aspects of the geotechnics of mining wastes and tailings dams martin fahey & tim newson...

ASPECTS OF THE GEOTECHNICS ASPECTS OF THE GEOTECHNICS OF MINING WASTES AND OF MINING WASTES AND

TAILINGS DAMSTAILINGS DAMS

ASPECTS OF THE GEOTECHNICS ASPECTS OF THE GEOTECHNICS OF MINING WASTES AND OF MINING WASTES AND

TAILINGS DAMSTAILINGS DAMS

Martin Fahey & Tim Newson

Geomechanics GroupThe University of Western Australia

Geomechanics Group, The University of Western AustraliaGeomechanics Group, The University of Western Australia

AcknowledgementsAcknowledgementsAcknowledgementsAcknowledgements MERIWA (Minerals and Energy Research Institute of WA)

and 12 gold-mining companies

Australian Research Council

Department of Minerals and Energy, WA (DOMWA) Hugh Jones, Roger Schultz, Jay Ranasooriya

Australian Centre for Geomechanics (Richard Jewell)

PhD Students (Yoshimasa Fujiyasu, Seng Huat TOH)

Technicians (Tim Smith, Mike McCarthy)

Visitor (Prof. Nimal Seneviratne)


““Companion” PaperCompanion” Paper““Companion” PaperCompanion” Paper CANALEX project in Canada examining liquefaction of

tailings

Directed by Professor Peter Robertson, University of Alberta, Edmonton

Invited to contribute section on this topic

This now appears as stand-alone paper: LIQUEFACTION IN TAILINGS AND ITS EVALUATION,

by P.K. Robertson and C.E. Wride


OutlineOutlineOutlineOutline Brief comments on waste rock dumps

Brief overview of tailings storages

Tailings consolidation consolidation properties (and their measurement) self-weight consolidation

Evaporation from tailings

Coupled evaporation and consolidation analysis

Case studies (parametric studies)

Conclusion


Waste DumpsWaste DumpsWaste DumpsWaste Dumps Short-term stability

generally not an issue - angle of repose

Long-term stability erosion (what is a stable land-form for 100 years? for 1000 years? erosion protection

Environmental aspects acid generation and methods of preventing it dust generation re-vegetation is “natural” appearance required?


Geotechnical Aspects of TailingsGeotechnical Aspects of TailingsGeotechnical Aspects of TailingsGeotechnical Aspects of Tailings Issues depend on

the type of tailings and the type of ore processing (gold, bauxite …) the climate (arid, tropical, temperate….) the location (near built-up area, agricultural land, forest, desert ...) seismic risk the soil conditions

permeable? impermeable? reactive?

groundwater regime above aquifer? fresh or “stock qaulity” water?

the regulatory environment


Tailings: Geotechnical Issues Tailings: Geotechnical Issues Tailings: Geotechnical Issues Tailings: Geotechnical Issues Sedimentation for sub-aqueous deposition

Beaching & segregation for sub-aerial deposition

Immediate “settled density” & short-term water return

Consolidation behaviour (time and amount) final density & strength profiles

Capping revegetation?

Erosion outer wall protection, especially if constructed of tailings


Geotechnical Issues (Environmental?)Geotechnical Issues (Environmental?)Geotechnical Issues (Environmental?)Geotechnical Issues (Environmental?) Escape of leachate

tailings may contain cyanide, heavy metals, high salinity, radioactive components …...

Dust from dry tailings can blow large distances visual impact, and health impact (on plant and animals/humans)

Acid generation (“Acid Mine Drainage”) capping, buffering, cleanup, neutralisation…..


Cyanide in Decommissioned StorageCyanide in Decommissioned StorageCyanide in Decommissioned StorageCyanide in Decommissioned Storage

5

7

9

11

13

15

17

0 20 40 60 80 100

Total cyanide (mg/kg)

De

p[t

h (

m)

Bh#1

Bh#2

Bh#3

(a)

0 20 40 60

Species concentration (mg/kg)

SCN Ni(CN)43-Co(CN)63- Cr(CN)63-Cu_(CN) Fe(CN)64-

Thiocyanate

Iron cyanide


Tailings StorageTailings StorageTailings StorageTailings Storage “Storage” (long term), not “disposal”

General requirements no (minimal) direct impact on people, fauna, flora

stability against catastrophic failure prevention of erosion “failure” prevention of dust (especially toxic dust - cyanide, salt, radioactive) prevention of groundwater contamination (acid, cyanide, salt etc)

acceptable visual impact can landform be created identical to surrounding landforms (usually no) what is acceptable?

Economic requirements provide safest, most cost-effective storage possible


Types of Tailings StoragesTypes of Tailings StoragesTypes of Tailings StoragesTypes of Tailings Storages “Paddock” storage

“Valley” and “hillside” storage

“In-pit” storage using mined-out open pit

“Co-disposal” with coarse waste (waste rock)

Underground backfill

Thickened tailings central thickened discharge “paste” technology


““Paddock” StoragesPaddock” Storages““Paddock” StoragesPaddock” Storages

Single cell

Multiple cell

Typically 30-40 m high, up to 100 hectares or more


Valley StorageValley StorageValley StorageValley Storage

Dam built across valley (eg. Boddington Gold Mine. SW of WA)

Tailings deposited from spigots on embankment, or around perimeter of valley

Seepage collection pond


““Upstream” Construction MethodUpstream” Construction Method““Upstream” Construction MethodUpstream” Construction Method

Construct starter embankment using suitable "borrow" material

Use dried material on beach to construct 2nd embankment "lift"

Continue to final height

(1)

(2)

(3)

(4)

Ring main and spigotsUse spigot system to deposit tailngs, forming "beach"


Water Management: Decant SystemWater Management: Decant SystemWater Management: Decant SystemWater Management: Decant System

Slotted concrete rings

Rockfill (filter)Drainage line to mill

Decant pond

Decant causeway

Spigots operating in this area

Drying on rest of beach

Decant pond

Tailings distribution line

PLAN

SECTION


Grading Curves for Gold TailingsGrading Curves for Gold TailingsGrading Curves for Gold TailingsGrading Curves for Gold Tailings

0

10

20

30

40

50

60

70

80

90

100

0.1 1 10 100 1000Particle size (m)

% f

iner

Higginsville

Mt Gibson

Ora Banda

Coolgardie Gold

Hill 50

Kaltails

Sons of Gwalia

Yilgarn Star

New Celebration

-

Clay (< 2 m)

Sand (> 2 m)

Silt


Consolidation Parameters: PermeabilityConsolidation Parameters: PermeabilityConsolidation Parameters: PermeabilityConsolidation Parameters: Permeability

1E-10

1E-9

1E-8

1E-7

1E-6

1 10 100 1000

Effective vertical stress 'v (kPa)

Pe

rme

ab

ility

k (

m/s

)

Higginsville

Ora_Banda

GrannyS_F

GrannyS_R

New Celebration

Hopes Hill

Kaltails

Eneabba (m.s.)

Yoganup (m.s.)


Rowe Cell for Consolidation TestingRowe Cell for Consolidation TestingRowe Cell for Consolidation TestingRowe Cell for Consolidation Testing

Apply pressure to top of sample

Bellofram jack

Base drain for two-way tests, or for direct permeability

measurement

Porous disks

Displacement transducer

Measurement of volume of water expelled

Pressure chamber

Sample (150 mm diameter)

Pore pressure transducer

Valvee

Rigid drainage tube (settles with sample)

Teflon lining

Grooved rigid plate


Tailings BehaviourTailings BehaviourTailings BehaviourTailings Behaviour When deposited on beach, tailings will

show some segregation sandier material deposited near discharge point silty and clayey material carried towards decant pond

develop a sloping “beach” beach angle, and degree of segregation depends on

grading coarse material forms steep beach, fine material forms shallow beach

concentration (% solids) dilute slurry gives shallow beach, thickened slurry gives steep beach

velocity at point of discharge (depends on number of spigots operating at a time) high velocity - flat beach; low velocity - steep beach

developing uniform beach requires good management of number and location of operating spigots


Segregation on BeachSegregation on BeachSegregation on BeachSegregation on Beach

0

10

20

30

40

50

60

70

80

90

100

1 10 100 1000Particle size (m)

Pe

rce

nta

ge

pa

ss

ing

Far from discharge point

From spigot

Near discharge point


CPT: Dormant Storage (Sons of Gwalia)CPT: Dormant Storage (Sons of Gwalia)CPT: Dormant Storage (Sons of Gwalia)CPT: Dormant Storage (Sons of Gwalia)

0

2

4

6

8

10

12

14

16

18

0 2 4 6

Cone tip resistance qc (MPa)

De

pth

(m

)

Near wall of storage

0 2 4 6


Between wall and decant

0 2 4 6


Near decant


CPT: Active Storage (Kaltails)CPT: Active Storage (Kaltails)CPT: Active Storage (Kaltails)CPT: Active Storage (Kaltails)

0 20 40 60 80 100

Shear strength su (kPa)

CPT: Nkt =12

Shear vane

0

2

4

6

8

10

12

0 0.5 1 1.5 2

qc (MPa)

De

pth

(m

)


CPT: CPT: u and salinity (Kaltails)u and salinity (Kaltails)CPT: CPT: u and salinity (Kaltails)u and salinity (Kaltails)

50 100 150 200

Salinity (g/l)

0

2

4

6

8

10

12

0 100 200 300

Penetration pore pressure (kPa)

De

pth

(m

)

hydrostatic line


Self-weight consolidationSelf-weight consolidationSelf-weight consolidationSelf-weight consolidation Tailings consolidation is due to

self-weight evaporation

Self-weight consolidation without evaporation in wet climate, or if tailings kept under water effectiveness depends on base drainage condition undrained base leads to very poor consolidation drained base - slightly better time for consolidation depends on d2 (drainage path length)

d for undrained base twice d for drained base: time increased by factor of 4 in-pit storage (e.g. Hopes Hill): undrained base, d = 80 m, fine-grained tailings

CONSOLIDATION MAY CONTINUE FOR MANY DECADES


Self-weight consolidationSelf-weight consolidationSelf-weight consolidationSelf-weight consolidation

420 kPa

-200 kPa 200 kPa

Pore pressure Final effective stresss

320 kPa

ot

1t

2t

3t

Pore pressure Final effective stresss

420 kPa300 kPa 120 kPa

ot

1t

2tt

t 30 m

Undrained base: low effective stress low shear strength low density

Drained base: higher effective stress higher strength higher density relies on suction

above water table


Net Pan Evaporation Rates, WANet Pan Evaporation Rates, WANet Pan Evaporation Rates, WANet Pan Evaporation Rates, WA

0 500 1000

Scale (km)

Tom Price

Mt. Newman

Marble Bar

Three Rivers

Wiluna

Wyndham

Fitzroy Crossing

Derby

Leonora

Esperence

Kalgoorlie

Albany

PerthMerredin

Geraldton

Carnarvon

Exmouth

PortHedland

MeekatharraCarnegie

Norseman

Cue

Gold-producing areas

2

3

4

3

3

1

24

Annual Net Pan Evaporation (m)

3

Telfer

Boddington/Hedges


Effect of EvaporationEffect of EvaporationEffect of EvaporationEffect of Evaporation Self-weight consolidation not efficient because:

weight not applied until material buried (remote from drained boundary), resulting in delayed consolidation

Evaporation “sucks” water from the surface consolidates the material on the surface, increasing the density if sufficient drying, tailings are sufficiently consolidated that:

no further consolidation will occur due to weight of overburden material maximum possible density achieved maximum possible strength achieved (important for upstream construction) maximum possible efficiency of the storage area no settlement after filling ceases strength sufficient for access to the surface for rehabilitation no further downward flow of water (+ contaminants) into the groundwater


Schematic of Micro-Lysimeter MethodSchematic of Micro-Lysimeter MethodSchematic of Micro-Lysimeter MethodSchematic of Micro-Lysimeter Method

One-way valve

(1) Install receptacle for micro-lysimeter

Wall thicknesses all 3 mm

24 hour evaporation not affected by sealed base

96 mm

(2) Take core sample in micro-lysimeter

(3) Place micro-lysimeter in prepared receptacle

Weigh micro-lysimeter before and after 24 hours of evaporation

155 mm

155 mm


Surface Energy FlowsSurface Energy FlowsSurface Energy FlowsSurface Energy Flows

Incoming shortwave

Reflected shortwave- depends on reflectivity

(albedo)

Incoming longwave

Reflected longwave

Sensible heat (H)

Flow into soil (G)

Rn = H + G + LeE

Net radiation = Upward flow (in air) + soil heat flow + energy used for evaporation


““Bowen Ratio” Weather StationBowen Ratio” Weather Station““Bowen Ratio” Weather StationBowen Ratio” Weather Station

Fibreglass pontoon Temperature sensors Heat flux plate

2.4 m2 m

1 m

Humidity and temperature

sensors Automatic rain gauge

PyranometerAnemometer

Net pyrradiometer

Solar panel

Data logger


Energy Measurements, Yoganup NorthEnergy Measurements, Yoganup NorthEnergy Measurements, Yoganup NorthEnergy Measurements, Yoganup North

-400

-200

0

200

400

600

800

1000

16-Dec-95 17-Dec-95 18-Dec-95 19-Dec-95

En

erg

y f

lux

(W

/m2 )

Total incoming radiation

Heat flow into soil Heat flow upwards (air)

Heat used for evaporation


Effect of Salinity on EvaporationEffect of Salinity on EvaporationEffect of Salinity on EvaporationEffect of Salinity on Evaporation

Elapsed time (days)

0%

20%

40%

60%

80%

100%

0 5 10 15 20 25 30 35 40 45

Non-saline

0

5%

10%

15%

Salinity(wt/wt)

20%

Evaporationas % of

Potential Evaporation

Ep


Effect of Salinity on Final ProfileEffect of Salinity on Final ProfileEffect of Salinity on Final ProfileEffect of Salinity on Final Profile

Void ratio (e)

0

10

20

30

40

50

60

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

5% salt(after 40 days)No salt

(after 42 days)

Height(cm)


Albedo of Kaolin Slurry (Lab. Tests)Albedo of Kaolin Slurry (Lab. Tests)Albedo of Kaolin Slurry (Lab. Tests)Albedo of Kaolin Slurry (Lab. Tests)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1

Cumulative water loss from fresh water drum (m)

Sh

ort

wa

ve

re

fle

cti

vit

y (

alb

ed

o)

White kaolin in saline water

White kaolin in fresh water

Salt crust removed from saline sample


Albedo of Fresh-Water Tailings (Red)Albedo of Fresh-Water Tailings (Red)Albedo of Fresh-Water Tailings (Red)Albedo of Fresh-Water Tailings (Red)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

18-Oct-95 17-Nov-95 17-Dec-95 16-Jan-96

Sh

ort

wa

ve

re

fle

cti

vit

y (

alb

ed

o)

Average value = 0.16

Red-coloured kaolinitic clay & fresh water:Only 16% of incoming radiation is reflected


Effect of Salt Crust on EvaporationEffect of Salt Crust on EvaporationEffect of Salt Crust on EvaporationEffect of Salt Crust on Evaporation

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

5-Oct-94 30-Oct-94 24-Nov-94 19-Dec-94R

elat

ive

evap

ora

tio

n r

ate

(E/E

p)

Salt crust in place

Just after salt crust removed

0

2

4

6

8

10

12

14

16

18

5-Oct-94 30-Oct-94 24-Nov-94 19-Dec-94

Eva

po

rati

on

rat

e (m

m/d

ay)

(b)

Salt crust in place

Just after salt crust removed

Potential (pan) rate


Salinity Increase at Evaporation SurfaceSalinity Increase at Evaporation SurfaceSalinity Increase at Evaporation SurfaceSalinity Increase at Evaporation Surface

0

50

100

150

200

250

300

0 0.05 0.1 0.15 0.2 0.25

Salinity Concentration, C (Wt. solute/total wt.)

De

pth

(m

m)

30 35 40 45 50 55 60 65

Moisture Content (%)

Salt concentration

Water content

Test in Large Tank:Ora Banda


E/EE/Epp along transect on tailings surface along transect on tailings surfaceE/EE/Epp along transect on tailings surface along transect on tailings surface

0

1

0

0.2

0.4

0.6

0.8

1

0 0.25 0.5 0.75 1

Normalised Distance to Decant

No

rma

lise

d e

va

po

rati

on

(E

/Ep

)

Decant pond

Decant causeway

Decant pond


Predicting Final StatePredicting Final StatePredicting Final StatePredicting Final State Amount of consolidation during filling depends on

potential rate of evaporation Ep(> 3 m/yr in Goldfields)

salinity of the process water (high salinity may reduce Ep to only 20% of freshwater value)

Consolidation process starts at “settled density” tailings settle out to density higher than pumped density

pumped at (say) 40% solids (e ~ 4.0; d ~ 0.2 t/m3)

settles to e ~ 2 to 3 for clayey tailings (d ~ 0.90 to 0.67 t/m3)

or to e ~ 1.5 to 2 for less clayey tailings (d ~ 1.08 to 0.90 t/m3)

sedimentation test in column to determine “settled density” can dry to < shrinkage limit if spread in thin layers, and high Ep can get very poor consolidation if clayey, and high salinity


Predicting Final State (Cont.)Predicting Final State (Cont.)Predicting Final State (Cont.)Predicting Final State (Cont.) Experience with similar material and similar operating

conditions may give good idea of final density achieved

Otherwise, sophisticated method of analysis required

Computer program MinTaCo developed at UWA for this purpose


The MinTaCo ModelThe MinTaCo Model((MiMine ne TaTailings ilings CoConsolidation)nsolidation)

The MinTaCo ModelThe MinTaCo Model((MiMine ne TaTailings ilings CoConsolidation)nsolidation)


FeaturesFeaturesFeaturesFeatures Uses large strain formulation

permeability and compressibility change with reducing void ratio Lagrangian coordinate system (i.e. material coordinates)

Can deal with filling at varying rates, with dormant periods possible between

filling periods change in material type at any stage of filling different base drainage conditions: drained, undrained, partially

drained (“leaky” base) varying rates of evaporation decantation of surface water


Typical Consolidation PropertiesTypical Consolidation PropertiesTypical Consolidation PropertiesTypical Consolidation Properties

0.6

0.8

1

1.2

1.4

1.6

1.8

2

1 10 100 1000

Vertical effective stress (kPa)

Vo

id r

atio

e

Fine

Mixed

Coarse

Symbols: dataLines: MinTaCo

1.E-09

1.E-08

1.E-07

0.6 1.1 1.6Void Ratio (e)

Per

mea

bil

ity

(m/s

)

Mixed

Coarse

Fine


Modelling EvaporationModelling EvaporationModelling EvaporationModelling Evaporation Evaporation from bare soil surface equal to potential

evaporation rate (Ep) while soil is saturated

Saturated conditions persist until shrinkage limit reached

Suction at this stage = air entry suction > 1000 kPa for tailings with significant clay content recent paper by Ward et al (Can. Geo. Journal):

suction = 3000 kPa for sand, silt or clay

Evaporation then reduces below Ep- “soil limiting” stage


Drying of Clayey Tailings (Saturated)Drying of Clayey Tailings (Saturated)Drying of Clayey Tailings (Saturated)Drying of Clayey Tailings (Saturated)

Water content

Volumeor

Length

No further increase in

density below

shrinkage limit

As deposited

Liquid limit

Plastic limit

Shrinkage limit (suction > 1 MPa)

Drying causes shrinkage


Drying of Fresh-Water TailingsDrying of Fresh-Water TailingsDrying of Fresh-Water TailingsDrying of Fresh-Water Tailings

0

2

4

6

8

10

12

14

4-May-95 26-Jul-95 17-Oct-95

8-Jan-96E

va

po

rati

on

ra

te (

mm

/da

y)

Potential (pan) rate

Measured rate (micro-lysimeters

0

20

40

60

80

100

120

140

160

180

200

4-May-95 26-Jul-95 17-Oct-95 8-Jan-96

Wa

ter

co

nte

nt

of

su

rfa

ce

ta

ilin

gs

(%

)

Shrinkage limit

Yoganup North: Red-coloured

kaolinitic clay & fresh water


Evaporation in MinTaCo ProgramEvaporation in MinTaCo ProgramEvaporation in MinTaCo ProgramEvaporation in MinTaCo Program Impose Ep as top boundary condition

assume evaporation pan gives good measure of Ep

use reduced value of Ep if tailings are saline

Impose Ep by adjusting surface pore pressure to give hydraulic gradient sufficient to keep water flow = Ep

Keep increasing surface suction until air entry value reached


Evaporation in MinTaCoEvaporation in MinTaCoEvaporation in MinTaCoEvaporation in MinTaCo Adjusting surface suction to give flow = Ep

Upward flow< Ep

Total head

Increase suction at surface

h h

L L

v = ki = khL


““Soil Limiting” Stage of EvaporationSoil Limiting” Stage of Evaporation““Soil Limiting” Stage of EvaporationSoil Limiting” Stage of Evaporation After suction reaches air-entry value

soil starts to desaturate permeability reduces rapidly suction increases dramatically

not sufficient to maintain evaporation = Ep rate of evaporation dictated by soil permeability - “soil limiting” stage

MinTaCo assumption soil stays saturated suction kept at air-entry value

no further consolidation no further reduction in permeability

rate of evaporation reduces dramatically


Field Tests at Yoganup NorthField Tests at Yoganup NorthField Tests at Yoganup NorthField Tests at Yoganup North Evaporation study by Fujiyasu at Yoganup North

Westralian Sands mineral sands operation clay tailings drying ponds (2-3 m initial depth) evaporation measured using Bowen Ratio weather station and

micro-lysimeters potential evaporation rate determine using Class A pan


MinTaCo Modelling of Yoganup NorthMinTaCo Modelling of Yoganup NorthMinTaCo Modelling of Yoganup NorthMinTaCo Modelling of Yoganup North

0

2

4

6

8

10

12

14

20-Sep-95 14-Nov-95 8-Jan-96 3-Mar-96

Ev

ap

ora

tio

n r

ate

(m

m/d

ay

)

Assumed Ep

MinTaCo output

Bowen Ratio

Micro-lysimeters

Measured Ep


Measured and Predicted Water ContentMeasured and Predicted Water ContentMeasured and Predicted Water ContentMeasured and Predicted Water Content

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 60 120 180Water content (%)

Dep

th (

m)

13-Oct-95

16-Nov-95

12-Jan-96

MinTaCo: linesData: symbols

12-Dec-95


Effect of Evaporation on Tailings:Effect of Evaporation on Tailings:MinTaCo PredictionsMinTaCo Predictions

Effect of Evaporation on Tailings:Effect of Evaporation on Tailings:MinTaCo PredictionsMinTaCo Predictions


Case StudyCase StudyCase StudyCase Study 20 m deep storage area

2.4 Mt/a dry weight of ore (= dry weight of tailings)

tailings at 40% solids

4.8 x 106 m3 slurry per year

Storage area = 100 Hectare (= 106 m2)

Equivalent to 4.8 m/yr slurry filling rate

Pan evaporation = 3.0 m/yr


QuestionsQuestionsQuestionsQuestions What would happen if only had 75 Ha? 50 Ha?

What would be effect of reduced evaporation rate 1.5 m/yr? (50% of Ep) - e.g. moderate salinity

0.3 m/yr? (10% of Ep) - e.g. high salinity

What would be effects on dry solids stored surface strength (access for rehabilitation)


Change storage area (for EChange storage area (for Epp=3 m/yr)=3 m/yr)Change storage area (for EChange storage area (for Epp=3 m/yr)=3 m/yr)

0

5

10

15

20

25

0 5 10 15 20 25 30Time (yr)

Su

rfac

e el

evat

ion

(m

)

4.8 m/yr: 100 Ha7.2 m/yr: 75 Ha9.6 m/yr: 50 Ha

Instantaneous filling

Filling rate (m/yr of slurry)7.29.6 4.8


Change storage area (for EChange storage area (for Epp=3 m/yr)=3 m/yr)Change storage area (for EChange storage area (for Epp=3 m/yr)=3 m/yr)

0

2

4

6

8

10

12

0 5 10 15Time (yr)

So

lids

hei

gh

t (m

)

7.2

9.6

4.8

Instantaneous filling

Filling rate (m/yr of slurry)


Change storage area (EChange storage area (Epp=3 m/yr)=3 m/yr)Change storage area (EChange storage area (Epp=3 m/yr)=3 m/yr)

Area (Ha)

Filling Rate

(m/yr)

Filling Time (yr)

Height Solids

(m)

Total dry weight

(M tonnes)Wt/Wt100

100 4.8 12.1 10.4 29.1 100%

75 7.2 4.8 6.4 13.4 46%

50 9.6 3.1 5.4 7.6 26%


Reduced evaporation (Area = 100 Ha)Reduced evaporation (Area = 100 Ha)Reduced evaporation (Area = 100 Ha)Reduced evaporation (Area = 100 Ha)

0

5

10

15

20

25

0 5 10 15 20 25 30

Time (yr)

Su

rfac

e el

evat

ion

(m

)

1.5

0.3

3.0

Ep (m/yr) Filling rate = 4.8 m/yr

No effect during filling


Shear strength of top 1 mShear strength of top 1 mShear strength of top 1 mShear strength of top 1 m

0

50

100

150

200

250

0 5 10 15 20 25 30Time (yr)

Sh

ear

stre

ng

th (

kPa

1.5

0.3

3.0

Filling rate = 4.8 m/yr

Ep (m/yr)

No crust during filling


Effect of Changing EvaporationEffect of Changing EvaporationEffect of Changing EvaporationEffect of Changing Evaporation

Area (Ha)

Evaporation Rate (m/yr)

Filling Time (yr)

Height Solids

(m)

Total dry weight

(M tonnes)Wt/Wt100

100 3 12.1 10.4 29.1 100%

100 1.5 7.25 6.2 17.4 60%

100 0.3 7.25 6.2 17.4 60%


Effect of Evaporation (filling @ 4.8 m/yr)Effect of Evaporation (filling @ 4.8 m/yr)Effect of Evaporation (filling @ 4.8 m/yr)Effect of Evaporation (filling @ 4.8 m/yr)

Ep = 3.0 m/yr - dries fully during filling very little post-filling settlement high surface shear strength

(function of air-entry suction assumed - 1000 kPa in this case)

Ep = 1.5 m/yr and Ep = 0.3 m/yr same filling rate (evaporation has no effect during filling) only 60% of solids stored compared to Ep = 3.0 m/yr

final amount of settlement same rate of settlement different rate of strength gain different


““Threshold” Filling RateThreshold” Filling Rate““Threshold” Filling RateThreshold” Filling Rate For particular (effective) rate of evaporation:

filling faster than a certain “threshold” rate

evaporation has no effect on tailings during filling evaporation rate is the actual (salt affected?) rate

For a certain filling rate an evaporation rate less than a “threshold” rate

evaporation has no effect on tailings during filling

threshold between 1.5 and 3.0 m/yr in previous example

“Threshold” rate of evaporation or filling depends on tailings consolidation properties


Evaporation & Water BalanceEvaporation & Water BalanceEvaporation & Water BalanceEvaporation & Water Balance

Active disposal area

(E = Ep ?)

Recently-active disopsal area (E = 0.9 Ep ?)

Decant pond

Decant structure

Decant causeway

Dry area (E = 0.2Ep ?)

Fresh-water tailings high Ep (3 m/yr?)

Saline tailings much lower rates than shown 70 - 80% reduction ?

Reduce E by 1 m/yr changes water balance by 1

million m3 for 100 hectare storage


Modelling Strategy for Valley StorageModelling Strategy for Valley StorageModelling Strategy for Valley StorageModelling Strategy for Valley Storage In a valley, depth varies from edge to centre

Could have material segregation coarser material near edge finer material towards centre

Could have different base drainage conditions in different parts of the valley

MinTaCo models a 1-D column of material (1m square) modelling of different areas required modelling different materials in different areas required


Modelling StrategyModelling StrategyModelling StrategyModelling Strategy

90 m

70 m50 m

30 m

CASE A CASE B CASE C CASE D

Final filled level: 462 m RL


Filling HistoryFilling HistoryFilling HistoryFilling History

A

B

C

D

370

390

410

430

450

470

0 2 4 6 8

Time (yr) from start of filling

Su

rfac

e el

evat

ion

(m

)

Filling curveCase ACase BCase CCase D

Filling rate reduces as valley widens


Output: “Fine” Material EverywhereOutput: “Fine” Material EverywhereOutput: “Fine” Material EverywhereOutput: “Fine” Material Everywhere

452

454

456

458

460

462

0 10 20 30 40 50

Time (yr) from start of filling

Su

rfac

e el

evat

ion

(m

)

A: undr.

A: dr.

B: undr.

B: dr.

C: undr.

C: dr.

D: undr.

D: dr.


Surface Profiles after ConsolidationSurface Profiles after ConsolidationSurface Profiles after ConsolidationSurface Profiles after Consolidation

452

454

456

458

460

462

30 40 50 60 70 80 90

Storage thickness (m)

Fin

al s

urf

ace

elev

atio

n (

m)

Fine

Mixed

Coarse

40 m fine, then 20 m mixed, then 30 m coarse


MinTaCo ModelMinTaCo ModelMinTaCo ModelMinTaCo Model Realistic modelling of consolidation and evaporation

behaviour of tailings

Can model complex geometry and soil conditions use range of geometries and material types to ensure behaviour

will be “bracketed”

Can track water flows (evaporation, decantation, base leakage

water balance studies possible

Can use it to carry out parametric studies (What if..?)


ImprovementsImprovementsImprovementsImprovements Explicit modelling of effect of salinity

currently allowed for by using lower Ep 10% of actual Ep for high salinity

Predict build-up of salinity with evaporation for low-salinity cases

relate E directly to surface salinity

Improve modelling of “soil limiting” stage of evaporation (from start of desaturation)

partially saturated water flow, but no volume change requires permeability-saturation relationship


ConclusionsConclusionsConclusionsConclusions Many geotechnical and geo-environmental aspects of

mine wastes

Have focussed on consolidation/evaporation behaviour

Environmental issues may be much more important

Modelling of tailings consolidation behaviour useful not an exact predictive tool (because of spatial variability in tailings) parametric study useful using upper bound, best estimate, and lower

bound values of material properties

Evaporation can be very beneficial for tailings management under some circumstances


ConclusionsConclusionsConclusionsConclusions The costs associated with tailings storage facilities

(TSFs) and their rehabilitation are now a significant part of the cost of a mining operation

Efficient and safe operation of the TSF requires: integrating the planning for the TSF into the planning for the rest

of the mining operation planning for the rehabilitation phase of the operation from the

start, otherwise rehabilitation costs may be excessive proper design & construction supervision of the TSF construction educating/training the personnel that operate the TSF, so that they

understand the principles of operation, and the end goals.


Conclusions (Cont.)Conclusions (Cont.)Conclusions (Cont.)Conclusions (Cont.) Savings on short-term costs often result in severe blow-

out of long-term costs “cash flow” is often important for many operations (particularly

small companies) temptation to “cut corners” with the TSF, particularly since it is not a direct money

earner

pressure from financial managers to keep size of the TSF to a minimum have only 1 cell rather than 2 or more cells use single-point discharge rather than a ring-main and spigot system poor site selection, preparation, and poor construction control of embankment

all these are contrary to efficient tailings management HIGH QUALITY DESIGN AND MANAGEMENT OF THE TSF IS THE KEY TO

MINIMISING COSTS IN THE LONG TERM


Thank You Thank You

Questions?Questions?

Thank You Thank You

Questions?Questions?

aspects of the geotechnics of mining wastes and tailings dams martin fahey & tim newson...

Documents

u erosion

u dust

edmonton u

topic u

liquefaction of tailings

u escape of leachate

u consolidation behaviour

repose u longterm stability