At first, oversized gravel and organic matter (grass roots) were removed from the natural cohesive soil. The soil samples were put in an oven to dry at a temperature of 105◦C to obtain the soil dry weight at a constant mass. To analyse the composition of clay minerals in the material an X-ray diffraction clay mineral analysis was performed. Type and amount of these clay minerals have a crucial influence on the binding force and therefore also on the bending tension and compressive strength. The content of clay minerals can be given as follows: 50% smectite (low binding force), 30% illite (high binding force), 10% kaolinite (high binding force) and 10% vermiculite (medium binding force). Fig. 1 illustrates the grain size distribution of the tested soil. The natural fibres were also oven dried at 105 ◦ C to obtain a constant mass.