Earth that holds its shape has a moisture content of 10-23% in its natural state. The more clay, the more moisture it can contain. Thoroughly dried walls have a moisture content of 3-5%. This means that earth with a naturally high moisture content will shrink considerably during drying. To assess the moisture content of the earth, a sample is weighed, dried, then re-weighed. The moisture content is the equivalent of the difference between the two weights.
Generally speaking, earth with a high moisture/clay content is best used for an air-dried earth block. Most of the shrinkage will have taken place before the blocks are laid. Through adding plenty of natural fibres, an earth rich in clay can be used for ramming as in the pisé technique.
8.3.4 The preparation of earth
Once the earth has been selected using the above methods, the topsoil, which contains organic matter and humus, is removed to a depth of 20-30 cm. The earth uncovered is then sieved through a steel net with mesh size of about 2.4 cm for ramming earth, or 1 cm for the production of blocks. If the earth has a variable moisture content, it must be well mixed and stored under a tarpaulin for three to four weeks.
Where necessary, stabilizers or extra sand or clay can be added either during sieving or later with an earth grinder. Mixtures stabilized with cement and lime must be used immediately; others can be stored, but they must be covered with a tarpaulin to preserve the moisture.
Any earth to be used for construction is transported straight to the building site without any industrial treatment. Here it is put into casts to make blocks, or rammed between shuttering to make walls. Mortars are also easily prepared on site. More industrialized products such as tiles and panels can be made in small local industries.
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