A laboratory experience on the effect of grains concentration and coarse sediment on the rheology of natural debris-flows

Not only solid volumetric concentration, but also coarse particle content play a relevant role on the rheology of soil mixtures involved in mud, debris, hyper-concentrated and earth flows. This paper is devoted to investigating the influence of bulk solid volume concentration and of coarse fraction on the rheological behavior of granular slurries. Laboratory activity is carried out involving different soils from the source area of real debris flows (from the Campania region in Italy). Experimental results demonstrate that the flowing behavior is the same as yield stress fluids with a static yield stress value larger than the dynamic one. A generalized Herschel–Bulkley model is considered, accounting for a consistent index which is a function of solid volume concentration. Experimental evidence demonstrates that the viscous characteristics (i.e., yield stresses and bulk viscosity) are very sensitive to coarse grain content, and the flow characteristics are deeply affected by small variations of both solid content and particle size. In effect, the tested materials show increasing yield stress (both static and dynamic) with solid concentration and decreasing yield stress for an increment of coarser solid fraction. To the contrary, assuming a constant fine grain relative content, the higher the coarse grain fraction is, the lower the yield stress results. Moreover, the typical transition from solid-like to fluid-like behavior, and vice versa, is strongly influenced by solid concentration and grain size distribution.