Tailings dams are man-made earth structures used for storing mining waste, comprised of water and fine minerals left behind from the extraction process. Tailings are usually non-plastic fine sand-and-silt-sized soils with angular particles that are often deposited in a loose state. The current practice mostly ignores the important differences among soils and how these differences influence CPT interpretation and calculation of residual shear strengths. Advances in CPT interpretation through still rely on CPT calibration chamber data on clean quartz sands. Case history based residual strength correlations do not account for differences among soils, how liquefaction was triggered, or system level differences.
Research done on various aspects of liquefaction assessment in tailings over the past few years at University of Toronto will be presented. This includes a new state parameter interpretation technique that provides soil-specific correlations by taking in Norsand’s parameters as input. It will be shown that this method estimates the in-situ state parameter more accurately than existing screening methods. Correlations between residual strength and state parameter are then presented based on laboratory tests on various gradations of tailings and natural soils. It will be shown that residual strength is soil-specific, and laboratory tests can be used in conjunction with case-histories to inform the level of conservatism in choosing residual strengths for design.
Mason Ghafghazi, PhD, PEng.
Mason is an assistant professor at the Department of Civil & Mineral Engineering at University of Toronto. His career includes working as a post-doctoral scholar, lecturer, and project scientist at the University of California Davis, and a geotechnical engineer at BC Hydro. His PhD work focused on estimating the in-situ state parameter by modelling the CPT. Evaluation of seismic performance, liquefaction consequences, and internal erosion susceptibility in embankment dams and their foundations, development of the instrumented Becker Penetration Test, and numerical modelling of mining activities are some of Mason’s past experiences. Mason’s areas of research are liquefaction assessment in tailings and gravelly soils, CPT modelling and interpretation, and advanced constitutive modelling of soil behaviour.
Lecture presented in Burnaby, BC, Canada on May 17, 2023. Video recorded and published with permission from Mason Ghafghazi.