The two most common forms of earth building are rammed earth and adobe block, however other methods of using the earth beneath our feet include pressed brick, poured earth, cast in situ, and cob.
All soils contain a mixture of particles from the finest being clay to the biggest being the underlying bedrock that make up our mountains.
The top soils which plants grow in contain a large portion of organic matter, be that the plant roots, decaying compost and the worms and other critters to make this an active eco system.
The subsoils below this organic top layer provide us with the material to build our homes with. This subsoil contains varying proportions of clay, silt, sands and gravels that we will utilise. The proportions of these materials influence how the soil is used, or what proportion of modifiers (usually grit in high clay soils) are needed to make it fit for purpose.
Clay is the active binder that glues all the other parts of soil together. Too much and the wall surface will spall off in sheets due to the clays swelling and shrinking when wetted and dried. Too little and the soils will not bind together to form strong enough walls.
Remember earthbuilding can be very low tech and by using locally found soils the carbon footprint of the house is reduced (carbon footprint is a better measure of green building).
Rammed earth has been used for millennia in dry climates where the soils are usually very gritty and low in clay content (from 20% down to 6%).
Heavy formwork, usually made a double layer of 20mm ply, is set up either side of the wall and stoutly locked together with bolts at close spacings.
The almost dry soil is shovelled in, about 150 deep at a time, and robustly rammed (tamped) down. Ramming may be done by using the traditional heavy foot on a pole or with modern pneumatic pogo stick like rammers – this layering continues to the desired height and the shuttering (formwork) is taken off and moved along the wall.
Reinforcing rods, to counter earthquake forces, are included from the foundations up, and are used to fasten the roof down.
Most rammed earth is improved by the addition of small quantities (around 10%) of cement. This not only increases the strength, for engineering design, but also improves weather resistance whilst permitting the use of non ideal soils.
Adobe Block, also known as adobes, adobe bricks, or mud bricks, use a puddling (or wet) mixing process to make a stiff mud that is shovelled into a simple mould, pressed into the corners and allowed to set for a few minutes before the mould is removed and the process repeated to make more adobes.
Adobe walls and floors create a delightfully softer texture for living in. Although the walls may be plastered to a smooth surface, depending on the wants of the client, they are often rendered and whitewashed to leave the outline of the bricks.
As with all earthbuilding methods, it is the clay that binds all the particles of the soil and any other fibre (e.g. strawbale) together. Soils of much greater clay content can be used as the shrinkage of drying occurs before the adobes are laid up to make the walls.
Excessive shrinkage can be controlled by the addition of modifiers or stabilisers. Modifiers include grit and sands, or traditionally short straw, to restore the clay ratio closer to the ideal of 10% – 15%.
Stabilisers act as “glue” to bind the solid particles together with cement being the most common stabiliser in New Zealand. Other stabilisers include (and this is not an exclusive list) bituminum emulsion, agave or other plant juices,
animal dung and other substances found locally that have proven over the centuries to work.
Natural fibers, such as short straw, may be used to bind the adobe mix together to add tensile strength to the mix. One criteria to consider is the durability and local availability of such fibres.
Adobes are mortared together, preferably with the same soils as the adobes, in a bond pattern for strength and stability with cross walls at regular intervals to provide the resistance (stiffness) required in New Zealand’s earthquake prone land. Vertical reinforcing rods, from foundation to top, increase this earthquake resistance – something the structural engineer will advise on.
Poured earth uses an adobe (or wet mix) with light weight shutters on either side of the wall to form the mud mix in, once set these shutters can be taken off and moved up to form another layer.
Smartform, which had been developed by Adobe South and widely used by our owner-builder clients and contractors. Plywood side boards are used with diamond shaped dividers, at regular intervals, tie the side boards together. This creates a large sandstone block looking wall.
The puddled soil is usually stabilised with 10% cement depending on its ratio of particle sizes (from clay, which is the finest, to stones of up to golf ball size) and well raddled into the framework thereby creating smooth wall surfaces.
Cast in Situ
Cast in situ uses the adobe mix with the mould (often of sheet steel) set on the wall so that the block is cast in place, and is finished to the final shape and texture while still malleable. A frequent comment is “this is the ginger bread home” and a distinct advantage of cast in situ is moving the “soil” the least!
Cast in situ walls are more at risk of cracking as the wall cures and dries out, so greater care when selecting and testing the soil mix is needed. Vertical steel reinforcing in the walls increases earthquake resistance.
Pressed Brick uses the almost dry soil mix of rammed earth, with the soil being placed in a mould which has top or bottom platens moving in to compress the soil into a tight, dense and solid brick.
There are several types of hand operated presses with the Cinva Ram being one well known press.