Morphological evolution of granular geomaterials under impact erosion

This study investigates the evolution of surface roughness and particle shape, two parameters that define the structure and mechanical behaviour of geomaterials, and shows that both can be controlled by tumbling. Two tumbling approaches were employed to study the evolution of the shape and texture of granite particles. Single-particle tumbling with abrasive grit was used to examine the controlled evolution of surface roughness, while multiparticle tumbling, with and without abrasive powder grits, was used to analyse the evolution of particle shape. The size and shape of the powder grits were characterised using optical microscopy with the Morphologi G3 system. A mechanism was hypothesised for different stages of single-particle tumbling and validated against the experimental evidence. Specifically, the large, elongated and angular grit particles used in the initial stages, which are harder than granite or any other natural geomaterial, alter the low-frequency features of the surface topography, whereas the small circular and rounded grits suppress the high-frequency features of the surface, i.e. the roughness. Additionally, multiparticle tumbling was carried out to study shape and texture evolution with time. The surface and shape of the granite ballast particles were characterised at different stages of tumbling using optical/specular reflectance, root-mean-square roughness, Fourier amplitude spectra or power spectral density, and peak-count distribution, in addition to roundness, circularity, and size distribution for multiparticle tumbling. This research highlights that tumbling a single rock particle with different grits produces a controlled evolution of roughness, whereas multiparticle tumbling results in a controlled shape evolution with randomly varying texture.