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Rammed Earth

rammed earth can be quite attractive if left unfinished

rammed earth can be quite attractive if left unfinished

Rammed earth is an age-old method of building walls by packing moist subsoil into formwork and then compacting it, either by machine or manually. The formwork is then struck and lifted to form the next layer. In terms of embodied energy  (0.45 MJ/Kg) and using sustainable materials the method is superb since the materials can usually be found on or near to the building site and very little energy indeed is used to shift, mix and compact them. It is also quick, as the preceding layer can be built on immediately.

It is however a rather difficult technique for the average UK self builder to access. You will almost certainly have to contract the work to a specialist. This is because sturdy specialised formwork is required for the process and this is not generally available because so little rammed earth building is done here. Chicken and egg! Also there will probably be a deal of soil testing required to get the right grade of material and this is not something most structural engineers specialise in.

Soil is very complex stuff, being made of mainly sand, humic materials, clay and gravel in varying proportions. It is important that the mix is correct. Too much clay and there will be shrinkage problems, too much humics and it will lack strength etc. Sometimes soils from different areas need to be blended. This sort of testing may be uneconomic for something as small scale as a single house.

If you do decide to use a specialist company then the question arises as to whether to go for the more traditional method (unstabilised) which uses no cement in the mixture or ‘stabilised’ which uses a small to medium amount of cement. This too can depend on the soil available and gets into quite contentious areas. It is interesting to compare the approach of Earth Structures who make a robust defence for using 6-7% cement (particularly here) with that of Ram Cast who have shunned it for reasons of the CO2 produced by cement manufacturing.

As with all earth building techniques you end up with rather thick walls which have very poor insulation values (k value about 0.8, depending mainly on the density of the earth) This is 23 times worse than mineral fibre for example so you need to add a considerable amount of insulation. This can create an extremely thick wall – maybe in the region of 750mm. This may have implications for the size of the building’s footprint and for window design.

Used as an internal wall, rammed earth will add considerable mass which is helpful with thermal stability.

Of course concrete foundations will normally be necessary and, depending on the thickness and density of the walls, they may be a significant factor in CO2 generation from cement manufacture.

rammed earth demonstration at ecobuild exhibition

The picture shows Amazon Nails displaying rammed earth formwork at the Ecobuild exhibition. Note the black steel side plates held together with threaded steel bars. The earth is rammed in from above using a punner.

There is a good write-up about rammed earth at lili.

EBUK (Earth Building UK) is the national charity promoting earth building and they are running Clayfest in 2018


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