Rheology of concentrated granular suspensions and possible implications for debris flow modeling

Granular suspensions are of major relevance for many geophysical flows. We investigated the rheological behavior of three samples taken from a debris flow deposit at varying solid concentration (from 38.0 to 54.2% by volume) and grain size distribution. Experiments were performed first on the fraction finer than 0.075 mm and then on suspensions with fine sand (up to 0.425 mm in size and percentages varying from 10 to 50%). A vane apparatus connected to a rotational rheometer was used, and experiments showed a reproducibility within +/- 12%. Results were processed by the Tikhonov regularization method in order to convert the shear stress (tau) versus rotation velocity (omega) curves to shear stress (t) versus shear rate ((gamma) over dot) g) flow curves. At shear rates lower than 3-5 s(-1), shear stress is rate-independent. At higher shear rates, the shear stress increases depending on the maximum grain particles included within the suspension. We used the Herschel-Bulkley and Bingham models to describe the monotonous increase of the flow curves. Varying the grain size distribution of the samples, the power and consistency indexes range within 0.75-1.35 and 0.44-40.8 Pa s(-n), where n is the exponent of (gamma) over dot; g; the viscosity coefficient varies from 1.0 to 47.2 Pa s; and the shear stress varies from 28 to 1959 Pa. For materials mainly composed of silt and clay with a variable sand content, (1) an increase in the particle size changes the rheological behavior from shear thinning to shear thickening, and (2) the percentage of sand affects both the frictional character of the flow and the magnitude of the rheological parameters.