Determining effective stress and permeability equations for soft mud from simple laboratory experiments

Applying models to describe the self-weight consolidation of soft mud soils requires specification of constitutive equations for the vertical effective stress and permeability. A wide range of constitutive equations exist, each involving soil-dependent coefficients. This paper presents a method for determining the constitutive equations for soft mud from low-cost and easy-to-do laboratory settling column experiments. The method assumes the validity of Gibson's consolidation equation and of the power law constitutive equations proposed by Merckelbach & Kranenburg. Analytical expressions, which take into account partial segregation that may have occurred during the suspension phase of the experiment, were derived for the initial settlement of the mud–water interface as a function of time and for the volume fraction of solids as a function of the depth below the mud–water interface when consolidation is (almost) completed. Determining the coefficients in the constitutive equations with these expressions requires only recording the interface height and a single density measurement at the end of the consolidation process. In addition, an expression for the final bed height is given. This expression can be used as an alternative for the expression for the volume fraction of solids as a function of the depth below the mud–water interface if no density measurement is available. The methods were applied to a data set of settling column experiments with a natural mud from the Dollard tidal basin (The Netherlands). The results were compared with conventionally determined constitutive equations for effective stress and permeability. It is concluded that the coefficients that are determined by new methods compare well with the coefficients that are conventionally determined, even if significant segregation occurred during the suspension phase of the experiment.