Rheological Properties of Cohesive Sediment Suspensions

In order to predict the stain and shear rate response of cohesive sediment suspensions to applied forces, their rheological properties have to be known. In this report the rheological properties of both China clay, an artificial mud, and Caland channel mud, a natural mud, are investigated. Four types of experiments are involved: equilibrium flow curves, stepwise changes in shear rate to investigate the time-scale of structural changes, harmonic oscillation experiments to monitor the visco-elastic behaviour of mud at low strains and experiments in which the increase in yield stress with rest time after pre-shearing was measured. For these measurements both shear rate and shear stress controlled instruments were used. Cohesive sediment suspensions exhibit strongly non-Newtonian and time-dependent behaviour caused by particIe interactions, which makes interpretation of experimental results complicated. Even for simple and well-defined systems of particIe suspensions it is difficult to deviate macroscopie properties from microscopie interactions, therefore all experimental results reported here are explained from a macroscopie point of view, adopting a thixotropie model including a structural parameter to account for structural changes within the material without bothering about the exaet type of interactions involved. The equilibrium flow curves revealed strong shear-thinning behaviour caused by structural changes. From the experiments with stepwise changes in shear rate can be concluded that the time-scale of these changes is in the order of seconds. As the samples were constantly sheared during these experiments, the presence of an interconnected aggregate structure within the samples is unlikely. Experiments in which the increase in yield stress with rest time after preshearing was measured showed that the time-scale for the build-up of such an interconnected aggregate structure is in the order of 10^4-10^5 s, much larger than structural changes within sheared samples. From harmonic oseillation experiments can be concluded that the range within which muds exhibit lincar visco-elastic behaviour is limited to strains smaller than approximately 1%