For Stabilised Rammed Earth to achieve its optimum strength and weather resistance,
it must be made from an earth that has a assortment of particles
that fit together like a jigsaw without leaving voids. The
sizes of the particles versus their percentages within the
earth when plotted on a graph is called a "Particle Size
Curve". The ideal curve for rammed earth building has been determined from numerous
laboratory tests and confirmed by years of experience on
site. An ideal earth mixed with the correct amounts of stabiliser
and water, then well compacted can achieve 98% solidity,
which is virtually solid stone
Can you use soil from the site for earth building?
is to be used, it must not contain any humus or vegetable
matter, so top-soil should be avoided in preference to sub-soil.
It is possible to use sub-soil from the actual earth building
site or from a site nearby, if it has the correct particle
size curve. If local sub-soils do not quite match the curve,
they can sometimes be modified and enhanced by the addition
of specified amounts of clay, sand or gravel.
Crushed Rock for earth building?
approach to earth building, which has become very common in Australia, is
to use material from a local quarry. The quarry can mix
different screened materials to achieve the ideal particle
size curve. Generally these are materials that would otherwise
be regarded by the quarry as waste. Consequently it is environmentally
beneficial to use this rather than digging fresh sub-soil.
It can also have the advantages that it is often available
at a good price and can be of consistent quality and colour.
Recycling - a Better Alternative?
is yet another alternative that has been investigated and
proven by Earth Structures. This is the use of recycled
building rubble. Bricks, stone and concrete from demolished
buildings can be crushed, on site by mobile crushing plants,
or at central collection depots, to the particle size curve
required. Red bricks and light grey concrete when crushed
can combine to make a pale pink sandstone-like material
with equal strength and weather resistance to the usual
crushed stone. An interesting incidental fact was noted
that this material required less stabiliser to be added
to the mix due to the cementatious effect of pozzolanic
material in the building rubble. This is surely an ideal
material to use from an environmental aspect. See Recycled
Building in the Image Gallery.
Experience and Testing
experienced earth builders are able to recognise whether
a sub-soil or crushed material has the right characteristics.
However it is wise to have a new material tested in a soil
laboratory to determine the particle size curve. The laboratory
can also test the proposed mix when it has been stabilised
and compacted to determine the compressive strength that
will be achieved.
Colour and Texture
colour of finished earth buildings will depend upon the colour of
the earth used and also according to whether grey or white
cement is added. White cement will brighten and lighten
the wall colour and is generally preferred for pale coloured
earths, whereas grey cement will give a softer and more
discrete final colour.
SRE samples made from different coloured
The texture of the finished walls of the earth building can be determined by varying
the maximum gravel size and the compaction technique. Smooth
walls with a fine granular finish are possible with a fine
screened mix, or, by using a coarser mix, the wall finishof the earth building
can be made rougher and more rustic in appearance.