Tailings and Mine Waste Conference

Characterizing the stiff clay foundation soil below a TSF in Western Australia Reid, David; Fanni, Riccardo; Chapman, P. M.


The recent tailings storage facility (TSF) failure at Mt Polley has highlighted the importance of the proper identification and characterization of stiff clay foundation soils. Specifically, the propensity for over consolidated stiff clays to transition to a normally consolidated state under vertical loading is a critical design consideration for TSFs constructed on stiff clay foundation soils. In many parts of Western Australia, TSFs are constructed on relatively stiff clayey soils. These materials can be either residual or transported, with materials of both origins found in relatively close proximity in some cases. Such soils may, under some loading conditions, and if saturated, exhibit contractive undrained behavior, giving mobilized shear strengths lower than drained strength. However, such soils are typically unsaturated prior to commencement of tailings deposition within the TSF, and may remain in such a state depending on the life of the facility and the influence of the hydrogeological conditions on seepage from the TSF. Further, residual soils are in some ways more difficult to characterize than transported soils, particularly with respect to defining a pre-consolidation stress and defining normally consolidated conditions. To characterize the behavior of a clay foundation soil below and adjacent to a TSF in Western Australia, a number of block samples were obtained in field investigations from near-surface locations, adjacent to the toe of the TSF. The material was then tested using a direct simple shear apparatus under a range of vertical effective stresses relevant to existing and future TSF loads. Preliminary results indicate that the material can exhibit contractive behavior when sheared under high vertical effective stresses, which are relevant to future TSF loading. Piezocone Penetration Testing undertaken near to the sample location provided results inconsistent with the laboratory testing. The reasons for potential divergence of the in situ and laboratory testing of the material are discussed.

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