10 February 2020
The exceptional mobility of tailings dam failures
I’m continuing to write my paper for the keynote at the 13th International Symposium on Landslides in Colombia this year (the paper is due this month, so the pressure is on). As I noted in a post last week, I’ve been looking at the impact of landslides in less developed countries; one key component of this is the impact of failures associated with mining.
As part of this work I’m taking a look at the mobility of tailings failures in relation to other major landslides. Tailings dam failures inflict a huge impact in terms of loss of life, environmental effects and social damage. It is well established that the impacts can extend for tens or even hundreds of kilometres downstream – the Ok Tedi tailings failure in Papua New Guinea for example extended for 1,000 km and disrupted the lives of 50,000 people. Of course much of this damage was caused by remobilisation of the tailings by the river, but the issue of the runout of the landslide itself is very pertinent.
A good way to analyse the runout of landslides is to examine the so-called Fahrböschung angle, which is the ratio between the vertical change of the landslide (from crown to toe) to the length of the landslides (again from crown to toe). More mobile landslides have a lower Fahrböschung angle.
Using case studies described in the World Mine Tailings Failures (2020) catalogue, and going back to original topographic and satellite data, I have been able to calculate the Fahrböschung angle for 27 tailings dam failures. I have then compared these with the mobility of large landslides using data presented in a famous paper (Legros 2002) a couple of decades ago. I have also included in the graph data for coal waste landslides:-
Taking the large landslides first, it is well-known that the Fahrböschung angle decreases as landslide volume gets larger – the reasons for this remain a little unclear. Interestingly the same effect is seen for coal waste landslides and for tailings landslides, both of which are more mobile than large landslides. But, most significantly, the tailings landslides have a far lower Fahrböschung angle than that of the large landslides, but with much greater scatter too. Indeed, in many cases the Fahrböschung angle is two orders of magnitude lower – in other words, tailings landslides travel far further than other large landslides.
The reason for this high mobility is likely to the nature of the materials that are released in the tailings dam failure. Typically, the failure involves materials that have been crushed and that, at the point of failure, are saturated and have undergone liquefaction. The extreme mobility at the time of failure was of course illustrated rather elegantly by the Brumadinho failure in Brazil. Interestingly, many investigations of tailings dam failures tend to focus on the failure mechanism, and to ignore what happens thereafter. This needs attention.