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TheUseofPondAshintheFillingofAbandonedCoalMinesUndertheIpswichMotorwayBillHolzADAAAPRIL1103PondashcoveredstockpileatthepastemixplantInllpasteowingdownoldroadwayphotofromremoteboreholevideocameraHigherviscositypasteusedtocreatebarrierwallsat89.9mbelowgroundlevelTheQueenslandDepartmentofTransportandMainRoadsDTMRcommittedtoupgradingtheIpswichMotorwaybetweenDinmoreandGoodnaD2G.Duringthepreliminarysiteinvestigationandthroughtheearlystagesofdesignabandonedcoalmineswerediscoveredundersectionsofthefootprintofthenewmotorway.Whilsttherewasknowledgeoftheexistenceoftheseminesthathadbeenoperationalfromtheearly19thCenturythroughuntil1987detailsastosizedepthandaccuratelocationwerelargelyunknown.TheDTMRgiventhecomplexityoftheprojectsrequirementsformedtheOriginAlliancetoconstructthemotorwayandtondsolutionstomitigatingthesubsidenceeffectsoftheoldcoalminesworkings.OriginAllianceformedaSubAllianceOriginKellerSubAllianceorOKSAwithKellerMineFillaspecialistminellcontractortotacklethetaskofaccuratelylocatingandtargetllingtheseminessoastoprovideprotectiontothemotorwayassetagainstafuturesubsidenceevent.TheMineFillingTeamadoptedtheuseofamixofpondashfromCSEnergysnearbySwanbankPowerStationincombinationwithGPcementcrusherdustandwatertoformapastetolltheminevoids.TheSwanbankpondashareadilyavailablesourceofapproximately7millioncubicmetresofashprovidedasupplyofpondashllertomakethepastemixmoreeconomical.Itiscriticalthatwastepondashcostremainnominalorthecostofllingofabandonedmineswillbecomesoprohibitiveastobenonviableeliminatingasubstantialreuseactivity.ThepondashwasminedandscreenedattheSwanbankPonds.Screeningbecamenecessaryduetothepresenceofrocksfromearlierroadbuildingactivitydiscoveredinsomesectionsoftheponds.Itisimportantthattheseobstructionsberemovedbecauseofthesubstantialdamagetheycausetothepaddlesinthemixchamberofthepastemixplant.Itwasalsoimportanttolimitthemoisturecontentoftheyashtokeepthematerialdensitylowandminimisetransportcosts.Alsoitwasimportanttoreducethepotentialforblockagesinthehopperbinsatthepastemixingplant.Tothisendtheuseofadrainedstockpileofpondashmaterialispreferred.RemediationofabandonedminesrequiredapprovalsfromtheQueenslandDepartmentofEnvironmentandResourceManagementDERMtheactivitybeingclassedasanEnvironmentallyRelevantActivityERA.Themineremediationtaskincludedtheinjectionofpastematerialintooodedminesduetothesensitiveenvironmentalissuesassociatedwithalteringthelevelofthewatertable.Waterwasthenpumpedfromtheminestoawatertreatmentfacilityduringpasteinjectionataratetomatchthepasteinjectionvolume.InthiswaytheERAinvolvednotonlyhandlingissuesrelatedtotheuseofthepondashbutalsothecontrolofwaterlevelstomaintainthematthenaturallevel.TheminewateralsohadtobetreatedwithaReverseOsmosiswatertreatmentfacilitysincetheminewaterisconsideredtobearegulatedwaste.Theapprovalsinvolvedstrictlimitationsonpondashtransportandstorageespeciallyinrelationtodustandrunoffmanagementfrompondashstockpiles.Thepondashstockpilewasmaintainedatthepurposebuiltpastemixplantlocatedadjacenttotheminesattheproposednewmotorway.Thestockpilewascoveredtoprotectagainsttheweatherandtolimitthepotentialfordustgenerationbyincorporatingasprinklersystemintotheroof.Inpracticetheroofandbinenclosurecombinedwiththeminedmoisturecontentofthepondashmostlyeliminateddustgenerationrequiringonlylimitedneedtoapplysprinklers.Becausepondashisclassedasaregulatedwastestrictconditionswereplacedonthepastemixdesignwithdesignchangesbeyonda10variationbyweighttoanyconstituentrequiringDERMapproval.MinelloperationscontinuedfrommidJuly2009throughtoNovember2010duringwhichtimeapproximately50000m3ofyashwasconsumed.Themotorwayisnowbeingconstructedsafeintheknowledgethatminesubsidenceriskshavebeeneliminated.BillHolzMineFillManagerKellerMineFill