Authors: Andy Rudy, Jeff Coffin
Conference: Tailings and Mine Waste 2023
Date: November 5-8, 2023
Following an exploratory site investigation campaign consisting of more than twenty sections on a series of historic tailings dams, it was clear that many, if not all, of the historic cells would require some type of risk mitigation. Over the facility’s long history, the tailings basin had been divided into cells, some of which were partially closed while others were receiving new tailings. All the cells had been constructed using the upstream method, with some cells showing interbedded layers of potentially contractive material.
A series of buttresses were proposed for the site; however, the buttressing project was extensive enough that it required annual construction for up to five years. Furthermore, portions of the historic dam were located directly adjacent to other infrastructure – so close, in fact, that a buttress in those areas was considered unfeasible. As such, a combination of tailings reprocessing, dam deconstruction, and buttressing was proposed to mitigate the risks on site.
These mitigations were proposed in phases, beginning with the highest risk facility. The highest risk facility was determined based on a series of seven parameters. Six of these were geotechnical parameters, and the seventh was a parameter to measure the unknowns remaining in the site investigation at the time.
The geotechnical parameters were the Average Cyclic Resistance Ratio (CRR), the Normalized Shear Wave Velocity (Vs1), the Average State Parameters, the geometric design parameters (called the Geometric Product), the Undrained Factor of Safety at Residual Strength, and the Presence of a Pond of Fresh Tailings in the Impoundment. These factors, along with a two-day risk assessment workshop, were used to prioritize the risk mitigation campaign and target areas for further investigation. This case study will show the approach to prioritizing and mitigating these risks.
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