you will probably add a cement end the size of a small coffee can to a sturdy pole).
Use soil that is just damp enough to ooze out a little when you tamp it. Tamp the bag, and drop it from 18” (50 cm) above the ground. It should not be crumbled all loose if it is a good soil. Tamp it again, and leave it to dry in the shade. (This could take 2 weeks in hot weather). A good soil that is cured won't break easily. You can hammer a nail in without chunks breaking off.
Pure sand stacked in earthbags will not make a strong wall. The bags will stay flexible and will want to slump and move. (If it is used in a wall it must be braced with temporary supports. It needs mesh and a rich cement mortar on both sides.)
A bag test will sometimes show that a very sandy soil will become hard. Some sand formed from coral or granite goes through a chemical hardening process after it is tamped. It makes very good fill for earthbags. Only a bag test will show this.
The tests below are quicker ways to get an idea how well the soil will work. If you test several different soils your first time, you will feel sure about what the test shows you. These tests can also tell you a little about how well the soil might work to absorb wastewater or hold up walls.
Bad soils or loose sand can be used in earthbags with lime and cement (or cement alone) added. But this can increase the cost a lot.
It can also make buildings less breathable. Buildings of cement are hotter and the walls get sweaty in warm humid climates. Different soils need different amounts of lime or cement. It is best to test bags with different amounts of cement and or lime.
Take a small handful of dry soil. Add water one drop at a time until it will hold a shape when you squeeze it in your palm.
This soil is very sandy. Dry it becomes a powder, but more water doesn't make it hold together.
It feels gritty when rubbed between your fingers. It will form a shape, but falls apart if bumped. This soil will probably need a lot of clay added.
Is there different soil available?
This soil feels a little gritty, but holds together better. It is a mixture of sand with some clay or silt.
Did the soil stain your hand?
Brush your hand off. If your hand is almost clean , the soil has more silt than clay. You can try the Ball Test below, but a different soil would be better.
Silt does not make strong earthbags. The soil particles are too round. You probably need to add clay to use this soil.
Even a little clay will stain your hand. It has to be washed off. It won't brush off when dry.
Do the Ball Test below to see how much clay the soil has.
This soil has a lot of clay that stuck together well when squeezed. Cut through it with a table knife. The clay in it will make the cut surface look smooth and shiny.
Do the Ribbon test (#5 below) to learn more about your clay. If this is a tropical soil, test it for swelling.
This will tell you if enough clay is in the soil.
Make balls 4 cm (or about 1 ½ inches) in diameter out of soil. Use soil that is just moist enough to hold together. Drop the balls one at a time from a 1.5 m height (about 5'). Most of the balls should act the same way.
The balls are too wet if they leave a big wet mark where they hit. Add dry soil until they stop leaving the wet mark.
Balls that are very shiny like this one also may be too wet.
This ball flattened with few or no cracks. It was a much drier mix than the wet ball shown above, but it only cracked a little.
This soil has a lot of clay. It is probably 15% or more clay. If it does not swell or crack it might work for earthbag. Sand could also be added to it. All tropical clays should be tested for swelling. Other clays may swell if they are very plastic and very hard.
(Very clayey soils may not drain well for wastewater and sometimes damage normal buildings because the soil can swell or shrink).
This ball split into just a few pieces when dropped. It contains enough clay to be strong in earthbags.
This is probably good soil for earthbag. If it is a tropical soil, test it for swelling.
This was a ball the same size. It broke into little pieces when dropped. It had to be squeezed hard to form a ball.
This soil has little clay. If it feels gritty it is a sandy soil. If it does not feel gritty between your fingers it is mostly silt with little sand.
A silty soil may not be very strong for earthbag. It also may not drain very well for wastewater.