What is rammed earth?

Rammed earth is a form of unbaked earthen wall construction in which walls are formed by compacting moist sub-soil inside temporary formwork. The moist soil is placed in layers of around 100-150 mm depth and compacted using pneumatic or manual rammers (traditional method). Once the soil has been adequately compacted the formwork is removed, often immediately, leaving the finished wall. Walls are typically 300-450 mm thick, but this can vary widely according to design requirements

Both loadbearing and non-loadbearing walls in a variety of structures have been constructed using this technique over many centuries. Rammed earth walls are found throughout the world, with many examples throughout Asia, including sections of the Great Wall of China, Africa (north Africa in particular), Europe (including France, Germany, Spain and the UK) and in both south and north America.

Rammed earth construction was (re)introduced into the UK in the later eighteenth/early nineteenth century following a revival of interest in France, led by François Cointeraux. Throughout the nineteenth century a number of rammed earth and rammed chalk buildings were erected in Wessex. Examples include a five storey loadbearing rammed chalk houses in Winchester built around 1840. Following WWI a series of experimental rammed earth and chalk houses were constructed in Amesbury, Wiltshire.

Rammed earth walls typically exhibit a distinctive layered form as a result of the construction process, corresponding to the successive layers of soil compacted within the forms. This attractive appearance is undoubtedly one of the appeals of rammed earth construction and as a result walls are frequently left without plaster or render. Transparent treatments, such as sodium silicate spray or carnauba wax solutions, are more commonly used.

Over the last 30 years there has been considerable growth in the number of loadbearing compacted earth buildings worldwide, including in Australia, Germany and the USA. In addition to the many housing projects applications have included churches, hotels, factories, schools, and exhibition centres.

Not all soils are suitable for rammed earth construction. Only sub soils, rather than top soils, should be used. Soil should be reasonably well graded between gravel to clay sized particles. The main soil constituent for rammed earth is sand (40-70%). Between 10-20% clay is generally sufficient for binding; higher clay contents may result in excessive shrinkage. In general maximum particle size should not exceed 10-20 mm, though stones up to 50-100 mm can be successfully used with care (depending on total proportion). Soils suited to cement stabilisation are generally sandier; cement replaces clay as the primary binder, and excessive clay content can inhibit effectiveness of cement.

In comparison with other forms of earthen construction, such as cob and adobe, rammed earth construction offers enhanced density (typically 1700-2100 kg/m3), compressive strength (1-3 N/mm2 for unstabilised rammed earth) and durability. Compaction at lower soil moisture content (8-14% depending on soil) significantly reduces shrinkage on drying. Enhanced material density, however, also increases thermal conductivity (around 1.0 W/mK) and the quantity of soil required for construction.

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