Northam has not fully adopted the Global Industry Standard on Tailings Management. We are, however, committed to make use of best practice and technological tools. This is demonstrated with the process followed with stability assessment and InSAR.

InSAR surface displacement analysis

In F2023, we appointed specialists to carry out historical InSAR surface displacement analysis for all our TSFs, including archived data from July 2018 to July 2022. The objectives were to:

• model and analyse the historical surface displacement for each of our TSFs;
• identify previously known instability events; and
• identify areas displaying current displacement anomalies that may have a potential future impact on stability.

No areas of concern were identified by the analysis.

InSAR image of the Zondereinde TSF cumulative vertical LOS displacement

(Jul 2018 – Dec 2021)

Booysendal North TSF stability assessments

In F2023, specialist geotechnical consulting engineers investigated the stability of Booysendal North’s TSF, in accordance with the 2020 ICOLD Tailing’s Dam Safety Bulletin Draft. The industry norm for stability analysis involves using the Mohr-Coulomb assessment, which we used for the initial screening stage.

We also carried out a complex and advanced stability assessment, made up of critical-state soil mechanics principles, which is not routinely used in South Africa. The NorSand constitutive model tests material behaviour in the stability analysis, instead of making assumptions. For example, a seismic event, such as an earthquake, can be simulated to test the tailings material’s response or we can test liquefaction as a response to a trigger. In F2023, we assessed various potential triggers, and found that a seismic or earthquake trigger event for 1:5 000 and 1:10 000 years return period (calculated in a site-specific probabilistic seismic hazard assessment) could induce liquefaction at planned future TSF elevations after 2026.

After completing the NorSand triggering assessment, new constitutive models better suited to dynamic or seismic modelling have enabled us to reassess TSF stability more accurately under dynamic loading. Further assessments were carried out in F2023. We are already implementing stabilisation measures in the form of a starter wall buttress. We will continue with our assessment work in F2024.

Letlhegonelo Judith Moeng, Construction assistant at Eland mine