Centre For Alternative Technology
Centre For Alternative Technology

What on earth?

"Rammed earth" is the name given to structures formed by compacting small layers (about 10cm) of moist sub-soil inside a temporary framework, which is then removed and the wall left to dry.Unlike bricks and cement, rammed earth is not fired and so has a much lower embodied energy. The high density of rammed earth gives it a good 'thermal mass', which means it regulates the temperature of a building by absorbing excess heat and re-releasing it when the building cools down. Earth is not a good insulator, so would need to be contained within an insulated structure.

Typically a rammed earth wall will be 300mm thick, and there are restrictions on the size and spacing of openings to ensure structural stability. With a team of three to four you can build 5-10 square metres of wall in a day. You can use local sub-soil, which can then be returned it to the earth at the end of the building's life. The soil must be well graded between gravel and clay sized particles and be low in organic content and salt (below 2%). Poor-quality sub-soils can be improved for rammed earth by careful mixing in of the correct size fractions; some rammed earth is "stabilised" by adding a small proportion of cement.

Water-related weathering, including erosion by rain and freeze-thaw, are the primary agents of decay in rammed earth buildings. These factors must be considered carefully at the design stage. Over a period of time a rammed earth wall will dry out and become as durable as sandstone, as long as it is waterproofed top and bottom. Any protective coatings applied to the surface must be permeable to water vapour.

The level and extent of materials testing you will need to do depends on the specific application and novelty of the material in use. For in-situ rammed earth, compliance tests are mostly undertaken on cylinders specially prepared for that purpose. In load bearing applications it is usual to undertake soil classification, moisture density testing, strength and shrinkage assessment. See 'Rammed Earth: Design and Construction Guidelines' by Peter Walker et al. (at http://store.cat.org.uk) for further details of compliance and structural tests. Recent experience across a number of local authorities in the UK demonstrates that rammed earth, when used correctly, is perfectly able to satisfy the requirements of modern building regulations.