The properties of loose materials such as grain shape, material moisture content and internal friction coefficient are discussed in the paper. There are two methods to consider the grain shape. One is based on the division of grain shapes encountered in the natural state into three groups only: spherical, irregular and sharp-edged grains. The second method consists in introducing a new grain shape factor (the so-called spherical) different than the ones met so far. Another important clement is the granular material moisture content and its effect on the behaviour of the granular bed during mechanical processing. It is proposed to replace moisture content by the internal friction coefficient. The last clement of the study is determination of the outflow function for screening of wet granular materials.