Shape of particles is one of the most fundamental physical properties of soil. It is defined by four parameters; sphericity, roundness, degree angularity of particles and roughness. Determination of every single factor influence on others physical and mechanical properties of soils is very difficult, but it can be summarised in one parameter. Up to now the roughness of particles was determined by some methods, but they are very difficult to be used in practice. A new method of roughness measurements is presented in this paper. Measurements of geometrical particles parameters are on photos from electronic scanning microscope. On the photo the length of particle contour, and length of polygon circumscribed on particle was measured with planimeter. In this method the area of polygon circumscribed on particle and area particle shape on the photo are the same. The results are used for calculation of asperity index I_a (equation 3). The experimental part of this method was realised on photos of about 150 particles of several soils, which shapes and roughness were very differentiated. The theoretically analysis of asperity index determined for imagined surface is based on assumption, that roughness of the surface is composed only with triangles, semiculars, or only with trapezoides forms (fig. 4). The value of maximal asperity index calculated for simplified shape very near to the maximal value I_a obtained for real shape of fly ash particle.

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