The effect of pH and the solids composition on the settling and self-weight consolidation of mud

A demand to use mud as fill material for land reclamations has emerged over the past years. Such reclamations start with pumping a muddy suspension into an area enclosed by dikes. The particles in suspension then settle to form a mud layer, so that the supernatant can be removed. This filling process may be repeated several times to completely fill the reclamation area. In 2016, an example of such a reclamation will be constructed in the Markermeer, the Netherlands. To make this work, it is essential to know the timescale of deposition and the properties of the formed soft mud layer. Formation of mud layers takes place in two steps. First, particles settle from a mud suspension and secondly, self-weight consolidation of a soft mud layer occurs. The goal of this thesis is to gain more understanding of factors influencing these two steps. More specifically, factors which are studied are the composition of the solid particles and the chemical composition of the water, together forming the mud mixture. Natural mud consists of different particles. Of these different particles, the clay particles are affected by the chemical composition of the water. This is because clay particles have a large specific surface area and surface charge, causing electrostatic interactions between them. The specific surface area and surface charge also mainly determine the plasticity of mud/soil. Under certain circumstances, clay particles flocculate, thereby forming flocs. Flocs settle more rapid than the individual particles, eventually forming a bed. Because these flocs contain large amounts of water, the formed beds are also very soft. The amount of water contained in the flocs is directly influenced by the size of the flocs (Winterwerp and van Kesteren, 2004) and the rate at which flocculation occurs (Mietta, 2010). The water content of the flocs also influences the settling and self-weight consolidation and thus, flocculation influences the formation of soft mud layers. From colloid chemistry, it is known that differences in the acidity of the water may influence the rate at which clay particles flocculate. To assess the impact of varying acidity on the settling and self-weight consolidation of mud, settling column experiments were set up in which the pH of the water was systematically varied. This was done for two artificial mud samples, i.e. a kaolinite and bentonite clay powder, and a natural clay sample, taken from the Markermeer. For the natural clay sample, the initial concentration was also varied. From the experiments, it can be concluded that there is an influence of pH on the settling and self-weight consolidation of mud, but mainly for samples with high to extremely high plasticities. For the kaolinite, a low plasticity mud, the effect of pH is rather limited. Hence, the influence of pH on the settling and self-weight consolidation of mud decreases with decreasing plasticity. This can be understood from a soil mechanical point of view: the factors determining the plasticity, namely the specific surface and surface charge, also directly determine the amount of electrostatic interactions between the clay particles. If the electrostatic interactions between clay particles in itself are small, their behaviour is also not noticeably altered by varying the suspension pH. For the natural sediment sample, containing large amounts of calcium carbonate, addition of hydrochloric acid is neutralized due to dissolution of calcium carbonate. However, the dissolution of calcium carbonate also leads to a release of Ca2+ ions, which also has a profound impact on flocculation. Because of the large neutralizing capacity of both the fresh water and the natural mud sample, no large changes in acidity are expected in the Markermeer environment, in spite of the presence of pyrite in the Markermeer bed. This confirms findings by Saaltink et al. (2016). The mineralogy of the present clay minerals exerts a much larger influence on the settling and self-weight consolidation process than suspension pH, which is reflected by large differences in gelling concentration and consolidated volumes between the three different mud samples. Self-weight consolidation marks the transition between fluid and soil mechanics. In fluid mechanics it is common to use the concentration or density to characterize mud layers. However, it is the relative water content of a suspension or mud layer, rather than the absolute water content (cf. density), that determines its settling and self-weight consolidation behaviour. This relative water content is given by the Liquidity Index, that is obtained by normalizing the water content of a suspension or muddy layer by its Atterberg Limits. Using the Liquidity Index enables us to compare different types of mud. The gelling concentration, a state parameter which is often used in cohesive transport modelling, is also related to a material property of mud, namely the Plasticity Index. A power law relation is proposed relating the gelling concentration to the Plasticity Index. Hence, a first estimate of the gelling concentration for different mud samples can be computed based on the Atterberg Limits of a mud sample. The undrained shear strengths of the consolidated samples, obtained through a shear vane test, are related to the respective average Liquidity Indices of these beds. For the natural clay, the Liquidity Index of the settled beds appeared to give a good indication of the undrained shear strength, and shows a fair resemblance with results obtained by Locat and Demers (1988) and Houston and Mitchell (1969). Summarizing: The suspension pH influences the settling and self-weight consolidation of mud, in particular for mud with high to extremely high plasticities. This can be attributed to the surface charge and specific surface area of the clay particles. Furthermore, it is found that the Atterberg limits are valuable material parameters, also from a fluid mechanical point of view. These Atterberg Limits can serve as a tool to predict the settling and self-weight consolidation of soft muddy layers.