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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Sadrekarimi, Abouzar

Abstract

The mechanisms through which mine tailings fail have been the interests of many studies recently. In this series of studies, the flow liquefaction and instability behavior of gold mine tailings were examined within the framework of critical state soil mechanics. In the first study, the behavior of a gold mine tailings in stress paths involving extensional consolidation or shearing in extension was evaluated. Monotonic tests were carried out on isotropically and Ko-consolidated samples to assess the effect of stress-induced anisotropy and mode of shearing on the static liquefaction characteristics tailings. In the second study, the drained instability of two gold mine tailings under lateral stress relief was investigated. Constant deviator stress (CDS) unloading tests were performed using a triaxial apparatus to simulate instability imposed by unloading in a drained condition. Several field failures have been associated to similar mechanisms. Different techniques were employed to determine the onset of instability in CDS tests and important affecting parameters were assessed. In the third study, the effect of the presence of different percentages of non-plastic fines on the static liquefaction behavior of a given gold mine tailings was examined. The variability of fines content during compaction of mine waste and deposition of tailings slurries, even in a single tailings impoundment, highlights the possible consequences of tailings’ gradation change and fine particles presence on the behavior of such materials. In the fourth study, the capability of a critical state-based model, NorSand model, in replicating the behavior of two gradations of a gold mine tailings was investigated. The purpose was to simulate liquidation susceptibility to tailings. The ability of the model was evaluated in a numerical model of a tailings dam.

Summary for Lay Audience

Mining industries are important to modern societies these days that provide vital mineral products and energy resources. Their by-product wastes, mine tailings, are usually stored in isolated conditions as deposits in impoundments under water behind dams to prevent them from exposure to atmospheric oxidation. However, these materials are prone to failure since they are usually deposited in loose and saturated conditions. Flow liquefaction is a known mechanism that can trigger these failures. To study the strength characteristics and behaviors of these materials, laboratory experimentation such as triaxial testing on reconstituted samples is a common method. However, these tests are usually carried out in a conventional way due to its simplicity and versatility, which includes consolidating samples isotropically (the simplest procedure of consolidation) and then loading (shearing) them in compression. In the first section, it is argued this is not necessarily the loading history applied on a soil element. The consolidation can be anisotropic, either by extension or compression, as well shearing, that can happen through extensive loading or compressive. These “stress-path” were applied on the tailings specimens and their behaviors were studied. Another triggering mechanics is lateral confinement relief. This can happen because of water table raise, snow melt, erosion, etc. As such, within the second part, special stress-paths were employed to simulate these conditions, and to study the instability behavior of tailings subjected to them. Also, particle segregation and separation can result in different gradations fabricated from a given mine tailings with different amounts of non-plastic fines. The difference in fines content can highly affect the behavior of these materials. Therefore, in the third part, this effect was studied through testing on different mixtures. Replicating the behavior of tailings using numerical methods, is crucial in design and analyzing geo-structures built from these materials. A critical state-based model called NorSand has been shown to perfectly capture the behavior of granular materials with liquefaction susceptibility. However, its capability in modeling tailings has faced challenges especially due to the distinctive magnitudes of parameters for tailings. In the fifth part, the feasibility of using this constitutive model in simulating tailings dams was assessed.

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