03 Feb 12 Techniques of Earth Construction (English)
Throughout history people have been using earth to build spaces and create a sense of place. Earth is arguable the oldest building material in the history of mankind and even to this day approximately 1.7 billion people continue living in buildings made of earth. Earth is also the most abundant material available to us on the planet and the section of earth we use for construction is not detrimental to agricultural needs (we do not use topsoil).
Examples of earth construction can be found worldwide and in a variety of notably distinct techniques. These techniques have probably developed due to different soil qualities and climatic conditions as well as different technologies that were available to each distinct culture at the time of development.
Depending on the state of hydration, soils can generally be workable in 3 different forms; monolithic, unit based, or as mixed structures. Within these 3 construction classifications there are 12 different techniques. These techniques and construction classification are often represented graphically as a ‘wheel’, an example of which can be seen in the image attached to this blog post (Image 00). TERRAVERSA´s logo is in fact based on the geometry of this wheel!
Simply defined the techniques are as follows;
- Earth sheltered space
- Cut blocks
- Compressed earth
- Direct shaping
- Stacked earth
- Moulded earth
- Extruded earth
- Poured earth
- Daubed earth
- Straw and Clay
The most common historic examples of these 12 techniques are adobe (moulded), CEB and rammed earth (compressed), cob (stacked), and wattle and daub (daubed).
Earth is dug out of the ground to create shelters much like a cave. In most cases this occurs in zones of soft soil. Dug-out spaces commonly occur on hillsides (Guadix, Spain) although there are some spectacular examples of communities being dug-out on cliffs (Cappadocia, Turkey) or in flat plains (Henan Province, China), each topographic situation lending itself to a different composition of interior spaces.
- Earth Shelter
This is the traditional technique nowadays commonly referred to as a ‘green roof’. For thousands of years soil has been traditionally used to cover roofs. In arid climates, either very hot or very cold, it regulates the inside temperature, due to its heavy thermal mass. In Scandinavia, the Vikings added grass and small plants to give cohesion to the soil which had the added advantage of increasing thermal mass thus making the inside temperature more consistent.
Traditionally with this technique humid soil is poured into a lightweight wooden structure usually framed by lattice (to sustain the soil), thus giving thermal mass and acoustic insulation. A modernised version of this system has been developed where synthetic textiles are used instead of lattice and kept in place by wooden or metal columns or poles driven and anchored into the ground. The Cal-Earth Institute, founded by architect Nader Khalili has done extensive work with the Fill-in technique by developing its own system, a series of long tubular synthetic bags filled with earth which are then wrapped into a dome formation, barbed wire is used as the ‘mortar’ to bind each successive layer of bags as they rise into the dome shape. They have rebranded there system as ‘super-adobe’ and have created some impressive results.
- Cut Blocks
In some instances, soil is naturally cohesive and has a unique ability to be cut in the shape of blocks. With contact to air and sun these blocks harden into a durable building material. Such examples are typically found in tropical areas where lateritic soils are abundant. The image of a laterite quarry attached to this blog post is from Goa in India. Parts of Angkor Wat in Cambodia were also built with lateritic cut blocks.
- Compressed Earth
Compressed Earth techniques fall into two categories, Rammed Earth and Compressed Earth Blocks, but they are essentially the same technique.
There are many great historic examples of Rammed Earth still standing today, a testament to the durability and strength of this technique, and clearly one of the reasons it has been accepted by the modern construction industry as a legitimate building material. With this technique, earth is slightly moistened with water to get a homogeneous humid mix which is then applied in layers within a rigid formwork and rammed with a heavy device to remove air and increase its density and compressive strength, once dried (cured) the formwork is removed and the solid wall revealed.
As with Rammed Earth, Compressed Earth Blocks are formed by creating a homogenous humid mix, but instead of ramming it into a formwork the mix is placed into a mould in a steel press and is compressed either manually or by machine. The first machines used to compress earth into bricks were developed in France in the 1800’s, but the descendant of the machines we used today was invented in 1952 by the engineer Raul Ramirez in Bogota, Columbia, thus making Compressed Earth Bricks one of the youngest techniques in the ‘wheel of earth construction’.
The input of soil stabilization (commonly with cement or lime) has allowed both Rammed Earth and Compressed Earth Blocks to achieve much higher compressive strength and water resistance than most other earth construction techniques and made them real contenders in the modern building materials market.
- Direct Shaping
Direct Shaping makes use earth in a very plastic state. Buildings and spaces are created much in the same way an artist would create a piece of pottery. The quality of the soil, its preparation and the water consistency are specialised knowledge and as such the labour required to build with this technique is highly skilled. This technique creates very particular type of architecture with unique forms. It is most commonly seen in parts of Africa where it is predominantly used to build granaries or other buildings to store goods.
- Stacked Earth
Plastic state soil is rolled into balls which are then stacked upon each other to create a wall. This technique has a long history in Europe and is commonly referred to as ‘cob’ in England.
Stacked Earth as a building technique is still common in many parts of Africa, India and Saudi Arabia, the example shown in the image attached to this blog is of the city of Shibam in Yemen, also known as ‘The Manhattan of the Desert’, which is listed as a world heritage site by UNESCO. Actually within the city can be seen amazing examples of both cob, adobe and rammed earth.
The construction of a large cob house (3000sqm) in Devon, England is featured in the British TV series Grand Designs, which gives good insight into how this technique is applied in construction.
- Moulded Earth
Sun dried clay bricks known as ‘adobe’ are without a doubt one of the oldest building materials on the planet and date back to around 7,000 BC. These bricks are made of thick malleable mud, often with straw added to increase compressive and tensile strength. After being cast either in a mould or shaped by hand they are left to dry under sun in the open air. There is a huge variety of examples worldwide and in recent years adobe has seen a resurgence in popularity.
- Extruded Earth
The extrusion technique has been used in the fired brick industry for quite a while. Stabilised earth, in a plastic state, is extruded through a machine giving the desired shape. The blocks are often hollow and are cut to the desired length. This technique is a recent development from the 20th century. The main issue encountered with this technique in terms of its use in earth construction is that the soil requires a much higher content of sand than is used in the fired brick industry (which is more clayey) and as such the raw material is more abrasive on the machine which gets worn out and damaged at a much faster rate.
- Poured Earth
This technique requires soil to be converted into a liquid state and poured like concrete into formwork. Traditionally water was added to create the liquid state but the high-water content caused shrinkage when the wall dried resulting in severe cracking thus reducing structural integrity.
In recent years there has been a significant amount of research and development done on this technique and it has been discovered that the introduction of a deflocculating agent (such as those commonly used in industrial ceramics) acts to disperse the colloidal fraction of earthen materials and release interstitial water trapped in the clay particles. The huge advantage is that little or no additional water has to be added and when the mix dries up again the water is reabsorbed at the molecular level and there is no shrinkage in the material, and thus no cracking and loss in structural integrity. In the next few years, we are likely to see this material break-through into the building materials market which could have a huge impact of the construction industry. It is being called the ecological alternative to concrete, in Spain being rebranded as ‘Hormigon de Arcilla’ (Clay Concrete) and in English-speaking countries as ‘Poured Earth Concrete’. There is still much investigation to be done, but it seems clear that this material could usher in a new era of ecological construction.
- Daubed Earth
A common version of this technique is known as ‘wattle and daub’ of which extensive examples can be found throughout Europe, especially in France and England, some examples dating back to the Iron Age (500BC – 332BC), usually circular shaped dwellings. The technique consists of vertical wooden stakes (wattles) being interlaced or woven with horizontal twigs and branches which is then ‘daubed’ with clay and mud and left to dry. In some well-preserved examples is has been discovered that horse urine was used as a stabiliser which also added some degree of waterproofing to the mix.
Lightweight internal walls with plasterboard infill, very common in the modern construction industry, could be said to be an evolution of the wattle and daub system.
- Straw and Clay
This technique requires a similar construction methodology to a straw bale house, but instead of compacted bales, straws is chopped into smaller pieces and mixed with very clayey soil in a liquid state. The mix is then placed inside a timber frame (the load bearing structure of the building) and as with Rammed Earth is then tampered (compacted with a heavy blunt instrument). This mix itself is lightweight and as such doesn’t have load-bearing capacity, but provides excellent thermal and acoustic insulation. This technique is very common in many parts of Germany where is can be seen as a loose filler material or in prefabricated blocks.