Towards Zero Harm

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TOWARDS ZERO HARM – A COMPENDIUM OF PAPERS PREPARED FOR THE GLOBAL TAILINGS REVIEW

TOWARDS ZERO HARM – A COMPENDIUM OF PAPERS PREPARED FOR THE GLOBAL TAILINGS REVIEW

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rate of instability over a normalised period of time; however, they do enable the comparison of general stability trends between facility types.

The volume of tailings under storage also varies with facility construction methods. Upstream facilities contain the highest total volume of tailings under storage, followed by downstream, hybrid and centreline, single raise, in-pit/natural landform,

dry-stack and other facilities (see Figure 6). The highest median volume of tailings stored per facility are hybrid facilities (18.3 million m 3 ), followed by centreline (7.3 million m 3 ) and upstream (5.9 million m 3 ).

same facility is not recorded. This may have the effect of undercounting the prevalence of stability issues for facilities prone to experiencing them. Due to this limitation, the findings are not a calculation of the

20

20

% of active facilities % of all facilities Standard error due to sampling

15

15

5 Total current stored volume (km 3 ) 10

Overall % for dataset

10

5

Percentage with a past stability issue

0

0

Other

In-pit/landform

Dry-stack

Single raise

Downstream

Centreline

Hybrid

Upstream

Other

Dry-stack

In-pit/landform

Single raise

Centreline

Hybrid

Upstream Downstream

Figure 7. Proportion of facilities with a past stability issue by raise type 14

Figure 6. Current volume of tailings under storage (in cubic kilometres) by raise type of active, inactive and closed facilities Note: shading indicates active facilities

It is possible that the incidence of past stability issues for any one particular construction method is not a function of the unique characteristics of these facilities, but rather, an artefact of the distribution of that facility type across other common characteristics known to influence geotechnical stability. For example, a particular construction method might have a greater proportion of facilities that are older, higher, larger, located in lower governance settings, in regions with a greater seismic hazard, or where rainfall is higher. These differentially distributed attributes might lead to these facilities demonstrating a higher or lower incidence of past stability issues, for reasons unrelated to the construction method. In the remainder of this section we will explore the influence of these factors on the past stability of the tailings facilities. At the conclusion of this section we return

to the question of whether the higher prevalence of past stability issues reported by upstream facilities is an artefact of the distribution of these facilities or a feature of the construction method itself. Tailings facilities located in OECD-countries, as well as those operated by ICMM-member companies generally reported a lower normalised incidence of past stability issue across those raise types that were elevated (see Table 1). This finding lends some weight to the view that tailings governance plays some role in ensuring geotechnical stability. However, the proportion of facilities reporting past stability issues for facilities located in OECD-countries and those operated by ICMM-member companies, remains high in absolute terms across a number of raise types (most notably upstream, hybrid and centreline).

3.4 INCIDENCE OF PAST STABILITY ISSUES Companies were requested to disclose any situation where a facility, ‘at any point in its history, failed to be confirmed or certified as stable, or experienced notable stability concerns, as identified by an independent engineer (even if later certified as stable by the same or a different firm).’ The reported issues ranged in seriousness from relatively minor to major issues. In total 10 per cent of facilities reported having experienced a past stability issue. The data exhibits distinct trends according to construction method, governance, age, height, volume and seismic hazard. Upstream and hybrid facilities were the most likely to report a past stability issue, when normalised against the frequency of each raise type. They were followed by centreline, downstream and single raise facilities (see Figure 7). The likelihood of a past stability issue

having been reported by active upstream facilities is twice that of active downstream facilities and six times as many as active dry-stack facilities. No active in-pit/natural landform facilities reported a past stability issue. From a geotechnical perspective the rate of past stability issues is significant (> 1 per cent) for most construction methods, highlighting the universal importance of careful facility management and governance. 13 One limitation of the dataset is that the occurrence of multiple instances of past stability issues at the 13. Construction practices that have been reported to improve geotechnical performance of conventional tailings facilities include: comprehensive characterisation of both the tailings and underlying soils, keeping the size of the decant pool as small as possible, allowing the development of long beaches to promote the desiccation and densification of tailings, and continuous monitoring of the disposal facilities (Williams, this volume; Santamarina et al. 2019).

14. Error bar lengths here, and in subsequent figures, are binomial confidence intervals for the subsample represented by each bar, showing +/-1 standard error (approximately 68%).