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Building With Earth

BUILDING WITH EARTH
Written by Cindy Harris for CAT's membership magazine, Clean Slate.
email: info@cat.org.uk; tel: 0845 3308373 or 01654 705989

Introduction

Earth constructions are buildings whose walls, sometimes floors, and occasionally roofs, are built with earth as the major raw material. The Earth might be mixed with straw, it may be compacted to varying degrees, and it can be dried in the sun or more slowly through simple evaporation.

Unlike the many fired earth products we are already familiar with - bricks, roof tiles, plant pots - earth buildings do not rely on materials heated to high temperatures in a kiln. Nor are they to be confused with earth-sheltered buildings, which are in effect underground reinforced concrete shells.

Earth is one of the most abundant, most locally available, cheapest and lowest impact materials it is possible to build with. Over one third of the world's population live in houses built from earth, and over 70% of theearth's landmass is either pure clay or laterite - a clay with some iron content. In many cases the earth can be excavated from the site itself.

History and Geography

There is archaeological evidence to show the existence of entire cities built of earth, such as Jericho and Babylon, some 10,000 years old. And we are not talking here of mud huts or other forms of primitive housing, but imposing temples and monuments, including the Tower of Babel which was seven stories high. Earth buildings have been excavated in China dating from the 7th century BC, and the greatest earthen construction of all, the Great Wall, was begun over 5000 years ago.

Different construction techniques were introduced into different countries by invaders who then settled. The conquering Roman armies introduced rammed earth building to the South East of France, in the area of the Rhone valley where the capital of Roman Gaul, Lugdunum, was situated, and where the foothills of the Alps provided earth of a suitable composition.

When the Moors invaded Spain from the South, they brought with them a kind of mud brick construction known as adobe, more suited to heavier clay soils. This same technique was later introduced to South America by the Spaniards, and it survives today in New Mexico, in the form of the much-cultivated Santa Fe style.

In Australia, the first earth buildings appeared during the gold rush period of the 1850s. For the vast treeless deserts of Central Australia, earth construction was often the most practical option.

It would be a mistake to imagine, however, that earth construction was limited to tropical or even Mediterranean climates. There is a long history of cob building in Devon, where the earth was protected from driving rain and freezing temperatures by an overhanging roof, usually of thatch, a good stone foundation built up above ground level, and several coats of lime- based render and limewash. There is an old Devonshire saying on cob buildings "Giv un a gude hat and a gud pair o' butes an 'er'll last forever"

Today, there is a growing interest in using more natural building materials such as earth or straw, although there are as yet very few modern examples of earth building in this country. At CAT, we built our new shop and information centre in 1998/99 with structural, load-bearing rammed earth columns and walls. As far as we know, it was the first ever large scale public building to be built from earth in the UK.

Environmental Advantages

In general, earth construction involves a very low energy input, and creates virtually no pollution. Historically of course, the energy required to dig up and treat sufficient earth for a house, was all provided by humans and animals. Nowadays, mechanical extraction and mixing is the more likely method, but assuming the earth is sourced fairly locally to the site, it is still a very low-energy form of construction.

The transportation of large amounts of earth is potentially an energy-intensive and polluting process. However, the distances involved are rarely that great and the overall energy cost is certainly much lower than the conventional alternatives of cement, sand and aggregates.

All earth buildings tend to have a high thermal mass, as earth is a dense material, particularly when compressed. This means that they can absorb and store solar energy, and re-release it in the form of heat when the building cools down, usually in the evening. They therefore have the effect of modifying temperature extremes internally, and are more comfortable and energy efficient than many conventionally built houses. High thermal mass is an essential component of passive solar design. Earth is also highly hygroscopic and so helps to regulate humidity.

When installed to a sufficient thickness, earth is a reasonably good insulator, and on demolition, it will simply revert to earth.

Different Techniques

Rammed Earth (also known by the French term pisé de terre): moist, loose earth is compacted in layers between shuttering or formwork. The forms are then moved along or upwards, to form a whole wall. The exact composition of the soil and the right degree of water are critical for the success of this method. However, a small proportion of cement or lime may be added to correct for any deficiencies. This 'stabilisation' is considered necessary for soil with a low clay content. Over a period of time, perhaps up to two years, a rammed earth wall will dry out and become as durable as sandstone, as long as it is waterproofed top and bottom.

Cob: Sub-soil is mixed with straw and water, and then pounded or trodden until it reaches a suitable consistency. It is then laid in horizontal layers, and again trodden down, to form free standing mass walls. The use of timber shuttering was a late development - from the 1820s onwards.

Adobe: Adobe or sun-dried earth blocks can be made from most types of sub-soil. Enough clay is required to bind the mix together, but not so much that the block cracks on drying. In the past, the earth was trodden to a paste (often by animals) then mixed with chopped straw, pushed or thrown into moulds and left to dry in the sun. The blocks were then laid and bonded with a mud and lime mortar and rendered with a mud and dung mix, and/or limewashed.

Mud bricks have the advantage of being simple to make and therefore appropriate for unskilled labour. They can be produced all at once, or in small batches, as and when time permits. The quality can be checked, and any suspect bricks rejected, before they are built into a wall.

Stabilised Earth Blocks: These are earth blocks made harder and more durable by the addition of small amounts of lime or cement (5 -10%). Bitumen can also be added as a water repellent. The blocks are compressed in a machine which exerts a large amount of pressure on the mould, to produce the blocks in standard sizes.

Stabilised compressed earth blocks perform at least as well as many commonly used bricks or blocks in terms of their load bearing capacity, long life and freedom from maintenance.

Relevance for Self Build

Building with mud brick or block n particular, requires little or no specialist skills. The process is labour-intensive and the work is often heavy, but it can be phased to suit both the weather and the availability of helpers. In Australia, where owner-builders have elevated 'muddies' into something of an art form, making one house worth of mud bricks is often done by a huge communal gathering over a weekend.

Earth can be seen as a 'patient' material, allowing time for trial and error, re-mixing and re-use, without threatening to 'go off' or set quickly. It is thus ideal for amateur experimenters and enthusiasts, and is easy to maintain and repair.

Self-builders tend to be rugged individualists and to demand the same qualities of their buildings. By its very nature, every element of earth construction will be unique in terms of colour, texture and finish.

Like all builders, self-builders have to satisfy the standards of Building Control Inspectors, and it is reassuring to know that earth can be tested and proven to possess the necessary performance characteristics relating to shrinkage, compression, weathering etc. Not all earth samples will pass these tests, but as more earth buildings get built in this country, a whole library of information will also be built up, concerning suitable earth-yielding sites and testing procedures.

Conclusion

It is obvious from what has been said already that earth buildings can withstand the tests of time, intensive usage and climatic extremes. We need to find alternatives to 'building as usual', both in terms of the energy and other non-renewable resources used, and the comfort and thermal efficiency of the finished house.

Earth buildings score well on both counts, and on top of that, have the aesthetic appeal of a natural material that is also superbly functional. "Have nothing in your house" said William Morris, "which is not beautiful or functional" - and to combine the two is satisfaction indeed.

Further Information

Cindy Harris is co-author of The Whole House Book, a complete reference guide for self-builders and architects. It gives comprehensive advice on choosing materials and designing a healthy, efficient, and low-impact home.

The Rammed Earth House (Easton)
Clear & comprehensive information for builders, contractors and architects.

Buildings of Earth and Straw (King)
Invaluable design guide for structural engineers and owner-builders.

Building with Earth (Norton)
Practical advice on whether to and how to build with earth. Includes soil selection, construction and maintenance.

These publications are all available by mail order, as are many more books covering earth-building methods. Visit the Building section of the Bookshop at www.cat.org.uk/shopping or call 01654 705959 for more details.

CAT runs several residential courses, covering areas including eco-building methods.
Tel: 01654 705981; web: www.cat.org.uk/courses

CAT's Resource Guides list suppliers, installers, publications and more. They are available to download from the pay-per-view page of our website.

You can also contact us with any further questions.

For more in-depth technical advice, many people find it useful to come here and run through their plans with our renewable energy experts. For further details, see the CAT Consultancy page.

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