Laboratory experiments on consolidation and strength evolution of mud layers

Many harbours in the world suffer from high siltation rates in their basins. To guarantee safe shipping, harbour authorities have to maintain the navigable depth by dredging large amounts of mud. Some authorities relate the navigable depth to the depth at which the density is equal to a certain value, e.g. 1200 kg/m3. However, the shear strength might be a more direct criterion to relate the navigable depth to. Presently, a research project, which is financed by The Netherlands Technology Foundation, is being conducted to develop a model that can be used to translate results from laboratory experiments to field conditions. To gain knowledge that is required for the model formulation, a first series of laboratory experiments was carried out at the University of Oxford and reported herein Continuous bulk density profiles were measured with a very accurate, non-destructive X-ray densimeter, which is available at the University of Oxford. At discrete levels the pore water pressure was measured. From these quantities accurate effective stress data and fairly accurate permeability data could be obtained. It turned out that both effective stress and permeability can be related to volume fraction of solids according to a power law. The concept of a fractal structure also implies power laws between the quantities mentioned. The fractal dimension obtained equals 2.71 ± 0.05. In contrast with the density and pore water pressure measurements, the vane tests are destructive. In order to study the evolution of yield stress in time, the three columns were started under the same initial conditions. After 8, 13 and 24 days, a constant shear rate vane test was carried out. The peak shear stress, calculated from the torques measured, shows a more or less linear relationship with the effective stress, and apparently exhibits a yield stress at zero effective stress, the so-called true cohesion. Because of the linear relationship with the effective stress, the peak shear stress reduced by the true cohesion can also be described by a power law holding for a fractal dimension of 2.71. The results of this study indicate that the structural properties that determine effective stress, permeability and peak shear stress, can be characterized by a fractal dimension