Thixotropic strength measurement of oil sand fine tailings

Oil sands fine tailings is a waste by product of synthetic crude oil production from the oil sands mining operations located in northern Alberta. The fine tailings is a high water content slurry, consisting of water, silt, clay, and bitumen, that consolidates slowly. One of the factors that may affect self-weight consolidation of the fine tailings is thixotropic gain in strength of the material. To arrive at a comprehensive long-term consolidation model of the fine tailings, the time-dependent strength properties of the material need to be understood. Cavity expansion tests have been developed to measure the thixotropic gain in strength in the oil sand fine tailings. Five different water content fine tailings were tested at time intervals ranging from 0 to 450 days. The thixotropic strength increased quadratically with the age of fine tailings and was still increasing at the end of the ten period of 450 days. Substantial thixotropic strength increases were seen for fine tailings with water contents less than 150%. When the water content decreased from 400 to 150%, the thixotropic strength gain was smaller compared with that from a water content decrease of 150 to 100%. The thixotropic strength ratio was at a minimum at 150% water content. The water content of the fine tailings significantly influenced the thixotropic strength development. The lower the water content, the higher the thixotropic strength. Results obtained from cavity expansion tests were compared with those obtained from vane shear and viscometer tests. The change in strength for the three test procedures followed similar trends, but the cavity expansion tests grave substantially higher strengths.