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Mining and the Environment UBC Mining and the EnvironmentMining and the Environment UBCUBC

Appropriate Concepts andCriteria For the Design of

Cover Systemsby

G. Ward Wilson

Appropriate Concepts andCriteria For the Design of

Cover Systemsby

G. Ward Wilson


Key Components & Criteria● Continuum across soil - atmosphere

boundary● Infiltration - mass transfer● Evaporation - heat & mass transfer● Surface properties● Vegetation, Runoff & Erosion● Design Issues● Material Science - Waste as a

Resource


The Soil-Atmosphere ContinuumThe Soil-Atmosphere ContinuumThe Soil-Atmosphere Continuum

Unsaturated Soil

PrecipitationPrecipitation

EvapotranspirationEvapotranspiration∆∆

Saturated Soil

Capillary Fringe

Ground SurfaceDry Soil

Two Phase Soil


Atmospheric Forcing EventsAtmospheric Forcing EventsAtmospheric Forcing Events

EvaporationTranspirationExfiltration

Infiltration RainfallSnowmelt


Infiltration and RunoffInfiltration and Runoff and Runoff

Time (sec)

Infil

trat

ion

Cap

acity

(m

/sec

)

K s

Runoff

Infiltration dueto Ponding Rainfall Intensity

Quantity of Infiltration

δh = Cw δ (Kw δh)δt δy δy


Conventional Methods Availablefor theEvaluation of Evaporation

●Thornthwaite Method●Penman Method●Priestley - Taylor Method●Complimentary Relationship

These methods are based onclimatic conditions only


AE

/PE

(%)

100

0

SandSlow Drying

Clay

Sand Fast Drying

MoistureAvailabilityField

CapacityPermanentWilt Point


AE / PEAE / PE vs vs. Suction. Suction

101011 101033 101055

Suction (kPa)Suction (kPa)101022 101044 101066

00

0.50.5

11

1.51.5

22

AE

/ P

EA

E /

PE

Beaver Creek SandCustom SiltRegina Clay

Beaver Creek SandCustom SiltRegina Clay


Relative Humidity & Suction

0

25

50

100 102 104 106

Suction (kPa)

R.H

. (%

) 75

100

R H e

WRT

v

. .=Ψ


Stem = 500 kPaRoot = 200 kPa

Soil = ? kPa

Leaf =1500 kPaEvaporation

Cohesion(in xylem)

WaterUptake(fromthesoil)

Air

Air

Air

H2OH2O

Atmosphere = 100,000 kPa(50% r.h. at 22oC)


Soil Atmosphere Model

●Climatic Conditions +

●Surface Characteristics

Soil PropertiesGroundwater Conditions

Vegetation


An Oxygen and Water BarrierAn Oxygen and Water BarrierAn Oxygen and Water Barrier


RunoffRunoff

EvapotranspirationEvapotranspiration

Reactive Waste

Soil Reinforced Water Cover

..

MinimizeNet Infiltration

O2


Store/ReleaseStore/Release


∆ ∆

EvapotranspirationEvapotranspiration

Net InfiltrationNet Infiltration

StorageStorageθr 100%

Effective Saturation


Soil-Water Characteristic CurveSoil-Water Characteristic Curve

VolumetricWater Content High

Storage

Low Storage

Controls: i) Gradationii) Silt and Clay Contentiii) Density and Structure

Suction

High AEV

Low AEV


Kidston - IntroductionKidston - Introduction

Reclamation at the Kidstonmine site presents a specialcase since the tailings surfaceand final cover system aresubject to extreme wet and dryconditions.


Precipitation and Potential Evaporation

1000

800

600

400

200

0July‘90

Sept.‘90

Nov.‘90

Jan.‘91

Mar.‘91

May‘91

Date

Flu

x (m

m/m

onth

)

Cumulative P.E. = 1907 mmCumulative Precipitation = 1349 mm

Cumulative P.E. = 1907 mmCumulative Precipitation = 1349 mm

PE > ppt.PE > ppt.PE > ppt.


Zero Flux Cover DesignAlternatives Considered for Kidston

Vegetation

1.5 mnon-compacted

oxide cap0.5 m compacted

oxide cap

waste rock

● Storage SystemVegetation

2.5 mnon-compacted

oxide cap

waste rock

● Barrier/Stora ge System

Mining and the Environment UBC Mining and the EnvironmentMining and the Environment UBCUBCSuction (kPa)

Vol

umet

ricW

ater

Con

tent

(de

c.)

SWCC

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.1 1 1e1 1e2 1e3 1e4 1e5 1e6

Non-compactedOxideCap

CompactedOxideCap


Vol. w/c versus Depth

Volumetric Water Content

Dep

th (

m)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.00 0.10 0.20 0.30 0.40

Day 100Day 200Day 300

5 mwaste rock

2 m oxide cap

0.5 m compactedoxide cap


Net Infiltrative Flux

Simulation NumberNet

Flu

x F

rom

Bas

e of

Cov

eras

a P

erce

ntag

e of

Tot

al R

ainf

all

yr. 2

-1%

0%

1%

2%

3%

4%

5%

yr. 1

yr. 1

yr. 2

Barrier/StorageSystem Storage System


Appropriate Concepts for TheDesign of Soil Cover SystemsAppropriate Concepts for TheAppropriate Concepts for TheDesign of Soil Cover SystemsDesign of Soil Cover Systems

EcosystemEcosystem - Biosphere- Biosphere

Living Soil SystemLiving Soil System - Organic/mineral media- Organic/mineral media(A and B horizons)(A and B horizons) - Buffer/Protector- Buffer/Protector

Barrier SystemBarrier System - Engineered Properties- Engineered Properties(C horizon)(C horizon) - Permeability/flow- Permeability/flow

- Change characteristic- Change characteristic water storage and volume water storage and volume

Isolated SystemIsolated System - Mine Waste- Mine Waste


Material ScienceMaterial ScienceMaterial Material CharacteristicsCharacteristics

TypeType PermeabilityPermeability Storage Storage StabilityStability CompressibilityCompressibility

ClayClay very lowvery low very lowvery low very low very low very highvery high

SiltSilt moderatemoderate highhigh very low very low moderatemoderate

SandSand highhigh lowlow high high lowlow

GravelGravel very highvery high very lowvery low very high very high very lowvery low

Well GradedWell Graded moderatemoderate low to low to highhigh lowlow

MaterialMaterial highhigh


Equity Silver Case StudyEquity Silver Case StudyEquity Silver Till is a Proven Performer

1.) Dense Well Graded Material

2.) Low Hydraulic Conductivit y and a High AEV

3.) High Shear Strength

4.) Resistant to erosion

5.) Good Workability

6.) Cost Effective


0

10

20

30

40

50

60

70

80

90

100

0.010.1110100

Per

cent

Fin

er T

han

Grain Size - (mm)

KidstonWaste Rock

Alaskan Till

KidstonOxide

Equity Till

Kidd CreekTailings

Grain Size DistributionGrain Size Distribution


Vol

umet

ric W

ater

Con

tent

(%

)V

olum

etric

Wat

er C

onte

nt (

%)

Suction (kPa)Suction (kPa)

0.000.00

0.080.08

0.160.16

0.240.24

0.320.32

0.400.40

0.480.48

101000 101022 101044 101066

Kidd Creek Tailings

Equity Till

Kidston Waste Rock

Soil-Water Characteristic CurvesSoil-Water Characteristic Curves

Kidston Oxide


Zone ofNegativeWaterPressure

Precipitation

RunoffEvapotranspiration

CoverWaterTable

Tailings

Zone ofPositiveWaterPressure

Drainage


Water contentWater contentw%w%

Net normal stressNet normal stress((σσmeanmean - - u uaa) )

Matric suction Matric suction ((uuaa - - uuww))

Constitutive SurfaceConstitutive Surface


Summary and ConclusionsSummary and Conclusions

● Soil Atmosphere Approach For Design● Well Graded Materials are Best● Using Waste as a Resource

– cost– construction equipment– construction materials

● Benefits

● Soil Atmosphere Approach For Design● Well Graded Materials are Best● Using Waste as a Resource

– cost– construction equipment– construction materials

● Benefits

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