Abstract:

The Kiena Mine, located west of Val d’Or, Quebec, was operating between 1984 and 2013 with mining operations ceasing in 2013. Slurry tailings was deposited into a Tailings Storage Facility (TSF) which is contained by perimeter dykes and flanked by Lac de Montigny, Petit Lac, CN railway tracks and Hydro-Quebec transmission lines. The TSF perimeter dykes comprised of initial phases of downstream raised embankments followed by later phases of upstream raised embankments. Subsurface soil conditions at the TSF are characterized by grey silty clay stratum underlain by a clayey silt stratum and till. As part of restart following the discovery of high-grade gold zone below the existing mine infrastructure, several tailings management options were assessed. Upgrading existing TSF to current dam safety requirements was chosen as preferred option. Significant upgrade requirement was to improve slope stability of the perimeter dykes. It was challenging due to several factors including, thick and soft silty clay to clayey silt foundation, close proximity to lakes and infrastructures and upstream raised configuration.

Extensive site investigation was performed which involved, borehole drilling and sampling, Nilcon Vane testing, CPT testing, and laboratory testing including cyclic direct simple shear testing on tailings and soft silty clay materials. A number of remediation options were assessed with due consideration for technical, economic and proximity to lake, infrastructure, etc. These upgrade options included combinations of downstream toe buttress, ground improvement and crest lowering. Crest lowering was found to be not effective and was ruled out. The downstream toe buttress was found to be cost effective, but not feasible or preferred for dykes segments with environmental and/or infrastructure constraints.

Optimized upgrade option for a dyke segment involved downstream toe buttress where possible and ground improvement combined with toe buttress. Deep Soil Mixing (DSM) was chosen as the preferred ground improvement method considering site conditions. Design analyses included liquefaction and cyclic softening assessments, slope stability and seepage analyses, and seismic deformation analysis using Flac. Laboratory bench scale testing was performed to establish soil-cement-water mix ratio for DSM.

Installation of DSM columns along the perimeter dykes’ toe was the most critical part of the TSF upgrade construction. Performance of the perimeter dykes during installation of DSM columns was monitored using Inclinometers, VW Piezometers and Survey Monuments. Pilot DSM columns installation program was performed to adjust and/or ensure mix design and quality control. Production of DSM columns started in September and complete in November 2021. The work included installation of 454 DSM columns over a total length of about 1,200 m of perimeter dyke segments. Quality control and assurance testing included specific gravity, viscosity and UCS testing, and sonic coring.

This presentation describes the Kiena TSF, analyses, upgrade design and construction with the focus on the challenges encountered during the design and construction phases. Selection of ground improvement option (DSM), bench scale mix design for DSM and installation of DSM columns are also discussed. Measures taken to optimize the upgrade design to minimize TSF dykes footprint and respect environmental constraints and requirements are highlighted.

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M. Guillaume Angers