Effect of specimen preparation and mechanical overconsolidation on the laboratory monotonic shear response of thickened gold tailings

Ainur Seidalinova, Dharma Wijewickreme

Dans les comptes rendus d’articles de la conférence: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Mining Geotechnics I

ABSTRACT: The mechanical response of gold tailings is assessed using constant volume monotonic direct simple shear (DSS) testing. The shear stress-strain response and excess pore water pressure development was observed for reconstituted gold tailings specimens initial normally consolidated to vertical stress levels between 50 and 400 kPa. The behavior of the same material mechanically overconsolidated to overconsolidation ratios of 1, 4 and 8 were also investigated. The observed monotonic behavior is similar to that has been typically noted for clayey soils. The results from the testing reconstituted tailings were also compared with that observed by testing of relatively undisturbed specimens obtained from a thickened tailings mass generated by pre-depositing the same material in a laboratory chamber.

RÉSUMÉ: La réponse mécanique des résidus miniers d'or est évaluée en utilisant un appareil de cisaillement simple monotonique à volume constant. La réaction contrainte-déformation et développement des pressions interstitielles excès est observée pour des spécimens des résidus miniers d'or reconstitués qui ont étés consolidés normalement aux contraintes verticaux entre 50 et 400 kPa. Le comportement mécanique du même matériau qui été surconsolidé mécaniquement aux rapports de surconsolidation de 1,4 et 8 est aussi inquiété. Le comportement monotonique observé est similaire au celui noté typiquement pour sol argileux. Les résultats des essais sur des résidus miniers reconstitués sont aussi comparés aux ceux sur des spécimens relativement non dérangés obtenus des résidus miniers épaissis générés par pre-déposition de le même matériau dans une chambre de laboratoire. 1. INTRODUCTION Thickened tailings deposition method is an attractive alternative to a conventional slurry deposition method, where the latter has many disadvantages in terms of performance as a mine waste storage method (ICOLD and UNEP 2001). Thickened tailings deposition allows optimal tailings management practice by improving physical properties, obviating the need for dams, increasing storage capacity and decreasing operating and closure costs (Theriault 2003); in this, the tailings material is dewatered to a point where it does not have a critical flow velocity when pumped, does not segregate during deposition, and produces minimal water bleed (Robnisky 1999). After the deposition, the paste continues flowing until it reaches enough strength to resist flow, forming gently sloped stacks. It is common for paste tailings to flow to a slope of the order of 3 to 10 degrees. A number of studies have shown that desiccation and consolidation are the primary mechanisms for shear strength gain (Kim et al. 2011; Simms et al. 2007; Simms and Grabinsky 2004). Static liquefaction is one of the contributory mechanisms in tailings dam failures, and it has the potential to cause devastating impact on local economies and ecologies, and loss of human life. Understanding the mechanical response of mine tailings especially under constrained drainage conditions is critically important in this regard. Assessing and comparing the mechanical response of thickened tailings to other similar fine-grained geomaterials would also give better understanding of this material and also contribute to developing analytical tools to model it. A limited number of publications on fine-grained tailings related to cyclic and monotonic response are available (James et al. 2011; Kim et al. 2011; Simms et al. 2007; Wijewickreme et al. 2005; Simms and Grabinsky 2004). It is of relevance to note that the mechanical response (i.e., shear-induced excess pore pressure development, stiffness and strength characteristics) of tailings as well as other geomaterials is governed by many parameters such as void ratio, initial water content, confining stress, fabric and stress-history, etc. In this paper, the results of a laboratory testing program carried out on a gold tailings material are presented. The subject gold tailings were tested with the following main objectives: (i) assess the effect of stress level on the monotonic response of the tailings; (ii) compare the mechanical response of tailings specimens derived from laboratory deposition/desiccation with those obtained from laboratory reconstitution; (iii) assess the effect of mechanical overconsolidation (stress-history) on

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