|Soil Block Presses (GTZ, 1988, 38 p.)|
Building with Earth
The advantages of building with earth are:
· availability in large quantities in most regions,
· hence low cost for excavation and transportation or no cost, if found on the building site,
· easy workability, usually without special equipment,
· suitability both as wall and roof construction material,
· fire resistance,
· favourable climatic performance in most regions, due to the high thermal conductivity and porosity, thus subduing extreme outdoor temperatures and maintaining a satisfactory moisture balance,
· low energy input in processing and handling unstabilized soil, requiring only 1 % of the energy needed to manufacture and process the same quantity of cement concrete (soil-cement requires up to 75 %),
· unlimited reuseability of unstabilized soil (in recycling of demolished buildings),
· environmental appropriateness (use of an unlimited resource in its natural state, no pollution, negligible energy consumption, no wastage).
In spite of these advantages, earth constructions have not found
the wide acceptance they deserve, largely due to the poor durability of various
traditional and wrongly constructed buildings. The main reasons are:
· excessive water absorption, causing
- cracks and deterioration by frequent wetting and drying (swelling and shrinkage)
- weakening and disintegration by rain and floods,
· low resistance to abraison and impact, thus rapid deterioration through climatic elements and human usage, and penetration by rodents and insects,
· low tensile strength, making earth constructions susceptible to destruction during earthquakes.
On account of these limitations, soil constructions still lack institutional acceptability in most countries, which is why building codes and performance standards often do not exist.
Contrary to common belief, building with earth is not a simple technology. The mere fact that natives of many countries have been building their houses with earth since thousands of years does not mean that the technology is sufficiently developed and known to everyone. It is indeed the lack of expertise that brings about poor constructions, which in turn gives the material its ill reputation. On the other hand, it must be remembered that some cultures give longevity least importance, eg in the case of nomadic tribes or those who abandon the house when the owner dies.
As stated above, the technology of soil construction is a complex one, requiring a deeper knowledge of the material, its limitations, methods of handling and proper design. The number of possible solutions is enormous, so that the problem is mainly one of making the best choice. In the following, the main procedural steps for a satisfactory soil construction are listed with brief comments. These operations are well documented in a number of publications (see Select Bibliography) and the reader is advised to refer to them for details.
· The variety of naturally occurring soils is immense and not all soils are suitable for all building purposes.
· They can differ in chemical composition and grain size
distribution, factors which determine their performance.
· Soil selection is not only a matter of experience, simple field tests and subsequent laboratory tests are vital.
· The list of tests is long and not all are needed for each soil type and use.
· The main field tests are by sight, smell, touch, by making balls, ribbons and threads, by sedimentation in a glass jar and by dropping.
· Laboratory apparatus is needed for particle size analysis by sieving, for determining shrinkage, plasticity, dry strength, compressibility, optimum moisture content and cohesion.
· Depending on the type of construction (eg rammed earth, adobe, compressed blocks) the soil mixture must satisfy certain requirements.
· These may call for: addition of sand or clay, crushing, sieving, dry and wet mixing, all these with regular checks to achieve uniform qualities. An optimum soil mix can compensate for a number of other deficiencies.
· To improve the qualities of the soil mixture, a large variety of stabilizers can be added. The main ones are:
· Fibres (such as straw, agricultural wastes, animal and synthetic fibres) to improve the tensile strength.
· Cement, to improve the compressive and tensile strength, dimensional stability, erosion resistance.
· Lime, dependent on the clay content of the soil, to achieve a similar improvement as with cement.
· Bitumen, mainly to reduce water absorption.
· Natural and synthetic resins, for increased load-bearing capacity, elasticity and impermeability.
· Animal (cow dung, blood, hair, casein) and vegetable products (oils and fats, ashes, sap).
· Commercial products, mainly for impermeability.
· All soil constructions require some form of compaction, either by throwing, vibrating, ramming or mechanical compression. The method and degree of compaction are directly related to the ultimate strength of the structure.
· This Product Information Portfolio deals with mechanical compression, which can achieve the highest level of compaction, usually in the form of blocks for masonry structures. The devices required for this purpose are described in "Equipment".
Drying and Curing
· All soil constructions have to dry for several days to gain strength, the time needed depending mainly on the weather conditions. Hence, controlled drying and protection from rain are essential.
· When cement or lime is used for stabilization, the material must be kept moist for the first 4-5 days (for curing).
· Depending on the degree of compaction, compressed soil blocks can be stacked (up to 5 layers) immediately or on the next day. Some block producers claim using the blocks in the building operations, without any drying beforehand. This technique is not widely accepted, as problems of durability are believed to arise.
Design and Construction
· For space limitations, the innumerable criteria for design and construction cannot be mentioned here. But an old saying puts these criteria in a nutshell: "All cob (soil) wants is a good hat and a good pair of shoes' that is, the base and the roof of a building are of greatest importance.
· A well designed building with well compacted earth walls can do without external rendering, though in humid climates some surface protection is usually recommended. Good quality compressed soil-cement blocks generally need no surface protection, thus saving costs, time and energy. If a render is applied, the choice is very wide, and some experience is needed to select the most appropriate one.