MEND/NEDEM

In recent years, experts from industry and academia have raised concerns about potential risks to the stability and physical performance of tailings storage facilities (TSFs) that may arise from, or be exacerbated by, geochemical processes occurring within tailings, construction and foundation materials, and drainage features over time. These geochemical processes are generally well understood; however, the ways in which they lead to physical changes and the geotechnical significance of those changes are not as well established. Although some geochemical processes may contribute to stability risks, there is limited published research and only a few case studies where these processes have been identified as specific contributors to TSF failures.
To address this knowledge gap, SRK Consulting (Canada) Inc. was retained by the International Network for Acid Prevention (INAP) and the Mine Environment Neutral Drainage (MEND) program to develop an industry-focused technical report summarizing key geochemical processes that should be considered when assessing potential risks related to dam stability. The project was divided into two phases:
• Phase 1: Develop the foundation for an industry-focused report that defines and communicates the observed or documented geochemical processes that could affect a TSF’s intended performance over its required service life.
• Phase 2: Assess the plausibility of these risks through development of a screening-level risk assessment tool that identifies key factors influencing a facility’s susceptibility to geochemically induced stability risks.
This report summarizes the work completed in Phase 1 and provides the technical basis for advancing Phase 2. Phase 1 consisted of an updated and refined literature review on geochemical processes and their effects on physical characteristics of materials, and an updated geochemical conceptual model of how physical changes in materials might occur within a tailings facility. The technical basis for Phase 2 was advanced by developing an example hazard assessment framework for one of the geochemically induced physical changes.
The literature review identified three main geochemical processes and eleven resulting physical changes. The findings are presented in tabular form, with a description, the potential geotechnical significance, examples, and factors or conditions that could enhance the plausibility, significance or timescale of each physical change. The geochemical conceptual models illustrate the potential occurrences of the physical changes within TSFs under different design and environmental conditions.
The example hazard assessment framework provides a basis for assessing the plausibility that geochemical processes may increase the likelihood of failure for a TSF and demonstrates that this approach could be used to develop a screening level risk assessment tool.
Some key findings are summarized as follows:
• Certain types of minerals and rocks are more susceptible to weathering than others. susceptible minerals include halides, certain sulfate minerals, ultra-mafic minerals, sulfides and hydrothermal alteration products that may hydrate upon exposure. Susceptible rocks tend to include one or more of the above listed minerals.
• There are many case studies that demonstrate physical changes occurring in response to geochemical processes. These case studies demonstrate how physical changes can be equated to changes in geotechnical properties such as permeability, shear strength, compressibility, etc.
• There are limited case studies available which cite geochemical processes or time-dependent physical changes as the sole cause of physical instability.
• Many of these processes evolve over extended periods, from decades to centuries, depending on site conditions, materials, and climate. This long-term perspective is critical for understanding how such changes may influence post-closure performance, even if they present limited risk during active operations.
• The influence and significance of the physical change will be specific to the characteristics of the facility in question and the location within the facility.
• Correlating all the physical changes to particular failure modes does not capture the nuances of conditional probabilities, likelihood, and facility susceptibility as discussed in Section 4.2.
• Increased likelihoods of geochemically induced changes can be tied to an increased uncertainty in material properties. If all else is equal, this increased uncertainty will lead to an increased probability of failure. Although the likelihood and consequence of physical changes resulting in geotechnical failures was not quantified at this stage, SRK has provided a framework that could be used for this purpose in a second phase of work.
Overall, this work aims to help practitioners to understand, define and communicate the observed and/or documented geochemically induced changes in material properties that could lead to tailings storage facility performance risks amongst both geochemists and geotechnical engineers involved with design, operation and closure of tailings storage facilities, ultimately contributing to safer and more sustainable tailings management practices.