Measurement of the shear strength of centrifuged oil sands tailings using different soil mechanics procedures

Shear strength is an important engineering property in the design and closure of tailings dams. The stability of a tailings dam depends upon the strength of the tailings. Alberta oil sands tailings dams have a footprint much larger than typical metal mines, averaging 4 km2, with the largest near 15 km2. Approximately 1.3 billion cubic metres of mature fine tailings (MFT) have been stored in these tailings dams. Significant dewatering and treatment of the MFT are required to achieve solids contents and strengths sufficient to support reclamation. Centrifugation is currently one of the potential technologies for converting MFT into a material with sufficient strength to support trafficability. In this research study, a large strain consolidation test (LSC) with shear strength, consolidated undrained triaxial test (UD-TR), consolidated drained direct shear (D-DS) test and a single-step Tempe cell test (SS-Tempe cell) were performed on an oil sands centrifuged tailings cake (Ccake) sample to measure the two shear strength parameters: apparent cohesion (c) and effective friction angle ('). The UD-TR and D-DS techniques provided comparable ' values of 20.7° and 22.9°, respectively, and this is because remoulding soil destroys any previous layering or structure that might have been present, and the resulting sample is more or less homogeneous. The LSC provided a lower value of ' of 17.8° due to the use of the vane shear device in the LSC test that destroyed the fabric and decreased the value of '. The SSTempe cell produced a non-linear shear strength envelope due to suction that increased ' (26°) and shear strength of the soil sample. The remoulded and consolidated C-cake sample tested in this study has a (c) of zero. The values of (c) and ' obtained indicated that the C-cake sample tested was very soft clay. Although these four different devices can provide similar shear strength parameter results, they have marked differences in complexity and cost. The DS and SS-Tempe cell devices are simple and cheap. The TR and LSC devices are complex, expensive and time-consuming. However, the UD-TR has the advantage over the other test methods in that specimen drainage can be controlled, pore pressure can be measured and the results are accurate. The results of the two strength parameters are crucial in any stability analyses of slopes against failure and landslides.