Influences of flocculation on bed properties for fine-grained cohesive sediment
Keywords: consolidation
clay

An understanding of the behaviour of newly deposited soil is important because of its direct applications in fields such as harbour siltation and storage of dredge slurries. In coastal regions the dominant mode for deposition of fine-grained cohesive material is through flocculation- the electrostatic aggregation of mud particles. The present study focuses on the measurement of both flocculation conditions and bed properties, towards the understanding of the influence of the former on the latter. A computerised laboratory setup, complete from sedimentation and flocculation to the long term consolidation allows each of these processes to be observed in instrumented perspex columns without the many variables present in an estuary. The bed deposition rates resemble those found in natural estuaries, and therefore this technique is a much improved method for examining natural processes than the method of slurry deposition, which is normally employed in soil mechanics. An image acquisition and analysis system has been designed specifically for this work. Image sequences are analysed in real time to give information about the dimensions and concentration of the particles, and the particle velocities. Floc sizes increase as the particle concentration increases in the column. Increasing the concentration further leads to an overall hindering of the sedimentation rate and breakup of the flocs. Images of beds formed through slow sedimentation clearly show aggregate features that are not present in slurry experiments. From low to medium sedimentation rates the bed height, normalised by the mass of sediment in the bed, increases. From medium to high rates of sedimentation, however, the normalised bed heights decrease. Overall slurry experiments have lower bed heights than slowly deposited experiments. Image analysis of the bed surface (top 0.5mm) has revealed that aggregates which are present immediately after deposition are broken down over tens of hours. The ISIS instrument has been modified to gather information about the resistance to erosion of the beds. A bed is more easily eroded after it has had a long period of consolidation. X-ray density measurements and imaging techniques are used to link the strength of the bed to biological factors. X-ray bulk density and pore pressure measurements allow calculations of void ratio, porosity, and effective stress. Bender element apparatus has been constructed to measure shear stiffness of the soil, and bed strength measurements are made using a shear vane. It is found that variations in the sedimentation conditions have significant effects on bed density and on void ratio. These effects continue to be visible in the consolidated bed, even after an order of magnitude increase in the total vertical bed stress (equivalent to the range of metres of overburden pressure). The variations in the bed structure can have profound effects on bed strength. Methods to compare floc measurements to bed measurements are complicated by the inherent variation in floc data. Possible techniques include the use of solids volume fraction, fractal dimension and intrinsic sediment properties. The usefulness of each of these is assessed. It is suggested that it is the frequency at which the bed is bombarded by flocs, and not the properties of the flocs themselves, that largely determines the bed properties.