Porosity: a parameter with no direction

The macroscopic properties of a granular material are controlled and manipulated by controlling its microscopic composition, i.e. the distribution of particle size and shape. The particles then combine to form an array which is usually characterized by the parameter of porosity, the fractional volume of the interstitial pore space. This parameter is a scalar variable which cannot describe important features of the array, such as anisotropy. The importance of the anisotropy is illustrated by the shear behaviour of a powder. Measurements made in a biaxial shear cell are reported. A tensor quantity that describes the array is proposed as well as the techniques for relating this parameter to the particle properties.