Evaluation of the wall friction angle of dry volcanic materials from laboratory experiments.

Granular materials are widely involved in several processes, from the food, cosmetic, and pharmaceutical industries to natural phenomena such as pyroclastic density currents and landslides. Therefore, the study and understanding of their behaviour and rheology are of paramount importance in terms of hazard assessment and for developing and implementing mathematical and numerical models. The simulations try to reproduce the complexity of the flows taking into account several parameters, but, despite the advances in the theoretical descriptions of granular flows, a gap still exists between the empirical models and the experimental observations. In this work, we present the results of the characterization of volcanic samples carried out with the FT4 Powder Rheometer (Freeman Technology). Shear tests, compressibility tests, and wall friction tests were performed to characterize the flowability of the powders. We particularly focus on the wall (or basal) friction angle, which describes the interaction of the particles with a variable-roughness substrate. Some glass beads have also been investigated, as a reference value. The results show how the ratio between the roughness of the surface and the average particle size of the samples influences the wall friction angle. Moreover, the comparison with the glass beads also reveals the influence of the irregular shape of the particles.