A geotechnical investigation of freeze-thaw dewatering of oil sands fine tailings

The production of synthetic crude oil from Alberta's oil sands has resulted in the accumulation of 350 million cubic metres of mature fine tailings (MFT) in tailings ponds. The containment structures required to store the MFT are a direct incremental operating cost and are a significant factor in the development of an environmentally acceptably reclamation plan. Previous research has demonstrated that freeze-thaw is effective in releasing the water trapped in the MFT, thereby reducing its water content and increasing its strength. This dissertation investigated the mechanisms responsible for the release of water due to the freeze-thaw process. These mechanisms were investigated in both laboratory experiments and a large scale field experiment conducted at the Suncor oil sands plant.;The laboratory investigation demonstrated that freeze-thaw overconsolidates the MFT as the freezing front advances and suctions are created when water flows to the vertical and horizontal ice lenses which form a three dimensional reticulate ice network surrounding blocks of overconsolidated MFT. During thaw the remnant ice fissures provide channels for fluid flow. The microfabric of particles was observed to change from an edge to face flocculated, disaggregated cardhouse fabric to a compact, aggregated structure. The latter microfabric retains less water which accounts for the significant increase in solids content. The altered fabric reduces the MFT compressibility and increases its hydraulic conductivity as high as 1000 fold.;The Suncor field freeze-thaw tests showed that the solids content of MFT was increased to as high as 70% with an accompanying settlement of 60% provided that release water was drained from the surface. Analyses suggested that freeze-thaw increased the solids content from 30% to around 55%. If the surface of the thawed MFT was allowed to dry, desiccation and seepage consolidation formed a crust up to 0.3 m. Under these conditions drying was calculated to reduce the volume by 20% and increase the average solids content from 55% to 70%.;Analytical procedures were capable of estimating freeze-thaw settlement and solids content provided a reduced compressibility curve was used. A simple model was used to estimate the settlement and solids content increase associated with drying. A finite strain freeze-thaw consolidation model overestimated the excess pore pressures because it did not properly account for thaw consolidation.