| Popis: |
This paper deals with the hindered settling of highly concentrated mud-sand mixtures. The suspension's settling characteristics and the settling velocity of the mud and sand fraction are determined. X-ray concentration profiles show that the mud-sand mixtures settle with only one interface, while mud-only suspensions at similar initial concentrations develop two interfaces during settling. This has a large influence on the resulting vertical concentration distribution. The disappearance of the lower interface is most likely caused by the disturbance created in the suspension by the settling sand grains. The suspensions' settling velocity is in the order of 0.02–0.3 mm/s, while the sand settling velocity, measured with particle tracking velocimetry and particle image velocimetry techniques, is in the order of 1–3 mm/s. The sand is strongly hindered by the presence of the highly concentrated mud suspension, and its settling velocity is strongly decreased compared to the Stokes' settling velocity of 10 mm/s. Once the suspension starts to gain strength, the sand can become arrested in the mud matrix and settle with the suspension's settlign velocity. However, we also observed that the sand grains follow the dewatering channels generated by water expelled during consolidation of the mud fraction. In that case, the sand particles settle at a larger velocity deeper into the suspension or bed. We anticipate that there exists a subtle interaction in the dewatering channels where the expelled pore water flow with the settling sand grains and the sand pockets affect the overall settling/consolidation behaviour of the mixture. |
