TSF Report

GROUP TSF MANAGEMENT STANDARD
DOCUMENT NUMBER: VERSION NUMBER: NEXT REVIEW: PREPARED BY: AUTHORISED BY:
JANUARY 2023
TSF 1
DRAFT 1.0_2022 DECEMBER 2023
GROUP HEAD OF TAILINGS CHIEF TECHNICAL OFFICER

GROUP TSF MANAGEMENT STANDARD
Contents
ACRONYMS
INTRODUCTION
PART A – GOLD FIELDS TSF MANAGEMENT STANDARD
4
6
7
Introduction 7 Implementation and conformance 7 Battery limits 7 Applicability and Deviation 7 Definitions 8
TOPIC 1 – PLANNING, TAILINGS MANAGEMENT PLAN 11
Introduction 11 A1 – 1 Project Conception 11 A1 – 2 Tailings Management Plan 12 A1 – 3 Project Execution Plan 13 A1 – 4 TSF Risk Management Plan 13 A1 – 5 TSF Re – mining Plan 13 A1 – 6 Design Codes, Guidelines and Standards 14 A1 – 7 Authorisation for Expenditure (AFE) 14 A1 – 8 Reports and Deliverables 15
TOPIC 2 – AFFECTED COMMUNITIES, WATER STEWARDSHIP AND CLOSURE 17
Introduction 17 A2 – 1 TSF Environment, Social and Governance (ESG) Management 17 A2 – 2 Free Prior and Informed Consent (FPIC) 17 A2 – 3 Human Rights 18 A2 – 4 Stakeholder Engagement and Communication 19 A2 – 5 Water Stewardship 19 A2 – 6 Closure Planning 20 A2 – 7 Reports and Deliverables 23
TOPIC 3 – SITE CHARACTERISATION AND INTEGRATED KNOWLEDGE BASE 25
Introduction 25 A3 – 1 Site characterisation and knowledge base 25 A3 – 2 Seismic Hazard Assessment 26 A3 – 3 Site geotechnical investigation 27 A3 – 4 Geological, engineering geological and geotechnical models 27 A3 – 5 Hydrogeological characterisation 28 A3 – 6 Tailings geotechnical characterisation 28 A3 – 7 Tailings and waste rock geochemical characteristics 29

A3-8 TSF water balance and water management 30 A3-9 Underground Mining Impact 31 A3-10 Blasting in close proximity to a TSF 32 A3-11 Reports and Deliverables 32
TOPIC 4 – TSF DESIGN 39
Introduction 39 A4-1 Options and Multi-Criteria Analyses 39 A4-2 Risk management, Failure modes and ALARP 40 A4-3 Dam break study 42 A4-4 Consequence classification 43 A4-5 Population at Risk (PAR) and Probable Loss of Life (PLL) 44 A4-6 Industry Standard Design Criteria and Design Basis 44 A4-7 Stability analyses 46 A4-8 Factor of safety (or prescriptive approach) 47 A4-9 Performance-based design 48 A4-10 Rate of rise and consolidation 49 A4-11 Tailings transportation and deposition plan 50 A4-12 Waste placement plan 50 A4-13 In-pit tailings deposition 51 A4-14 Cyclone deposition 51 A4-15 Placement of tailings filter cake 53 A4-16 Special design considerations 54 A4-17 Pond control and size 56 A4-18 Instrumentation and monitoring plans 56 A4-19 Progressive Rehabilitation and Closure Design 57 A4-20 Recommissioning an Inactive or Closed TSF 58 A4-21 Design Reports and deliverables 58
Gold Fields will retain ownership of all Intellectual Property Rights in any general skills, expertise, knowledge and know- how arising from using and applying this Standard. Should the consulting firm want to, or be requested to share the detail of this report with any third party, written consent will be required from Gold Fields and Gold Fields is under no obligation to provide such consent. Gold Fields reserves all of its rights to proceed with legal proceedings in the event that there is unlawful dissemination of the information.
"Intellectual Property Rights" means copyright and related rights, rights in designs, rights in computer software, database rights, rights to preserve the confidentiality of information (including know-how and trade secrets) and any other similar intellectual property rights, in each case whether registered or unregistered and including all applications (or rights to apply for and be granted), renewals or extensions of, and rights to claim priority from, such rights and all similar or equivalent rights or forms of protection which subsist or will subsist now or in the future in any part of the world.
Version
Date
Author
Approved by document owner
Brief description of changes
Rev 0
January 2023
Johan Boshoff
Original Copy
GROUP TSF MANAGEMENT STANDARD
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GROUP TSF MANAGEMENT STANDARD
Contents
TOPIC 5 – CONSTRUCTION 61
Introduction 61 A5 – 1 Contract Documentation 61 A5 – 2 Technical Adjudication 61 A5 – 3 Constructability Review 61 A5 – 4 Construction Risk Assessment 62 A5 – 5 Level of Construction Quality Assurance 62 A5 – 6 Quality Control and Quality Assurance 62 A5 – 7 Deviations from design 63 A5 – 8 Construction Records 64 A5 – 9 Deviance Accountability Report (DAR) 64 A5 – 10 Construction Reports and deliverables 64
TOPIC 6 – OPERATION 67
Introduction 67 A6 – 1 Operation, Maintenance and Surveillance Manual 67 A6 – 2 TSF Commissioning 67 A6 – 3 Trigger Action Response Plan (TARP) 68 A6 – 4 Updating the Tailings Risk Assessment 68 A6 – 5 RACI Matrix 69 A6 – 6 Potential Material Changes 70 A6 – 7 Operation Reports and deliverables 70
TOPIC 7 – EMERGENCY RESPONSE PLANNING 73
Introduction 73 A7 – 1 Emergency Preparedness and Response Planning 73 A7 – 2 Emergency Preparedness and Response Plan (EPRP) 75 A7 – 3 Testing and EPRP 75 A7 – 4 Notifications and Warnings 75 A7 – 5 Health and Safety 76 A7 – 6 Deliverables 76
TOPIC 8 – INDEPENDENT REVIEW 79
Introduction 79 A8 – 1 Dam Safety Inspections (DSIs) 79 A8 – 2 Dam Safety Review (DSR) 80 A8 – 3 Independent review 81 A8 – 4 Group TSF Standard review and Third – party Verification 81 A8 – 5 Internal Gold Fields Governance Reviews 82 A8 – 6 Regulatory review 82 A8 – 7 Deliverables 82
2

TOPIC 9 – TSF MANAGEMENT AND GOVERNANCE 85
Introduction 85 A9 – 1 ICMM TSF Position Statement 85 A9 – 2 Global Industry Standard on Tailings Management (GISTM) 85 A9 – 3 ICMM Tailings Management Good Practice Guide 86 A9 – 4 ICMM Conformance Protocols 86 A9 – 5 Change Management 86 A9 – 6 Roles and Resources 87 A9 – 7 Key appointments 87 A9 – 8 Tailings Awareness Training 91 A9 – 9 Deliverables 91
TOPIC 10 – CLIMATE CHANGE 93
Introduction 93 A10 – 1 Climate Change and Water Management Structures 93 A10 – 2 Process for Incorporating Climate Change Adaptation into Decision-Making 94 A10 – 3 Deliverables 95
PART B – REGIONAL TSF DESIGN AND OPERATIONAL CONSIDERATIONS 96
B1 – Western Australia 96 B2 – South Africa 96 B3 – Ghana 96 B4 – Peru 96 B5 – Chile 96
PART C: SURVEILLANCE MINIMUM REQUIREMENTS
97
C1 – Design Considerations for a Surveillance Program 97 C2 – Performance Management 98 C3 – Analysis of Surveillance Results, Communications, and Decision-Making 99 C4 – Critical Controls 100 C5 – Incidents 100 C6 – Trigger Action Response Plan 100
TERMS REFERENCES
102 117
GROUP TSF MANAGEMENT STANDARD
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GROUP TSF MANAGEMENT STANDARD
Acronyms
AE Accountable Executive
AEP Annual Exceedance Probability
AFE Authorisation for Expenditure
AFPR Annual Facility Performance Report
ALARP As Low As Reasonably Practicable
AMD Acid Mine Drainage
ANCOLD Australian National Committee on Large Dams
ARD Acid Rock Drainage
CDA Canadian Dam Association
CDIV Construction vs. Design Intent Verification
CHM Conceptual hydrogeological model
CPTu Cone Penetrometer Testing with pore pressure measurements
CRR Construction Records Report
CSL Critical state line
CTS Corporate Technical Services
CQA Construction Quality Assurance
DAR Deviance Accountability Report
DBE Design Basis Earthquake
DBR Design Basis Report
DSI Dam Safety Inspection
DSR Dam Safety Review
EoR Engineer of Record
EPRP Emergency Preparedness and Response Plan
ESA Effective stress analysis
FMEA Failure Mode and Effects Analysis
FPIC Free Prior and Informed Consent
GISTM Global Industry Standard on Tailings Management
ICMC International Cyanide Management Code
ICMM International Council of Metals and Mining
IDF Inflow design flood
I&MP Instrumentation and monitoring plan
ITRB Independent Technical Review Board
LL Liquid Limit
LoM Life of Mine
MCE Maximum Credible Earthquake
MOC Management of Change
MQA Material Quality Assurance
4

OBE
Operating Basis Earthquake
OMS
Operations Maintenance and Surveillance
Pa
Pascal
PAR
Population at Risk
PEP
Project Execution Plan
PFMA
Potential Failure Mode Analysis
PI
Plasticity Index
PLL
Probable Loss of Life
PMF
Probable Maximum Flood
PPV
Peak Particle Velocity
PSHA
Probabilistic Seismic Hazard Assessment
PTE
Potentially toxic elements
RTFE
Responsible Tailings Facility Engineer
SHA
Seismic Hazard Assessment
SIR
Senior Independent Reviewer
SQRA
Semi-Quantitative Risk Analysis
SWI
Standard work instruction
TARP
Trigger Action Response Plan
TMP
Tailings Management Plan
TMS
Tailings Management System
TSF
Tailings Storage Facility
QA
Quality Assurance
QC
Quality Control
USA
Undrained stress analysis
WAD
Weak acid dissociable
GROUP TSF MANAGEMENT STANDARD
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GROUP TSF MANAGEMENT STANDARD
GIntroduction
old Fields Limited (Gold Fields) is committed to the safe, stable, and sustainable
operation of its tailings storage facilities (TSFs). The Group TSF Management Standard (Standard) sets the minimum Gold Fields requirements for managing TSFs.
Therefore, the reference to a "TSF" in this document can generally be assumed to incorporate the TSF with its associated infrastructure, e.g., delivery and distribution piping, decant facilities, decant water storage, and return pumping systems. This Standard comprises three parts, namely:
• Part A – TSF Management Standard: consists of leading practices and principles which must be adhered to during the life cycle of all TSFs in the Group. This part of the Standard consists of the core directives. The Group Audit Criteria are discussed and included in Attachment A.
• Part B – Regional TSF Design and Operational Considerations: provides some background to regional specific requirements associated with the design or operation of TSFs.
• Part C – Tailings Surveillance Guideline and Minimum Requirements: This leading practice surveillance guideline defines the minimum requirements for an effective surveillance program to support the safe and efficient management and operation of TSFs.
In addition, the following attachments are included:
• Attachment A – Group Assessment and Third-party Audit Criteria • Attachment B – Consequence Classification Table
• Attachment C – Minimum Requirements for Surveillance
• Attachment D – Dam Safety Inspection Frequency Guideline.
6

Part A
INTRODUCTION
The purpose of the Standard is to define the minimum
The purpose of the Standard is to define the minimum requirements for Tailings Storage Facilities (TSFs) throughout their lifecycle, from early studies and site selection, through design, operation, closure and post-closure.
This Standard provides a framework for safe tailings facility management while affording the regions and sites flexibility regarding how best to achieve this goal. Unless otherwise specified, the requirements of the Standard are directed
to the sites but, in many cases, will require significant external input from our Engineers of Record and specialist consultants.
The Standard is underpinned by 10 Topics comprising a list of requirements and deliverables. The 10 Topics are intended to facilitate improved risk management and TSF performance, achieve the design intent, and meet legal requirements and corporate policy.
This Standard includes greater governance and accountability than was previously required.
This Standard includes greater governance and accountability than was previously required.
BATTERY LIMITS
GOLD FIELDS TSF MANAGEMENT STANDARD
IMPLEMENTATION AND CONFORMANCE
This Standard is effective from January 2023. Gold Fields expect that each of its operations (including Joint Venture operations), new TSF designs, projects, and closed and legacy sites work towards achieving and sustaining full conformance against this Standard, together with the associated assessment criteria, where appropriate and applicable.
Facilities with Very High and Extreme consequence categories must fully conform to this Standard (this document) by Q2 2024. All other TSFs must conform by the end of 2025.
In supporting our current high level of governance, the Accountable Executives (AEs) and Responsible Tailings Facility Engineers (RFTEs) from the regions and sites must demonstrate at all times that initiatives are in place and progress is made towards implementing the requirements of this Standard.
All new studies, investigations, designs, construction activities etc., must conform to this Standard from January 2023.
The battery limits, in the context of this Standard, for a TSF are:
• The inlet flange of the tailings delivery pumps at the process plant
• The discharge point of the return water pipe at the process/raw water tanks or ponds or the point of environmental release.
• The final filter discharge point at the plant is the battery limit for filtered tailings.
It is acknowledged that battery limits for certain facilities may need to differ from those stated above. In such cases, the operation must apply for a variance to the Standard and obtain approval from the Group Head of Tailings.
APPLICABILITY AND DEVIATION
This Standard applies to all sites and TSFs in all mine life cycle phases, including exploration, design, construction, operation, closure and closure.
This Standard aligns with the Global Industry Standard on Tailings Management (GISTM). However, it must be noted that this Standard supplements the GISTM, and the GISTM requirements will also need to be adhered to.
Examples of application of this Standard include, but are not limited to:
• An existing TSF where design is changed for any reason, such as modification of storage capacity, realignment of an embankment, addition of a stability buttress, etc.
• Closure designs for TSFs.
• Construction and/or operation of internal/separation embankments/buttresses
• Construction of a new TSF (above surface and in-pit)
• Expansion or raising of an existing TSF
• Facilities in a state of care and maintenance and closed TSFs
• Filtered stacks with soil-like material behaviour; and
• Operation of an existing TSF, including active and inactive facilities
• Planning and design of a TSF at the project development/study stage (pre-feasibility (PFS) or higher)
• TSFs under rehabilitation and facilities in post-closure status (including legacy facilities)
GROUP TSF MANAGEMENT STANDARD
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GROUP TSF MANAGEMENT STANDARD
Due to some special/unique conditions, it is acknowledged that it may not be feasible for a specific operation to conform with a particular topic/clause of this Standard. In such cases, the operation could apply for a variance to specific clauses of the Standard and obtain written approval from the Group Head of Tailings.
The application for the variance must be supported by a justification, risk assessment, and a review of alternative options.
Additional and potentially more stringent requirements than this Standard must be considered and applied if required by the location of the TSF and the prevailing laws in the country of operation.
DEFINITIONS
Variance in terminology can hinder common understanding of key issues, so definitions of Tailings, Waste Rock, in-pit TSF and TSF status are provided here.
Tailings Storage Facilities
A TSF is a facility designed and managed to contain the material produced by the tail end of a processing plant, ‘tailings.’
Generally, a TSF comprises:
• The entire impoundment structure, including the embankments, internal embankments, surface water diversions, spillways, storage basin, placed tailings (including filtered stacks and cycloned embankments), foundations, drainage and liner systems, and access roads/ ramps on the facility
• The area directly downstream as delineated in the Gold Fields TSF Incident Standard2
• The tailings delivery and distribution systems and return water piping and decant structures, and
• TSF monitoring equipment, seepage management system, and collection ponds.
In-pit TSFs
This Standard also applies to in-pit TSFs. However, these facilities must also consider the safety aspects of the Corporate Technical Services (CTS) Geotechnical Management Guidelines3.
1Tailings Storage Facility Incident Reporting and Classification Standard. October 2019.
2Corporate Technical Services. Geotechnical Management Guidelines for Open Pit Mines. Version 1.1. 1 January 2020.
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Tailings
Table 1 provides generic definitions/classifications for tailings materials based on solids concentration and Yield stress.
Table 1: Generic definitions for each tailings type
Classification
% solids (w/w)
Yield stress (Pa)
Transport
Conventional tailings slurry: Mixture of finely ground rock and water, low solids content with water being the transport mechanism, no or little mechanical dewatering applied.
It has a critical flow velocity and no yield stress, segregating where coarse material forms a beach on deposition and fines/slimes migrate towards a large pond.
<50
<5 to 20
Centrifugal pump
Thickened tailings slurry or high-density thickened slurry: Higher solids content than conventional slurry with additional mechanical dewatering applied.
Fully saturated, homogeneous, non-segregating, may have critical flow velocity, high slump value (~254 mm) and yield stress, may have some water bleed, and may result in a small pond.
50 to 60
20 to 100
Centrifugal or positive displacement pump
Paste tailings: Typically free-standing, possess yield stress, has no critical flow velocity, exhibits plug flow characteristics, has a measurable slump (178 to 254 mm), is homogeneous and non-segregating, minimal to no water bleed, fines are the transport mechanism, small or no pond.
60 to 75
100 to 500
Positive dis- placement pump
Filtered tailings: A cake-like material that retains a level of partial saturation, the degree of which is con-trolled by the filter performance.
Typical moisture content of less than 20% is achieved using a combination of belt, drum, horizontal and vertical stacked pressure plates and vacuum filtration systems. Additional drying/drainage may be required to meet the material's optimum moisture content to achieve adequate compaction.
>75
>500
Truck or conveyor
Waste rock
Waste rock can be a highly variable material, and it may be helpful to define various parameters to help classify the waste. Table 2 provides generic definitions/classifications for waste materials based on gradation, plasticity, and geochemistry.
Finer grain size typically results in increased rates for mineral reactions (including oxidation of acid-generating sulphide minerals and dissolution of acid-neutralizing carbonate/silicate minerals) due to increased specific surface area (surface area of grain normalized by volume) of the materials.
Hence, for the same reactive mineral contents, fine-grained materials typically have higher mineral reactivity and higher potential for acid generation/metal leaching when compared to coarse-grained materials.
Using these ranges, the waste material can be adequately classified.
For example:
• Mixed-grained, low plasticity, highly reactive waste rock with moderate ARD potential would represent a material that
is 15% fines, 20% greater than 75mm, LL<35 and PI<10, with moderate (or unknown) potential for chemical breakdown, oxidation, and generation of ARD/ metal leaching.
GROUP TSF MANAGEMENT STANDARD
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GROUP TSF MANAGEMENT STANDARD
Table 2: Generic definitions for each waste material type
Gradation
Very fine-grained
Mixed grained
Very coarse-grained
% fines (passing #200 sieve, <0.075mm)
>50%
25-50%
10-25%
5-10%
<5%
% greater than 75mm
<10%
10-25%
25-50%
50-75%
>75%
Plasticity
Highly plastic fines, LL>50, PI>20
Moderately plastic fines, LL 35-50, PI 10-20
Low plasticity fines, LL<35, PI<10
N/A
Chemical stability
Highly reactive
Moderately reactive
Low reactivity
ARD/metal leaching potential
High potential for chemical breakdown/sulphide oxidation/ generation of ARD and metal leaching
Moderate (or unknown) potential for chemical breakdown/ sulphide oxidation/ generation of ARD and metal leaching
Low potential for ARD/metal leaching
TSF Status
The status definitions of TSFs, for this Standard, are presented in Table 3 below.
Table 3: Status definition
Status
Definition
Study phase3
The facility is in the design stage at the PFS level or a higher stage
Construction
The facility is in the detailed design stage and/or under construction for initial start-up or re-start-up
Active
The facility is actively used for the deposition/placement of tailings
Inactive
The facility was active in the past but is currently not active – with a plan to be active in future (could be under care and maintenance)
Closed
The facility is currently not active with no plans to be active in the future; rehabilitation has not commenced
Closed Active
The facility is Closed with rehabilitation work underway
Closed Passive
The facility is Closed with rehabilitation work completed
10
3Gold Fields Study Guidelines for Scoping, PFS and FS

INTRODUCTION
TOPIC 1
PLANNING, TAILINGS MANAGEMENT PLAN
The Gold Fields’ corporate policy on Tailings Management forms the basis for establishing all systems, information and plans relevant to the current and future lifecycle phases of a TSF at an appropriate level of detail.
The tailings management and project teams responsible for each TSF shall ensure full design integration between the different plant, mining, TSF, environmental, social, and sustainable development discipline areas and that battery limits, roles, and responsibilities are well defined.
This section discusses the following topics associated with Tailings Management and Planning:
• A1-1 – Project Conception and Alternatives Analyses
• A1-2 – Tailings Management Plans (TMP)
• A1-3 – Project Execution Plans (PEP)
• A1-4 – TSF Risk Management Plans (RMP)
• A1-5 – Re-mining Plans
• A1-6 – Design Codes and Guidelines
• A1-7 – Authorisation for Expenditure (AFE) • A1-8 – Reports and Deliverables
Figure 1: TSF life cycle
A1-1 PROJECT CONCEPTION
The Project Conception or study phase (Project Conception) involves developing and analysing a range of options and alternatives for a proposed TSF (e.g., options for the location of a new TSF and technologies to be applied). Interdisciplinary stakeholders must review the options. A design basis report, site characterisation and risk assessment, multi-criteria and options analysis and final study report should be prepared as part of the Project Conception and Alternatives Analysis phase. The primary output is the final, approved selection of the preferred alternative and associated cost estimates in accordance with corporate requirements.
The Project Conception phase is a process of making some of the most important decisions about tailings management, some of which will be difficult or impossible to reverse once the Design phase has been completed and executed.
Thus, Operators should carefully consider the Project Conception phase before the Design phase is initiated.
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TOPIC 1

GROUP TSF MANAGEMENT STANDARD
It is important to emphasise that the Project Conception phase is not relevant only to new TSFs facilities; it plays a key role in all TSF-related activities not limited to expansions, raises, re-purposing and re-mining. Furthermore, it is a recurring activity through the lifecycle and can also be applied to planning for:
• Potential material changes in design (depending on complexity), such as:
• Extensions to the life of an existing TSF beyond its initial design capacity.
Key activities in the Project Conception may include:
• Identification of relevant interdisciplinary stakeholders.
• Risk identification and analysis.
• Preliminary dam breach modelling to estimate the extent of inundation zones and level of impact.
• Modifying the design of a TSF, such as the strengthening of embankments or reductions in water levels.
• Re-activation of an existing TSF for mine re-opening. • Closure and Post-Closure phases.
• Site characterisation.
• Definition of performance objectives and design criteria.
• Identification of potential sites, alternatives, development of preliminary designs, and multi-criteria alternatives analysis to select the preferred alternative.
An integrated approach to mine planning is essential to safe tailings management and involves the full integration of planning across the lifecycle of all aspects that can impact tailings management. An integrated approach is essential.
A1-2 TAILINGS MANAGEMENT PLAN
A Tailings Management Plan (TMP) must be developed, implemented, and maintained for any tailings project, including a TSF and any site with operating TSFs. The TMP should be prepared at a site level for each individual asset. For example, a TMP is not required for a closed passive or legacy TSF unless ongoing capital works are planned, but reference should be made to known legacy TSFs in the site-level TMP.
The TMP is a site-specific plan for all existing and future facilities developed in collaboration with the EoR. This can be a brief document or memorandum summarising the life of mine tailings deposition plan, indicative storage locations, the lead times required for investigations, studies, designs, stakeholder engagement, regulatory approvals, and critical associated risks. In addition, the document should reference other supporting TSF documentation, such as the site characterisation or design basis report.
The TMP shall, at a minimum:
• Include planning, resources, and capital forecasts to manage tailings in the short term (1 to 2 years), medium-term (5 years), Life of Mine (LoM), including rehabilitation and closure, or a Master deposition plan.
• Be developed in tandem with the LoM plan using production rates and the mining schedule.
• Include lead times and schedules for investigations, design, review, regulatory approvals, and construction of new storages (including expansion of existing storages) relative to the estimated time of filling existing storages.
• Include key risks to achieving the storage schedule to highlight what needs to be tracked and measured, and aim to allow contingency storage of at least 6 months.
• Include the range of tailings production data from the process plant, the expected ranges of the slurry solids content, throughput, particle size distribution, and mineralogy.
o This data forms a fundamental part of the TSF design
criteria.
o Therefore, the impact of any data changes on the design or operation of the TSF must be thoroughly evaluated before implementation.
• Identify critical risks to the TSF and its performance and include discussion of viable closure options, and, where possible, reference a closure design and/or plan.
• Be agreed upon and signed off by the Accountable Executive (AE)4 and Responsible Tailings Facility Engineer (RTFE)5.
• Be compared with actual TSF performance on at least an annual basis, such as tailings throughput, insitu density, water return rates, pond control, freeboard, rate of rise and exposed beach length.
o The plan(s) shall be updated using the management of change (MoC) process.
• Be revised when a projection of future mine production (reserve declaration) indicates an impacting change in TSF storage capacity requirements.
• Be submitted as part of a TSF-related Authorisation for Expenditure (AFE) application.
The TMP shall be reviewed annually, and any changes to the plan agreed upon and signed off by the RTFE and AE. In addition, the TMP shall be reviewed and updated as part of the annual Engineer of Record (EoR)6 dam safety inspections.
A TMP includes the arrangements/concepts and analyses of viable storage options, schedules for investigations and testing, the design, regulatory approvals and permitting, construction activities, and high-level capital and operating cost estimates.
4GISTM Requirement 8.4 5GISTM Requirement 8.5 6GISTM Principle 9
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TOPIC 1

A1-3 PROJECT EXECUTION PLAN (PEP)
A Project Execution Plan (PEP) is a standalone document that describes the execution of a TSF’s detailed design, procurement, construction, and commissioning elements. In addition, a PEP describes how the project will be organised, executed, managed, controlled, and delivered.
A PEP is thus the basis for directing the engineering and construction stages of a TSF project, should the project obtain approval. Any changes to the PEP shall be agreed upon and signed off by the RTFE.
The PEP shall be reviewed by interdisciplinary stakeholders and Corporate Technical Services (CTS) before an AFE submission.
A1-4 TSF RISK MANAGEMENT PLAN
TSF Risk management plans (RMPs) are intended to describe the Principle Hazards and risk controls or mitigation to reduce the risks identified, as well as actions, persons responsible for completing the actions, and timelines for action completion. The risk management plan is specific to TSF risk.
The development of an RMP should begin during Project Conception and be refined and developed in greater detail during design, construction and operation. The risk analysis, assessment, and risk management plan should be reviewed and updated regularly throughout the different stages of the facility's lifecycle. A live site-specific tailings risk assessment may constitute an RMP.
The RMP shall:
• Be developed in line with the Gold Fields Group Enterprise Risk Management Guideline7.
•
Ensure that the level of risk management is commensurate with the identified risks and the organisation’s appetite for risk.
Identify who has direct ownership or accountability for managing the risk and who implements the mitigation plan.
Any changes to the plan shall be agreed upon and signed off by the RTFE.
The RMP should also be reviewed and updated in the event of changes that were not anticipated at the beginning of mine life, such as mine life extensions, temporary suspensions of mine operations (depending on the duration of the suspension), changes in the ore being processed, process and technology changes, etc.
A1-5 TSF RE-MINING PLAN
A TSF re-mining plan (where applicable) shall be developed and implemented where the tailings are reclaimed through either mechanical or hydraulic means.
The re-mining plan shall:
• Be incorporated into the TMP, described in section A1-2 of this Standard.
• Be developed in tandem with the LoM plan using production rates and the mining schedule.
• Include key risks identified as part of the RMP to achieve the mining schedule to highlight what needs to be tracked and measured.
• Acknowledge the insitu state of the facility, especially the level of the phreatic surface and the pore pressure regime.
• Identify stormwater management criteria, safe slope angles, offset distances from the TSF perimeter embankment, freeboard requirements, etc.
• Be agreed upon and signed off by the RTFE and EoR.
Any changes to the plan shall be agreed upon and signed off by the RTFE and EoR. In addition, the re-mining plan shall be reviewed and updated as part of the annual EoR dam safety inspections.
• reviews •
• Be agreed upon and signed off by the AE and the RTFE.
• Be reviewed and updated as part of the annual EoR operational
7Gold Fields Group Enterprise Risk Management (ERM) Guideline. January 2022.
GROUP TSF MANAGEMENT STANDARD
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TOPIC 1

GROUP TSF MANAGEMENT STANDARD
A1-6 DESIGN CODES, GUIDELINES AND STANDARDS
The management of TSFs is an integrated process that relies upon implementing and managing all applicable design criteria and principles during the facility's life cycle. These minimum requirements may be exceeded where appropriate to ensure the risk of the facility remains acceptable.
Due to some special/unique conditions, it is acknowledged that some variations in guidelines may exist. In such cases, the RTFE and EoR should use engineering judgement and the operation could apply for a variance and obtain approval from the Group Head of Tailings.
At a minimum, Gold Fields expect the sites and operations o
Conformance Protocols. Global Industry Standard on
to comply with the following design codes, guidelines and Standards:
• Gold Fields
o TSF Management Policy Statement
o Tailings Management Standard (this document) o Tailings Storage Facility Incident Reporting and
Classification Standard, October 2019.
o Corporate Technical Services. Geotechnical Management
Guidelines for Open Pit Mines. Version 1.1. 1 January 2020. o Study Guidelines for Scoping, PFS and FS.
o Group Enterprise Risk Management (ERM) Guideline,
January 2022.
o Capital Estimating Guideline. Revision 0, November 2017.
November 2016.
o Group Water Management Guideline. Version 2019,
October 2019.
o Gold Fields Guidance Notes (referenced throughout the
Standard)
• The Global Industry Standard on Tailings Management (GISTM), August 2020
• ICMM
•
•
•
• • •
Tailings Management, May 2021.
o Tailings Management Good Practice Guide, May 2021.
ANCOLD
o Guidelines on Tailings Dams – Planning, Design, Construction, Operation, and Closure – Revision 1 (July 2019)
o Guidelines for Design of Dams and Appurtenant Structures for Earthquakes (July 2019)
o Guidelines on the Consequence Categories For Dams (October 2012)
CDA
o Dam Safety Guidelines published by CDA (Canadian Dam Association) in 2007 and revised in 2013.
o Application of Dam Safety Guidelines to Mining Dams, CDA, 2014.
Ghanaian Minerals and Mining (Health, Safety, and Technical) Regulations, 2012 (L.I. 2182)
SANS code or Practise for Mine Residue Deposits SANS 10286) The International Cyanide Management Code (ICMC)
Local and national design codes and operational guidelines
Where an applicable Design Code or Guideline is updated, the operation should progressively meet the new updated code. Part B of this Standard provides a full list of guidelines and Standards.
The selected Design Code or Guideline will define the minimum design, construction, and operational requirements for the TSF.
A1-7 AUTHORISATION FOR EXPENDITURE (AFE)
Tailings management is one of Gold Fields' strategic priorities. As tailings-related projects require significant capital expenditure, proactive, forward planning is critical. As a result, tailings management is a long-term activity.
By taking a long-term view of controlling risk, value can be maximised, people allocated efficiently, and confidence in delivery increased.
Gold Fields recognise that maintaining TSF integrity is an ongoing process of continuous assessment that needs to be maintained for the life (including closure) of each TSF. The operation, surveillance, maintenance, effective monitoring, and review practices must align with the TSF design requirements and be quality-assured and controlled to ensure the TSF is functioning as intended.
Where a TSF's integrity is compromised, the facility is not functioning as intended, or there is a need for capacity expansion or design modification, the AFE process should be used to seek capital approval.
Proactive management and visible project information are key to streamlining the TSF-related AFE process.
Ongoing communication of critical project information allows for the continuous review of a project's risk exposure and allows decision-makers to respond quickly to internal and external changes.
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TOPIC 1

The following supporting documentation is required in support of TSF-related AFE submissions:
• A
• A
Tailings Management Plan.
short-term plan referencing the TMP and comprising the following documents:
o Preliminary Project Execution Plan (PEP).
o Risk Management Plan.
o For modifications to an existing TSF (i.e., new lift, toe buttress, extension, etc.):
 A feasibility-level design for the specific task, technical specification, and design drawings. o For a new TSF:
 A feasibility-level design in accordance with the Gold Fields Study Guidelines.
 A Feasibility-level project cost estimate and schedule for the total internal and external project expenditures. The cost estimate
will be based on tendered prices.
o Basis of cost estimate. Multiple tenders must support the cost estimate following the Gold Fields Capital Estimating Guideline8.
The review of a TSF-related AFE will not progress without any of these supporting documents. Therefore, submitting a draft copy of the proposed AFE to the Group Head of Tailings is recommended three months before submission.
Gold Fields undertake business and operational planning activities a year in advance. The capital associated with TSFs should be submitted as part of the activities, and the AFE process is used to finalise the request for expenditure.
A1-8 REPORTS AND DELIVERABLES
Each site, led by the RTFE, shall develop and maintain the following planning documents and deliverables:
Project Conception and Alternatives Analysis
• Cost estimate of the preferred option
• Design Basis report
• Multi-criteria Alternatives analysis.
• Site characterisation and risk assessment.
• Study reports (Scoping, PFS or FS)
Tailings Management Plan (TMP)
• A current version of the Tailings Management Plan document, as discussed under Section A1-1.
• A comparison of the TMP with actual TSF performance annually and annual revision of the TMP as part of the annual EoR operational review.
• Submission of the TMP as part of a TSF-related Authorisation for Expenditure (AFE) application.
• Agreeance of the TMP, signed off by the Accountable Executive (AE) and Responsible Tailings Facility Engineer (RTFE).
• Evidence that the RTFE has communicated the TMP and its contents to the AE and other stakeholders.
Project Execution Plan (PEP)
• A Project Execution Plan document as discussed under Section A1-3.
• The preliminary PEP needs to be updated and describes how the project will be organised, executed, managed, controlled, and delivered.
• The PEP is thus the basis for directing the engineering and construction stages of the project, should the project obtain approval.
TSF Risk Management Plan (RMP)
• A TSF Risk Management plan document as discussed under Section A1-4.
TSF Re-mining Plan (where applicable)
• A TSF re-mining plan document as discussed under Section A1- 5.
• Agreeance of the TSF re-mining plan, signed off by the RTFE and EoR.
• Annual review of the re-mining plan, updated as part of the annual EoR dam safety inspection.
Authorisation for Expenditure
• AFE submissions, where required, per the requirements set out in Section A1-7.
8Gold Fields Capital Estimating Guideline. Revision 0. November 2017.
GROUP TSF MANAGEMENT STANDARD
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GROUP TSF MANAGEMENT STANDARD
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TOPIC 2

TOPIC 2
AFFECTED COMMUNITIES, WATER STEWARDSHIP AND CLOSURE
INTRODUCTION
Gold Fields is committed to building on its leading commitment to Environment, social and Governance (ESG.) In this topic, the Standard discusses the following:
• A2-1 – TSF Environment, Social and Governance Management
• A2-2 – Free Prior and Informed Consent (FPIC)
• A2-3 – Human Rights
• A2-4 – Stakeholder Engagement and Communication
• A2-5 – Water Stewardship
• A2-6 – Closure Planning
• A2-7 – Reports and Deliverables
A2-1 TSF ENVIRONMENT, SOCIAL AND GOVERNANCE (ESG) MANAGEMENT
The RTFE of a TSF must demonstrate a commitment to environmental, social and governance (ESG) and manage TSFs in accordance with leading practice and in-country regulatory compliance.
TSFs are generally governed in the ESG space by an Environmental, Social, and Health Impact Assessment (ESHIA), Environmental Management Plan (EMP) or local license conditions. In addition, a supporting monitoring plan, typically defined by license conditions, is typically managed by the site-based sustainable development teams. The plan considers how to address, monitor, manage and report site- specific environmental and social issues.
The GISTM promotes an interdisciplinary and collaborative
approach. As such, the RTFE and the sustainable
development (SD) team must collaborate to monitor a TSF’s
ESG performance and make informed decisions regarding ESG impacts, not limited to community, social or environmental topics. The RTFE must become familiar with the conditions set out in the mine’s license conditions and aim to integrate TSF monitoring plans with those performed by interdisciplinary stakeholders. Section A8-5 of this standard discusses the ESG topic in further detail.
A2-2 FREE PRIOR AND INFORMED CONSENT (FPIC)9
Free Prior and Informed Consent (FPIC) is a new requirement of the GISTM but is embedded in how we operate at Gold Fields. The FPIC process is generally managed by our Community and Social teams at both a site and Corporate level. The RTFE shall consult with the Community Relations Site, Region and Corporate representatives. In summary, recognising the potential vulnerability of host communities, including Indigenous Peoples, Gold Fields is required to:
• Respect the rights, interests, special connections to lands and waters, and perspectives of Indigenous Peoples, where mining projects are to be located on lands traditionally owned by or under customary use of Indigenous Peoples.
• Adopt and apply engagement and consultation processes that ensure the meaningful participation of indigenous communities in decision-making through a process consistent with their traditional decision-making processes and based on good faith negotiation.
• Work to obtain the consent of Indigenous Peoples where required by this position statement.
• Maintain timely, ongoing and inclusive engagement with stakeholders through appropriate processes that provide a platform for dialogue, understanding of stakeholder views and our impacts on those around us.
• Work to obtain the free, prior, and informed consent of communities in accordance with the ICMM Position Statement on Indigenous People and Mining, including where projects are located on lands traditionally owned or under customary use by Indigenous Peoples and are likely to have significant adverse impacts on Indigenous Peoples.
9GISTM Requirement 1.2 and Gold Fields Community Policy Statement, 2021
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GROUP TSF MANAGEMENT STANDARD
FPIC comprises a process and an outcome. Through this process, Indigenous Peoples are:
• Able to freely make decisions without coercion, intimidation, or manipulation.
• Given sufficient time to be involved in project decision-making before key decisions are made, impacts occur and
• Fully informed about the project and its potential impacts and benefits.
Gold Fields supports the United Nations Universal Declaration of Human Rights, the UN Guiding Principles on Business and Human Rights and the Voluntary Principles on Security and Human Rights.
A2-3 HUMAN RIGHTS
Across the globe, Gold Fields:
• Encourages diversity and inclusivity in our workplaces
• Respects the human rights and interests, cultures and customs of communities surrounding mining activities
• Provides training and guidance for all relevant staff, including security staff and contractors
Evidence of Gold Fields’ commitment to protecting Human Rights is demonstrated by the board's endorsement of a Human Rights policy.
The GISTM has introduced the need to demonstrate respect for human rights under the United Nations Guiding Principles on Business and Human Rights (UNGP), conduct human rights due diligence to inform management decisions throughout the TSF lifecycle and address the human rights risks associated with TSF credible failure scenarios.
Generally, a TSF would trigger a human rights issue if it has the potential to impinge on:
• The right to life (i.e., TSF failure could lead to fatalities)
• The right to a healthy environment; and
• The right of people to practise a cultural life of their choice (cultural heritage resources, incl. graves).
• Undertake human rights due diligence
• Provides site-level grievance mechanisms for its workforce and communities; and
• Works to raise awareness of human rights issues with our vendors and collaborate with them to address identified concerns
As such, the RTFE is encouraged to discuss potential human rights issues with an interdisciplinary team and capture the discussions or build them into a site-specific tailings risk assessment or similar.
For existing facilities, the Operation can initially opt to prioritise salient human rights issues under the UNGP.
Salient human rights include:
• Health and Safety •
• Water •
• Human resources •
• Resettlement •
Procurement Transportation Mine closure Public security
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A2-4 STAKEHOLDER ENGAGEMENT AND COMMUNICATION
Stakeholder engagement is key to the success of any project or operation.
The RTFE and the community relations team must ensure that all potentially affected stakeholders living or working within a hypothetical TSF dam breach inundation zone
or run-out region (based on credible failure modes) are meaningfully engaged throughout the TSF lifecycle.
Stakeholder engagement should be carried out meaningfully, following the GISTM10 and the Gold Fields Community Relations, Stakeholder Engagement Handbook11, and Stakeholder Relationship and Engagement Policy, 2020. Potentially affected stakeholders – including communities, employees and contractors, public sector agencies and other relevant organisations.
Community engagement related to tailings management should be integrated with broader community engagement activities, although engagement more specifically targeted to tailings management may be appropriate on some topics (e.g., community input during the Project Conception phase).
Engagement should be coordinated with and conducted
in collaboration with personnel with specific expertise in community engagement, but tailings specialists involved in community engagement should receive appropriate training.
A2-5 WATER STEWARDSHIP
Stakeholder engagement is critical in developing and testing Emergency Preparedness and Response Plans (EPRPs).
The EPRPs should be updated and tested at all phases of the TSF lifecycle at a frequency established in the plan, or more frequently if triggered by a material change to the TSF or the social, environmental, and local economic contexts. The EPRP topic is described in further detail in section A7-1 of this Standard.
Water stewardship is defined as using water in a socially equitable, environmentally sustainable, and economically beneficial way. This is achieved through an inclusive stakeholder process that involves site and catchment-based actions.
Good water stewards understand their own water use, catchment context and shared risk regarding water governance, water balance, water quality and important water-related areas. A water stewardship champion has been nominated at each Gold Fields operation globally. The RTFE, along with the sustainable development team, Water Champions and Water Management Groups, should:
• Consider water management requirements, risks, and costs in the planning cycle, including improved consideration of water management in early project planning phases (exploration to feasibility) and provision for water management after closure.
• Ensure adequate flood planning and protection.
• Ensure that the quantity and quality of water discharges are controlled to minimize the impact on the receiving environment and water assets.
• Ensure the operation complies with regulatory requirements and obligations relating to industry rules, codes, and standards we subscribe to.
• Invest in research, technology, and infrastructure (including natural) to manage water quality, increase efficiency, and create sequential and shared use opportunities.
• Maintain a long-term water balance and evaluate usage across the project life cycle.
• Proactively reduce social and environmental impacts and risks.
• Promote and match appropriate water quality sources with the operational activity wherever technically and economically feasible.
• Promote operational water efficiency – minimize, reuse, recycle.
In addition to ESG, the RTFE and the sustainable development team must demonstrate that the local surface and groundwaters' beneficial uses are not being impacted by the TSF and that groundwater quality and levels are being managed as expected and required.
10GISTM Principles 1 and 2
11Community Relations and Stakeholder Engagement Handbook: Summary. 2015
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GROUP TSF MANAGEMENT STANDARD
A2-6 CLOSURE PLANNING
Planning for rehabilitation and closure and operating a TSF in a manner consistent with the closure objectives are activities that crosscut the entire lifecycle. Thus, while the Closed Active and Closed Passive phases can be regarded as distinct phases of the lifecycle, planning and design for these phases begin at the outset of Project Conception and continue throughout the lifecycle. The sustainable development group manages the Closure Planning process at a site and corporate level.
This section of the Standard considers three stages of closure:
• Closure planning
• Active Closure
• Post-Closure
The requirements for the detailed engineering and design of a TSF’s closure landform are further detailed in section A4-18 of this Standard.
Closure Planning
The liability or estimated rehabilitation and closure costs for planned closure, early or unscheduled closure, reclamation, and post-closure management and associated obligations related to the TSF and its appurtenant structures shall be reviewed periodically following the Gold Fields Group Mine Closure Planning Guideline12 to confirm that adequate financial capacity (including insurance, to the extent commercially reasonable) is available for such all stages of the TSF lifecycle.
The minimum requirements are:
• TSFs shall be planned and designed, from the outset, with the Closed Active and Closed Passive phases in mind.
• The closure plan should clearly define the proposed vision, principles, and objectives of closure and should become more detailed and elaborated during the design phase.
• The closure plan and objectives shall be considered in the multi-criteria alternatives analysis conducted during the Project Conception phase of the tailings facility. In addition, they should be a key consideration in the facility design and location and in the technology decisions of the facility.
• The development of the closure plan may lead to changes in current practices or the adoption of newer technologies to reduce risk and better position the tailings facility for closure. Therefore, regular review of such opportunities is central to continual improvement for any tailings facility.
• The closure plan should then be refined, elaborated, verified, and updated periodically during the Operations phase of the lifecycle and in preparation for the transition to the Closed Active phase.
• Many operations deal with existing facilities and legacy facilities. The guidance provided here expresses the importance and urgency around the acceleration of a timeline for the guidance that follows to arrive at the goal of a final closure plan that will achieve a stable landform status.
• The development of conceptual closure plans for existing TSFs that do not have closure plans shall begin as soon as possible.
The execution of the closure plan can be a period of rapid change. Therefore, it is vital to have established performance objectives and success criteria to establish metrics and achieve designated goals during the Closed Active phase when the plan is executed.
12Group Mine Closure Management Guideline. Revision 4. November 2016.
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Active Closure
It is essential that the operation continues to be diligent through the transition from closure planning to active closure and that the operation does not become complacent about tailings safety because tailings are no longer being produced and deposited in the tailings facility.
In particular, for closed TSFs:
• Overall governance structures should remain in place, with accountability and responsibility appropriately assigned.
• Change management should remain robust as processes and • personnel transition from the Operations phase to the Closed
Active and Closed Passive phases.
• The Tailings Management System (TMS) should continue to •
rehabilitation and closure plan, adhering to design specifications and quality management requirements.
Independent Review continues to focus on implementing the progressive rehabilitation and closure plan and preparations for the Closed Passive phase.
Community engagement continues.
be implemented to the extent appliable, and this is revised to reflect post-closure activities.
• The Risk Management Plan (RMP) or Tailings Risk Assessment should be updated for progressive rehabilitation and closure, and the risk management plan should be updated accordingly.
• The Operating, Monitoring and Surveillance (OMS) manual should be updated for progressive rehabilitation and closure and implemented to meet the requirements of the Closed Active and Closed Passive phases.
• Construction activities should be carried out per the progressive
Post-Closure (or passive closure)
• Emergency Response and Preparedness Plan (EPRP) documentation should be updated to reflect closure conditions, including a potential change in the operation’s role and third parties responding to an emergency as the Operation’s on-site resources change.
The Post-Closure (or passive closed) phase begins when changes from the Active Closure phase have been fully implemented, and the facility enters a period of long-term maintenance and surveillance.
A TSF facility in the Closed Passive phase may require the same level of care as it did in earlier phases of the lifecycle. This can be a challenge since the operation will have few on-site personnel. In addition, depending on the location, there may be more limited access to a power supply, communication infrastructure, etc. This should be considered when determining the closure liability or closure cost estimate.
For the Closed Passive, the operation should:
• Ensure a governance structure remains, with accountability and responsibility appropriately assigned.
• Continue to implement the TMS, although the frequency of Identifying Actions to Improve Performance and reporting to the Accountable Executive may be decreased.
• Maintain the TSF site characterisation and knowledge base.
• Periodically update the risk assessment, particularly if changes in the facility performance or external changes could impact the risk (e.g., increased population in the potential area of inundation).
• Update the risk management plan as appropriate.
• Update the OMS manual and review periodically through
the Closed Passive phase and update as appropriate. There may be a more significant role for community engagement in surveillance in the Closed Passive phase.
• Continue to conduct independent reviews until the facility is deemed to be in a safe state of closure, i.e., the facility is physically and geochemically stable.
Closure design is a key component of TSF design which must be integrated into Closure Plans. The Closure Design topic is discussed in detail in section A4-18 of this Standard.
GROUP TSF MANAGEMENT STANDARD
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GROUP TSF MANAGEMENT STANDARD
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TOPIC 2

A2-7 REPORTS AND DELIVERABLES
The GISTM addresses the ESG documents and deliverables listed below. In general, at a minimum, each site, led by the RTFE, shall develop and maintain the following:
Environmental and Social Management
• An Environmental, Social, and Health Impact Assessment (ESHIA) and management plan and monitoring plan for each TSF, addressing site-specific environmental and social issues identified.
• The RTFE and the sustainable development team shall monitor key items to understand the system and make informed decisions regarding any pollution potential from the TSF and environmental and social impacts.
• The annual governance audits must include an environmental and social component to demonstrate compliance.
• Community engagement should be coordinated with and conducted in collaboration with personnel with specific expertise in community engagement, but tailings specialists involved in community engagement should receive appropriate training.
• Community engagement related to tailings management should be integrated with broader community engagement activities.
• Resettlement risks and impacts must be captured in the site- specific tailings risk assessment.
Human rights
• The human rights policy must be up to date.
• A procedure describing human rights monitoring and evaluation, due diligence process and mitigation strategies.
Stakeholder engagement
• Stakeholder engagement plan.
• Stakeholder engagement and negotiation records. • Stakeholder agreements.
Water Stewardship
• Consider water management requirements, risks, and costs in the planning cycle, including improved consideration of water management in early project planning phases.
Closure planning
• The closure plan and objectives for each facility shall be considered in the multi-criteria alternatives analysis conducted during the Project Conception phase of the tailings facility.
In addition, they should be a key consideration in the facility design and location and in the technology decisions of the facility.
• Closure risks need to be captured in the site-specific tailings risk assessment.
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GROUP TSF MANAGEMENT STANDARD
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TOPIC 3

TOPIC 3
SITE CHARACTERISATION AND INTEGRATED KNOWLEDGE BASE
INTRODUCTION
In this topic of The Standard, the following components related to the site characterisation and the development of an integrated knowledge base will be discussed:
• A3-1 – Site characterisation and knowledge base •
• A3-2 – Site-specific Seismic Hazard Assessment •
• A3-3 – Site Geotechnical Investigation •
• A3-4 – Geological, engineering geological and geotechnical •
A3-6 – Tailings geotechnical characterisation
A3-7 – Tailings and waste rock geochemical characteristics A3-8 – TSF water balance and water management
A3-9 – Underground Mining Impact
models
• A3-5 – Hydrogeological characterisation
• A3-10 – Blasting in close proximity to a TSF
• A3-11 – Reports and Deliverables
A3-1 SITE CHARACTERISATION AND KNOWLEDGE BASE
The site characteristics at each operation are unique. The site teams and Engineer of Record must have a robust understanding of the site conditions before commencing any detailed design activities.
All site characterisation information should be well documented to form the foundation of a knowledge base. This knowledge base must be updated routinely and consider the full TSF life cycle, including closure and will be owned and managed by the RTFE on site.
Site characterisation is an iterative process that is initiated during Project Conception and continues throughout the lifecycle. It involves collecting and compiling a wide range of information about a site and the adjacent environment and developing a site characterisation model.
A summary of all technical site characteristic information should be collated into a single document, a Site Characterisation Report. This stand-alone document will play a crucial role in future designs, audits and lessons learned and will reference relevant key technical studies and supporting documents.
The site characterisation report13 shall:
• Be refined and updated based on updated site characterisation information, as-built conditions, and surveillance results.
• Include site-wide information, the outcome of site investigations, key performance drivers and regulatory requirements.
• Include a description of the following:
o The current state of knowledge related to the climate,
geomorphology, hydrological, geotechnical, and
hydrogeological settings in which the TSF is developed.
o Known regional and local geology in detail and the related issues of seismicity and hydrogeology associated with the
selected site
o The historical state of knowledge related to legacy issues
and decommissioned infrastructure such as pipelines and solution corridors.
•
o Aquifers, water levels, range of ground conditions that are likely to be encountered, excavatibility
o Environmental geochemistry. o Future ore reserves.
Document knowledge about the TSF's social, environmental and local economic context, using approaches aligned with international best practices.
o Update this knowledge at least every five years and
whenever there is a material change either to the tailings facility or to the social, environmental and local economic context.
o Capture uncertainties due to climate change.
13GISTM Requirement 2.2.
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GROUP TSF MANAGEMENT STANDARD
A3-2 SEISMIC HAZARD ASSESSMENT
Earthquake and seismic loading conditions play a critical role in evaluating the stability of a TSF structure. Therefore, it is important for design teams to have a robust understanding of any potential seismological hazards and loading criteria.
The GISTM defines the seismic recurrence intervals design engineers must apply when preparing a TSF design. It should be noted that the recurrence intervals defined by the GISTM are relatively new, and site-specific hazard analysis (SHA) will likely be required for each site. A SHA defines the seismic loading criteria to be applied to a TSF.
A design engineer may consider two approaches when completing a SHA:
• Probabilistic seismic hazard analysis (PSHA), and
• Deterministic seismic hazard analysis (DSHA) sometimes called scenario-based seismic hazard analysis.
In both approaches, the seismic hazard analysis must identify the potentially active faults and other seismic sources that could affect the TSF site. The approaches are described further below.
Approaches to Site-Specific Hazard Assessments
The selection of appropriate model parameters, earthquake source and recurrence models, and ground-motion models (i.e., ground-motion prediction equations) can be complicated and typically requires significant engineering judgment and expertise.
The earthquake loading criteria, developed using the approaches described above, must be commensurate with the consequence classification of the facility defined by the GISTM and ANCOLD guidelines and defined in the Design Basis Report (DBR). Table 4 presents which approach should be applied per the consequence classification.
Table 4: SHA approach
Probabilistic Seismic Hazard Analyses
A Probabilistic Seismic Hazard Analysis (PSHA) evaluates the ground motion level that will be exceeded at a specified frequency or annual probability.
In a PSHA, all historic earthquake events, seismic sources, and resulting ground motions are considered, and the annual exceedance probabilities (AEPs) for the range of ground-motion levels are estimated.
The PSHA is useful to inform the decision about fault activity since the full range of known fault parameters is considered.
A Seismic Hazard Assessment (SHA) defines the level of earthquake ground shaking across a site that has a likelihood of being exceeded in a given time period. The most common reason for a SHA is to determine the seismic loads to be considered in the earthquake-resistant design of a TSF or embankment.
Deterministic Seismic Hazard Analyses
Where there is an active fault(s) in the vicinity of the TSF site, the deterministic method will assess the ground motion at the TSF site for the Maximum Credible Earthquake (MCE) on the active fault(s).
A DSHA evaluates the ground motions considering the worst-case earthquake size and location but without considering the seismic hazards from non-controlling seismic sources. A DSHA typically assigns a maximum earthquake magnitude for a particular seismic source, referred to as the Maximum Credible Earthquake (MCE). Based on the minimum distance from the site to the fault source, the level of ground shaking at the site is estimated.
Consequence classification (GISTM)
Risk-based approach (PSHA)
Deterministic approach (DSHA)
Extreme

–
Very High

–
High

–
Low and Significant
–

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A3-3 SITE GEOTECHNICAL INVESTIGATION
Geotechnical investigations must be carried out for all TSFs. The information must be referenced in the site characterisation report.
The extent and detail of the geotechnical investigation should be determined by the EoR responsible for the design of the facility. The site investigation should be guided by an assessment of the typical soil profile made from the existing information, which could include the interpretation of stereoscopic aerial photographs of the site and a limited amount of fieldwork. The geotechnical investigation should be carried out by appropriately qualified and experienced personnel.
As a minimum requirement, the investigation should be sufficiently detailed to provide the following:
• Characterisation of the relevant engineering properties of the foundation soils.
• As a minimum, these should include the assessment of the following: o strength characteristics; and
o drainage characteristics.
• Characterisation of the surface profile, tailings and confining embankment materials over the entire area covered by the TSF and associated infrastructure – the characterisation should define the spatial extent and depth of different soil horizons;
With regard to foundation investigations, the objective shall be to determine the following:
• The permeability and potential erodibility of the rock in the foundation.
• The potential for internal erosion and piping.
• The presence of karst features and/or sinkholes.
• The presence of low-strength defects or seams in soil foundations (including fissures, relict defects from weathering, and pre-sheared surfaces) and, if present, their shear strength or influence on the shear strength of the soil mass.
• The presence of low-strength, contractive soils and brittle soils in the foundation.
• The presence of low-strength strata or seams in the rock foundation and, if present, the shear strength.
• The presence of potentially liquefiable soils and their properties which control the likelihood they will liquefy and the liquefied strength.
• The shear strength, compressibility, and permeability of soils and rock in the foundation.
• The stratigraphy and structure of the foundation soils and rock.
• The potential effects of ground movements and subsidence resulting from mining and the effects of blasting.
The behaviour of the underlying foundation material is often important to the stability of an above-ground TSF. However, given the variable nature of foundation soils between sites, the methods to characterise a foundation can be more rigorous than those relevant to classify tailings material which is relatively more consistent it is characterisation.
A3-4 GEOLOGICAL, ENGINEERING GEOLOGICAL AND GEOTECHNICAL MODELS
The EoR shall use the site investigation information described in section A3-3 of this Standard to develop a detailed engineering geological and geotechnical model (e.g. 3D Leapfrog model) for all TSFs of all Consequence Classifications.
Geological models emphasise the geological aspects significant to the project. The geotechnical model builds on the engineering geological model by characterising the geotechnical properties of critical components of the TSF, including impounded tailings and embankment raises. The EoR uses it to analyse stability, groundwater flow, erosion potential, etc.
The geological, engineering geological and geotechnical model should be updated with additional information collected during site investigations and following periodic TSF embankment geometry changes (TSF embankment raises or downstream waste rock placement).
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GROUP TSF MANAGEMENT STANDARD
A3-5 HYDROGEOLOGICAL CHARACTERISATION
Potential interactions between groundwater and tailings must be considered from the site selection process through design, construction, and operations and into the Closed Active and Closed Passive phases periods.
The primary role of hydrogeology in TSF design is to:
• Demonstrate that the facility will comply with environmental compliance criteria
• Reduce the risk and uncertainty in the design process by providing a clear and quantitative understanding of how the TSF is expected to interact with the natural groundwater system.
The EoR shall develop a conceptual hydrogeological model (CHM) for all TSFs, of all consequence classifications, combined with numerical modelling, to reduce design uncertainty and minimize the risk around hydraulic gradients in the groundwater system, hydraulic properties of foundation materials, heterogeneity of tailings and natural materials, and the operational performance of engineered drainage systems.
The CHM is integral to all phases of TSF development, including site selection, design, operations, and closure. Hydrogeologic investigations and CHM development can often be coupled with geotechnical investigations in the early design stages.
When utilised at the design stage, a good CHM can
help mitigate the risk of water-related issues during TSF operation. Design decisions that rely on the CHM may include selecting a liner system if required, seepage collection and control systems, and design and optimisation of the site monitoring program.
Notably, the CHM should be updated throughout the life of the TSF as new information and data become available. The CHM is key to successfully designing groundwater monitoring systems. Often monitoring programs are designed around monitoring bore networks installed during design phase investigations.
However, in complex geology, reliable prediction of seepage flow pathways and migration rate can be challenging, and additional piezometers and groundwater sampling wells may be required as the understanding of the CHM evolve over the facility's life. The EoR should consider this.
In support of the proper consideration of hydrogeology during TSF design, operation, management, and closure, the EoR and RTFE are encouraged to:
• Engage a qualified tailings hydrogeologist as a key member of
the design and investigation team from the beginning, through operations, to closure.
• Identify groundwater occurrence, key hydrogeologic features, and aquifer geometry early in the design phase and develop a conceptual hydrogeologic model of the groundwater system.
• Evaluate how preferred design elements may interact with the natural groundwater system.
• Using models as appropriate, use these hydrogeologic assessments to feed into the design and operation of the TSF.
• Consider how TSF-groundwater interactions will change
over the life of the facility as loading increases and hydraulic gradients change, and then also during the Closed Active and Closed Passive phases.
The EOR and RTFE may consider integrating the hydrological model into the geological and geotechnical model. All hydrological studies and information shall be referenced in the site characterisation report.
A3-6 TAILINGS GEOTECHNICAL CHARACTERISATION
The geotechnical parameters of the proposed and existing tailings material play a key role in evaluating the stability of a TSF structure. Therefore, the minimum requirements when considering the geotechnical characterisation of tailings residue are as follows:
• Cone Penetrometer Testing with pore pressure measurements (CPTu) in conjunction with laboratory testing shall be carried out sufficiently early prior to the design and construction of upstream lifts.
• CPTu tests shall be carried out annually for Extreme, Very High, and High consequence facilities that are not yet in a safe state of closure, irrespective of the wall-building approach. However, due to some special/unique conditions, it is acknowledged that it may not be feasible for a specific operation to conform to this topic. In such cases, the operation could apply for a variance and obtain written approval from the Group Head of Tailings.
• Laboratory testing is most successful if used to refine and enhance the results obtained from the CPTu. In other words,
a synthesis of in situ and laboratory tests provides the best currently available means to characterise tailings. Please refer to Gold Fields Guidance Note 414.
The scope and rigour of geotechnical investigations and laboratory testing campaigns, including field and laboratory testing, must be commensurate with the facility's consequence class, risk profile and complexity. This information will be summarised in the site characterisation report, but it shall be referenced in the Design Basis Report (DBR.)
All field investigations and laboratory testing campaigns shall follow Gold Fields Guidance Note 4 . The primary aim of Guidance Note 4 is to outline a framework that can be applied when undertaking field investigations and laboratory testing at Gold Fields TSF sites to enable a consistent and credible assessment of potential risks.
14TSF Guidance Note 4. Minimum Requirements for Field and Laboratory Testing. Rev 1. February 2022.
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A3-7 TAILINGS AND WASTE ROCK GEOCHEMICAL CHARACTERISTICS
Geochemical considerations shall be evaluated during the conceptual and preliminary feasibility stage of all TSF projects. Geochemical risks are challenging to reverse; thus, Gold Fields strives to be proactive in this regard. The potential environmental hazard of tailings is largely determined by their acidity, alkalinity, and susceptibility to leaching and/or oxidation.
Tailings geochemical characterisation should be carried out in conjunction with process water quality testing. The testing should recognize the potential for longer-term geochemical changes. The EOR and RTFE may also consider undertaking sampling for geochemical testing during proposed site investigation works.
To assure safe closure conditions, it is essential to evaluate the acid mine drainage (AMD) potential in the long term and understand the factors that may control the mobility of potentially toxic elements (PTE).
The minimum requirements are:
• Geochemical testing and characterisation shall be completed on tailings and waste rock or ore before deposition on a TSF or use as a construction material.
• Additionally, geochemical testing and characterisation shall be completed on soil and rock materials used to construct a TSF, including environmental geochemistry.
• Where applicable, tailings solutions shall meet the site-specific ICMC requirements for weak acid dissociable (WAD) cyanide (CN), typically being ≤ 50-ppm or lower concentration, and/or as determined by legal compliance criteria and/or risk-based approach at the point of discharge into the TSF.
• Assessment of the tailings geochemistry, including acid-base accounting, metals analysis, leachate testing and reaction kinetics under oxidising conditions, and the presence of process reagents.
• Assessment of potential attenuation characteristics of the tailings, the TSF embankment and foundation and the groundwater geochemistry.
• Static geochemical tests should be conducted on representative tailings samples as they provide information on bulk geochemical characteristics of materials. However, they do
not provide information on rates of chemical processes or how weathering products are released.
• Kinetic testing should also be conducted at a minimum,
where the static testing results indicate the potential for acid generation from the tailings. Kinetic testing is a procedure used to measure the magnitude and/or effects of dynamic processes, including reaction rates, material alteration, drainage chemistry, and loadings from waste material resulting from weathering.
Unlike static tests, kinetic tests measure the performance of a sample over a prolonged period. Material composition and/ or environmental conditions are often simplified or controlled to permit measurement of the physical, chemical, or biological characteristics, processes, or relationships of interest. Kinetic tests have many different forms and locations, including lysimeters, field test pads, leaching columns, and humidity cells.
In Acid Mine Drainage (AMD) studies, the most common form of kinetic test is laboratory procedures designed to determine the quality of water and rates of reaction resulting from the interaction of water and the mined material. Laboratory kinetic testwork should be supplemented with longer-term in-field or on-site kinetic tests for potentially acid-generating materials.
The MEND Report 1.20.1. ‘Prediction Manual for Drainage Chemistry from Sulphidic Geologic Materials’ (Price 2009) is a comprehensive document that can guide collecting, presenting, and interpreting geochemical characterization data of tailings, waste rock, and blended materials. This includes presenting data from elemental, mineralogical, and ARD testing methods.
The Global Acid Rock Drainage Guide (INAP 2014) is another comprehensive source and guidance document on these topics.
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GROUP TSF MANAGEMENT STANDARD
A3-8 TSF WATER BALANCE AND WATER MANAGEMENT
As overtopping is a leading failure mode to be considered, the presence of water exacerbates the consequences of a potential failure, even if the water is not an initial failure trigger. Water, hydrological and hydrogeological considerations should be summarised and referenced in the site characterization report.
This section of the standard discusses Water Balances, Water Management Plans and the impacts of storing water on TSFs.
Water balance
Develop, implement and maintain a water balance model and associated water management plans for the TSF, taking into account the knowledge base, including climate change risk as a result of increased rainfall, upstream and downstream hydrological and hydrogeological basins, the mine site, mine planning and overall operations, and the integrity of the TSF throughout its lifecycle. In addition, the water management program must be designed to protect against unintentional releases.
The inextricable linkage between tailings and water management necessitates a good understanding of all water inflows and outflows to a tailings facility, including variations over time and uncertainties in those variations. Many credible failure modes for tailings facilities are rooted in water management.
A water balance for the mine site as a whole and the TSF
shall:
• Consider quantifying inflows and outflows of water to the site and flows within the site.
• Outline critical risks and opportunities for a site (and given facility) with respect to water management, as identified and explained using the water balance model for critical facilities and consideration of regulatory, social, and environmental aspects of the broader catchment.
• Consider risks and be integrated with broader risk processes, and an action plan should be developed and executed.
• Recognise and evaluate potential implications of uncertainty with the complexities of TSFs. Perform sensitivity analyses to evaluate the impacts of uncertainty in inflow and outflows.
Water management plans
• Consider the impacts of climate change, including:
o Gathering reliable data is essential for developing a life-
cycle plan for current and future operations incorporating
adaptation to climate change.
o Extended periods of high rainfall.
o An increase in the intensity and frequency of extreme
precipitation events, like the increased intensity of a
Probable Maximum Precipitation (PMP) event.
• Allowance for redundancy in decant systems and stormwater
diversion systems
• Collection of reliable data to develop a life-cycle plan for current and future operations incorporating adaptation to climate change.
A water management plan may also be referred to as a surface water or hydrological plan, and subsurface water management plans may be referred to as a hydrogeological plan. The water management plan may be a stand-alone document or may be fully integrated into the site’s water management plan.
When developing a water management plan for a TSF (within the context of a site-wide water management plan), the plan shall:
• Clearly define site-wide strategies and objectives for water management, including relevant legal requirements and any additional social and environmental commitments the operation has made, such as protecting against unintentional releases.
• Be underpinned by a regularly updated water balance model.
• Consider surface and subsurface, hydrological and hydrogeological properties.
• Define operational rules and performance indicators for given facilities and performance indicators and incorporate these rules into the OMS manual.
• Include three types of climate scenarios:
o Historical scenario with historical climate inputs to calibrate and validate the model.
• •
o Deterministic forecasting scenarios, including average, relevant wet/dry, and user-defined climate conditions, typically a mixture of wet/dry and average climate conditions.
o Stochastic forecasting to provide an understanding of climate/hydrologic variability, including the potential for climate change and the risks to current and planned water management scenarios.
Present results that include graphs comparing modelled versus monitored data to allow for model validation at each update.
Be updated, calibrated, and validated on an annual basis.
Runoff generated outside a TSF shall be diverted from the facility unless approved for collection as make-up water. Temporary and permanent TSF-related stormwater structures shall be designed and constructed to convey the design storm event determined and documented by the risk-based design approach with site-specific data or otherwise specified by local regulatory requirements, whichever is more stringent.
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During the water balance development process, the input shall be gathered from site personnel responsible for managing the site's water aspects, such as mining, processing plants, engineering, environment, tailings, and water management.
In addition, the TSF water balance must be updated throughout the operation to reflect changes in mine plans, ore geochemistry, processing, and operations annually. The EOR and RTFE shall refer to the Group Water Management Guideline19.
The risk of storing water on TSFs
TSFs shall not be used to store or hold excess water beyond the design limits during operations or closure unless designed and required for special circumstances, e.g., Cerro Corona, where excess water is stored to provide operational water during the dry season. However, this clause does not preclude the development of supernatant ponds with clarification as the core function.
Large water ponds within in-pit TSFs lead to poor tailings consolidation resulting in lower storage capacities, impacts on the geotechnical stability of the pit slopes, increased risk to potential groundwater migration and contamination, and greater challenges when attempting to rehabilitate the facility where capping of the final tailings surface is required.
The control of the supernatant pond is probably one of the most critical procedures in managing a TSF. Inadequate pond control can result in overtopping, increased pore pressures, freeboard reduction, high seepage rates, and embankment settlement.
The minimum requirements are:
• Where TSFs have a volume of water stored outside the limits defined in the site water balance or OMS (such as due to storm surges and/or upset conditions), a risk assessment must be completed with participation from the RTFE and the Engineer of Record to identify the additional risks resulting from excess water storage.
• In such cases, actions would need to be taken immediately to remove the excess water as soon as practicable. In addition, a review of possible alternative water storage options must be included.
• In some cases, additional water storage may be required to limit sulphide oxidation. In such cases, attempts must be made to limit the volume of stored water, and the facility must be designed to safely accommodate the additional water.
Where storage of excess water is necessary for above-ground TSFs due to process commissioning, topography, hydrological and/or geochemical issues, an application supported by a justification, risk assessment, and a study of alternative options is required.
A3-9 UNDERGROUND MINING IMPACT
The proximity of underground workings to surface TSFs, in-pit TSFs and the surrounding geological and hydrogeological conditions must be accurately assessed and described as part of the site characterisation process. In addition, the potential for tailings slurry and supernatant water inflow into the underground workings under high hydraulic heads needs to be evaluated and documented. This information should be summarised and documented in the site characterisation report.
The stability of underground mines in the vicinity of an in-pit tailings facility may be jeopardised and should be investigated. For example, liquefied tailings may rush into underground voids resulting in catastrophic consequences, or the increasing weight of the overlaying tailings may cause convergence in underground roadways.
The RTFE must work with the Mining Engineers to meet this requirement.
19Group Water Management Guideline. Version 2019. October 2019.
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GROUP TSF MANAGEMENT STANDARD
A3-10 BLASTING IN CLOSE PROXIMITY TO A TSF
Managing the vibration impacts of blasting is a challenge for mine planners and operators. Production blasting is integral to operations in an open-pit mining environment. TSFs are part of the mine infrastructure as they often form part of waste stockpiles produced from the waste and overburden mined from the pit. Consequently, TSFs are often subject to blasting impacts.
For the operation, the main questions are, “how close can mine blasting progress towards the TSF?” and “what is the maximum vibration that the structure can be safely subjected to?” For the EoR, the key concerns are potential modes of failure, the consequence of failure, the likelihood of failure, and risk management.
A3-11 REPORTS AND DELIVERABLES
The RTFE and the EoR shall define the acceptable blasting limits for TSFs, impacts on various facility elements and failure mode analysis. In addition, risk mitigation measures shall be considered, including design, operation, and monitoring controls. This information should be summarised and referenced in the site characterisation report.
As a minimum requirement, for upstream raised facilities, the ground vibration at the toe of the TSF due to a blast shall not exceed a peak particle velocity (ppv) of 50 mm/s. The ppv for downstream raised facilities shall not exceed a ppv of 100 mm/s.
Each site, led by the RTFE, shall develop and maintain, at a minimum, the following deliverables:
Site Characterisation Report16
• Produce a site characterisation report and update this knowledge at least every five years and whenever there is a material change to the tailings facility or the social, environmental and local economic context.
• One site characterisation report shall be prepared and maintained where there are multiple tailings storage facilities. In addition, the tailings' physical and chemical properties shall be characterised and updated regularly to account for ore properties and processing variability.
Seismic Hazard Assessment
• Develop a seismic hazard assessment commensurate with the consequence classification of the facility.
Site Geotechnical Investigation
• Carry out a geotechnical investigation and produce a subsequent report. Update the site characterisation report and the geological and geotechnical models with this latest information.
• The EoR shall develop a detailed 3D engineering geological and geotechnical model for all Consequence Category facilities.
Geological, engineering geological and geotechnical models
• The EoR shall develop a detailed 3D engineering geological and geotechnical model for all TSFs.
Hydrogeological characterisation
• A CHM needs to be developed for the TSF and/or site.
• The CHM needs to confirm that the TSF complies with legal and permitting conditions and does not impact beneficial uses of the local surface and groundwaters
• The proposed groundwater monitoring system needs to be underpinned by the CHM.
Tailings Geotechnical Characterisation
• Characterise the tailings in accordance with Gold Fields Guidance Note 4 and produce a subsequent report.
• CPTu tests shall be carried out annually for Extreme, Very High, and High consequence facilities that are not yet in a safe state of closure, irrespective of the wall building approach.
16GISTM Requirement 2.2
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Tailings and Waste Rock Geochemical Characterisation
• Characterise the tailings and waste rock and produce a subsequent report.
TSF Water Balance and Water Management
• Develop a water management plan and water balance for the TSF.
• The TSF water balance must be updated throughout the operation to reflect changes in mine plans, ore geochemistry, processing, and operations annually.
• The site-specific tailings risk assessment needs to capture water management risks and costs.
• If excess water is stored on a TSF, an application supported by a justification, risk assessment, and mitigation strategy for storing excess water must be submitted in writing to the Engineer of Record.
Underground Mining Impact
• Provide evidence that the stability of underground mines in the vicinity of an in-pit tailings facility is not compromised.
Blasting in Close Proximity of a TSF
• The RTFE and the EoR shall define and document the acceptable blasting limits for TSFs, impacts on various facility elements and failure mode analysis.
• Risk mitigation measures, including design, operation, and monitoring controls, shall be considered.
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GROUP TSF MANAGEMENT STANDARD
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TOPIC 4

INTRODUCTION
TOPIC 4
TSF DESIGN
Topic 4 of this Standard discusses the various elements involved in the design of a TSF. This Standard aligns with the guidance, legislation and Standards listed in sections A1-6.
• A4-1 – Options and Multi-Criteria Analyses
• A4-2 – Risk management, Failure modes and ALARP
• A4-3 – Dam break study
• A4-4 – Consequence classification
• A4-5 – Population at Risk (PAR) and Probable Loss of Life (PLL)
• A4-6 – Industry Standard Design Criteria and Design Basis
• A4-7 – Stability analyses
• A4-8 – Factor of safety (or prescriptive approach)
• A4-9 – Performance-based design
• A4-10 – Rate of rise and consolidation
• A4-11 – Tailings transportation and deposition plan • A4-12 – Waste placement plan
• A4-13 – In-pit tailings deposition
• A4-14 – Cyclone deposition
• A4-15 – Placement of tailings filter cake
• A4-16 – Special design considerations
• A4-17 – Pond control and size
• A4-18 – Instrumentation and monitoring plans
• A4-19 – Progressive Rehabilitation and Closure Design • A4-20 – Design Reports and deliverables
A4-1 OPTIONS AND MULTI-CRITERIA ANALYSES
Stakeholders involved in the design and planning of a new TSF or raise shall use the knowledge base and site characterization information to undertake a multi-criteria alternatives analysis17 of feasible sites, technologies, and tailings management strategies. These alternatives shall at least be considered at scoping and PFS.
Options analyses aim to select an alternative that minimises risks to people and the environment throughout the tailings facility lifecycle and minimise the volume of tailings and water placed in external tailings facilities.
For existing tailings storage facilities, options and multi- criteria analyses provide a means of integrating a wide range of relevant information into the decision-making process and provide a basis for documenting outcomes that can then be used to demonstrate the basis for decisions to:
• Senior management
• Regulatory agencies
• Investors and insurance providers
• Potentially affected communities.
The process allows for the transparent consideration of environmental, technical, socio-economic, and project economics factors and allows testing the outcomes under different assumptions.
The evaluation of alternatives can be used to inform a range of decisions, such as the selection of the preferred options for:
• Tailings management technology to be used.
• Increasing the capacity of existing tailings facilities.
• A material change in tailings facility design (change in wall raising strategy and technology).
• Re-activation of an existing tailings facility.
• Progressive rehabilitation and/or closure design.
To be effective, it is essential that the evaluation of
alternatives:
• Be conducted by a multi-disciplinary team to be able to interpret and assess the full range of information considered in the process.
• Have input from Independent Review during the design of the evaluation of alternatives and through the steps in the process.
• Be appropriately scaled and scoped to the planning decision to be made.
• Have input from potentially affected communities when appropriate (e.g., new tailings facilities, closure planning).
• Consider the performance objectives and risk analysis and integrate those into decision criteria to evaluate alternatives.
• Consider Opex and Capex across the entire life cycle.
• Consider all aspects of the project, direct or indirect that may contribute to evaluating each alternative (e.g., design of the mine and ore processing to the extent that they would impact tailings production, water management, and treatment).
• Consider and integrate a wide range of information about the characteristics of each alternative being evaluated and relevant to the planning decision to be made, such as:
• Technical considerations (e.g., geotechnical, geochemical, mine operations).
• Environmental considerations (e.g., potential impacts on terrestrial and aquatic ecosystems) and Socio-economic considerations (e.g., potential impacts on communities and other economic, recreational, spiritual, or subsistence activities).
• Project economics (e.g., short-term, and long-term capital and operating costs).
Consider each alternative across the relevant phases of the lifecycle of the tailings facility (e.g., for new tailings facilities, consider the lifecycle implications of each alternative from construction through to closure and post-closure).
17GISTM Requirement 3.2
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GROUP TSF MANAGEMENT STANDARD
A4-2 RISK MANAGEMENT, FAILURE MODES AND ALARP
Site-specific tailings risk assessments
A Risk Management Plan or tailings risk assessment addresses all risks associated with a proposed or existing TSF and
its supporting infrastructure. This RMP or risk assessment should document technical, project and ESG-related risks and hazards. The assessments and plans should be developed by an interdisciplinary team and used as the basis for engaging with the external community. The tailings risk assessments must capture human rights, community, environment and other ESG-related risks in addition to project and technical risks. This assessment should be formed at the Conception phase of any TSF project and grow over time.
Failure modes
The next risk identification step is identifying site-specific potential failure modes using a Potential Failure Mode Analysis (PFMA). A potential failure mode is a cause of failure, a chain of events (event tree), or one possible way a system can fail.
Once site-specific potential failure modes have been identified, they should be characterised to determine if they are credible and then determine the likelihood of occurrence if they are credible using a Semi-Quantitative Risk Analysis (SQRA).
Credible failure modes are failure mechanisms that are technically feasible given the materials present in the tailings facility and its foundation, the properties of these materials, the configuration of the tailings facility, drainage conditions and surface water control at the tailings facility throughout its lifecycle under the static and transient loading conditions the facility may be subject to over that lifecycle.
The term ‘credible failure mode’ is not associated with a probability of this event occurring, and having credible failure modes is not a reflection of facility safety.
Credible failure mode = credible mechanism AND credible initiating event AND credible failure process.
Credible failure modes typically vary during the lifecycle of a tailings facility as conditions change. Not all tailings facilities have credible failure modes, and not all credible failure modes could lead to a catastrophic failure.
The minimum requirements are:
• A potential failure mode may be non-credible if ruled out categorically during initial screening (PFMA, SQRA, etc.).
• For each credible failure mode that still exists, the likelihood of the event leading to specific consequences shall be estimated, including the likelihood of the specific loading condition and the likelihood of an adverse structural response to the event.
• Event trees help illuminate the likelihood of an event occurring (along with an adverse structural response).
• Quantitative estimates of the probability of failure and the consequences can be obtained using an event tree.
• Such credible failure scenarios fall into two basic categories,
Risk credibility
based on the behaviour of the material if a failure occurs, and thus the methods used to conduct a more detailed analysis of potential consequences:
1) Credible failure scenarios that would include a flow of materials – water alone or water and solids (i.e., tailings and other entrained solids such as soil) – into the downstream environment. Flow failures are the failure mode most often associated with catastrophic consequences when failures occur.
2) Credible failure scenarios with potentially catastrophic consequences but not related to a flow of materials into the downstream environment (e.g., a slump of tailings solids with limited water).
Once site-specific potential failure modes have been identified, they should be characterised, first to determine if they are credible and then to determine the likelihood of occurrence if they are credible.
A Failure Mode and Effect Analysis (FMEA) shall be used to evaluate potential failure modes for a tailings facility.
An FMEA:
• Is a structured, logical framework that allows informed operatives and specialists to systematically use available knowledge and information to lead to and understand the sources of risk in a tailings facility or a tailings containment system.
• Determines the effect(s) of a potential failure mode on the facility's operation, for each basic element of the system (and subsystems), along numerous paths until the effect on the overall system function is known.
• The severity of the consequences due to each failure is then classified, and the probability is estimated.
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Risk analysis
Semi-Quantitative Risk Analysis (SQRA) is a process to evaluate the significance of potential failure modes from a risk perspective. SQRA is a risk categorization system that assigns likelihood and consequence categories to potential failure modes based on existing data and available consequence estimates.
Uncertainty is the result of imperfect knowledge about the present or future state of a system, event, situation or population under consideration. To manage risk, uncertainty should be acknowledged, assessed and considered.
Risk estimates will have a degree of uncertainty that should be characterised. This includes acknowledging subjectivity in estimating risk, reflecting various factors such as the experience and expertise of those involved in developing the estimate, the models used, and the comprehensiveness of available site characterisation information. Uncertainty may be represented by assigning ranges to estimates of both likelihood and consequence.
Risk evaluation
Risk evaluation compares the outcomes of risk analysis for existing conditions to determine if risks are within acceptable limits, whether present risk measures and controls are adequate, and what additional alternative risk reduction measures could be considered.
The process typically considers the following aspects: robustness of design, past and future performance monitoring, site context, and practicality of any remediation considered.
Guidelines from regulatory agencies, governing bodies, other industries associated with tailings facility safety, and corporate governance should all be reviewed to determine what risks are within normal operating limits. Understanding environmental, social, cultural, ethical, political, and legal considerations should also be included in risk evaluation.
The team typically considers risk mitigation alternatives at this stage. The risk assessment outcome includes recommendations for actions deemed justified by the team.
ALARP
A fundamental principle in achieving tolerable risk is “reducing risks as low as reasonably practicable” (ALARP). The ALARP level is reached when the time, trouble and cost of further reduction measures become unreasonably disproportionate to the additional risk reduction obtained.
Accordingly, the following factors are commonly taken into account in making a judgment on whether risks are ALARP:
• The level of risk in relation to the tolerable risk limits. What can
be done – what is possible to ensure health and safety?
• The disproportion between the cost (money, time, trouble and effort) of implementing the risk-reduction measures and the subsequent risk reduction achieved
• The cost-effectiveness of the risk-reduction measures
• Compliance with well-established practice
• Risk importance measures; and
• Societal concerns revealed by consultation with the community
and other stakeholders.
Thus, the ALARP evaluation and demonstration are qualitative and quantitative. Consideration of the cost-effectiveness of achieving life-safety risk reduction relative to the life-safety benefit achieved is a quantitative aspect.
However, it introduces the consideration of cost only to justify further risk reduction below tolerable risk limit(s) and not to justify achieving the limits in the first place.
The ALARP evaluation and approach for a specific TSF will be collectively agreed upon and decided upon by the RTFE, EoR and Independent Technical Review Board (ITRB).
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GROUP TSF MANAGEMENT STANDARD
A4-3 DAM BREAK STUDY
Following the completion of an FMEA, the team must develop and document a breach analysis for the tailings facility using a methodology that considers credible failure modes, site conditions, and slurry properties. The results of the analysis shall estimate the physical area impacted by a potential failure.
Different levels of assessment can be completed (initial, intermediate, or comprehensive) depending on the likelihood of credible failure modes and the anticipated level of consequence. This is influenced primarily by the population exposed to the potential dam failure hazard (termed the population at risk (PAR)), the amount of downstream development, and the anticipated severity of the flooding (warning time, depth, velocity, and duration).
In cases where there are credible flow failure scenarios, in other words, loss of containment is possible; a breach analysis should be conducted to assess in more detail the potential consequences if a failure were to occur.
Initial dam break assessment
An initial dam-break flood hazard and consequence assessment may be sufficient to determine the impact of a TSF from existing knowledge or a qualitative estimation of the magnitude of a potential dam-break flood and a visual inspection of the flood path, and its potential impacts on people, property and the environment.
Such an assessment should be completed conservatively and is often restricted to considering a hypothetical, instantaneous and complete breach failure mode (except for long embankments or ‘turkey nest’ storage embankments where a realistic breach length should be nominated).
However, the initial assessment may raise uncertainties (e.g., in open flat areas and where buildings/infrastructure are close to the edge of inundation) that can only be resolved by undertaking an intermediate or comprehensive assessment.
Intermediate dam break assessment
An intermediate dam-break flood hazard and consequence assessment require a more quantitative assessment of the magnitude of a potential dam-break flood and downstream consequences than an initial assessment.
Therefore, an intermediate-level assessment should consider potential failure modes and include the estimation of breach parameters to a greater level of detail (e.g., considering breach size and development time appropriate to the dam type and the nature of the foundation), the likely extent of flood inundation, the impact on the population at risk, and the level of possible damage and disruption.
Comprehensive dam break assessment
Comprehensive dam break studies shall be conducted following the CDA Technical Bulletin18. A comprehensive assessment is typically required for TSFs with higher consequences and, therefore, requires detailed outputs for emergency planning and preparedness or developing risk reduction measures.
The process for a comprehensive dam-break flood hazard and consequence assessment is similar to an intermediate assessment.
However, the completion of a comprehensive assessment usually requires the identification and consideration of potential failure modes, dam-break flood routing, mapping of the extent of flood inundation, and evaluation of the peak flood depth, flow velocity, time of flood arrival, time of flood peak and inundation duration at key locations (e.g., buildings and infrastructure). It would usually also require the completion of a detailed damage and loss assessment
Suppose the anticipated level of consequence is high and credible failure modes exist, or the TSF has a Consequence Classification of 'High,' 'Very High,' or 'Extreme.' In that case, a comprehensive assessment should be completed.
On the other hand, if credible failure modes are unlikely, the anticipated level of consequence is low, and the hypothetical dam break is confined to a well-defined flow path, then a low or intermediate assessment may be appropriate.
18CDA Technical Bulletin: Tailings Dam Breach Analysis (2021 Edition).
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For credible failure scenarios that would include a flow of
material, a breach analysis shall be conducted to estimate:
• The physical area that a potential failure would impact.
• Flow arrival times at various downstream locations (e.g., communities, bridges).
• Flow depth and velocities at various downstream locations.
• Duration of flooding.
• Depth of material deposition.
For credible failure scenarios unrelated to a flow of material, conduct an appropriate analysis (e.g., simplified deformation analysis) to estimate the potential consequences of a failure. The key requirements are:
A4-4 CONSEQUENCE CLASSIFICATION
• Dam break studies shall be conducted whenever there is a material change to the facility or the physical area impacted. Whether the change impacts the breach result or not shall be discussed between the RTFE and the EoR and endorsed by either an independent reviewer or the ITRB.
• Where two TSFs are adjacent to each other, within the area
of inundation or run-out influence of one another, or share a common embankment, dam break analyses, run-out analyses, and consequence classifications shall include potential impacts from one facility onto the other.
• Cascading flow scenarios shall be considered in specific circumstances and based on credible failure modes.
• Dam break studies shall be based on topographical survey data not older than 2 years.
Once credible failure modes have been identified, a preliminary analysis shall be conducted by the EoR to identify and assess the scenarios that could develop and the potential consequences of those scenarios, including impacts on human health and safety, the environment, and infrastructure.
The assessment and selection of the consequence classification shall be based on credible failure modes and shall be defensible and documented.
Consequences inside the inundation area could be directly caused by flowing water and debris, typically injury and loss of life and damage to structures and infrastructure. Consequences outside the inundation area could be indirectly caused by a dam failure, such as unemployment caused by the closure of a damaged business inside the area.
Consequences outside the inundation area will be indirect and result from disruptions in the provision of services or the lack of a commodity normally provided by assets in the inundation area.
The consequence classification (Extreme, Very High, High, Significant or Low) is based on the potential for damage and loss in the unlikely event of a dam failure. In addition, the consequence classification considers the extent of the dam breach inundation zone and Population at Risk (PAR). The consequence classification analyses are completed independently of the probability of occurrence of the unwanted event that could trigger the failure.
Several variables are used to estimate the loss of life, including flood depth and velocity in downstream populated areas, the location/type of impacted structures, the amount of available warning time, and the likelihood and effectiveness of warnings.
While the loss of life is always paramount in dam safety, dam failures have a myriad of consequences. Therefore, evaluation of consequences other than loss of life gives the dam owner a complete picture of the risk of their dam.
The consequence classification determines the design criteria for a particular dam, with dams with higher failure consequences having higher design criteria. The consequence classification is then used to inform the external loading component of the design criteria and the level of engineering and governance required during the TSF life cycle.
The GISTM Consequence classification dictates the conformance deadline for a TSF against the GISTM. However, the impact on the Owner’s business (production loss, supply chain impacts, financial losses, reputational damage etc.) is not directly covered by GISTM. Instead, the GISTM refers to the financial impact on the communities and the businesses in the communities.
Dam Safety guides/standards typically are geared toward protecting People and the Environment and are not about protecting the Owner’s business.
At our discretion, as the Owner, we may increase the Consequence Classification to address the impact on our business.
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Although not mandated, also consider the cost of the following under the Infrastructure and Economics column (GISTM consequence classification table):
• Clean up of a failure, which will likely include compensations/fines, etc.
• Mine or processing infrastructure.
• Business interruption costs. Consider the cost of not operating for 3 to 6 months (like Cadia) or 4 to 5 years (Samarco, etc.).
• For example, if people in the community would lose employment because of a plant shutdown, that is included.
• Conversely, economic loss to the company due to a plant shutdown would be a separate consideration.
The Consequence Classification shall be reviewed at each Dam Safety Review (DSR) and at least every five years or sooner if there is a material change in the social and/or environmental context.
If a change to a Higher Consequence Classification occurs, design upgrades of the TSF to accommodate the new Consequence Classification as determined by the DSR or third-party review shall be completed within three years. The Consequence Classification should be documented in the Design Basis Report. (DBR)
A4-5 POPULATION AT RISK (PAR) AND PROBABLE LOSS OF LIFE (PLL)
The PAR represents an estimated number of people who would be present within the inundated regions assuming they took no action to evacuate. A key feature of this definition is that evacuation and warning are not considered when evaluating PAR. It should also be noted that the presence of the Operator’s staff and contractors in the inundation area should be considered.
The evaluation of PAR should include both permanent and temporary populations. Permanent populations are those linked to a fixed location on a permanent basis (e.g., their legal residence or their place of work). Temporary populations are considered those who do not usually live or work in the dam-break flood area but are temporarily visiting.
It is well established that permanent and temporary populations in a given area will vary according to the time of day, day of the week and season as they move for work, school or recreation. Therefore, to deal with the potential changes in PAR, it is required that PAR is identified for the greatest period of risk.
The Potential Loss of Life (PLL) should be assessed following the ANCOLD Guidelines on Consequence Categories for Dams (2012) and Graham (1999)19 .
The PLL is the incremental number of people from among the PAR who would potentially lose their life in case of an unwanted event, even after emergency response and preparedness plans have been in place and activated. The incremental number is defined as the additional potential loss of life caused by a dam breach/structural failure/loss of containment/surface or groundwater contamination compared to the same climatic event occurring without the dam breach or structural/containment failure of loss.
The PAR and PLL should be documented in the Design Basis Report. (DBR)
A4-6 INDUSTRY STANDARD DESIGN CRITERIA AND DESIGN BASIS
Design Basis and Criteria
The design basis and criteria shall be adopted in a TSF’s design and must be developed in accordance with the mine plan, process flow sheet, license conditions, and other operational requirements. In addition, the design criteria must meet all applicable regulatory and permitting obligations and leading practice principles.
The design basis lists the key characteristics and parameters adopted in the design to achieve the requirements listed in the design criteria.
Design criteria are the key elements of a safe design and are typically related to the main failure modes. In addition, the design criteria should consider the consequences of plausible failure modes and have a technical basis for their occurrence. Design considerations, including scheduling, interaction with other structures, constraints, or limitations, borrow sources, etc., are also listed with the design basis.
The design criteria should be summarised and documented in a Design Basis Report (DBR). All stakeholders shall agree upon the content of this report before advancing the design. This report will be independently reviewed by an Independent Review or Independent Technical Review Board (ITRB.)
19A Procedure for Estimating Loss of Life Caused by Dam Failure, DSO-99-06, US Department of Interior, Bureau of Reclamation: Denver Colorado.
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The design criteria are explicit goals and/or defined targets that the design must meet/achieve throughout its lifecycle, including closure and post-closure phases.
General Considerations
General considerations are:
• Include the design criteria and design basis in the Design Basis Report (DBR) and submit it to the RTFE, independent reviewer(s), and the Independent Technical Review Board (ITRB) for approval before the commencement of the design. Any changes in the design basis must be subject to a Management of Change (MoC) process.
• The engineering properties of tailings are essential for the safety evaluation and engineering design of tailings storage facilities. However, the characterisation of mine tailings is
a difficult and an underestimated problem in geotechnical engineering. The engineering properties must be referenced in the site characterisation and DBR report.
Flood design criteria
Flood design criteria shall be defined during the design basis development and shall be documented or referenced in the DBR. The minimum requirements are:
• Investigations and modelling shall consider the risk to the facility
of extreme precipitation and/or drought events.
• The minimum dam safety design criteria for flood design for TSFs shall not be less than that provided in Table 5. These criteria are intended to prevent failure of the facility resulting in the tailings and/or supernatant water release.
Seismic design criteria
Seismic design criteria, as discussed in section A3-2 of this Standard, shall be defined during the design basis development. The criteria shall be summarised and referenced in the site characterisation report and documented in the Design Basis Report. The minimum requirements are:
• A seismic hazard assessment (SHA) is required to determine the magnitude and return periods of earthquakes considered in the design.
• The SHA shall consider probabilistic and deterministic approaches in line with the facility consequence classification, select design criteria that meet jurisdictional requirements and reduce operational and closure risk.
• The SHA should consider developing results for the “Extreme” classification to support future increases in the consequence classification per the GISTM.
• The minimum dam safety design criteria for seismic design for TSFs shall align with the guidance provided in Table 5.
• A deterministic approach will be adequate for Low and Significant Consequence Category facilities. (Table 4)
• A probabilistic approach (PSHA) shall be adopted for High, Very High and Extreme Consequence facilities.
• The behaviour of the underlying foundation material is often crucial to the stability of a TSF. However, the methods to characterise a foundation can be more variable than those relevant to the tailings, as the foundation materials are
usually more variable between sites. This information shall be summarised in the site characterisation report and referenced in the DBR.
• Inundation or dam breach study results used to define the consequence classification should be included in the DBR.
• For filtered stacks, target moisture contents for safe transport and geotechnical stability must be evaluated. In addition, the potential for stack saturation, swell potential, and compaction requirements in the field to ensure material maintains dilative behaviour under the full stack height shall be considered.
• All operating and new TSFs must be able to safely store or release at least a 1:2 500 Annual Exceedance Probability (AEP) 72-hr flood through an emergency spillway to protect the facility.
• The emergency spillway must be designed for a minimum 1:2 500 AEP peak flow developed from critical storm derivation.
• Operational spillways (different from emergency spillways) must be designed according to the applicable regulatory requirements.
• Operational criteria shall be adopted for serviceability requirements based on risk assessment. Operational criteria are intended to limit the damage so that the facility remains functional, and operations can continue after repairs to the facility. The Operating basis earthquake (OBE) is an example of operational criteria.
• The earthquake design criteria in Table 5 are for the maximum design earthquake (MDE) or design basis earthquake (DBE) and not for the OBE.
• MCE is based upon a deterministic seismic hazard assessment considering various scenarios.
• The OBE is generally expected to cause limited damage/ deformations that could be repaired without significantly disrupting operations. The OBE must not be less than a 1:250- year event.
• The selection of an AEP of 1:2475 as a minimum design earthquake for High Hazard is based on the typical design earthquake for buildings in certain building codes, the application of this value for dam safety in multiple countries, and its inclusion in the GISTM.
• The hydrologic and earthquake criteria presented in Table 5 are not intended to be applied retroactively. Instead, a risk assessment must be completed to verify that the risk of staying with lower criteria is tolerable.
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Table 5: Minimum hydrologic and earthquake design criteria for TSFs
Consequence classification (GISTM)
Min AEP for IDF* (Active Care)
Min DBE* return period (Active Care)
Min Design earthquake/ IDF return period (Passive Care)**
Extreme
PMF
10 000 years or 84th Percentile MCE
1:10 000
Very High
1: 5 000
5 000 year or 50th Percentile MCE
High
1:2 475
2 475 year
Low and Significant
1:1 000
1 000 year
Notes:
*1) For existing TSFs, the EoR, with review by the ITRB or an independent reviewer, may determine that the upgrade to this design criteria is not feasible or cannot be retroactively applied. In this case, the Accountable Executive shall approve and document the implementation of measures to reduce both the probability and the consequences of a TSF failure to reduce the risk to a level ALARP. The basis and timing for addressing the upgrade of existing TSFs shall be risk-informed and carried out as soon as reasonably practicable.
** 2) The concepts of PMP and PMF are acceptable for assigning flood loading if they meet, or exceed, the requirements above for Extreme Consequence Classification facilities and/or facilities at the Post-Closure (or Passive Care Closure) phase.
The Maximum Credible Earthquake (MCE) is part of a deterministic approach that can govern in some areas. Therefore, the method that produces the most appropriate ground motion for the facility's safety should be used for the design.
Acronyms: AEP – Annual Exceedance Probability, IDF – inflow design flood for freeboard allowance and spillway design, DBE – Design Basis Earthquake, MCE – Maximum Credible Earthquake (has no associated AEP value), PMF – Probable Maximum Flood
Note the following:
• The earthquake design criteria in Table 5 are for the maximum design earthquake (MDE) or design basis earthquake (DBE) and not for the OBE.
• MCE is based upon a deterministic seismic hazard assessment considering various scenarios.
• The OBE is generally expected to cause limited damage/deformations that could be repaired without significantly disrupting operations. The OBE must not be less than a 1:250-year event.
• The selection of an AEP of 1:2475 as a minimum design earthquake for High Hazard is based on the typical design earthquake for buildings in certain building codes, the application of this value for dam safety in multiple countries, and its inclusion in the GISTM.
• The hydrologic and earthquake criteria presented in Table 5 are not intended to be applied retroactively. Instead, a risk assessment must be completed to verify that the risk of staying with lower criteria is tolerable.
A4-7 STABILITY ANALYSES
A stability analysis must be undertaken for each TSF as part of the design process; the inputs to the stability analysis must align with those defined in the site characterisation and design basis report. The minimum requirements are:
• The Stability analyses shall account for the undrained behaviour of contractive materials that could generate excess pore pressures on shearing.
• Relevant material properties and input parameters used to develop the stability analysis methods must involve an assessment of the following aspects:
o State parameter Ψ estimate techniques, characteristic state value selection, and assigning of layers
o Peak undrained shear strength selection
o Residual/liquefied shear strength selection.
• In addition to Effective Stress Analysis (ESA), contractive materials must always be modelled with the Undrained Strength Analysis (USA) approach (Ladd, 1991)20, known as the Su/σv’ approach or equivalent.
• An accurate estimate of the in-situ effective stresses must be made in applying the USA approach to assess the σv’ term. This includes an accurate estimate of pore pressure distribution in
the TSF, especially under undrained conditions and for under- consolidated or loose tailings.
The following aspects shall also be considered:
• For the construction loading case, pre-construction shear strengths must be used for materials expected to demonstrate undrained behaviour (unconsolidated-undrained loading). In addition, a staged construction approach (Ladd, 1991) shall be used for staged impoundment/embankment raises.
• The primary assumption is that the increase in pore water pressures (pore pressures) during construction is equal to
the added total stress leading to no change in the effective stress and no gain in shear strength. However, the potential for construction-induced pore pressure dissipation should also be considered in cases where adequate information is available.
• Pseudo-static stability analyses are not appropriate for the determination of TSF seismic stability. Instead, the post-seismic stability must be assessed using post-seismic, post-peak, or residual strengths, where appropriate. Post-peak or residual strengths can be assessed following the methodology outlined in Jefferies and Been (2016)21.
20Ladd, C.C., “Stability Evaluation during Staged Construction” ASCE Journal of Geotechnical Engineering, Vol. 117, No4, April 1991. 21Jefferies, M and Been, K. Soil Liquefaction: A Critical State Approach. CRC Press. 2nd Edition, 2016.
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• Shear strength estimates used in the design analyses must be based on site-specific field and/or laboratory measurements. Combined with a series of laboratory tests (mainly testing to determine the Critical State Line (CSL) and other properties) on each material type, the in situ ψ of the critical materials is determined.
• Once the in situ ψ has been characterised, laboratory testing should focus on the anticipated and expected engineering behaviour of the TSF.
• Strain incompatibility occurs when the potential failure surface passes through materials in which shear resistance is mobilised at significantly different strains due to differences in initial
stiffness. Under these circumstances, applying peak shear strength values for all materials would overestimate the stability because the materials will not contribute equally to the overall slope stability resistance.
• For example, such situations can occur where softer, more ductile materials (clay-like) are interlayered with zones of stiffer, more sand-like materials. When strain incompatibility
is identified, an appropriate analytical method that considers the stress-strain dependency of the various materials can be used to assess the slope stability under both static and seismic loading conditions.
A4-8 FACTOR OF SAFETY (OR PRESCRIPTIVE APPROACH)
General
The term “Factor of Safety” (FoS) is commonly used to express the safety margin of slopes on embankment dams. An FoS is often misinterpreted as a sole measure of safety. It is based on the premise that a higher FoS reduces the likelihood of failure.
However, an FoS is not a measurable value; it is an outcome based on inputs derived by the designer based on site data, laboratory testing, and modelling. Natural variations in site and laboratory data give rise to uncertainty around the calculated FoS. However, FoS values are rarely reported with uncertainty limits.
Two of the most important considerations that determine appropriate magnitudes for the factor of safety are:
• Uncertainties in the input parameters, including shear strengths.
• Consequences of failure or unacceptable performance.
The target factors of safety, if met, are viewed as acceptable practice. Where these targets are not met, the risk exposure must be managed through further investigations and analyses, supplemented by the comprehensive use of the observational method to reduce uncertainty. Difficult ground conditions and high potential consequences of a failure may warrant higher target factors of safety.
Performance indicative of a potential failure mode is identified and monitored to validate whether the design basis remains sound and, if not, to initiate mitigation measures. The amount by which variances from the expected performance can be tolerated is often then supported by additional design calculations and judgement. The minimum recommended targets for the factors of safety against the instability of TSFs are provided in Table 6.
Table 6: Target FoS for TSF embankment stability
1 For maximum design earthquake (MDE) and not for operating basis earthquake (OBE). For OBE, a higher minimum Factor of Safety (FS) may be needed to limit the damage and keep the facility in a serviceable state.
2 Post-peak strength is not necessarily residual strength. Judgement has to be based on a realistic expectation of shear strains.
3 To be related to the confidence in selecting shear strength. 1.0 may be adequate for use with well-considered lower-bound results. If the Post seismic FS<1.1, deformation analysis is required to validate the design.
Loading condition
Min AEP for IDF* (Active Care)
Static drained and/or undrained with a potential loss of containment
1.5
Static drained and/or undrained with no potential loss of containment. Also, during the construction of embank- ment raises with no potential for uncontrolled release.
1.3
Post seismic1. Also, for post-peak2 conditions.
1.0-1.13
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Considerations for a prescriptive approach
To support a prescriptive approach, the following aspects shall be considered:
• Brittle behaviour (a rapid reduction in shear strength from peak to residual), a characteristic of materials susceptible to static liquefaction, must be investigated, assuming triggering occurs.
• It must, however, be noted that not all contractive materials are susceptible to static liquefaction. For example, many tailings materials in a loose state will exhibit contractive undrained behaviour during shearing. However, if there is no rapid reduction in shear strength from peak to residual, such materials should not be characterised as susceptible to static liquefaction.
• Where avoidance of materials susceptible to static liquefaction is not viable, the stability of the facility shall be checked using a dual approach:
o Aim for a minimum FoS of 1.5, using peak undrained
strengths
o Aim for a minimum FoS of 1.1 using post-peak (generally not
residual) strengths.
• The magnitude of shear strains expected within the materials shall be considered in estimating the post-peak strengths. Arbitrarily assigning residual shear strengths to all contractive materials is not realistic and should be avoided. Instead, the rate of decay from peak shear strength to residual shear strength must be adequately assessed.
• Assessing post-peak behaviour can be somewhat complicated, and some judgment is often required to select an appropriate large-strain strength if the shear resistance is still decreasing within the limits of the testing method used.
A4-9 PERFORMANCE-BASED DESIGN
• For facilities with consequence classification High or higher (as per Table 2), deterministic and probabilistic analyses shall be completed. Probabilistic analyses shall be completed to add insight into the deterministic analyses, help reduce uncertainty, and assist in focussing the effort to reduce risk and implement risk controls. However, probabilistic approaches alone must not be used as stability criteria.
• For deterministic approaches, sensitivity analyses must be completed, especially for high consequence and/or complex ground conditions.
• Care must be taken with over-consolidated soils that become normally consolidated when loaded beyond the pre- consolidation pressures. Undrained strength ratio changes (and potentially the transition from dilative to contractive behaviour) of the foundation material under the new loading of a TSF shall be assessed.
• Where warranted, more sophisticated Finite Element or Finite Difference methods such as FLAC22 or Plaxis23 shall be used with appropriate and well-documented input parameters. For example, such analyses could be required for the following: o Investigating the potential for static liquefaction by
evaluating the magnitude of shear strains.
o Overcoming strain incompatibility issues in limit equilibrium
analyses and
o Estimating post-seismic deformations for Maximum Design
Earthquake (MDE) and/or Operating Basis Earthquake (OBE) conditions.
The designers and EoRs should follow a performance-based design (PBD) approach for new TSFs, new embankment lift designs, and modifications to existing TSFs. This section discusses considerations for the design process and considerations during the Construction, Operations, Closure, and Post-Closure phases.
During the design process:
The performance-based approach adopts a proactive procedure for managing tailings facility performance data. This is accomplished by defining performance objectives using sequential forecasts of the tailings facility behaviour through
all lifecycle phases and verifying that the performance is behaving as intended throughout the lifecycle. In addition, the approach requires an understanding of the facility risk profile based on its current performance and an understanding of the impact of potential changes and future development of the facility risk profile. This understanding assists in making risk- informed decisions throughout the TSF life phases.
• Assess current behaviour. Identify the key failure modes for
the tailings dam that could lead to uncontrolled release or inoperability of the tailings dam. These will be informed by
the risk assessments described earlier. For each failure mode, identify performance parameters (e.g., achieved density of construction material and tailings, pore pressures, deformation, freeboard, etc.) that impact the risk profile of the TSF and hence should be monitored and reviewed during dam construction and operations.
• Use site characterisation data (geotechnical, geologic, hydrogeologic, seismic, climate) to establish performance objectives for the TSF. These objectives should focus on the critical elements affecting safe construction, operation, and closure.
• Forecast behaviour as part of the design process to inform the evolution and finalisation of the design to meet the performance objectives. Forecasting tools (or digital twins) are selected dependant on the complexity of the challenges and the questions that need to be answered.
22FLAC, Fast Lagrangian Analysis of Continua, is numerical modelling software for advanced geotechnical analysis of soil, rock, groundwater, and ground support in two dimensions. FLAC is used for analysis, testing, and design by geotechnical, civil, and mining engineers. It is designed to accommodate any kind of geotechnical engineering project that requires continuum analysis. FLAC utilizes an explicit finite difference formulation that can model complex behaviours, such as problems that consist of several stages, large displacements and strains, non-linear material behaviour, or unstable systems (even cases of yield/failure over large areas, or total collapse).
23PLAXIS 2D WorkSuite integrates the powerful and user-friendly finite element and limit equilibrium analysis capabilities of PLAXIS applications for the design and analysis of soil, rock, and associated structures.
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• The forecasting tools may be relatively simple analytical models, but, where appropriate, forecasting may utilise advanced numerical techniques such as finite element or finite difference models.
•
However, these tools are often initially constructed using case study inputs and the somewhat limited site characterisation data available during the Design phase. Consequently, the ability
to calibrate many of these models during the Design phase is limited.
During the Construction, Operations, Closure, and Post-Closure phases:
During the construction, operations, closure and post-closure phases, some design elements may need to be adjusted to align with actual performance. The following points shall be considered:
• Calibrate and re-forecast the performance of the facility, • comparing it against the performance objectives. If the re-
forecast does not meet the performance objectives, changes to
the design and/or performance objectives may be required, as indicated by the EoR.
• Inform the need for potential changes to the design to improve facility safety and test proposed changes to the design by • predicting future behaviour if the proposed changes are implemented. Then, based on understanding the performance criteria and the impact on the risk profile, establish appropriate triggers and a Trigger Action Response Plan (TARP).
The EoR determines whether the facility’s embankment and foundation are adequately robust to meet the performance objectives. In some cases, the PBD approach can compensate for the limitations of limit equilibrium analyses, and the facility's safety can be described in terms of how well the dam performs and the Factor of Safety.
A4-10 RATE OF RISE AND CONSOLIDATION
Rate of rise considerations are critical for upstream-raised TSFs, and centreline-raised TSFs where the upstream slope stability is critical and dependent on the beach strength characteristics.
‘Rate of rise’ is the height difference by which a tailings beach increases over time. It is often used in tailings design practice as a ‘limiting rate,’ whereby too high a rate of rise can cause concern. The rate of rise is one of the most critical construction- related factors controlling upstream-raised TSF stability.
For gold tailings TSFs raised in an upstream manner, an annualised rate of rise of less than 2.5 m/year has been widely adopted as a maximum acceptable value (e.g., Chamber of Mines of South Africa 1996 and ICOLD 1995)24, with significantly lower rates of rise adopted for other materials, such as 1 m/year for laterite nickel tailings (Williams 2002)25.
The EoR is responsible for recommending detailed design criteria and utilising a Design Team with adequate, relevant experience compatible with the complexity of the assignment. In addition, the EoR and the Operation must always recognise legal requirements applicable to the design process and the selection of design criteria.
The EoR and the Design Team are expected to formulate the procedures and the material property characterisation required to initiate the design process.
• Relative to a precautionary approach, the required instrumentation is expanded to maximize performance validation to the degree considered to be of value.
The following shall be noted:
• For upstream raised facilities, the rate of rise must be low enough to ensure that the outer zone tailings attain adequate shear strength and are sufficiently consolidated to reduce the risk of liquefaction (i.e., maintained unsaturated and in a dilated state) and do not recharge the phreatic surface within the facility.
o However, if a cell with an embankment height of 2 m is filled over a period of 3 months and then left fallow for 9 months, the marginal rate of rise would be 8 m/year (2 m divided by 3 months, or 0.25 years), but the annualised average rate of rise would remain at 2 m/year.
This is important in the context of allowing sufficient fallow time for the tailings to consolidate and gain sufficient strength to construct an embankment raise. Thus, while a marginal rate of rise of 8 m/year may be acceptable over short periods, an annualised average rate of 8 m/year may not provide sufficient consolidation time.
It should also be noted that the consolidation time is directly related to the square of the thickness of a layer. That is, if the layer thickness is doubled, the time for consolidation may take up to four times as long.
• • The difference between marginal rate of rise and annualised
average rate of rise.
o The marginal rate of rise is the increase in beach height
divided by the deposition time.
o The annualised average rate of rise is the average increase
in beach height over a year.
o So, for example, if a cell with an embankment height of 2 m
is filled over a period of 1 year, the marginal rate of rise and annualised average rate of rise would equal 2 m/year.
•
24Chamber of Mines of South Africa, 1996. Guidelines for Environmental Protection, Volume1, 1979 (Revised 1983 and 1985), pp170. 25Williams, D.A., 2002. Overcoming the Challenges of Raising the Embankment of a Laterite Nickel Tailings Storage Facility in an Upstream Direction. IEAust presentation, Perth 10 September 2002.
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• A more rapid loading on the underlying material arising from a \ higher rate of rise has three potentially significant effects:
o Prolonged periods of excess (greater than hydrostatic) pore
pressures in the underlying tailings, thus adversely affecting
the slope factors of safety.
o Tailings remaining in a loose, saturated state, raising
their susceptibility to liquefaction under adverse loading
conditions.
o Inadequate desiccation of the tailings at the surface,
impacting the practicability of persisting with the desired method of embankment construction.
• The maximum allowable rate of rise for un upstream TSF shall be adjudicated considering the tailings state, with reference to the representative critical state line (CSL) for the tailings. The aim shall be to maintain the tailings in the outer zone in a dilative state (sufficiently below the CSL).
• The CSL represents a useful reference point for soil and
tailings behaviour. Generally, materials that are looser than
the representative CSL exhibit contractive behaviour during shearing while those denser than the representative CSL exhibit dilation.
• Further, significantly dilative materials are unlikely to exhibit static liquefaction, which occurs when contractive materials are loaded (often in a drained condition) to a shear-stress ratio such that contractive undrained shearing may occur with a negligible trigger.
• A risk assessment must be completed with participation from the RTFE and the EoR to identify the additional risks resulting from exceeding the maximum allowable rate of rise. This may also be integrated into a site-specific tailings risk assessment or Risk Management Plan.
A4-11 TAILINGS TRANSPORTATION AND DEPOSITION PLAN
Tailings material can be placed using a range of methods, via pipelines to above-ground of in-pit facilities, using a cyclone system or by placing filter cake.
The tailings transportation and deposition plan are integral to selecting the tailings management technology and the site-specific conditions of the tailings facility.
A tailings transportation and deposition plan shall:
• Be developed during the Project Conception phase and refined • during the design phase.
• Be developed in accordance with the requirements of the “International Cyanide Management Code for the Manufacture, Transport, and Use of Cyanide in the Production of Gold”
(Cyanide Code). •
• Integrate with the design approach for the tailings facility and the overall plan for ore extraction and processing.
• Describe how tailings will be transported to and deposited in the tailings facility and how the tailings facilities' capacity will be increased over the life of the mine.
• Integrate into the OMS manual, implemented, regularly reviewed, and updated during the Operations phase of the lifecycle.
A4-12 WASTE PLACEMENT PLAN
Be developed, implemented, and updated in a manner aligned with the closure concept, progressive rehabilitation and closure plan to ensure that the final tailings surface topography at the end of the Operations phase facilitates the closure plan and post-closure land use.
Be developed considering a range of site characterisation information. These characteristics should be validated
and updated periodically throughout the lifecycle. If some characteristics do not meet the design specifications or intent, then these deviations' potential impacts and risks should be assessed, and appropriate actions should be taken to address them.
• Integrated with the water management plan.
Downstream wall raising typically comprises the loading and haulage of waste rock (shot rock) from the mining face in the pit directly to the tailings disposal site (the final resting position). The waste rock is removed from the open pit as economically and quickly as possible and is used to form engineered embankments progressively raised to ‘keep up’ with tailings production.
The RTFE and the EOR shall ensure that the TSF waste demand schedule (waste type and schedule) is in sync with the waste generation schedule and the site’s mine planning requirements.
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A4-13 IN-PIT TAILINGS DEPOSITION
As the name suggests, in-pit tailings storage is simply the process of backfilling abandoned open-pit surface mines with tailings. An advantage to in-pit storage is that the tailings do not require retaining walls; thus, the risks associated with embankment instability are eliminated.
As part of in-pit tailings storage designs, the following aspects should be considered and allowed for:
• Hydrogeological studies are required to:
• Evaluate hydraulic containment
• Characterise the behaviour and potential environmental impacts of the in-pit facility, including tailings deposition and consolidation, baseline hydrogeology and geochemistry, and a groundwater control system and closure design.
• Confirmation of the extent of the groundwater rebound based on current mining and production schedules, the prediction of the groundwater recovery with tailings deposition from the base of the pit and the potential impacts and mitigation measures required.
• Rapid rates of rise occur particularly in the early stages of deposition when the pit is at its deepest and the exposed surface area the smallest. This reduces the tailings' solar drying and desiccation potential, resulting in low strength and poor consolidation properties.
• Poor consolidation can result in long durations of surface deformation after a pit has been filled. This is mainly due to the
A4-14 CYCLONE DEPOSITION
The design of a sand dam or sand embankment should
consider the following key aspects:
• Seismic stability. This means that the dam or embankment must maintain the containment function of the deposited tailings even under the design event.
• Construction of the facility or embankment as part of the impoundment operation.
• The production of sand is a daily task that must be adjusted to the features of the tailings provided by the flotation plant. This condition does not exist in civil construction, where the construction material is selected and processed until the required quality is obtained, transported by trucks, and placed on the dam.
• In the case of a cyclone facility, the material is classified in
the cyclone station or on the embankment, transported by
pipe, hydraulically discharged from the dam crest, and then mechanically compacted. All of these present challenges derived from (i) the inherent variability of the tailings (in gradation, solids content and tonnage); (ii) the height and length of the dam and (iii) the large volumes to be managed, and (iv) variability in cyclone performance.
The seismic stability of a sand dam depends on three
factors of a different nature:
• The intrinsic parameters of the material: density, strength parameters, and the level of saturation.
• Characteristics of the seismic forces: frequencies, duration of the event, and maximum accelerations; and
low solids content of the tailings and the depth of the stored material.
• Therefore, pits will have to be continuously topped up with tailings until the consolidation rates are minimal and a rehabilitation cover can be implemented and contoured successfully.
• Water return rates must be maximised to maintain a relatively small pond and exposed beaches to maximise insitu densities.
• To achieve the objective of in-pit disposal over the LoM, the density of the tailings must be maximised to minimise the volume occupied. Additionally, to isolate the tailings mass from the groundwater flows, the permeability of the tailings is to be minimised.
• The stability of the pit walls shall be assessed, and safe stand- off distances will be defined along the pit perimeter.
• Safe access down to the decant point shall be provided, taking cognisance of falling ground, loose rock and toppling failures.
• The geometry of the dam: especially the height of the dam and slope angle.
Of all the factors mentioned above, the most critical
factor for stability is the level of saturation of the sand fill. Therefore, the most critical issue to address during design is seepage control through the TSF embankment and its foundation.
The design and construction of a sand facility involve the
following challenges:
• Production of sand with a grain size distribution that complies with the minimum design permeability.
• Hydraulic transport of the sand to facilitate the designed discharge sequence.
• Producing and placing the required quantity of sand to maintain the minimum freeboard for the tailings/slimes impounded and the geometry of the dam (slopes and crest width), and
• Compaction of the placed sand to reach the minimum design density.
Each one of these aspects has different solutions depending on the project characteristics. This section will describe the shear strength of cyclone underflow material, the geometric/ volumetric requirements, material balance analysis, underflow split and stacking angle to consider for cyclone deposition.
GROUP TSF MANAGEMENT STANDARD
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GROUP TSF MANAGEMENT STANDARD
Shear strength of cyclone underflow material
The typical specification for underflow with high shear strength and adequate drainage characteristics is a maximum of 15%, passing the 75 microns (200 mesh) sieve size. For hard rock ores, this specification will invariably guarantee an underflow that adequately meets shear strength requirements for an embankment for even the stringent conditions encountered in areas with seismic activity. In addition, compaction of the underflow may be required to resist significant deformation under seismic loading.
Geometric/volumetric requirements
The underflow specification, driven by shear strength considerations, clearly limits the maximum theoretical mass of underflow to the proportion of the total feed that exceeds 75 microns plus 15%. In addition, ores have been milled finer and finer to improve recovery efficiency, so has reduced the proportion of tailings available for underflow.
As underflow becomes scarcer, it becomes essential to manage the cyclones and the placement of underflow more carefully to minimise the loss of coarse tailings to overflow and minimise the inclusion of out-of-specification fines in the underflow. In addition, the split between underflow
and overflow that dictates the quantity of underflow, and the stacking and drainage, must be controlled to meet specifications.
Material balance analysis
On the cyclone product output side, two defining parameters are required, the solids split and a size specification on one of the two products. The two parameters in this application are the sand yield and quality.
The relatively free-draining characteristic of the sand is defined by the mass fraction of particles of less than 75μm.
The sand yield defines the efficiency and, therefore, the cost and viability of the sand production at the right quality. While the largest contributor to the sand yield is the coarseness of the original tailing stream, much of the sand yield is also determined by how the cyclones are set up and operated.
The staging of cyclones in series can improve the sand yield or sand quality. The first cyclone stage aims to ensure coarse particle recovery, while the second stage aims to polish the product, ensuring product quality.
Managing the split to underflow
The practical maximum proportion of underflow produced is not equal to the theoretical maximum due to cyclone inefficiency, bypassing, pressure variations, and variations in the tailing's rheological properties. It is seldom possible to produce more than 50% to 75% of the theoretical maximum underflow whilst meeting the specification.
The most important factors influencing the actual split are the performance of the infrastructure (pumps, pipelines and cyclones) and the skill of the operators. Therefore, optimisation on a site-specific basis is always required to achieve the best performance.
Stacking angle
Any underflow placed outside the specified minimum dimensions of the embankment is effectively ‘lost’ and reduces the proportion of useful underflow produced. It is therefore also important to control the placement of the underflow carefully.
The angle at which the underflow stacks is therefore important, and whilst being primarily a function of the underflow solids concentration, is also impacted by underflow rheology, particle size distribution and discharge rate.
If not factored into the design and operational considerations, these parameters can further reduce the portion of useful underflow produced.
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