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IntroductionBackground to ICOLD Tailings Committee

Last ICOLD Bulletin on Tailings Dam Safety (1989) & Improving Tailings Dam Safety (2011)

Technical aspects of safe design

Governance principles affecting dam safety

Tailings Subcommittee – Tailings Technology UpdateOttawa Workshop – Bulletin Release

Bulletin Working Group:

H. McLeod (Canada), A. Bjelkevik (Sweden), T. Alexieva (USA), D.Grant-Stuart (S.Africa), D. Brett (Australia), M.

Cambridge (UK), J. Pimenta de Avila (Brazil)

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Introduction & Agenda

• Purpose of the Bulletin

• Tailings Properties – Lab based

• Tailings Properties – Field

• Tailings Process Technologies

• Tailings Containment Technologies

• Summary

Purpose of the New Bulletin

• Update of Bulletin 106 (1996)

• Technologies have changed (engineering, environmental, equipment)

• Regulations are changing (EU Extractive Waste Directive, Brasil)

• International practice is changing (ICMM and Investors Group (Church of England and Swedish Pension Fund)

• The Bulletin is prepared for tailings dam designers

• The Bulletin is a technical document and does not address the Governance aspects (which are very important)

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TAILINGS PROPERTIES –Tailings Types

• Tailings characteristics (types) dependent on ore mineralogy and processing

• % clay fraction is a dominant controlling factor in the geotechnical properties

• Specific gravity of tailings varies from 1.5 t/m3 (coal) to 3.5 t/m3 (sulphidic hard rock);

• With segregation and variations in ore mineralogy you may have several types of tailings in the TSF

Tailings Type Symbol Description (compare) Example of mineral/ore

Coarse tailings CT Silty SAND, non-plasticSalt, mineral sands, coarse coal

rejects, iron ore sands

Hard Rock tailings HRT Sandy SILT, non to low plasticityCopper, massive sulphide, nickel,

gold

Altered Rock tailings ARTSandy SILT, trace of clay, low

plasticity, bentonitic clay content

Porphyry copper with hydrothermal

alteration, oxidized rock, bauxite.

leaching processes

Fine tailings FTSILT, with trace to some clay, low to

moderate plasticity

Iron ore fines, bauxite (red mud), fine

coal rejects, leaching processes,

metamorphosed/weathered

polymetallic ores

Ultra Fine tailings UFTSilty CLAY, high plasticity, very low

density and hydraulic conductivity

Oil sands (fluid fine tailings),

phosphate fines; some kimberlite

and coal fines

TAILINGS PROPERTIES – Index Tests

• Atterberg limits can be used to classify type of tailings

• Liquid limit and Clay activity help identify F and UF tailings and problematic clays

Phosphate fines Highly oxidized ore Oil sands fines Mine-water (acidic) sludges

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TAILINGS PROPERTIES -Gradation

• % clay is a dominant indicator

• % silt influenced by degree of grinding in the mill circuit – less dominant than clay effects

TAILINGS PROPERTIES – Settling and Consolidation

• Laboratory jar settling tests

• Good indication of likely consolidation properties

• Indicates quantity of initial water release and initial water recovery potential

• Large strain tests needed to quantify consolidation characteristics up to 10 kPa

• Test segregated tailings to understand differences with the TSF

• Consolidation test

• Determine time for consolidation and density of tailings

• Consider consolidation variability in TSF with segregation and laminations

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TAILINGS PROPERTIES – Permeability

• Controlled by effective stress and % silt and % clay

• Segregation results in significant variations within the TSF. E.g. HR tailings could have C tailings near the spigot points and AR or F tailings near the decant pond.

• Dominated in the tailings pile by interlayering and kh/kv ratio –typically 10:1 or higher

TAILINGS PROPERTIES – Permeability

• % fines (silt and clay) control permeability

• Cyclone sand permeability influenced by % clay

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TAILINGS PROPERTIES – Geochemistry

Static Testing

• Acid base accounting

• Shake flask (leach test)

• NAG test

• Mineralogy and carbon speciation

Kinetic Testing

• Humidity cell tests for industry comparison

• Field barrels to reflect actual site conditions

• ARD generally not a concern if kept saturated

• Testing to simulate field behaviour (e.g. slow leaching, runoff, etc.)

• Accumulation of contaminant buildup over life of mine. (E.g. sulphate)

• Field barrels to reflect actual site conditions

TAILINGS PROPERTIES – Field Based

Beach Slopes

• Dependent on % solids (thickening) and % fines

• Local variation from spigot point(s)

• Length of beach places limitations on density of discharge

• Below water slopes follow sedimentation principles (steep then flatten)

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TAILINGS TECHNOLOGIES – Spigotting

• Deposited tailings settles, drains and consolidates in thin layers on an exposed beach

• Natural segregation of tailings particles that is dependent on the distance from the point of deposition

• Number, spacing and size of operational spigots is controlled to ensure that the flow velocity of the slurry on the beach is low enough to allow the heavier (coarser) particles to settle out close to the point of deposition, while the finer particles are carried further out onto the beach

• Suitable for upstream construction

• Good for dust control

TAILINGS TECHNOLOGIES – Cycloning

• Developed in the mid – 1960’s

• Objective is to produce a construction material for the dam a not use external borrow materials (land disturbance), and reduce storage requirements

• % clay and requirements for compaction -control selection of cyclone system

• % sand recovery can vary depending on the field requirements

• Cyclo-wash system may be required

• Single or double stage cyclone

• Can be used in conjunction with other technologies, such as filtering

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TAILINGS TECHNOLOGIES – Thickening

• Conventional: 50% solids by weight

• High rate and high efficiency thickeners up to 63%, centrifugal pumping for Conventional TSF

• High compression feed filtration system for Filter TSF

• Deep cone thickeners feed paste plant for underground backfill

• Practical limitations

Require high level of operator technology

Flocculants breakdown in transport

Variations in ore mineralogy can “upset” system

TAILINGS TECHNOLOGIES - Filtered

• Used since 1970’s

• Vacuum belt filters

• Compression filters achieve lower moisture content

• Fine tailings are more difficult to filter

• Costs – Capital

• Costs – Operating Power

Maintenance, repair & operator

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TAILINGS TECHNOLOGIES – Processing

• Desulphurization to allow separate storage of sulphide tailings

• Cycloning to produce separate products for filtering

• CN destruction

• Process research into producing “benign” tailings

Wheal Jane minewater treatment plant generating low density sludges for deposition in the tailings dam

TAILINGS CONTAINMENT – Closure: An opportunity?

Goals of Closure Design

• Create a new land – if possible – for a new purpose useful for people. “If the land becomes useful – then someone will look after it.”

• Create a long term stable landform

LADY OF THE NORTH, UK

ASKERSUND GOLF COURT, SWEDEN EDEN PROJECT, UK

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TAILINGS CONTAINMENT – Design for Closure

Landform Design

• Remove potential to pond (store) water to reduce consequence of failure and long-term liabilities

• Covers to control erosion and simulate natural environment

• Redundant spillways and erosion protection

• Saturation control and seepage control to mitigate ARD/ML effects

TAILINGS CONTAINMENT - Centerline

• Dam safety is the same as a Downstream dam

• Conventional technology since mid – 1960s

• Core sometimes used for seepage control (environment)

• Materials can be cycloned sand, mine rock or borrow material

• What is “New”?

• Deletion of pervious zones upstream of core to reduce gradients and piping risk

• Maximizing use of cyclone sand (both upstream and downstream of core) to reduce piping risk

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TAILINGS CONTAINMENT - Downstream

• Conventional water dam technology

• Can be constructed of cycloned sand, mine rock or borrow material

• CFRD, asphalt faced, earth core

• What is “New” ?

• Keep core on upstream slope to reduce gradients and risk of piping – do not have pervious zones between the core and the tailings

• Recognition of tailings/geomembrane liner systems to reduce risk of piping

TAILINGS CONTAINMENT - Upstream

• Containment structure is constructed over drained and consolidated beach

• Suitable in regions with high evaporation and low rainfall which promote consolidation of deposited tailings

• Not suitable in regions of high seismic activity

• Not suitable for retention of large supernatant pond

• Cost effective

• Facilitates concurrent rehabilitation

• Can be constructed by paddocking, cycloningor spigotting

• “Engineered” upstream – compaction and drains used to ensure stability

Compacted (by dozers) upstream tailings dam

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TAILINGS CONTAINMENT - Upstream

• Closure of old upstream dams need to ensure drained conditions for seismic stability

• Need to understand dam breach parameters for partially saturated TSFs

Clemows Valley Tailings Dam interim restoration of d/s face & final cross-section

TAILINGS CONTAINMENT – Central Cone & Central Decant

Central cone

• Deposition from a central point to minimize dam height

• Surface water will collect along dams

Central decant

• To discharge water from ring dyke impoundment

• Central water pond to maximize beach length

• Culvert required under/through dam wall

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TAILINGS CONTAINMENT - Filtered

No one size fits all!

• Reduces consequence of failure by not having water stored on tailings surface

• Scale considerations from 20,000 tpd to >100,000 tpd

• Climate challenges

• Costs relative to the ore value is very high for low grade copper mines

• Risk of mudflows

• Need to control runoff water

TAILINGS CONTAINMENT – Cell Construction

• Used to limit evaporative area to maximize water reclaim

• Tailings discharged at maximum rate in each cell progressively

• Water release calculated with settling/consolidation data (large strain consolidation tests)

• Post placement: when water release (consolidation release) < evaporation rate - water recovery stops

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TAILINGS CONTAINMENT – Liner Systems

Underliner Flow (L/day) – 10 mm dia hole(tailings 10-7 m/s)

6 x 10-6 m/s 5

Geotextile/6 x 10-6 m/s 8

1 x 10-5 m/s 4

Pea gravel Piping failure

Geotextile with pea gravel 6

6 x 10-7 m/s 3

• Tailings dramatically reduce seepage through liner defects

• Landfill/heap leach liner use of over drains and liner “protection” is not appropriate

• Further research on filter designs for tailings

• Do not place drains or pervious soils over the liner

TAILINGS CONTAINMENT - Other Technologies

• Mud Farming (Amphirol –Archimedes screw tractor (AST)

• Air Drying

• Co-Disposal – mix with mine rock

• Typically difficult and costly

• Co-disposal of filtered tailings with mine rock

• Co-placement – place ARD rock in the TSF

Air drying examples of minewater sludge & conventional Pb/Zn thickened tailings

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TAILINGS TECHNOLOGIES - Reducing the Risk of Tailings Dams

• Design for long term use of the land

• Landform design and “dry” closure

• Filtered tailings can reduce consequence of failure

• Core Zone and Filters

Place core on upstream slope

Gradient reduction of tailings/core zone system

• Risk reduction

Robust filters – use of cyclone sand upstream and downstream of core

Geomembranes and geotextiles to reduce piping risk

Beach min. 1.5 x H upstream of dam crest

Simple design, simple operation

TAILINGS TECHNOLOGY UPDATE - Summary

• Index tests on tailings can provide basis for classification and preliminary geotechnical properties assessment

• Technologies for improved design:

Thickening and filter systems to reduce the water content

Process technologies to separate “benign” tailings and improve water quality

Dams that use the lower hydraulic gradient benefit of the tailings

Liner systems that maximize benefit of the tailings

Geometric technologies to optimize the TSF design

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Tailings Subcommittee – New Bulletin Tailings Dam Safety

Introduction

• Need for a common International Guidance

• Minimum requirements for dam safety

• Governance aspects affecting dam safety

• Important references

• Cooperation with ICMM, UNE and other stakeholders

Dam Safety Guideline that provides requirements for design of tailings dams inclusive of design basis, geotechnical, hydrotechnical,

environment risk management and closure for safe dams

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Bulletin Table of Contents

• Introduction

• Governance

• Design

• Construction

• Operations

• Closure

• Emergency Response

Governance

• Summary of Governance Principles

• Risk register and risk controls

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Governance – Responsibility and Accountability

• Dam Owner (accountable executive) – Direct corporate responsibility

• Responsible Person – Operations person (engineer) responsible for oversight of TSF and coordination of all aspects influencing safe design and safe operations

• Engineer of Record (Qualified Professional) responsible for safe design

• Independent Reviewer(s) – Review Boards, Peer Reviews

• Corporate HSEC audits “checking” dam design and governance processes

Governance – Risk

• An engineering failure modes effects assessment (FMEA) will be carried out for the TSF and associated facilities.

• Preventative controls for the dam failure mechanisms need to be documented. Monitoring requirements for confirming that preventative controls are implemented need to be documented and implemented.

• Critical preventive controls should be determined using the ICMM framework. Monitoring methods for critical controls and trigger action response plans (TARPS) need to be documented.

• Mitigative controls developed

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Design

• Alternative (site and technology) assessment and selection

• Consequence determination

• Site characterization and site conditions

• Design basis/criteria

• Design Component• Geotechnical

• Hydrogeological

• Hydrotechnical

• Environmental

Design Alternative Assessment and Selection

• Alternative site and technology assessments

• Stages of design and risk reduction (likelihood and consequences)

• Design for “ultimate” dam height (life of mine planning)

• Design for closure

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Design – Dam and Site Characterization

• Bedrock geology

• Surficial geology

• Geotechnical parameters

• Hydrogeologic conditions

• Seismo-tectonic conditions

• Dam Foundation Guideline (Engineers & Geoscientists of BC, 2016)

Design – Consequence Assessment

• A semi-quantitative dam break analysis may be carried out for lower consequence TSF and detailed numerical models for high consequence TSFs.

• Consideration of water transported tailings and “liquefaction” flow

• Dam break analyses will be carried for “sunny” day and “flood” conditions

• Inundation maps for the dam break conditions

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Consequence Classification – example only

• IN PROGRESS

Flood and Seismic Criteria – example only

• IN PROGRESS

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Factors of Safety – example only

• IN PROGRESS

• Factors of safety commensurate with behavior of materials and uncertainty of material properties

• Eurocode 7 – limit state approach and load factors

Design

• Design basis memorandum (example)

• Geotechnical

• Hydrogeological

• Hydrotechnical

• Environmental (e.g. geochemical)

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Construction

• Engagement of designer/engineer/EoR --- QA

• As constructed documentation

• Ongoing processes with continual change

• Specifications for fill and tailings placement

• Engagement, competence of operators

• Foundation preparation

• Deposition requirements

Operations

• Operations Maintenance & Surveillance Manual implementation

• Data collection, records

• Design (EoR) verification and interaction with operations

• Scheduling of raises

• Water management

• Monitoring and critical controls

• Responsibilities (technical training)

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Closure

• Design for closure

• Closure design should be feasibility level (comprehensive) and updated regularly (e.g. five years)

• Design to transition to “land form” or “mine waste structure”

• Repurposing of the facility

• Progressive closure

• Executive summary of ICOLD Bulletin (153) on Closure

Emergency and Response Planning

• Specifically developed for dam failure modes

• Identifying dam failure sequences and what you do at stages of escalation

• Identifying what equipment, resources, etc. are required for different types of failure

• Linkage to trigger, action, response plans (TARPs)

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Path Forward

• Integration/compatibility with ICMM/UNE and others working towards the same objective

• Accelerated preparation of the Guideline

• Initial Draft in 4 months and Draft by year end