1. Professor, Dept. of Geotechnics, Faculty of Civil Engineering, Czech Technical Univ., Thákurova 7, 166 29 Praha 6, Czech Republic.
2. Head, Dept. of Particulate Media, Institute of Theoretical and Applied Mechanics of the Academy of Sciences of the Czech Republic, Prosecká 76, 190 00 Praha 9, Czech Republic (corresponding author).