Testing Soils for Earthen Building
If you decide to build with earth there is one thing that you will have to find out about the dirt you are using. With earthen building techniques like adobe, cob and rammed earth there are critical percentages of sand and clay that needs to be met so that your building is strong and durable.
Different sources will tell you slightly different things about the needed composition of these soils but in general they will be pretty close to each other.
The above table is typically of adobe construction and can usually be applied to rammed earth and cob as well. But before you can start building you will need to test you local soils to find out if they have the correct composition.
This can be done quickly and easily with a combination of soil texturing and the jar test. Soil texturing consists of wetting the soils and putting them through a series of tests that will give you a good idea of the percentages of sand, silt and clay. For further information on this check out the associated article Using a Soil Texture Triangle.
You should also run a sample of soil through what is called a jar test. A jar test is really simple but can take anywhere from a couple hour to a couple of days to fully run. For further reading on that check out the Jar Test for Soil Composition article here on the website.
Now you have a good idea what composition your soil is it is time to make a few test blocks. These blocks should be made following whatever method of earth you are building be that adobe, cob or rammed earth. Let these blocks fully dry and then inspect them for large cracks or other weaknesses. You should also test them for strength and cohesiveness by dropping them from heights and hitting them with sledgehammers. A final test is hit them with lots of running water or drop them into large tubs of water and see how they hold up. Good blocks with lose very little material to the running water or will not fall apart quickly in the tubs.
Above all use your best judgement and start small if you have never built with earth before.
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