In Alberta (Canada) bitumen is extracted from the mined oil sands ore by a process that uses hot water. This process produces large amounts of tailings, which is a mixture of water and sediment particles. These tailings are stored in large ponds to allow the sediments to settle out of suspension. The water is recycled in the hot-water process and the solids are used for land reclamation in the mine site areas that have been mined out. The fine particles in the tailings settle very slowly so companies are trying to find ways to accelerate the dewatering process. A dewatering process consists of three phases: sedimentation, consolidation and ripening when the sediment layer is exposed to air. Samples of the tailings were shipped to Delft University of Technology to perform dewatering experiments. From one of the first experiments it was suspected that the moisture content of the samples was already too low for the sedimentation phase to precede the consolidation phase. The goal of this project was to find out if there is a relation between the moisture content at the transition between the sedimentation and consolidation phases and the liquid limit and liquidity index of the soil. To be able to compare the results with another type of soil, the same tests were performed on river clay.
Three sedimentation columns with different moisture contents were prepared for each of the materials and standard tests (BS 1377) were performed on the soils to determine the liquid and plastic limit from which the liquidity index was derived. To further characterize the materials, hydrometer tests were performed. From the British soil classification chart was deduced that the thickened tailings were to be classified as an intermediate plasticity clay and the river clay as a high plasticity clay. The major conclusion that may be drawn from the test results is that the liquidity index at the boundary between sedimentation and consolidation for both materials is around 18 and 19 for both soils. Furthermore the moisture content at the transition from the sedimentation phase to the consolidation phase is around ten times higher than the liquid limits of the materials. Another conclusion is that the consolidation phase shows no acceleration on a logarithmic time scale when it is preceded by the sedimentation phase, where this acceleration is visible when the sedimentation phase does not occur.