Design considerations for geosynthetic clay liners (GCLs) in heap leach pad liner systems

Designconsiderationsforgeosynthetic clay liners (GCLs) ingeosyntheticclayliners(GCLs)inheapleachpadlinersystemsC. Athanassopoulos, PECETCO, USA

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M. Smith, PERRD International Corp, USA

September25,2013

Liner Applications in Heap LeachingGeomembraneshaveaproventrackrecord,butarevulnerabletopuncturedamage

LinerApplicationsinHeapLeaching

InHeapLeachPads,puncturesserveasopenpathwaysforlossofPregnantLeachSolution(PLS):

SmithandWelkner (1995)estimatedlinerleakageratesrangingfrom5to10,000L/ha/day

ThielandSmith(2003)reportedupto2,000L/ha/dayforonevalleyfillwithhighheads

Liner Applications in Heap LeachingLinerApplicationsinHeapLeachingToreduceleakagethroughdefectstheGMcanbeplacedoveralowpermeability soil as part of a composite liner system:permeabilitysoil,aspartofacomposite linersystem:Compactedclaywithk=106 cm/sor107 cm/s;ORGeosyntheticclayliners(GCLs),sodiumbentonitebasedlinerswithky y ( )=5x109 cm/s

Claycomponentwillplugdefectsorpuncturesinthegeomembrane,reducingoverallleakage

Low Permeability SubgradeLowPermeabilitySubgrade

Smallerwettedarea,limitsPLSfromspreadingoutl lllaterally.

High Permeability SubgradeHighPermeabilitySubgrade

Landfill Liner Leakage RatesLandfillLinerLeakageRates

Source: Bonaparte, Daniel, and Koerner. (2002) Assessment and Recommendations for Optimal Performance of Waste Containment Systems, EPA/600/R-02/099. USEPA, ORD, Cincinnati, OH

NOT like LandfillsNOTlikeLandfillsScaleofoperationsisawesome: Finalheapheightsashighas200mFinal heap heights as high as 00 m Resultinginenormousnormalstresses(upto3500kPa) Steepslopes1.4H:1V,approx.36

Combinationofhighnormalstresses,overstressingofGMnexttopipes,largeangularrockinoverlinerandsubgrade,hightemperatures,andaggressiveleachingsolutions

Pushesthelimitsoflinermaterials

Requiresspecialdesignconsiderations

HeapLeachPadTechnicalConsiderations

GCL Chemical Compatibility with

Considerations

GCLChemicalCompatibilitywithMineLiquids(includingpHeffects)

GeomembraneandGCLPuncturePerformance

GCLShearStrengthandSlopeStabilityStability

GCL Chemical CompatibilityASTMD6766,StandardTestMethodforE l ti f H d li P ti f

GCLChemicalCompatibility

EvaluationofHydraulicPropertiesofGCLsPermeatedwithPotentiallyIncompatibleFluids

ModifiedversionofD5887andD5084(shoppressandfixedwallpermeameter)

Termination criteria for chemicalTerminationcriteriaforchemicalequilibrium:E.C.In E.C.OutpHIn pHOut

GCL Permeability with Gold/Silver PLS

1E-7

GCLPermeabilitywithGold/SilverPLS)

GoldPLS NaCN = 600 ppm

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NaCN 600ppm pH=10to11 K

Compatibility with extremepH LiquidsCompatibilitywithextreme pHLiquids

BentonitecanhandleawidepHrange3 12

Strongacidattacksthel t it tclayorconvertsittononswellingform

MetalsaremoresolubleatlowpH

Jo H Y Katsumi K Benson C H and T Edil (2001) Hydraulic Conductivity and Swelling of NonprehydratedJo, H.Y., Katsumi, K., Benson, C.H., and T. Edil (2001), Hydraulic Conductivity and Swelling of NonprehydratedGCLs Permeated with Single-Species Salt Solutions, Journal of Geotechnical and Geoenvironmental Engineering, 127 (7): 557-567.

GCLPermeabilitywithCopperPLSy pp

pH=1.7p ElectricalConductivity=

37,000uS/cm Aluminum = 5,044 ppmAluminum 5,044ppm Calcium=262ppm Copper=800ppm

I 1 788 Iron=1,788ppm Magnesium=498ppm Zinc=198ppm

Source: Athanassopoulos et al (2009).

BentonitePolymer AlloysBentonitePolymerAlloysNewclaypolymernanocompositeswithimproved 1.00E-04

tolerancetolowpH,highionicstrengthsolutionshavebeendeveloped

1.00E-07

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RecenttestswithUraniummilltailingssolution(pH

HighLoadStaticPunctureTesting16staticpuncturetests,atloadsof:

Testing

1300kPa2580kPa3450 kPa3450kPa5170kPa

FS = 1 5 for ore heights of:FS 1.5fororeheightsof:45m90m120m180m

Source: Athanassopoulos et al (2009)

BenefitofGCLUnderneathGM(5172kPa,or180m)

GMalone8.5%typicalstrain25.7%peakstrain

3

GM/GCL1.6%typicalstrain3.0%peakstrain03punctures 0punctures

BenefitofGCLunderGM:Deformation MeasurementsDeformationMeasurements

GeomembraneDamageduringShearing

Constructionloading,oreplacement,seismicforces,orsettlementcancausesheardisplacement

TestingatUCSanDiegofoundGMis vulnerable to shearinducedisvulnerabletoshearinducedpuncturedamageathighnormalstressI l di GCL b t th IncludingaGCLbetweenthegeomembraneandthesubgradesoilcanessentiallyeliminatesuchd t l tdamage,evenatnormalstressesashighas4144kPa(150moforewithFS=1.5)

Athanassopoulos et al (2012). Shear-Induced Geomembrane Damage due to Gravel in the Underlying Compacted Clay Liner Underlying Compacted Clay Liner. GeoAmericas 2012.

DirectShearTestingofGeomembrane/GCL

SheardeviceatUniversityofCaliforniaSan DiegoCalifornia SanDiego

Testchamber=152x1067mmMaxnormalstress=4,144kPa

Doublenonwoven,needlepunchreinforcedGCL

testedagainstBlownfilmcoextrudedtexturedGM

Source: Athanassopoulos, Fox, and Thielmann (2012), EuroGeo5

Sand(Subgrade)

GCL HDPEGeomembrane

Gravel(Overliner)

GM/GCLInterfaceatUltrahighloads(4,145kPa)

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LDfrom2013study 200mmPeakfrom2009studyLDfrom2009study 75mm

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NormalStress,n (kPa)Thielmann et al (2013);Athanassopoulos et al (2009)

ApplicableStressRanges

Liner Systemf,2n 2

f,1n,2n,1

f,2

f,1c2,

c1,1n,2n,1

LowLDfrictionangleswilloccurunderthedeepestpartoftheheap.Therewillbemuchhigherf i i l i h i i l bili d h d h f ifrictionanglesinthecriticalstabilityzonetowardsthetoe,andthereforegreaterresistancetosliding.

BreitenbachandSwan(1999)( )

IncreasedShearStrength(~5degrees)duetoGMDimplingunderHighNormalStress

ImprovingHeapLeachPadStabilitySpeedBumps

Source:BreitenbachandAthanassopoulos(2013)

Source: Comanco

Example stability berm configuration from a heap leach pad liner projectExamplestabilitybermconfigurationfromaheapleachpadlinerproject.

ImprovingHeapLeachPadStabilityp g p yStairStepberms

Source:BreitenbachandAthanassopoulos(2013)

Source: Allan Breitenbach

Stair step pattern on back slope

Source:AllanBreitenbach

Dependingonlocalterrain,incorporatingnaturalundulationsofStairsteppatternonbackslope.sitesubgradeintogradingplanmayreducecost.

HLPFeasibilityStudy ExampleGM/compacted

soilGM/GCL

Leakage (L/ha/day) 1 929 203Leakage(L/ha/day) 1,929 203

CopperinPLS(ppm) 3,000 3,000

Copper lost due to leakage 2 113 222Copperlostduetoleakage(kg/ha/yr)

2,113 222

Copperprice(Sept2013)($/kg) $7.06 $7.06pp p p g

Costofcopperlost($/ha/yr) $14,910 $1,590

Gain in Revenue ($/ha/yr) $13,340GaininRevenue($/ha/yr) $13,340

BenefitCostRatio(PV,i =10%,n=15years,claycost=$4.30/m2,GCLcost=$5.90/m2) 6.3

GCLsinHeapLeachPads Summary ChemicalCompatibility.GCLsarecompatibilitywithCNleachsolutions.GCLpermeabilitycanbeaffectedbylowpHsulfuricacidleachsolutions;

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however,increasednormalstressmitigatestheseeffects.

PunctureProtection.GCLscancushiongeomembrane from stones in both the overlinergeomembranefromstonesinboththeoverlinerandsubgrade.

ShearStrengthandSlopeStability. GM/GCLinterfaceshavehighpeakstrengths,butg p g ,potentiallylowLDstrengths.However,incorporatingnonplanarfeaturesinthesubgrade(e.g.,speedbumps)canimprovestability,eveninworstcase conditions (7 degrees)worst caseconditions(7degrees)

FeasibilityStudy.ImprovedPLScontainmentprovidedbyGCLoverlifeofprojectwilloftenmorethanoffsettheinitialcostofGCL.

ThankyouQ ti ?Questions?C.Athanassopoulos,PECETCO,[email protected]

M.Smith,PERRDInternationalCorp,[email protected]