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Thesis Format

Integrated Article


Doctor of Philosophy


Civil and Environmental Engineering


Abouzar Sadrekarimi


Canada has the world’s third-largest crude oil reserve, with an estimated recoverable oil reserve of 171 billion barrels (10.3% of world proven oil reserve). However, this is primarily stored in oil sands deposits and should be extracted through mineral processing. The remaining tailings following the extraction of bitumen from oil sands consists of sands, fine particles, water, and residual bitumen, which is deposited behind a tailings dam. Tailings dams are particularly susceptible to liquefaction and exhibit a strength loss when loaded without allowing their volume to change in loose saturated conditions. A tailings dam must withstand both static and seismic loads to remain stable for a very long time. Lateral stress relief and effective stress reduction also have been recognized as the culprits in flow failures of several mine tailings dams and can be simulated in the laboratory as drained constant shear unloading (DCSU) tests. This research examines the instability of coarse oil sands tailings (CST) in both constant volume (CV) and DCSU stress path through a series of monotonic direct simple shear tests. A detailed investigation is carried out to observe the cyclic, post-cyclic liquefaction behaviour and dynamic properties of coarse oil sands tailings (CST). A tailings dam is made up of coarse sands tailings (CST) and fluid fine tailings (FFT), which can be mixed in various quantities and have significantly different properties than either CST or FFT. Laboratory specimens were prepared using moist-tamping (MT), dry deposition (DD), and slurry deposition (SD) techniques to investigate the effect of sample reconstitution method and tailings fabric on CST’s instability behavior. The effects of consolidation stress, void ratio, initial shear stress, and unloading rate on the drained instability of CST specimens were also investigated. The effectiveness of a naturally driven biological process known as microbially induced calcite precipitation (MICP) on the monotonic and cyclic shear response of oil sands tailings is also assessed. Sporosarcina pasteurii is employed to precipitate calcite (CaCO3) by its metabolic action to alter the engineering properties of CST.

Summary for Lay Audience

Many tailings dams have failed in recent decades due to a phenomenon known as liquefaction, in which saturated loose sands and silts lose strength and behave a flow like liquids when subjected to static and cyclic loads. Canada has the world's third-largest crude oil reserves, and the extraction process of bitumen generates massive amounts of waste slurry, which is typically dumped in tailings dams. These tailings comprise of coarse sands tailings (CST), fluid fine tailings (FFT), water and residual bitumen. To explore the liquefaction behaviour of oil sands tailings, a series of monotonic and cyclic direct simple shear tests were conducted. In addition, the efficiency of a soil strengthening treatment that relies on a specific metabolic activity of specific bacteria is assessed for the mitigation of liquefaction-induced strength loss.

The undrained static liquefaction behaviour of CST was investigated through direct simple shear tests, which involved shearing the specimens without allowing any changes in volume to occur. The strength obtained from these tests were correlated with field measurable shear wave velocity. The shear wave velocity was measured using a pair of bender elements, which is the velocity at which small-displacement shear waves travel through the specimens. The instability of this CST was also investigated considering that it has already a constant static load (representing slope of a tailings dam) and subsequently sheared as a results of overburden pressure reduction caused by lateral extrusion, water table rise, and heavy rainfall. The resistance of this CST subjected to cyclic loading was also investigated with or without considering the volume changes. The dynamic properties (shear modulus and damping ratios) are the required parameters for designing of tailings dam subjected to dynamic loading such as earthquakes and vibrations, were also assessed. The CST and FFT are blended in varying percentages during deposition, resulting in a distinct liquefaction behaviour than the CST and FFT alone. Hence, the static and cyclic liquefaction behaviour of the CST-FFT mixes was also investigated. Finally, the effectiveness of a strengthening treatment including the byproducts of a specific metabolic process of certain species of bacteria in enhancing CST resistance through soil particle carbonate bonding.

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