|Soil Preparation Equipment (GTZ, 1991, 18 p.)|
For every earth construction technique there is a preferred set of characteristics of the material. When the earth is excavated, it does not necessarily possess these favourable characteristics and can thus be the cause of unsatisfactory performance or limited durability of the end product. Therefore, special attention must be given to soil preparation and handling, prior to building construction.
Raw earth can have two major deficiencies:
- Texture deficiency: in the case of earth containing an unsatisfactory grain size distribution, that is, for instance, too much sand and gravel with too little silt and clay to obtain a compact and cohesive material, or containing too much clay, which leads to excessive swelling and shrinkage. The texture is also deficient when the earth contains too much plant material (eg in the form of roots) or when the soil is excavated from different layers, each containing different proportions of coarse and fine material.
- Structural deficiency: in the case of earth containing lumps of consolidated material, or being too wet.
The need for preliminary operations to correct these deficiencies should not be underestimated. They ensure the quality of the end product and subsequently the quality and durability of the buildings. They also guarantee that the structure is built economically in the sense that they optimize the proportions of the materials and the additives, if the earth is stabilized. If sieving, pulverizing and mixing is properly done, up to 50 % of the stabilizing agent can be saved without reducing the crushing strength of the end product, or in other words, the effect of the stabilizer in providing crushing strength and resistance to abrasion and moisture penetration can be greatly increased.
While all earth construction techniques need soil preparation, the emphasis here is on compressed soil block production.
For the self-builder, the advantage of earth construction is that the raw material is obtained directly on the building site, or close by, depending on the suitability of the soil. After removing the top soil, it is usually dug out with simple tools.
In large scale undertakings, the problems involved in excavating earth for construction purposes are similar to the excavation of materials for the ceramics and cement industry, in stone quarrying and even for agriculture and road building. In these cases, the methods of excavation depend upon several factors that are not specific to earth construction, for instance, geological and engineering aspects, economic and legal considerations. With regard to earth construction, the only relevant aspect of excavation is the suitability of the soil.
Soil Testing and Quality Control
The suitability of the soil has to be tested, not only before commencing full-scale excavation and when changing the excavation site, but also at different points within a single excavation pit, as the type and composition of earth can vary even within distances of a few metres.
There are basically two types of tests for approving the excavation of a soil deposit:
- Indicator or field tests: which are mainly by sight, smell, touch, by making balls, ribbons and threads, by sedimentation in a glass jar and by dropping, for the preliminary selection of soils, before conducting
- Laboratory tests: which require special apparatus, for instance, for particle size analysis by sieving, for determining shrinkage, plasticity, dry strength, compressibility, optimum moisture content, cohesion, etc, for an accurate characterization of the soil.
The list of tests is long and not all are needed for each soil type and use. However, for the day-to-day running of a production unit, two types of tests have to be conducted regularly:
- Control tests during production: which serve to check the quality of of the production process at different stages, that is, delivery of new raw material, moisture content and uniformity of the mixture, and quality of the compaction. These tests should be conducted at least twice for each new mixture.
- Acceptance tests on the final product: which serves to verify the conformity of the production process or products in comparison with the required performances of a reference sample. The tests are mainly visual inspection to check dimensional tolerances, surface and edge smoothness, presence of lamination or cracks, etc, and compressive strength tests to determine the quality of compaction and stabilization. In the ease of soil blocks, for instance, at least 5 samples should be tested for each delivery of 2000 blocks. If the samples do not pass the tests, the complete production must be rejected. It is therefore important to clearly identify each daily production with standardized indicators, and not to mix the production of different days in the storage area.
Earth is usually excavated in a dry state, in which the clay fraction normally occurs in the form of lumps that can attain sizes of 200 mm and more. Since this is not acceptable for any form of earth construction, it is necessary to break up these lumps.
Three cases have to be considered when pulverizing earth:
- If the grain size distribution of the soil is correct or acceptable, the larger homogenious particles (pebbles, gravel) should not be crushed.
- If the quantity of large particles exceeds the required proportion, pulverization must be followed by sifting out the excess particles.
- If the proportion of gravel and stones is correct, but the particles are larger than the permissible size, they may be crushed to the required size during pulverization.
Sifting is indispensable either when the earth has texture defects (that is, when it contains oversized particles or organic matter) or when pulverization is unsatisfactory. The most appropriate grain diameter depends upon the construction technique aimed at and the specifications of the construction project.
For most earth construction techniques, the maximum grain size is 20 mm, although for making better quality soil blocks 10 mm should be the upper limit, whereas for rammed earth stones of 50 mm diameter are acceptable.
Unless the earth can be used directly after excavation, pulverization or sieving, some form of mixing is usually required, and can be of three types:
- mixing of the ingredients of the same soil, which was separated out into distinct fractions;
- mixing of different soils (as in the case of soil improvement); and
- blending of other materials (eg water, stabilizer, fibres, waterproofing additives, etc) with the above two mixes.
Only dry materials should be mixed with each other or with a liquid. Mixing dry and moist materials together rarely gives good results. Therefore when stabilization is carried out with powdered hydraulic binders, such as cement and lime, preliminary dry mixing is essential. Water is only added afterwards by progressive sprinkling of small quantities either in gushes, fine spray, droplets or in steam under pressure, according to the level of sophistication employed.
For mechanical mixing, it is usually necessary to provide 2 to 4 minutes of mixing, depending on the efficiency of the mixing process. In any case, the colour of the mix should be uniform at the end of the process.