Evaluation of the effect of shape of granular materials on uniaxial compressibility behavior by analytical and experimental methods

Granular materials used today in many engineering projects such as rockfill dams and railway have a wide variety of shapes. This shape variation ranges from very sharp to perfectly rounded.  The shape of the aggregates affects the mechanical properties of the grain, including fracture strength and internal friction angle. As a result, the mechanical behavior of the mass of granular materials depends on the shape of the grains. In order to investigate the effect of this property, different types of grains in the shapes of spheres, cylinders, cubes and pyramids, which include a wide range of shapes of natural aggregates, were made artificially in the size range of 1.5 to 2.0 cm. Small-scale uniaxial compressibility tests were performed on each of the grain shapes under the same conditions including initial porosity ratio and maximum stress and after each experiment, the stress-strain behavior and the amount of breakage were obtained using Hardin breakage factor. Then, the results evaluated using an analytical model proposed by McDowell et al. based on the law of conservation of energy. This model has 7 parameters which depend on the initial conditions of the grains, material, shape, size and fracture strength of the grains. Comparison and evaluation of the results indicates the ability of the analytical model in predicting the compressibility behavior of pyramidal grains. As the grains become angular, the compressibility and breakage of the materials increase. Also, with increasing the fracture surface energy of material, the effect of shape on compressibility decreases.