The hydraulic gradient as a function of mixture velocity has thus been measured for calibrated beads of two specific masses (2500 kg m 3 and 3650 kg m 3), with large particle to pipe diame-ter ratios of 5%, 10% and up to 15%, as well as for mixtures of cal-ibrated beads and for real stones of similar physical properties. The tests have been conducted in an horizontal pipe. The main results are the following: