III.1. Preliminary on-site tests
Soil samples from zone 2 and zone 3 soils (obtained from test holes) should be tested in the way described below:
Smell test: Damp soil emitting a musty odour indicates the presence of organic material and is therefore not suitable for block making. Such soil should be discarded.
Colour appearance: The dark brown crumbly humus in the soil is organic matter. Soil of this colour should in general be discarded. Light brown to black colouring indicates that the soil contains at least a small proportion of organic matter but that it may be suitable for stabilising. The colour test does not, however, work in all cases. For example, black cotton soils are dark brown to black in colour but do not contain much organic material.
A reddish to dark brown colour indicates the presence of iron oxides which are acceptable for soil stabilisation purposes. White to yellow colouring is an indication of the predominance of lime-based compounds or sand. This type of soil can be stabilised.
Pale brown colouring is characteristic of the presence of clay; lime might be needed as a stabilising agent for this type of soil.
Shine test: A small piece of dry soil is rubbed with the back of a finger nail in order to identify the main component in the sample. The soil surface is abrasive to the touch and the soil remains dull if sand or silt is predominantly present. On the other hand, a sample containing clay shines and is smooth to the touch.
Thread rolling test: This test requires adding sufficient water to a small quantity of soil so that the sample can be easily moulded by hand. The soil sample is then rolled out on a flat clean surface into a thread with the palm of the hand or the fingers (see figure II.10). The reduction of the thread to about 3 mm in diameter indicates the presence of a high clay fraction. On the other hand, the breaking of the thread at a larger diameter indicates the presence of a moderate sand fraction. This test is also used to determine the plastic limit of a soil (see section III.3).
Hand moulding test; After having removed stones and any foreign bodies larger than about 6 mm diameter, the soil sample is moistened and formed into a cube with an edge of about 2.5 cm. If a cube is formed easily, a high clay fraction is present. Although good adhesion and mouldability of such soil are advantageous in the block making process, too much clay will make the soil sticky to work with, and its high shrinkage may lead to cracks within the manufactured soil blocks.
Next, the moulded test ‘cube’ is allowed to dry out in the sun for one day. The occurrence of any surface cracks indicates a high clay fraction, which may give similar cracking problems in the blocks. On the other hand, the splitting of the cube into several pieces indicates the presence of too much sand or silt. Blocks produced from such soil may also fall apart.
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