III.2. Further soil testing procedures
The preliminary on-site testing methods described above will indicate whether a soil is likely to be suitable for stabilised soil block production. These tests may not, however, be sufficient. Other tests may be necessary, especially if the preliminary tests are not conclusive.
Sophisticated laboratory methods of soil testing, including chemical and sieve analysis and determination of the plastic limit, liquid limit and the optimum moisture content for maximum soil density have all been evolved by soil engineers. However, these laboratory tests are expensive and time-consuming and are only deemed necessary for large-scale projects. For a small project, fairly effective but simple on-site tests requiring simple equipment which may be locally manufactured can be conducted.
After preliminary on-site tests on soil samples obtained from test holes, the holes producing a priori good quality soil should be opened up in order to collect a larger sample for more detailed examination. The following on-site tests may then be performed:
Particle size distribution: This test gives a quantitative measure of the individual soil fractions. It requires four sieves and a tray similar to those illustrated in figure II.11; these sieves nest onto one another for proper site sieve analysis.
The four sieves must have different wire mesh sizes (e.g. 6 mm, 2 mm, 0.2 mm and 0.06 mm). The 0.06 mm mesh may be difficult to obtain and could be replaced by an open weave cloth. The fifth container is a catchment tray. The test should be performed according to the steps noted below.
A sun-dried soil sample of 2 kg is first weighed out and placed inside the 6 mm sieve located on top of the nest of sieves. By shaking the nest of sieves simultaneously, all the fine particles pass through this sieve and, depending on their fineness, some will rest on intermediate sieves, while those passing the 0.06 mm sieve will fall into the catchment tray.
Once the transfer of material from one sieve to another has ceased, the separated fractions of soil lying on top of each sieve and in the catchment tray are removed, weighed and recorded. A simple particle size distribution is thus obtained for soil sampling.
The fraction of soil retained on the sieves may be classified as follows:
The results of the sieve analysis give an indication of the type of stabilising agent best suited for the soil. Ideally, there should be an even distribution of each soil fraction in order to manufacture good-quality stabilised soil building blocks. If this were to be the case, about five per cent cement would be needed as a stabilising agent. In practice, it is generally found that one fraction is larger than the others. For example, if there is a high fraction of coarse and medium sand and a low silt/clay fraction (e.g. less than about 20 per cent), about four to six per cent cement should be used to stabilise the soil. Conversely, if the silt/clay fraction is high, (e.g. above about 30 per cent), about six to eight per cent lime can be used as a stabilising agent. However, there may be a high proportion of silt present which would affect the linear shrinkage properties of soil; in this case, cement may be required.
Sedimentation bottle test: This test gives more information on the finest particles contained within a soil sample. It is performed in the manner noted below.
A wide-necked, straight-sided and flat-bottomed bottle or jar is needed for this test. The bottle is first filled to one-third with clean, uncontaminated water (see figure 11.12(1)). Approximately the same volume of dry soil (which has passed through the 6 mm sieve) and a teaspoonful of common salt are added. Salt facilitates the dispersion of soil particles (see figure 11.12(2)).
The lid is then firmly fixed on the bottle and the contents well shaken. When the soil and water have been mixed, the bottle is placed on a flat surface for about half an hour. Then, the bottle should be shaken again for two minutes and replaced on the level surface. Two or three minutes later, the water will start clearing. The finer particles fall more slowly and are thus deposited on top of the larger size particles. Two or three distinct layers will be observed, with the lowest layer containing fine gravel, the central layer containing the sand fraction and the top layer containing the combined silt and clay fraction. Figure 11.12(3) illustrates this layer formation in a bottle. The individual percentages can be determined by direct measurement of the depth of each layer.
Linear shrinkage mould test: This test indicates the linear shrinkage of a soil sample as it dries. This information will help determine the best type and amount of stabiliser required. This test requires first the construction of a linear shrinkage mould with the following internal dimensions: 40 mm × 40 mm × 600 mm. Figure 11.13 illustrates the mould required together with leading dimensions.
The first step in this test is to lubricate the internal faces of the mould with some type of oil or grease. Ideally, silicone grease is preferred but any type of mould release oil or grease could be used. The lubricant reduces soil drag on the internal faces of the mould occurring as the soil sample dries out and shrinks.
The soil sample which passed through the 6 mm sieve is mixed with water until a wet puddingy mix is obtained (this occurs near the liquid limit - see section III.3). This mix is then packed into the mould cavity, ensuring that the mould is completely full (absence of air pockets) and the top open surface is smooth. The mould is then placed to dry either in the sun for about five days or under shading for about ten days. In either case, it must be protected from rain.
If the soil has a high clay content, the sample will shrink and hog up out of the mould. This is illustrated in figure 11.14 which shows the shrinkage properties of black cotton soil. A soil sample which shrinks and cracks across the width of the mould (see figure 11.15) indicates a high sand fraction and low silt and clay fractions.
The linear shrinkage can be determined by subtracting the length of the dry soil sample from the length of the mould cavity. This shrinkage is usually expressed as a percentage of the original mould cavity length.
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