2019-06-14

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Challenges with Upstream Tailings Dams

Harvey N. McLeod, P.Eng., P.Geo., FECKlohn Crippen Berger Ltd.

Brumadinho – A Tragic Lesson in Upstream Dams

• Iron ore “slimes” tailings• Failure took less than a minute• Over 250 lives lost

• Why was it so sudden and catastrophic?• Mechanism of static liquefaction leading

to a “fluid like” flow

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Talk Outline

• What are upstream dams and how are they constructed?

• Tailings properties and sensitivity to static flow liquefaction

• Case examples of static liquefaction and consequence of failure

• Challenges with assessing the safety of upstream dams

What makes Upstream Dams Different?

• The safety of the structure depends upon the strength of the tailings within the “structural zone”

• Tailings are spigotted to form a beach and are in a loose state (contractant)

Advantages of upstream dams• Less cost• Less “borrow material” which often needs to be

sourced from external areas which causes disturbance

• Can be progressively reclaimed

Disadvantages of upstream dams• Very limited QA/QC over placement of tailings• Changes in ore types, spigot locations, thickener

efficiency, drying times, etc. lead to very significant variability

• Loose tailings, when saturated, susceptible to liquefaction under changes in static stresses or due to seismic loading.Partially

Saturated

Saturated

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Types of Upstream Dams – Coarser Outer Zone

Cyclone sand shell to promote drainage- Africa

Segregation on the beach to promote drainage - Asia

Segregation on the beach

Types of Upstream Dams – Desiccation of Outer Slopes

Mud farming (Amphirols)Australia red mud (bauxite)

machine exposes tailings to drying which promotes consolidation

“Day Wall” Smelter Slimes Africa – Outer Zone is placed in

cells and allowed to dry –strengthen outer slope and

provide freeboard for flood control

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Types of Tailings Dam – Partial Desiccation

• Partial desiccation may result in different undrained shear strength ratios

• Example shown su/sigma’ varies 0.23 to 0.4 to 0.6 for peak undrained.

• Uncertainty with “brittleness” of desiccated tailings – does it collapse under stress and trend back towards 0.23?

Types of Upstream Dam – Compacted Tailings -Canada

Compaction of the beach with dozers Underdrains may also be used to promote downward gradients and desaturation

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Tailings Properties and Types (Coarse to Ultra Fine)Tailings Type Symbol Description Example of mineral/ore

Coarse tailings CT Silty SAND, non-plastic Salt, mineral sands, coarse coal rejects, iron ore sands

Hard Rock tailings HRT Sandy SILT, non to low plasticity Copper, massive sulphide, nickel, gold

Altered Rock tailings

ART Sandy SILT, trace of clay, low plasticity, bentonitic clay content

Porphyry copper with hydrothermal alteration, oxidized

rock, bauxite. leaching processes

Fine tailings FT SILT, 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

Simple “jar” field test of coal

tailings

Sensitivity of Tailings to Liquefaction• Linked to the basic properties of the tailings.

Plasticity Chart

Plastic Limit (rolls like plasticine) and Liquid Limit (soils start to creep/flow)

• Liquidity index (wc-PL)/PI

• Tailings with in situ water content higher than the Liquid Limit

• Low density soils have higher void ratio

Zone of potential high Liquidity Index

1.5 - 2

Fundao slimes tailings

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Sensitivity of Tailings to Liquefaction

• Indications from Cone Penetration Testing (CPT)

• Robertson (2016) classifies a zone as CCS (Contractant Clay Sensitive)

• Very low residual shear strengths e.g. < su/sigma’ < 0.05

1

10

100

1000

0 1 10

No

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lize

d C

on

e R

es

ista

nc

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Normalized Friction Ratio, FrRobertson (2016)

Typical “Hard Rock” Plot

Fundao slimes tailings

Tailings Properties Influence Dam Break Assessments

With failure and static liquefaction what is the extent and rapidity of tailings flow?

Kolontar Hungary: Flow due to both water and intermediate liquefied tailings flow

Mt. Polley, Canada, Flow due to large water pond, but limited liquified tailings flow

Sullivan Mine, Canada -Dry & limited flow

Cadia, Australia - Dry & limited flow

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Dam Break Assessments

• Prime mechanism is water erosion of tailings with the critical condition typically being the “rainy day” extreme flood event and the volume water that can be stored/released. “Rule of thumb – 1 m3 tailings released for each 1 m3 of water”

• However, for sensitive tailings the extent of static liquefaction flow can vary tremendously; with Fundǎo and Brumadinhobeing at the extreme edge of the spectrum

• Designers need to assess potential for both static and seismic liquefaction when determining consequence of failure.

Challenges with Seismic Liquefaction Assessment -Amplification

• Amplification for dams 20 m to 40 m high due to the natural period of the dam

• Static bias which accounts for the stresses induce by the sloping dam

3455

3460

3465

3470

3475

3480

34850 1 2 3

Ele

vati

on

(m

)

Factor of Safety Against Liquefaction, FSliq

CPT102, amax=0.12, alfa=0CPT102 amax=0.12, alfa=0.1CPT102, amax=0.2, alfa=0CPT102, amax=0.2, alfa=0.1Series4

FSliq =1.0

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Challenges with Assessment of Upstream Dams

• SCPTu is the best available technology for assessing properties of tailings, laboratory testing supplements the understanding

• Interpretation is very challenging

• Use of mean values is not appropriate – use of the lower quartile and lower should be assessed

• Dynamic pore pressure responds reflects dilative versus contractive state

• “Micro” interpretation of layers is not appropriate

High variability in strength

Challenges with Assessment of Upstream Dams

• Zones of low factor of safety may initiate progressive failure

• Challenging to confidently predict soil behavior and stress state – and to model

Stress concentrations as dam gets higher

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Summary

• Upstream dams have a long history in the mining industry and there are thousands of them in the world

• Are they all “unsafe” – No, but their actual factor of safety is less than what Owners think!

• “Triggers” for static liquefaction are difficult to predict due to uncertainty in material properties

• A factor of safety of > 1.5 for static peak undrained strength may be adequate, however not in all cases!!!!

• Preventative best practice is to assume liquefaction could occur and adopt the post seismic condition (post liquefaction strength with a factor of safety > 1.1)

• Safe Design• Understand the Type of tailings and its properties• Understand undrained response of contractant tailings• Recognize the inhomegeneity of tailings• Recognize variability and challenges in predicting actual conditions• Assume parameters that account for the uncertainties

Thank You

Harvey N. McLeod, P.Eng., P.Geo., FEC

Klohn Crippen Berger Ltd.

hmcleod@klohn.com