Experimental study on consolidation behaviour and shear strength gain for saturated/unsaturated treated fluid fine tailings

• Author / Creator

  Bereket Tsegai Fisseha

• Mining industries face enormous challenges during handling and managing hydraulically placed soft tailings such as fluid fine tailings (FFT) throughout the operations and closure period. These soft deposits require sufficient bearing capacity along the profile that can support machinery and cover materials (i.e., sand capping) for the specified period. The presence of unsaturated zones, and its influence towards an increase in effective stress for the soft FFT has been theoretically defined and established as integral part of soil mechanics principles.

  A new meso-scale column apparatus was designed, constructed, and assembled to carry out the consolidation behaviours of deposited FFT using increments of applied suction method. The column apparatus was designed to mimic field condition by allowing process of desaturation of deposited FFT due to dewatering process using applied suction method. The column apparatus was designed to incorporate variations of moisture flux that frequents near the ground surface due to either rainfall or evaporation using the desaturation or wetting processes. The process of consolidation and desaturation due to applied suction were aligned with shear strength measurements for the FFT materials under investigation. The presence of negative pore-water pressure (suction) contributed towards an increase in effective stress distribution calculation.

  Experimental tests were carried out to investigate consolidation and shear strength behaviours of treated fluid fine tailings (FFT) using newly developed column apparatus and vane shear measuring equipment. The column apparatus used increments of applied suction through its bottom boundary to characterize the consolidation behaviours of FFTs. The FFTs constitutive relationships during the consolidation process were defined using independent stress state variables such as effective stress, suction and void ratio. Shear strength of the FFTs were characterized using vane shear measuring device simultaneously while the consolidation process progressed. The results provided a basis to establish the constitutive relationships of FFTs in terms of stress–strain states. In addition, the column apparatus attempted investigating formation of interlayer or transitional zone under controlled environment as the process of saturation/desaturation developed to simulate effects of evaporation during field trials.

  Two treated FFTs were used to conduct the experimental work, namely flocculated FFT and flocculated centrifuged tailings cake (FCTC).

  Results of settlement, change in void ratio, rate of excess pore pressure dissipation, suction distribution within the deposited FFT profile and developed shear strength during the experimental investigation were monitored, and analysed. Plausible agreements were found between characterization for the FFTs between standard laboratory and meso-scale test apparatuses.

  During the self-weight consolidation results of FFTs both from the experimental work were agreed with results predicted using numerical modelling exercises. In addition, the experimental results were in good agreement with results reported in literature for FFTs. The results of consolidation using applied suction (isotropic method) provides 30% higher strain rates compared to the consolidation using step load method (k0-loading). The numerical modelling process for the consolidation using applied suction stage show further refinement to achieve credible agreement. Undrained shear strength measurement for the FFTs show linear correlation with the corresponding effective stress providing an average slope of 0.27 to 0.37 for both flocculated FFT and centrifuged FFT. The linear correlation shows FFTs behaviour to natural soils such as clays. Higher undrained shear strength can be obtained due to drying and desiccation process of tailings, by initiating the development of higher negative pore pressure on the surface of the tailings. In addition, effects of scaling were compared between standard laboratory testing and meso-scale testing. The results were evaluated, consistent and agreeable results were observed between the standard and meso-scale testing.

  In conclusion, the research study provides significant contributions in designing, assembling and developing the column apparatus, which can be used to characterize FFTs and soft tailings to mimic effects of moisture fluxes being observed near the surface during field condition. These research findings can be further studies by applying field scale experiments. Furthermore, the findings from the experimental research investigation can be integrated in versatile numerical modelling software to analyze and refine the saturated/unsaturated behaviours of deposited FFTs which are central during the design of mine waste structures.

• Subjects / Keywords

  • Consolidation of fluid fine tailings
  • Saturated/unsaturated fluid fine tailings

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-swaj-6e48

• License

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