Experiments on the effects of fine-coarse grains content on the yield stress of natural debris flows

The yield stress is an important parameter to describe flow characteristics of natural flows, such as slurries, mud and debris flows, which causes fatalities, injuries and landscape changes worldwide. Many studies have been carried out devoted to the determination of yield stress of granular-fluid mixtures, and some progress has been made in recent years. It is not surprising that the interaction between solid and liquid phase could approach different behaviour. In fact the effects of Brownian motion and colloidal forces control the finest solid fraction, whereas, frictional and collisional contacts and hydrodynamic forces govern the coarser particles content. Therefore, the bulk behaviour of viscous suspensions is very complex and the effects of many parameters (e.g., solid volumetric concentration, size and shape of the particles, nature of the interstitial fluid, etc.) must take into consideration to analyse their hydrodynamic behaviour. Yu et. al. (2013), show that sediment volumetric concentration and coarse particle size distribution have much larger effects on the yield stress than the finer sediment (i.e., clay). Previous experimental works on coarse particles dispersed in a clay suspension (e.g. Ancey & Jorrot, 2001) and on natural debris flows mixtures (e.g. Coussot & Piau, 1995), confirm that the amount of finest fraction influences the main rheological parameters of the entire suspensions in terms of fluid-like flow behaviour of the slurries (Pellegrino & Schippa, 2018), whereas the yield stresses strongly vary with the amount of coarse particles (Major & Pierson, 1992). The present experimental work aims to enlarge the knowledge on the contribution of grain size distribution (referring to the coarser particle content) on the rheological behaviour of debris flow mixtures with particular attention to the transition process from a solid-like to fluid-like flow behaviour.