The effect of grain size on the viscosity and yield stress of fine-grained sediments

In debris flow modelling, the viscosity and yield stress of fine-grained sediments should be determined in order to better characterize sediment flow. In particular, it is important to understand the effect of grain size on the rheology of fine-grained sediments associated with yielding. When looking at the relationship between shear stress and shear rate before yielding, a high-viscosity zone (called pseudo-Newtonian viscosity) towards the apparent yield stress exists. After yielding, plastic viscosity (called Bingham viscosity) governs the flow. To examine the effect of grain size on the rheological characteristics of fine-grained sediments, clay-rich materials (from the Adriatic Sea, Italy; Cambridge Fjord, Canada; and the Mediterranean Sea, Spain), silt-rich debris flow materials (from La Valette, France) and silt-rich materials (iron tailings from Canada) were compared. Rheological characteristics were examined using a modified Bingham model. The materials examined, including the Canadian inorganic and sensitive clays, exhibit typical shear thinning behavior and strong thixotropy. In the relationships between the liquidity index and rheological values (viscosity and apparent yield stress), the effect of grain size on viscosity and yield stress is significant at a given liquidity index. The viscosity and yield stress of debris flow materials are higher than those of low-activity clays at the same liquid state. However the viscosity and yield stress of the tailings, which are mainly composed of silt-sized particles, are slightly lower than those of low-activity clays.