The influence of desiccation and overconsolidation on monotonic and cyclic shear response of thickened gold tailings

Hyunseung Kim, Farzad Daliri, Paul Simms, Siva Sivathayalan

In the proceedings of: GEO2011: 64th Canadian Geotechnical Conference, 14th Pan-American Conference on Soil Mechanics and Geotechnical Engineering, 5th Pan-American Conference on Teaching and Learning of Geotechnical Engineering

Session: Mining and Rock Mechanics

ABSTRACT: : Regulators are naturally concerned with remobilization of unbounded deposits of mine waste residuals, such as thickened tailings stacks. Consequently, the monotonic and cyclic behaviour of thickened tailings is of significant interest to practitioners of mine waste geotechnique. Thickened tailings are known to gain strength through a combination of hindered settling, desiccation, and consolidation post-deposition. This paper reports on the effects of over-consolidation ratio (OCR) and desiccation, using a simple shear apparatus (NGI type). Samples were alternately overconsolidated by either mechanical loading, or by desiccation followed by rewetting and consolidation. Mechanical overconsolidation highly influenced the monotonic and cyclic resistance of thickened tailings specimens and both monotonic and cyclic resistance exponentially and continuously increased with increasing OCR for desiccated tailings, the relationship between increasing OCR and increasing cyclic resistance is more complicated and, surprisingly, could be converse with excessive desiccation beyond the shrinkage limit.

RÉSUMÉ: Des régulateurs sont naturellement concernés par la remobilisation des dépôts illimités des résidus miniers, tels que les piles des résidus épaissies. En conséquence, le comportement monotone et cyclique des résidus épaissis est intérêt significatif aux praticiens du geotechnique. Des résidus miniers épaissis sont gagner la puissance par une combinaison de sédimentation, de la dessiccation, et de consolidation. Ce document rend compte des effets du rapport d'au-dessus-consolidation (OCR) et de la dessiccation, utilisant un appareil de cisaillement simple (type de NGI). Des échantillons sur-consolidés alternativement par le chargement mécanique, ou par la dessiccation suivie de ré-humidification et de consolidation. OCR mécanique influence fortement la résistance monotones et cyclique des spécimens et résistance monotones et cyclique exponentiellement augmentation avec l'augmentation d'OCR mécanique. Pour les résidus miniers, le rapport entre augmenter OCR par dessiccation, et augmenter la résistance cyclique est plus compliqué et, étonnant, pourrait être inverse avec la dessiccation excessive au delà de la limite de retrait. Key words: Thickened tailings, cyclic resistance ratio, desiccation, overconsolidation, simple shear test 1 INTRODUCTION To remove or reduce reliance on dams and minimize the risk of dam failure, an increasing number of mines are employing thickened tailings technology. This technology dewaters the tailings to the point where they exhibit a yield stress, which allows the tailings to form gently sloping deposits. Thickened tailings deposition has been increasingly used since 1970™s (Robinsky, 1999). It is known that thickened tailings may gain shear strength through desiccation and self-weight consolidation under subsequent layers of deposition (Simms et al. 2007, Simms and Grabinsky, 2004). The increase in density due to desiccation not only decreases the total volume of the stack layers, but also likely increases the resistance of tailings stack to earthquake loading. Therefore, stress history in terms of matric suction as well as mechanical loading may influence the cyclic resistance of the tailings. Figure 1 shows a possible volume-stress history of a thickened tailings layer in a cyclic deposition scheme, which initially shrinks due to drying by matric suction, is subsequently rewetted by the placement of an additional layer, and then undergoes consolidation as deposition continues. It has been suggested that if thickened tailings are dried to their shrinkage limit, that they will not exhibit flow liquefaction, and so pose minimal risk of significant deformation during seismic events (UNEP and ICOLD 2001). However, there has been only limited experimental evidence to back up this supposition. Al-Tarhouni et al. (2011) investigated the influence of desiccation on cyclic behaviour of thickened gold tailings. In their study, samples were allowed to desiccate to various water contents at and below the shrinkage limit. The samples were then rewetted to near saturation before consolidation to a relatively low confining stress (50 kPa) and cyclic testing in a simple shear device. When compared to samples that were normally consolidated to 50 kPa, the desiccated samples did show a substantial increase in cyclic resistance. Nevertheless, sizable deformations would still be possible through cyclic mobility and post-cyclic strains, for a sufficiently strong earthquake. The present paper continues the line of investigation started in Al-Tarhouni et al. (2011). In this paper, a

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Cite this article:
Hyunseung Kim; Farzad Daliri; Paul Simms; Siva Sivathayalan (2011) The influence of desiccation and overconsolidation on monotonic and cyclic shear response of thickened gold tailings in GEO2011. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GEO11Paper765,author = Hyunseung Kim; Farzad Daliri; Paul Simms; Siva Sivathayalan ,title = The influence of desiccation and overconsolidation on monotonic and cyclic shear response of thickened gold tailings,year = 2011}