Horizon Dyke 10 Geotechnical Design

Richard Sisson, W.K. Chan, Catherine Fear

In the proceedings of: GeoManitoba 2012: 65th Canadian Geotechnical Conference

Session: T1C - Design of Earth Dams

ABSTRACT: Canadian Natural Resources Limited (CNRL) has developed Horizon Oil Sands, an open pit mine, extraction, and upgrading facility, 80 km north of Fort McMurray, Alberta. The facility requires an External Tailings Facility (ETF) that is formed by a horseshoe-shaped dyke designated as Dyke 10. The dyke will be constructed entirely from local overburden from mine stripping and interburden, will have a maximum dyke height of about 64 m (locally, up to 71 m), and a crest length of about 13 km. Foundation conditions at the site typically consist of about 2 to 20 m of Quaternary Holocene and Pleistocene soils overlying Cretaceous Clearwater (clay shales) and McMurray (oil sands) Formations. The Clearwater Formation is a marine deposit that includes silty clay to clay shales with weak shear zones. The presence of these weak shear zones in the Clearwater Formation has been well documented at other oil sands projects and is known to have a significant impact on slope stability. Two key geotechnical considerations for the design of Dyke 10 are foundation stability considering low mobilized shear strengths in the Clearwater Formation, and internal seepage control measures considering potential core cracking and internal erosion (piping) of core materials. The observational method (Peck, 1969), a proven effective risk management tool in the design and construction of major tailings storage facilities worldwide, was adopted to address these considerations. The observational method was (RWC) conditions for stability and seepage considerations. To address foundation conditions, flat overall downstream slopes (12H:1V) were adopted for much of the dyke, along with a contingency for a stabilizing berm. An overburden waste area has been located along the downstream side of one portion of the dyke, and this will result in even flatter overall downstream slopes in this area. Because of the very flat downstream slopes of Dyke 10, dictated by the weak foundation, adopting a conventional design with a blanket drain along the base of the dyke would require large quantities of sand borrow, resulting in significantly high construction costs. Therefore, the horizontal drainage blanket was located some distance above original ground. Detailed analyses were performed to assess the risks associated with this novel design. A mitigation strategy was adopted including the potential for installing pumping wells in the filter. Construction has been initiated. The use of the observational method has allowed design optimization and modification during construction in response to observed performance.

RÉSUMÉ: Canadian Natural Resources Limited (CNRL) a développé le projet de sables bitumineux Horizon, une mine à ciel ouvert, 80 ktransport du minerai, ainsi que ceux de récupération et de valorisation du bitume. Un bassin de sédimentation des Digue numéro 10 (Dyke 10). La digue, construite essentiellement avec le stérile provenant de la fosse, culminera à une hauteur de 64 à 71 m km. Le sol supportant la digue consiste m recouvrant à leur tour des formations de schistes argileux de la période crétacé (Clearwater) et de sable bitumineux (McMurray). La formation argileuse de type Clearwater est un dépôt marin comprenant argiles limoneuses et schistes argileux caractérisé par des zones de faible résistance au cisaillement. La présence de ces faibles zones de cisaillement dans la formation de schistes argileux Clearwater a été largement documentée par les exploitants de sables bitumineux environnants, et est considérée comme ayant un impact majeur sur la stabilité des talus. Les deux principaux éléments de géotechnique considérés lors du design de la Digue numéro 10 sont la stabilité du sol sous-jacent, considérant la faible résistance au cisaillement mobilisée de la formation de schistes argileux Clearwater, ainsi que le contrôle interne des eaux considérant le fractionnement du noyau de la digue. La méthode observationnelle (Peck, 1969), un outil prouvé et efficace de gestion des risques dans la conception et construction de grands bassins de résidus miniers dans le monde entier, a été adoptée pour prendre en charge ces deux derniers éléments. La méthode observationnelle a été spécifiquement appliquée à la conception de la Digue numéro 10 en établissant les scénarios suivants: le plus probable, défavorable de la fondation, une pente de 12H:1V pour la partie en aval de la digue a été adoptée pour la majeure partie de la structure, avec provision pour une berme stabilisatrice au besoin. Une halde à stérile a été désignée à un e à cet endroit. À cause de la pente très peu prononcée de la partie aval de la Digue numéro 10 tel que dicté par la

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Richard Sisson; W.K. Chan; Catherine Fear (2012) Horizon Dyke 10 Geotechnical Design in GEO2012. Ottawa, Ontario: Canadian Geotechnical Society.

@article{Paper130,author = Richard Sisson; W.K. Chan; Catherine Fear,title = Horizon Dyke 10 Geotechnical Design,year = 2012}