Upgrade of a Tailings Dam using Ground Improvement

Samuel Briet, Jordan Hunt, Jeff Scott, Hubert Guimont, Gilles Costa

In the proceedings of: GeoEdmonton 2018: 71st Canadian Geotechnical Conference; 13th joint with IAH-CNC

Session: Soil Mechanics and Foundations (Ground Improvement)

ABSTRACT: This article represents a case-study of the upgrade of a nickel tailings dam in Thompson, Manitoba. In order to maintain the utility of the dam, and keep the tailings covered by water, a significant raise of the existing dam was required. As part of the upgrade process, two forms of ground improvement were carried out: Rapid Impact Compaction and a Cement Bentonite Wall. The initial dam raise consisted of the placement of two waste rock berms. A slag material was advanced between the rock berms, placed by truck and bulldozer. In consideration of the loose state of the slag, Rapid Impact Compaction was used in order to densify the material in order to improve the overall stability of the dam. In order to reduce the permeability of the dam, a cut-off wall was required. As an alternative to a traditional cut-off wall such as sheet piling, a Cement Bentonite (CB) Wall was implemented. A 1 meter wide cut-off wall with the required hydraulic conductivity properties was installed within the dam, keyed into the low permeability clay layer below. This article further describes the particularities of the project including its geotechnical conditions, the design parameters, the ground improvement techniques implemented, as well as the testing and other quality control measures in place.

RÉSUMÉ: Cet article représente une étude de cas sur l™amélioration d'un barrage existant de résidus de nickel à Thompson, dans le Manitoba. Afin de s™assurer que le barrage demeure fonctionnel et les résidus submergés, une élévation significative du barrage existant s™avérait nécessaire. Deux techniques d'amélioration des sols ont donc été mises en œuvre: la Compaction à Impactes Rapides et une Paroi Étanche en ciment bentonite. L™élévation initiale du niveau a été réalisée par la mise en place de deux bermes rocheuses. Un matériau scorie, mis en place entre les bermes rocheuses au moyen de camions et bulldozers, était constitué majoritairement de fines. Compte tenu de son état lâche, la Compaction à Impactes Rapides a dû être utilisée pour augmenter la densité et améliorer la stabilité globale de l™ouvrage. Par ailleurs, une coupure étanche était nécessaire afin d™assurer l'étanchéité du barrage. Comme alternative à un rideau de palplanche traditionnel, une Paroi Étanche en ciment bentonite a été choisie. Cette paroi peut être assimilée à un mur de 1 mètre de large, exécutée au sein du barrage, et permettant d™obtenir les propriétés de conductivité hydraulique désirées. Cet article décrit les particularités du projet, en l™occurrence ses conditions géotechniques, les paramètres de calcul, les techniques d'amélioration des sols ainsi que les tests et autres mesures de contrôle de qualité effectués. 1. INTRODUCTION 1.1 Project Description The third phase of a dam upgrade project in Thompson, Manitoba consisted of the raising of a tailings dam, Railway Dam. The tailings, a by-product of the nickel mining operations in the area, react with the air forming an acid run-off that may result in environmental issues. As a preventative measure, the designed solution involved raising the water level within the tailings pond area, covering the tailings and isolating them from the air, essentially halting the acid generating process. In order to accommodate the increased level of water, the existing dam needed to be raised, and seepage rates need to be reduced under the design hydraulic head differential. Two ground improvement methods were implemented in order to facilitate the raised dam™s construction: Rapid Impact Compaction and a Cement Bentonite Slurry Wall.

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