DanO’Connor,TeamLeaderTunnels,GHD

NightcapWTPOutletTunnelRehabilitateorReplace?DDO’Connor1 ,SRMacklin2

Overview• BackgroundonNightcapWPT

• HistoryandsettingoftheNightcapWPTOutletTunnel

• TheProblem

• Successcriteria

• Constraintsontheproject

• Solutiondevelopment

• Solutionoutcomes

Background

• Owner– RousCountyCouncil

• RousCountyCouncilistheregionalwatersupplyauthorityofNorthernRiversregionofNSW.

• NightcapWaterTreatmentPlantprovidesapproximately90%ofthewater.

• BulkwaterpassesthroughaDN 750cementlinedmildsteelpipe,locatedwithintheunlinedOutletTunnel.

Construction

• DesignedbyGHDinlate1930s.

• Originalconceptconsistedof:• ~25mhighdam• Waterintaketower• CastIronDN500(mm)watertransferpipe• 1200mmwidex1800mmhighunlinedtunnel• Massconcretetunnelplug• Tunnelventilationintake

• Anecdotallythisreflectstheschemethatwasconstructedandfirstimpoundedin1953

Upgrades

• Upgradeswerecarriedoutin1984:• Wideningofthetunnelto2200mmwide• InstallationofmildsteelDN750watertransferpipe

• Removalofrockfalldebris• Installationofsectionsofrocksupport• InstallationofintakeshaftforfutureWaterTreatmentPlant

• ConstructionofWaterTreatmentPlantin1992/3

• ConstructionofNewWaterIntakeFacilitiesearly2000’s

TheProblem

• 2013safetyconcernsraisedregardingleaksaroundthemassconcretetunnelplug.

• Inspectionofthetunnelconsideredthatriskstothetunnel,watertransferpipesandplugwerehigh.

GHDengagedin2014

• ADN150ventilationpipeofunknownmaterial/conditionthatpenetratestheplug.

SuccessCriteria

• Whatisasuccessfuloutcome?

• CriticalReview=Listening+Reflect

Twoseparablesuccesscriteria:

1. Isolationofthetunnelfromthereservoir;and

2. ImprovingtheriskratingoftheDN 750pipetotheyear2060andbeyond.

Constraints

1. Access– Limitedaccesstothepipelineandtunnelportal

2. Operation- Criticalnatureofflowsintheexistingpipe

3. WorkHealthandSafety• Underground,and• Confinedspace.

Access

1. Thetrackwaywasinpoorcondition

2. Necessarytoupgradetheaccesstrackforsignificantworksatthesouthernportal.

3. Crossedprivatelandwithariskthataccesscouldberestricted.

Operation

1. Anyoutageofgreaterthan24hrswouldbecatastrophictoRous

2. DN750pipeandthetunnelhadtoremaininalmostcontinualoperation

3. Surfacebypasspumpingexpensiveandwouldresultinapipethatwouldpassunder/overtheonlyentrancetothewatertreatmentplant.

WorkHealthandSafety

WorkHealthandSafetyAct(2011)requires:“personswhohaveadutytoensurehealthandsafetyto‘managerisks’byeliminatinghealthandsafetyriskssofarasisreasonablypracticable,andifitisnotreasonablypracticabletodoso,to

minimisethoseriskssofarasisreasonablypracticable”

1. Confinedspace

2. Unlinedrocktunnelthathadsufferedsignificantrockfallsintherecentpast.

ConfinedSpace

ModelConfinedSpace– CodeofPractice(2014)• Reliesonthe“selfassessmentofrisk”• Owner,Rous,andtheirofficershaveadutyofcaretoensureworksusetheappropriatemeasuretoeliminateorminimiserisks.Inparticular:

“shouldeliminatetheriskof engulfment”

Concernsincluded:1. Uncertaintieswithregardtothebulkhead2. Restrictedmeansofescapeduetothepipesinthe

invert.

Unsupportedground

Unsupportedgroundinexistingtunnels:• Therearenoregulations,AustralianStandardsorCodesofPractice

• Guidanceisgivenbysomerecentauthors:1. GilchristandLee,20142. Rosin,2005or3. RosinandBrown,2005

Thecombinationofrisksindicatedthatrehabilitationworkswithinthetunnelwouldpresentanunreasonablelevelofrisktopersonnelandthewatersupply

SolutionDevelopment

• Conventionalmethods• Barringdownandrockbolting• Groutingtunnel

• Earlyisolation• Permanentbypasspumping

• ConstructabilityReview• 16differentsolutions• MCAwitha30/40/30Split

betweenCost,ConstructionRiskandResidualRisk• ConfirmedbyECIprocessandcompetitivetender

SolutionOutcomes

• First:Permanentduplicationofthewatertransfersystem(pipejacking).

• Second:Newtunnelplugfromanadit(Conventional)

Pipejackiscomplete;Newtunnelplugduetobeginshortly.

ConcludingRemarks

• Thereisawiderangeoftechnologiesthatcanbeusedintherehabilitationandreplacementoftunnels.

• Assessmentofriskconstraintsisvitalforsuccessfuloutcomes

• Itisvitallyimportanttocriticallyreviewtheobjectivesoftheworkattheoutsettomakesureweare“solvingtherightproblem” This work was conducted in collaboration with

Michael McKenzie Rous County Council Mark Stalhut of the NSW Public Advisory.