covers for mine waste disposal facilities by andy robertson and shannon shaw

COVERS FOR

MINE WASTE DISPOSAL FACILITIES

By

Andy Robertson and Shannon Shaw

Review of Cover Purpose

• Access• Physical Isolation• Erosion Protection• Vegetation Establishment• Oxygen (oxidation) Control• Infiltration (leaching) Control

Soil Covers

• Types of Covers:

• Simple– Permeability depends on grain size– Compaction– Oxygen diffusion depends on moisture content

• Compound

• Complex– Variable– Multi-layered

waste

low density

high density

waste

moisture

Grey Eagle Tailings Cover

Trends in Cover Design

• Infiltration barrier design for wet climates (water barrier)• Evaporation enhancing designs for dry climates (water

balance/store and release)• Thick HDPE geomembrane covers to resist disruption

Typical comparison of alternative covers

A ) N o C o ve r

B ) M od erate Ve g etation C ove r

C ) G oo d Veg etation C o ve r

D ) L ow P erm ea bility Till S o il C o ve r

E ) H ig h P erm ea bility T ill S o il C o ve r

N o te : (“”) N u m b e rs are pe rce n t o f to ta lA n nu al precip ita tion

F a irG rass

G oodG rass

F a irG rass

F a irG rass

1 m

1 m

P rec ip ita tion E vapora tion R un o ff

S eepage

P rec ip ita tion E vapora tion R un o ff

R un o ff

S eepage

S eepage

Ta il ingsk= 6x10 cm /s-5

Ta il ingsk= 6x10 cm /s-5

Ta il ingsk= 6x10 cm /s

-5

P rec ip ita tion E vapora tion R un o ff

S eepage

k= 1x10 cm /s

T ill S o ilH igh P e rm eab ility

-4

Ta il ingsk= 6x10 cm /s

-5

Ta il ingsk= 6x10 cm /s

-5

P rec ip ita tion E vapora tion R un o ff

S eepage

k= 2x10 cm /s

T ill S o ilLow P e rm eab il ity

-6

Typical comparison of alternative covers

I) S o il B a rrier w ith S ynthe tic M em brane L iner,Lace ra l D ra in and S oil C over

H ) S o il B a rrier w ith La cera l D ra in and R ip rap C over

F ) R ip rap E rosion R esis tan t C ove r

G ) S oil B a rrie r w ith Lace ra l D ra in and S o il C ove r

0 .5 m

1 .0 m

0 .3 m

1 .0 m

P rec ip ita tion E vapo ra tion R un o ff

S eepage

P rec ip ita tion E vapo ra tion R un o ff

S eepage

La te ra l D ra inS lope0 .5%

Ta il ingsk= 6x10 cm /s

-5

P rec ip ita tion E vapo ra tion R un o ff

S eepage

k= 1x10 cm /s C ov e r S o il-4

P rec ip ita tion E vapo ra tion R un o ff

S eepage

k= 1x10 cm /s B a rrie r S o il-7L inea r

S lope0 .5%

0 .3 m

0 .6 m

1 .0 m

F a irG rass

S lope0 .5%

0 .3 m

1 .0 m

1 .0 m

F a irG rass

La te ra l D ra in

La te ra l D ra in

k= 5x10 cm /s-3

Ta il ingsk= 6x10 cm /s

-5

k= 1x10 cm /s R oc k/G rave l-2

k= 1x10 cm /s B a rrie r S o il-7

k= 5x10 cm /s-3

k= 1x10 cm /s C ov e r S o il-1

k= 1x10 cm /s B a rrie r S o il-7

k= 5x10 cm /s-3

Ta il ingsk= 6x10 cm /s

-5

Ta il ingsk= 6x10 cm /s

-5

k= 1x10 cm /s R oc k/G rave l-2

N o te : (“”) N um b ers are pe rcen t o f to ta lA nnual precip itat ion

mm

Dry Cover Costs

• Oxygen Control• Migration Control

• Simple Covers– Soil covers

• Complex Covers– Multiple soil– “Cefyl”– Clay layered– Geomembrane*

* Perpetual replacement @ 50 year intervals ~22% of present value hence long term fund $0.40-0.60/tonne

$2 - $6/m2

or $0.03-0.10/tonne waste rock

$20 - $30/m2

Or $0.33-0.50/tonne waste rock

Water Cover Costs

• New Rock Waste– In natural water bodies

• Haulage costs– In man-made impoundments

• $0.25 - $1.50 + haulage costs

• Old Rock Waste– With treatment of storage products– >$2.75 - $4.00/tonne

Suitability of Cover Types

Tailings Waste RockPotential Problems

Risk inDesign

Risk duringConstruction(slimes)

Long-termRisk ofDisruption

Risk inDesign

Risk duringConstruction(slopes)

Long-termRisk ofDisruption

Access

Slumping

Erosion Sheet

Channel

Consolidation

Biota

Infiltration

Oxygen Flux

High Risk Elements of Cover Performance

Minimizing Risk of Cover Failure1. Materials

– Maximize use of natural, durable materials– Minimize use of geosynthetics which deteriorate– Rip-rap control of erosion in dry climates

2. Climate• In dry climates:

– consider evaporative cover designs– increase use of rip-rap for erosion protection– provide for sedimentation control

• In wet climates:– consider infiltration cover designs– consider water covers

Minimizing Risk of Cover Failure

3. Biota– Beaver (beaver control or wide channels)– Burrowing animals (bio-intrusion layers)– Ants & insects (bio-intrusion layers)– Anthropogenic activity (institutional control)– Roots (control vegetation species)

4. Monitoring & Maintenance– Erosion– Settlement– Sedimentation (& Glaciation)– Biotic factors (fauna, flora, man)