Tailings, produced when rock is crushed to recover metals, are normally discharged as slurry of predominantly siltsized particles into storage areas that are created using dams. These dams have a poor safety record with billions of dollars in damages over the past decade alone, making reliable engineering of silt an important economic and safety issue for the mining industry. Equally, engineering of silts is challenging, as understanding of soil behaviour relates mostly to 'sands' or 'clays'. Undisturbed silt samples suffer substantial densification between sampling, transfer to element test and reinstatement of in situ stresses. Hence, silts require a sand-like approach that combines laboratory tests on reconstituted samples with in situ cone penetration test (CPT) soundings. This paper presents calibrated spherical cavity expansion in a general critical-state soil model to simulate the CPT in silt. The developed methodology is numerical, accurately captures calibration data and allows determination of the in situ state parameter in silts from CPT data. A validation is presented for a large tailing impoundment using stacked thickened tailings. Open-source software implementing the methodology is provided on the journal website as supplementary material.
Finite-element analyses using critical state theory proved necessary to understand the development of static liquefaction during three recent large tailing dam failures at Fundao (in Brazil), Cadia (in Australia) and Brumadinho (in Brazil). However, the complexity of these events prevents these analyses being viewed as a complete validation of the methodology. Here the authors evaluate a far simpler case of static liquefaction: the 1974 Tar Island slump (in Canada). This upstream slump involved a rapid drop of 5 m during construction of a 12.5 m high upstream raise over loose tailings. While not a dam stability issue, the event has the attraction for validation of being load-induced, with simple geometry, and with known material properties and in situ state. The computed liquefaction develops from a prior drained condition before propagating rapidly undrained – there are similarities to the video record at Brumadinho (an animation is provided as online supplementary material to illustrate this). A range of scenarios are explored, with the base case of taking reported conditions at face value giving deformations close to those measured. An important aspect was using elastic shear moduli determined by geophysical methods. The analyses were carried out with commercial software (Plaxis) and used critical state theory with largely familiar soil properties measured by standard methods.
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