Civil Engineering April 2022 | Vol 30 No 3

20 April 2022 Civil Engineering Stellenbosch Uni vers i ty geotechni cal research abstracts Tailings storage facility breach parameter predictions A popular area of research is the prediction of tailings storage facility breach volume and runout distances for catastrophic failures. A review of prediction methods is provided by Gildeh et al. (2020). Notable empirical predictions have been developed by Rico et al. (2008), Larrauri and Lall (2018), and Rourke and Luppnow (2015). The most comprehen- sive collation of case studies to date is that of Rana et al. (2021). METHODOLOGY & FINDINGS Total impounded volumes and released volumes for 41 failures are shown to natural scales in Figure 1a. Included are relationships proposed by Rico et al. (2008) and Larrauri and Lall (2018). While reported R 2 values are above 0.85 for both methods, estimates can only be considered order of magnitude estimates. Rourke and Luppnow (2015) showed accuracy improved by including the ratio of pool to total area. However, a small database is used and excludes contradictory cases such as Feijao. As a large and detailed database is (hopefully) unlikely, safe estimates are likely to rely on ratios of released to total volumes. Figure 1b shows that the ratio of released to total volume is less than 0.4 for 64% of cases and less than 0.75 for 90% of cases. Loots and Coetzer (2018) suggested reducing scatter by matching pertinent geological and geographical aspects. For example, the ratio of released to total volume is less than 0.23 for all southern African cases in the database. Insufficient data is available to accurately predict runout distances and inundation areas. Attempts by Rico et al. (2008) and Larrauri and Lall (2018) to estimate runout distance and by Ghahramani et al. (2020) to estimate inundation area had R 2 values less than 0.6. Rana et al. (2021) reviewed available satellite imagery to build a database of runout distances and inundation areas. Rana et al. (2021) defined the runout distance as the length before tailings entered a flowing water course. A distinction was also made between flows constrained by topography (e.g. dry riverbeds) and those not constrained along any length of the flow. Recorded runout distance is plotted against inundation area for the two topography groupings in Figure 2. It is suggested that for unconfined flows, the maximum runout distance and inundation area (3.2 km and 1.3 km 2 respectively) can be used for scoping purposes. For confined flows the range in inundation zones can be reduced by considering an estimate of release volume. Release volumes less than 1 Mm 3 had an average runout distance of 7 km and inundation area of 0.5 km 2 , whereas release volumes greater than 1 Mm 3 had average runout distance of 25 km and inundation area of 7 km 2 . Tailings flowing into perennial rivers within these runout distances will likely flow significant dis- tances downstream; however, reliable estimates of this distance are difficult. INTERPRETATION Results from this study can be used to obtain a first (or order of magnitude) approximation of potential tailings storage facility Dr Charles John MacRobert Pr Eng Senior Lecturer | Department of Civil Engineering Stellenbosch University macrobert@sun.ac.za Niel Marais Geotechnical Consultant (Tailings) SRK Consulting (Kazakhstan) nmarais@srk.kz Stellenbosch University has compiled a series of geotechnical research abstracts outlining research undertaken by the Department of Civil Engineering. The fourth abstract examines runout distance for tailings storage facilities. Figure 1 Prediction of volume released during a tailings storage facility breach event: a) Scatter plot and b) Histogram of release to total volume ratios (data from Rana et al. (2021))