As with concrete, it is an advantage to have an even proportion of different sized particles in the earth, and no larger stones. Awell-graded clay binds better since smaller particles fill the gaps between the larger ones.There are usually two tests to assess the binding tensile strength. In both tests, moist earth samples are kept under a damp cloth for 6-12 hours before testing:
• The ball test tests stiffness. A sample of 200 gram of earth is rolled into a ball, which is then dropped from a height of 2 m on to a glass surface. If the diameter of the flattened ball is less than 50 mm after impact, then the earth is good enough.
• The figure-of-eight tests the cohesion between the particles and can also be used for testing concrete. The earth is knocked into the figure eight form in three layers with a wooden hammer (see Figure 8.4). The mould has specific proportions and can be made of either hardwood or steel. The thickness of the mould is 2.23 cm. The hammered piece of earth is taken out and hung between two circular steel rings. It is then loaded with weight in the form of sand in a small vessel at a rate of not more than 750 g per minute.The pouring is stopped when the sample breaks. The weight under which the sample breaks, divided by the section of the sample, which is 5 cm2, indicates the tensile strength 'binding power' of the earth. At least three samples should be made to achieve a precise picture of the strength properties. An earth with a binding power of less than 0.050 kp/cm2 is unusable.
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