| Popis: |
The formation of a cohesive sediment bed is a combination of settling and consolidation processes. These processes strongly influence the structure, properties and erodibility of the sediment bed. At low values of the bed shear stress, suspended sediments, individual particles or flocs, deposit onto the bed. Subsequently, during the consolidation process, the flocs and aggregates rearrange themselves to form a denser structure. The pore water, initially supporting the particles, is being expelled. In an extensive set of laboratory experiments the influence of sand on the settling and the consolidation of mud has been studied. The deposition of mud/sand mixtures has been closely followed in settling column experiments for different types of mud at different sand contents. During the experiments settling rates, density profiles and pore water pressures have been measured. The presence of a sand fraction in the initial suspension has a large impact on the bed formation processes. The results show that the heavier sand particles settle faster and form a separate layer at the bottom of the column as long as the mud does not form a continuous network structure that prevents this segregation. A continuous mud matrix is formed at low sediment supply rates or at high initial suspension concentrations, when the concentration of the mud fraction in the mixture exceeds the gel point density. The settling rates of the mud/sand suspension increase with increasing sand content. The local bed densities within the consolidating bed increase with increasing sand content of the initial suspension, even for the sand-free top layer. Sand addition also speeds up the consolidation process and permeability increases. However, these effects seem to be limited to a maximum sand content above which no additional effect is found. Using the experimental results, guidelines for modelling the settling and consolidation of mud/sand mixtures have been formulated. |
Full Text from ScienceDirect