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Soils for different types of earthen walls

All soil is made of different proportions of microscopic clay particles, round silt particles (like flour), and larger grains of sand. The top of the triangle is soil of only clay, the bottom right would be a soil of only silt.

Good blends of all three kinds of particles makes stronger soil. The soils in the center of the triangle have some of everything.

Soil that contains a lot of a very strong type of clay is likely to shrink when dried. Shrinking 5% or more can cause problems. If your soil is a clay that feels smooth between your fingers, test for shrinkage.  

CEB (compressed earth blocks) are made of a special kind of sandy clay loam soil so that with just a hand press they will compact into very strong blocks. More soils can be used for rammed earth, and sandier soils for adobe blocks.

Earthbag can be made out of more types of natural soil than any other earthen technique. But for good strength when tamped lightly in bags, resilient Contained Earth (CE) can't have too much silt. To be easy to work, it shouldn't have too much clay.

The clay loam, loam or sandy loam soils that work well for resilient CE can contain very weak types of clay or very strong types of clay. For seismic hazard areas, any earthen building materials should be strength tested.

Soil triangle graphic modified from R Wheeler and image below by Rajan Journalist from Nepal; both from Wikimedia Commons and used with permission.

You don't discover strength of dried soil from:
  • The soil color
  • How sticky it is
  • The name of the soil type
  • How much clay it contains

Know soil strength before you build.