Consolidation and drying of slurries

Abstract

The Marker Wadden project aims to improve the ecosystem of Lake Markermeer (The Netherlands) by constructing a wetland with sediment from the lake. Sediment is dredged from the bed, and the resulting slurries are pumped into the project area. During this process, segregation and oxidation of the sediment may occur. The native sediment composition and changes induced by the construction process affect the mechanical behavior of the wetland. The initial stress state of the sediment is another variable affecting the behavior. Over time, vegetation may colonize the wetland, also influencing the mechanical properties of the sediment. These factors were studied in physical and numerical experiments as part of this thesis. First, consolidation experiments in settling columns at low initial concentrations below the gel point (virgin consolidation) were performed, and the material parameters were obtained. These parameters were different from the parameters obtained from the Seepage Induced Consolidation (SIC) test because of over-consolidated initial conditions induced by mixing. Numerical simulations were performed with a 1DV consolidation model to quantify the effect of over-consolidation and material parameters on the consolidation behavior. Incremental Loading and Constant Rate of Strain (CRS) tests were performed to analyze the compressibility behavior, and Fall Cone tests were used to determine the undrained shear strength. The fractal theory was found to be a useful tool to normalize and identify the different behavior of samples across all tests. The drying behavior was analyzed with using a Hyprop test, and the Soil Water Retention Curves obtained were fitted with a van Genuchten model. The model parameters were found to be more influenced by the type of organic matter than by its total amount. Finally the effect of Phragmites australis (i.e. common reed) on the consolidation and drying was assessed with a newly-designed column device. The reed acted as an ecological engineer, draining the sediment. However, no differences in the thickness of the sediment layer were found presumably because of armoring by roots. The general conclusion is that over-consolidated initial conditions can be induced by different processes such as mixing and atmospheric drying. Furthermore, the composition of the sediment may change when exposed to segregation and oxidation. In particular, the type of organic matter affects the mechanical behavior of fine sediment at all stages (settling, consolidation, drying) and needs to be characterized. Consequently, the material parameters need to be determined for actual project conditions.