Sylvania Platinum : Tailings Dam Risk Mitigation Measures

Risk Profile and Management of Sylvania Tailings Storage Facilities in South Africa

Short Report Prepared by Senior Management of Sylvania Platinum

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Contents
1	Introduction ....................................................................................................................................	3
2	Technical Causes of Failure - a Case Study of Feijão Dam I in South America ...............................	3
3	Acceptable Risk in South Africa.......................................................................................................	4
4	Risk Drivers / Triggers .....................................................................................................................	5
5	Legal compliance (South Africa)......................................................................................................	7
6	Vale / Sylvania (SA) comparison......................................................................................................	8
7	Conclusions ...................................................................................................................................	10
Figures
Figure 1 Acceptable Risk (Wium) ............................................................................................................	5
Tables
Table 1 Slope Failure Triggers for an Appropriate Monitoring and Response Strategy .........................	6
Table 2 Vale Incident Comparison with Sylvania TSF Operation and Risk..............................................	8

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1 Introduction

Following some unfortunate disasters associated with tailings dam failures in South America during recent years, there has been widespread international concern about the safety of such and similar facilities in South Africa. These concerns are often linked to the impact of a flow slide failure of a Tailings Storage Facility ("TSF"), which could result in significant environmental damage, damage to property, loss of production and/or loss of life.

Based on input and recommendation from the Sylvania Board, a study was launched to compare Sylvania's existing management systems and risk factors to those of typical South American tailings dams in order to ensure that learnings from recent TSF failures are taken into account and that the Company's existing strategy in terms of TSF management and safety remains adequate and effective.

The purpose of this document is to report on the risk profile, risk drivers and relevant management and controls on the Tailings Storage Facilities ("TSFs-") of the Sylvania Dump Operations in South Africa. The document includes a review of the relevant lines of responsibility and accountability, liability, legal framework, continuous review of operating performance linked to the risk profile on the TSFs.

This document deals specifically with the Chromite Tailings Dams operated and managed by Sylvania in South Africa.

2 Technical Causes of Failure - a Case Study of Feijão Dam I1 in South America

Shortly after the TSF failure incident at Córrego do Feijão Iron Ore Mine in Brazil on 25 January 2019, a panel of four experts in geotechnical engineering with special expertise in water and tailings dams was commissioned by Vale to investigate the technical causes of failure of the TSF.

The Panel was instructed to use its expertise and professional judgment to review and assess requested relevant data and technical information to determine the technical cause(s) of the Dam I failure.

The Panel relied on assistance from consultants to review historical data and documents, evaluate specific subject areas, conduct field and laboratory testing, and engage in computer modelling.

The following summary of findings was included in the report, explaining the history that created the conditions for instability in the dam:

• A design that resulted in a steep upstream constructed slope;
• Water management within the tailings impoundment that at times allowed ponded water to get close to the crest of the dam, resulting in the deposition of weak tailings near the crest;
• A setback in the design that pushed the upper portions of the slope over weaker fine tailings;
• A lack of significant internal drainage that resulted in a persistently high water level in the dam, particularly in the toe region;
• High iron content, resulting in heavy tailings with bonding between particles. This bonding created stiff tailings that were potentially very brittle if triggered to become undrained; and

1 Extract from Report of the Expert Panel on the Technical Causes of the Failure of Feijão Dam I, by Peter K. Robertson, Ph.D. (Chair), Lucas de Melo, Ph.D., David J. Williams, Ph.D. and G. Ward Wilson, Ph.D.

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• High and intense regional wet season rainfall that can result in significant loss of suction, producing a small loss of strength in the unsaturated materials above the water level.

The Panel found that the failure and resulting flow slide was the result of flow liquefaction within the tailings in the dam. The history described above created a dam that was composed of mostly loose, saturated, heavy, and brittle tailings that had high shear stresses within the downstream slope, resulting in a marginally stable dam (i.e., close to failure in undrained conditions). Laboratory testing showed that the amount of strain required to trigger strength loss could be very small, especially in the weaker tailings. These were the main components that made flow liquefaction possible.

The Panel concluded that the sudden strength loss and resulting failure of the marginally stable dam were due to a critical combination of ongoing internal strains due to creep, and a strength reduction due to loss of suction in the unsaturated zone caused by the intense rainfall towards the end of 2018.

This followed a number of years of increasing rainfall after tailings deposition ceased in July 2016. The calculated pre-failure strains from this combination of triggers match the small deformations of the dam detected in the post-failure analysis of satellite images from the year prior to the failure.

The internal strains and strength reduction in the unsaturated zone reached a critical level that resulted in the observed failure on January 25, 2019.

3 Acceptable Risk in South Africa

TSFs are large geotechnical structures and in Southern Africa these facilities are managed on Civil Engineering principles and in accordance with approved engineering codes and standards. The term "Acceptable Risk" is often used when referring to TSFs. Acceptable Risk in the Civil Engineering Industry was quantified by D.J.W. Wium (The Civil Engineer in South Africa, December 1988). From this study, risk profiles acceptable to the South African society are indicated in Figure 1.

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RISK OF DEATH	ATTITUDE	ACTIVITY
	ACCEPTABLE	BEING STRUCK BY LIGHTNING
1 IN 1,000,000
		NATURAL DISASTERS
1 IN 100,000
	SOME
	WARNINGS
1 IN 10,000		ALL INDUSTRIAL WORK
		TRAFFIC ACCIDENTS
1 IN 1,000		ALL ACCIDENTS

1 IN 100	UNACCEPTABLE

Figure 1 Acceptable Risk (Wium)

It is common practice in the South African tailings industry to manage TSF slope failure (not flow slide failure) at P(failure)< 1:500 000. Experience has indicated that only 1:50 slope failures in South Africa results in a flow slide of any magnitude.

4 Risk Drivers / Triggers

Risk drivers / triggers are directly related to the South African (and African) method of TSF construction and operation. The following should be noted regarding the Sylvania TSF operations:

• Southern Africa is a net evaporation area, i.e. in most cases the annual evaporation is almost double the annual precipitation. This phenomenon creates soil suction, having been measured up to 2kPa in soils that classifies as Sandy-Silt (USCS system), typically the classification of the outer perimeter tailings;
• A combination of down-stream (at start-up of a new TSF) and up-stream deposition methods are used;
• Tailings placement is done with on-wall cyclones; the cyclone underflow is used for perimeter construction and to contain the cyclone overflow, layer thickness being less than 500mm, with the cycle time being longer than the consolidation time;
• Supernatant water is decanted with a standpipe in-dam system (penstock with concrete rings). This configuration allows for the water pool depth on the TSF at any given time to be less than 300mm.

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