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

Abstract

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

The Effects of Sediment Size and Concentration on the Rheological Behavior of Debris Flows

“…Experiments on debris flows reconstituted mixtures analyzed in the present study involves pyroclastic soils presenting a very small clay fraction. They show a behavior of non-Newtonian fluids with yield stress according to several other previous works [15,22,44]. Stress-strain curve significantly depart from Bingham idealization, and varying the solid volume concentration, the mixtures show dilatant or pseudoplastic flow behavior, depending on the granular concentration, no matter of the considered soil characteristics.…”

“…The works carried out in the last decades on a large amount of collected data indicate the Hershel-Bulkley model as the most appropriate to describe the rheological behavior of simple yield stress fluid in a large range of shear rate (i.e., 10 À1 -10 2 s À1 ) [21]. Referring to the steady state flow-like regime of debris flows, rheological parameters may be expressed as a function of the bulk volume sediment concentration, whereas the yield stress greatly depends not only on the sediment concentration but also on the relative content of finer and coarser grain [22]. This chapter presents recent experiments [22] on reconstituted debris flows mixture, stressing the effects on the rheological behavior due to the sediment concentration and the presence of coarse-grained fraction.…”

“…Referring to the steady state flow-like regime of debris flows, rheological parameters may be expressed as a function of the bulk volume sediment concentration, whereas the yield stress greatly depends not only on the sediment concentration but also on the relative content of finer and coarser grain [22]. This chapter presents recent experiments [22] on reconstituted debris flows mixture, stressing the effects on the rheological behavior due to the sediment concentration and the presence of coarse-grained fraction.…”

“…Materials investigated and experimental methods are widely described in [22]. The investigated materials come from the source area of two real debris flows event occurred in May 1998 (soil B-Montefiorino Irpino) and in March 2005 (soil A-Nocera) in Campania region (southern Italy), which involved the pyroclastic terrains, originated by the volcanic activity of Somma-Vesuvio mount, covering the mountains of that region.…”

“…The tray is progressively inclined until a threshold inclination corresponding to a blatant motion of the mass front, and the experiments were carried on until the full stoppage of the flowing mixture. Accounting for the still, threshold and stoppage condition, and according to the lubrication assumption (i.e., still material thickness much smaller than its longitudinal extent; [29]), it may be assumed a uniform flow condition for the slurry, and momentum balance gives the shear stress distribution within the mixture [22,29].…”