Assessing the strength and bearing capacity of tailings for oil sands reclamation
Creators: Paul, Abigail L
Publisher: name: University of Alberta Library
Year: 2024

<p>Reclamation of oil sands mines in northern Alberta presents a significant challenge for mine operators, particularly the reclamation of tailings deposits that are produced by the mine waste stream. A proposed approach to reclaiming tailings deposits as upland or wetland landforms is capping which involves placing material such as tailings sand or petroleum coke on the tailings surface. Critically, the underlying tailings deposit must have sufficient strength, density, and bearing capacity to support the cap as well as the equipment and personnel required to place it. Otherwise, equipment can “punch through” the cap into the underlying tailings, posing a significant hazard for the equipment and operator.</p>

<p>Clay minerals play a significant role in the challenging geoenvironmental behaviour of oil sands tailings, and therefore must be considered in the design and implementation of capped deposits. A well-established method for quantifying clay behaviour in geotechnical engineering is the Atterberg limits, which define the water contents for which clay will exhibit plastic behaviour. Atterberg limits can also be used to develop correlations between the liquidity index and remoulded undrained shear strength. Atterberg limits are currently used to characterize oil sands tailings, however, there are unique challenges to applying existing measurement methods to these materials compared to natural soils. There is also no relationship between remoulded strength and liquidity index for strong, dense tailings that are being targeted as capped deposits, though relationships exist for natural soils and fluid, low-density tailings. Current practice to evaluate deposits is to predict bearing capacity from peak undrained shear strength and apply an appropriate factor of safety.</p>

<p>A laboratory testing program and a review of existing published data was undertaken to investigate the Atterberg limits, strength, and bearing capacity of oil sands tailings. A series of Atterberg limits tests in which material properties, preparation method, and test procedure were varied were completed. It was determined that these factors influenced the measured Atterberg limits, though it was challenging to determine the effect of individual factors compared to the quantified variability of the tests. Air-drying the tailings from above the liquid limit to the plastic limit at low temperatures is proposed as a standard preparation method as this preserves the properties of the as-received tailings and is straightforward to perform. Atterberg limits and strength measurements determined in the test program were also used to determine a mathematical correlation between the remoulded strength and liquidity index of high-density tailings. Model footing tests at the benchtop scale demonstrated that existing methods of predicting bearing capacity from peak strength are appropriate. The sensitivity ratio was used to apply the proposed correlation between remoulded strength and liquidity index to the model footing results and propose a new method for predicting bearing capacity from index properties. The results of this research program support the idea that index properties such as Atterberg limits can provide a cost-effective method for long-term monitoring and the preliminary design of capped deposits. </p>