21 - 26 April 2018 Dubai International Convention

& Exhibition Centre, UAE

ITA - AITES WORLDTUNNEL CONGRESS

ORAL PAPERPROCEEDINGS

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TBMTunnellingbelowtheSuezCanalinChallengingGroundConditionscontainingMethaneGas

DrAhmedFouda1,DerekWilliams2,AhmedSheta3,MamdouhElSayeed4,Dimitrios

Rizos5,TarekAmin6,DrKlausRieker7

1EngineeringAuthorityArmedForces,8EhsanAbdElkodous,Heliopolis,Cairo,ArabRepublicofEgyptemail:ahmed.fouda@eaafeg.com

2Systra,1191CornicheElNile,WorldTradeCentre,Cairo,ArabRepublicofEgyptemail:dwilliams@systra.com

3EngineeringAuthorityArmedForces,8EhsanAbdElkodous,Heliopolis,Cairo,ArabRepublicofEgyptemail:ahmed.sheta@eaafeg.com

4EngineeringAuthorityArmedForces,8EhsanAbdElkodous,Heliopolis,Cairo,ArabRepublicofEgyptemail:mamdouh.sayed@eaafeg.com

5OrascomConstruction,2005ACornicheElNil,NileCitySouthTower,Cairo,ArabRepublicofEgyptEmail:Dimitrios.Rizos@Orascom.com

6OrascomConstruction,2005ACornicheElNil,NileCitySouthTower,Cairo,ArabRepublicofEgyptEmail:Tarek.Amin@Orascom.com

7Wayss&FreytagIngenieuerbauAG,EschbornerLandstrasse130-132,FrankfurtamMain,Germanyemail:klaus.rieker@wf-ib.de

ABSTRACT

ThePort SaidRoadTunnelsProjectprovides a vital transport crossingof the SuezCanal, aspart of the Suez Canal DevelopmentMega-project in the North Sinai. The twin-tube, 2.8km longtunnelsareexcavatedbytwo,mix-ShieldTBM’swithaboreddiameterof13.05mandatdepthsupto 57m below ground level. Each tunnel tube affords a dual-carriageway road, an emergencyevacuation-way below the road-deck and the tunnel tubes are connected by cross-passages(excavated using ground-freezing). The TBM’swere launched east of the canal and driven to theretrievalshaftswestofthecanal.

Exploratory boreholes along the alignment of the tunnel encountered methane gas, insufficientquantitiesandpressuretocauseaminor ‘blow-out’.Suchpotentiallyexplosive,and/orflammablequantitiesofgas,clearlypresentedasignificantrisktothetunnellingworks.Methanegasis mostly associated with a water-bearing sand layer, which the TBM’s encountered forapproximately1700m(60%)ofthetunneldrives,includingthecriticalsectionbelowtheSuezCanal.

A surface drilling programmewas initiated to vent the gas prior to TBM arrival, alongwithgeophysicalsurveystoinvestigatepotentialgaspockets.AstheTBM’sweredesignedandbuiltpriorto thegasencountered,certainmodificationsweremadeto theTBM’s in-situ, tomitigate thegasrisk, along with special working procedures being adopted during tunnelling and cutterheadinterventions.Thegashazardwasassessedinaccordancewithinternationalcodesandstandards,toensuretheriskswereassessedandaddressedaccordingtointernationalbestpractice.

The soil conditions included a very soft clay layer, whose consolidation status wasundetermined, with possible significant implications for the long-term deformation of the tunneltubes. An extensive exploration programme was carried out to evaluate the clay, and alsoinvestigate the cutterhead clogging potential of this and a further clay layer, which could beencounteredbelowtheSuezCanal.Priortocommencementoftunnelling,modificationsweremadetotheTBMcutterheadstoreducetheriskofclogging.

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The requirement for hyperbaric interventions, potentially at pressures up to 6 bar, wasreduced by the construction of soil substitution safe-havens at selected points, allowinginterventions in free-air, or at much reduced hyperbaric pressure, including an interventionsuccessfullycarriedoutat60mdepthatahyperbaricpressureof2.8bar.Differentialmovementsofthe tunnel tubes, between the clay ‘free-field’ and the soil substitution blocks, required theconstructionoftransitionzonestomaintaintubedeformationwithinacceptablelimits.

OutsidetheCanalZone,thetunnellingwascarriedoutlargelybelowunoccupiedgroundwithlittle infrastructure. However on the West side, the tunnels passed below a railway line, majorhighway,gaspipelineandirrigationcanal–thelattertwowithTBMcoverlessthatonediameter.

TBMexcavationhasconcludedwithbothdrivessuccessfullycompletedinDecember2017andJanuary2018,withoutincident.

1. INTRODUCTION

The Port Said Tunnels Project comprises twin-tube, road tunnels crossing the Suez Canal with aplannedcapacityof2100mixedvehicles/hourineachdirection.Thedual-carriagewaytunnelsarepartoftheSuezCanalRegionDevelopmentPlan.

TheClient,theEngineeringAuthorityoftheEgyptianArmedForces(EAAF)appointedSystraastheirConsultant and the EPC Contract is executed by a Joint Venture of two local Contractors, ArabContractorsandOrascomConstructionIndustries.ProjectdesignisbySenersupportedbyAmberg.Wayss & Freytag Ingenieurbau AG of Germany is supporting the JV and providing technologytransfer to theMain Contractor in the operation of both TBM’s. The projectwas identified as ofstrategic importance by the Government, and was developed in a ‘fast-track’ process to a verydemandingtimeschedule.

Project Description: The project is located (Fig. 1) 20km South of the City of Port Said, near theNorthernentranceoftheSuezCanal.Thetunnelscomprisetwo,2.85kmtubes(withcross-passagesat1000mspacing),andcut-and-coverandU-sectiontrenchesconnecttotheat-graderoadways.

Figure1.ProjectLocation

Thetubesdeclineat3.3%toreachamaximumdepthof57mbelowgroundlevel,withaminimumclearanceof18mbelowthebaseoftheSuezCanal.Thetunnelsarespaced35mapartatthestartandendoftunnelling,reducingto17.4mwithincreasingcoverandimprovedgroundconditions,inordertoreducecrosspassagelength.

Excavation isexecutedby two,Herrenknecht,SlurryshieldTBMsof13.05mdiameter.The tunnelsare lined with 600mm thick concrete segments (8+Key) manufactured on site to give a finished

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internaldiameterof11.4m.Laterworks includethe installationofasegmented,pre-castconcreteroad-deckwithassociatedcivilandMEPfit-out,includingafixedfire-fighting(water-mist)system.

Figure2.TunnelTypicalCross-section

Twocrosspassages (foremergencyservicesaccess),willprovideapressurisedemergencyconduitbetweenthetwotubes.Thecrosspassagesareexcavatedusinggroundfreezingforsoilstabilisation.The tunnelevacuationconceptusesanevacuationpathbelow the road-deck (Fig.2), accessedbyemergencystairsat250mintervals.ThepermanentTunnelOperationalfacilitiesateachportalarelocated at-grade and include a security area, toll-booths and technical buildings, together withfacilities foroperationsandmaintenance staff and theemergency services.A single,mainControlBuildingislocatedontheWesternsidewithanauxiliarycontrolcentreattheEastside.

Figure3.TBMS959andtheLaunchingShafts

The TBMs were launched from the canal East side, as land was more readily available for siteinstallations, allowing time for West side land expropriation. Both Launch Shafts (Fig. 3) wereconstructed as 5 intersecting circular shafts (25m diameter, diaphragm wall support) to allowunhinderedmachineassemblyinfullconfiguration.TheReceptionShaftscomprisetwosimilarcellsforsequentialTBMdis-assemblyandremoval.Shaftsbreak-out/inaresecuredbysoilsubstitution(plasticconcrete)blocks.

The TBMswere launched in December 2016 and January 2017 and both driveswere successfullycompletedoneyearlater,inDecember2017andJanuary2018,withoutincident.Maximumadvancerates(Fig.4)of24m/daywereachieved,withaverageratesof7.5m/dayoverallforbothtubes.The

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TBM‘swereoperatedbyacombinationofex-patriateandlocalpersonnel,whichallowedsuccessfultransferoftunnellingtechnologytolocalContractors.

Figure4.TBM2–performancediagram

2. GEOLOGICAL–GEOTECHNICALCONDITIONS

Theproject is locatedintheEasternNileRiverDelta–alow-lying(+0.5–3.0mmamsl)plainunderlain by geologically recent, thick clay and sanddeposits. Threemain lithological layersweredefined,comprising(fromsurface):

• Clay Layer 1 (CL 1) – a highly plastic, very soft to medium stiff, silty-clay with an averagethicknessof42m(range34–45m).NsptValues<5upto20mdepthandgraduallyincreasingtotheSL2.Theupperpartoftheclay(10-15m)isverysoftwithveryhighplasticity(ConsistencyIndex0.2to0.5;Plasticity Index30 to98%).Below30mdepth, theclay ismediumstiffwithaConsistencyIndexbetween0.5-0.75.

• SandLayer2(SL2)–Verydense,finetomediumgrainedquartzsandofvaryingthickness(70m-120m).Thesand,comprising95%quartz,isabrasive.

• Clay Layer 3 (CL 3) – a hard, silty-clay present as layers and lenses (0.5 – 3m thick andexceptionallyupto11m)withintheSandLayer2.TheclayhasanaverageConsistencyIndexof1.0withaPlasticityIndexsimilartoCL1.

The geotechnical profile along the tunnel (Fig. 5) has a high degree of confidence based on 3campaigns of Soil Investigation boreholes, enhanced by information provided by the drillingexecutedformethanegasventing(seebelow).ItwasnotpossibletoconductanydrillingbelowtheSuezCanalduetorestrictedconditionsfromtheShippingFreightTrafficRules.Forthefirst0.8km,thetunnelsaredrivenintheCL1horizon,followedby1.15kmmainlyintheSL2,thena0.35kminmixedsoilconditions(CL3/SL2)andfinally0.55kmagaininCL1.TheTunnelsbelowtheSuezCanalwereexcavatedmostlyinSL2withoccasionalCL1andCL3atthecrownandinvertrespectively.

Groundwater:GroundwaterintheCL1isat+1.0maslontheEasternandslightlylowerat0.0mamslon theWestern side of the canal. A confined aquifer occurs in SL 2 with +1.0 m to +2.0 mamslground water head. Groundwater chemical analysis reveals very high aggressivity with very highsalinityof3.8%,sulphatecontentof0.4%andaChloridecontentvaryingfrom1to5%.

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Figure5.GeologicalProfile

3. METHANEGAS

InAprilandMay2016,duringdrillingworksonbothsidesoftheSuezCanal,boreholespenetratingintotheSL2sandlayerencounteredmethanegas,whichinoneinstanceignitedatsurface(Fig.6).The occurrence in BH 16 ofmethane in substantial quantities under pressure, implied the risk of‘gas-flooding’totheTBMandtunnelwithpotentiallydisastrousconsequences.

Gasmonitoringconfirmedthatmethanewaspresent,invaryingquantities,inallboreholes,onbothsidesoftheSuezCanal,whichcouldhaveasignificantimpactonthesafetyofthetunnelworks.

Figure6.MethaneGasFireatInvestigationBorehole.

3.1. ImplicationsfortheTunnellingandFurtherInvestigations

A Working Group, comprising all parties (including the Client and TBM manufacturer), wasestablished to evaluate the gas hazard and themitigationmeasure required. TheWorking Groupwastaskedwiththe:

• Reviewtheapplicablesafetycodesandstandardstoprovideaframeworkforclassifyingthegashazard,andthereforerecommendationstobeappliedfrominternationalbestpractice.

• Further investigations required to define the gas hazard, including an estimate of thecompositionandquantityofthegaspresent.

• Review of the requirements and options for TBM modifications and the tunnellingprocedures. As both TBM’s were being delivered to site at the time of methane gasdiscovery, clearly any TBMmodificationswould have to be carried out retrospectively atsite.

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3.2. ApplicableCodesandStandards

Various international codes and standards were reviewed to provide a framework of regulatoryguidanceforthemanagementofthemethanegashazard.Allpartiesagreedthatitwasessentialtofollow international best practice inmanaging the hazard. The following appropriate codes werecompared:

• SUVA (SwissAccident Insurance Institute)defined in ‘PreventionofAccidentsdue toFiresand Explosions during the Construction of Underground Works in Rock FormationsContainingNaturalGas’.TechnicalDataSheetdatedMarch2002.

• ATEXWorkerProtection(Use)Directive1999/92/ECasBSEN60079-10:2015,whichcoversthesafetyofworkersatriskfromexplosiveatmospheres.

• BS6164:2011,theBritishStandardforSafetyinTunnelling.

The abovedirectives all attempt to define thehazardor risk level and the actions to be taken tomitigatetheriskofanexplosionorfire,intermsofprevention(eitherofthegasentrytothetunneland/orsourcesofignition),monitoringandequipmentprotection.ThestudybytheWorkingGroupdetermined that no one Standard covered all the likely requirements for this particular tunnel –whileSUVAandBS6164arethemostdirectlyapplicabletotunnellingworks,theATEXDirectiveisparticularlyapplicabletotheTBMandtunnelelectricalequipment.Thegashazardwasassessedforeachdirectiveandtheresultsandrequiredmitigationmeasurescompared.

An initialandveryconservativeassessmentof thegashazardzoninggaveaSUVAClassificationofZone 3, based on the possibility of ‘gas flooding’. However, in parallel to this assessment, theWorking Group determined that the adoption of the suitable mitigation measures (listed below)would allow the hazard to be re-classified to Zone 1: i) Additional geophysical and boreholeinvestigations to locate ‘gas pockets’ and remove gas by surface borehole venting; ii) EnhancedventilationintheTBMandtunneltoprovideanairvelocityofminimum0.5m/sec.;iii)Theworkingchamber will generally be kept full of slurry; iv) Additional gas monitoring (fixed and portableinstruments) will be installed; v) Gas monitoring will be carried out either side of the TBMsubmerged wall prior to and during interventions; vi) Appropriate working procedures will beadopted;vii)ModificationswouldbemadetotheSlurryTreatmentPlant(STP).

Furthermore,while theabovemitigationwould allowa SUVAClassificationof Zone1, theClass 3requirements could be met by the mitigation and TBM modifications proposed. Implicit in themethods for the classification of hazardous areas, it is required that mitigation actions areundertaken, including the capability to operate the tunnel ventilation at all times. The WorkingGroup therefore determined that ventilation should be capable of operating in the presence ofmethane.

There are differences in the action levels between the SUVA and ATEX approaches; however, incombinationwith BS6164, itwas determined thatwith the actions anticipated and the gas levelslikelytooccur,thetunnelscouldbeexcavatedandoperatedsafely.

3.3. GasOriginandEstimatedQuantity

TheClientengagedPetrobel (fromtheMinistryofPetroleum)whose investigationsconcluded themethanewasoflocal,biogenicorigin(hencere-chargeofthegasisveryunlikely)andthemethaneisdissolved in the groundwater at amaximumof8.5% /m3,with any gas ‘pockets’ underpressurelikelytobeoflimitedextent.

3.4. GasVentingandGeophysicalInvestigations

Anearly and relatively easy to implementmitigationmeasurewas to vent the gas present in theground along the alignment, by drilling boreholes to below the tunnel invert, ahead of TBM

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excavation.Uponcompletion,theholeswerecappedwithavalveandthegaswasmonitoredonadaily basis. This provided a valuable indicator for the extent ofmethane in the Sand Layer 2 andprovidedausefulcomparativetoolforgasatelevatedquantities.

Petrobel selected Electrical Resistivity Tomography (ERT) to investigate the occurrence of gas‘pockets’,whichcouldalsobeusedtoinvestigatethesectionbelowtheSuezCanal.Anyareaswithpotentialforgaspocketswouldbethesubjectoftargetedboreholes,toventthegaspriortoTBMarrival. The surveyswere carriedout in sections and the results indicatednoobvious gaspocketsalongtheTBMalignment.

3.5. TBMModifications

The potential for methane gas entering into the atmosphere of the TBM and tunnel is partlymitigatedbythechoiceofTBM.Inthiscase,themix-shieldslurryTBM’saremorefavorable,asthegroundwater containing methane is evacuated through the TBM slurry circuit to the Slurry-TreatmentPlant(STP)onsurface.ModificationstotheTBMwerecarriedoutduringTBMstoppageatasoilsubstitutionblocksafe-havenSH6(intheCL1),wherethemethaneriskwaslower.

MethanecouldprincipallyenterthetunnelandSTPatmosphereby:

• Escapingfromtheexcavationchamber.

• Fromaroundthetailskin.

• Duringslurrypipeextensioninthetunnel.

• FromtheSamsonValveoutlet.

• AttheSTPwhenslurryexitstheslurrypipeintotheplant.

ThemodificationstotheTBMwereseenasacomponent intheriskmitigation(ofanexplosionorfireinthetunnel),andwerecomplimentarytoothermitigatingmeasures–theinvestigationsforgaspockets and venting gas by the drilling of boreholes and the adoption of the safe workingprocedures.

In the evaluation of the improvements required on the TBM, it was recognised that therequirements should be assessed holistically with all the mitigation measures to be adopted. InregardtotheTBMoperation,bymaintainingthefacesupportpressureabovethewaterandearthpressure, combinedwith keeping theexcavation chamber full of slurry, the evolutionofmethanegas that could enter the TBM and the tunnel would be limited. In addition, an industrial airconnectionwasinstalledforcontinuousflushingoftheairbubble/workingchamber.

3.5.1. SamsonValveOutlet

TheSamsonvalveoutletregulatesairbubbleover-pressureintheexcavationchamber.TheoutletisinstalledontheTBMis intheshieldareaandpresentedanobviousmeansofmethanegasescapeintotheTBM.ThevalveoutletwasextendedtotherearoftheTBMback-up,inabetter-ventilatedareaandagasmonitoringdeviceinstalled.

3.5.2. VentilationImprovements

The primary ventilation fans on surface were un-changed; however the 2.4m diameter flexibleductingwasreplacedbyanti-staticducting.

The existing secondary ventilation fan on the TBM back-up was enhanced by the addition of asecondlinefromtheprimaryventilationoutlet,feedinganadditionalducttothein-byeendoftheTBM.Thesizeofthesecondaryductswasalsoincreasedallowinganincreasedairvelocity.

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In a gas event requiring the shut-down of the secondary ventilation fan, an airflow of 0.3m/seccouldbemaintained in theTBMbycontinuedoperationof thesurface fanandtheadditional lineinstalled.

3.5.3. ElectricalEquipment

Modifications included the installation of explosion-proof systems to equipment at risk.However,for the Refuge Chamber on the TBM back-up, an over-pressurised box for the batteries (locatedoutsidetheChamber)wasinstalled,preventingtheingressofgasatatmosphericpressure.

Forotherelectricalcomponents(e.g.theErectorRemoteControlbox),proceduralsystemswereputinplacetoensureremovalofthoseelementsduringtunnelevacuation.

3.5.4. Multi-ServiceVehicles(MSVs)

Modificationsreducedthepossibilityofgasignitionbyspecialisolationoftheexhaustlinetoreducethetemperature,isolationofthebatteryandfittingasparkarrestor.Gasmonitoringwasinstalledtoprovideanalerttothedrivers.

3.5.5. SlurryTreatmentPlant(STP)

TheSTPwasanobvioussourcefortheescapeofmethane.Thenaturalventilationwasincreasedbyremovingside-panelstoachieveaconstantairflowthroughtheplantandeliminateenclosedareaswheregascouldaccumulate.GasmonitoringwascarriedoutbyfixedmonitorsandaGasInspectorwaspresentwhocarriedoutconstantgasmonitoringatdefinedlocationsintheplant.

3.6. SafeWorkingProcedures

Intrinsic to work in any tunnelling operation is the adoption of robust, safe working procedures,amplified in method statements and risk assessments, taking account of the unique risks andhazardspresentedbytheparticularoperation.Specialproceduresweredeveloped, intheeventofencounteringmethanegasrequiringtheevacuationofpersonnelfromthetunnel,shut-downoftheTBM,isolationofthepowersupplyandensuringsafeconditionsfortunnelre-entry.

Theearlydetectionofthepresenceofmethane(orotherharmfulgases)isvitaltothemitigationofthe risk. Fixed gasmonitoring deviceswere installed on the TBM and tunnel, at locations chosenaccording toa riskprofile, taking intoaccount thatmethane is lighter thanair andwould tend tomigratetothetunnelcrownorenclosedareasontheTBM.

3.6.1. GasInspector

AtrainedGasInspectorcarriedoutconstantandroutinemonitoringintheTBMandtunnel,inareasdefined as a location for gas inflow or accumulation, and at random. TheGas Inspector,workingcloselywiththeTBMShiftEngineerunderadefinedprotocol,couldsuspendanyactivityifgaswasdetectedinpotentiallyhazardousconcentrations.

3.6.2. TBMandTunnelCoreProcedure

TheselectionofthegaswarningdetectionlevelswasestablishedinaccordancewiththeSUVAandATEX guidelines and a core procedure developed for routine and emergency scenarios. The coreprocedurewasprominentlydisplayed in theTBMand tunneland featured in the tunnel inductionprocessforallemployees.

3.6.3. TunnelRe-entryProcedure

Theabilitytocontinueventilatingthetunnelandremotemonitoringatsurfaceofcertain,fixedgasmonitors(throughtheTBMIRISsystem)enabledaninformeddecisionofwhenacautiousre-entrytothetunnelcouldbecarriedout.

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TheRe-entryProcedurecomprisedre-entrybyagroup(includingtheGasInspector,ShiftManagerand Tunnel Foreman) equipped with self-rescuers, gas monitors and an anemometer so thatconstant gas and air velocitymonitoring could be carried out. Upon reaching the TBM, a carefulcheck formethanearoundall electrical switchesandmotorswouldbe carriedout.RestorationofthepowertotheTunnelandTBMwasonlyallowedifallreadingswerebelow1%andallpersonnelwereremovedfromthetunnelpriortopowerbeingrestored.

3.6.4. TBMMaintenanceInterventions

InregardtotheTBMcutterheadinterventions,theriskfrommethanegaswascarefullyevaluated.Interventions, under hyperbaric conditions or free-air at safe havens, in the possible presence ofmethane gas presented a particular risk. The risk mitigation approach was to reduce therequirementforthehigherriskhyperbaricinterventions,throughtheuseofsafehavens,combinedwith the venting of methane gas from boreholes, all contributed to controlling the risk to anacceptablelevel.

However, therequirementforhyperbaric interventionscouldnotbeeliminatedentirely, thereforespecific work procedures were developed. For man-entry the procedure required constant airflushingoftheworkingchamberandverifyingtheabsenceofmethanebymonitoringattheSamsonValveoutlet.Theflushingwasmaintainedatalltimesduringtheintervention.

Uponentrytotheman-lock,ahand-heldmonitorwithaprobewasusedtodetectpossiblemethaneand power to the working chamber was energised and man-entry allowed, only after theconfirmationoftheabsenceofmethane.

Gasmonitoringwascontinuedduringthefulldurationoftheinterventionwithreplacementdevicessuppliedasrequiredthroughthemateriallock.Anyincreaseinmethaneconcentrationwouldcausetheinterventiontobeaborted.Forthesafehavens,aSafeWorkingProcedurewasdevelopedwithagasmonitoringregimesimilartothatforhyperbaricinterventions.

4. GEOTECHNICALCHALLENGES–MITIGATIONMEASURES

ThegeotechnicalconditionspresentedsignificantrisksandchallengestoTBMdrivingandthetunneldesign, which together with tunneling below the important international waterway of the SuezCanal,combinedtogiveuniquechallengestothetunnelingteams.

4.1. VerySoftClay(CL1)

TheupperCL1claylayerisaverysoftclayfortheinitial12-15mbelowgroundlevel,withverylowmechanicalproperties,graduallybecomingstifferwithdepth.Thetunnelingchallengesincludedthefollowing.

4.2. ConsolidationStatusofClayCL1

The clay consolidation status (either under-consolidated, normally consolidated or over-consolidated) could impact significantly on the design concept, in terms of expected grounddeformation (creep) and consequently the behavior of the tunnel structure in the short,mediumandlongterms.

A thorough investigation program was initiated to verify the consolidation status, comprisinglaboratory testing and numerous in-situ investigations - CPTU 100% dissipation tests and theinstallationofalargenumberofVibratingWirePiezometers(VWP).Relativelyrecentdredginganddumping of material on the East bank of the Suez Canal was found to have influenced theconsolidationstatusoftheclay,andconcludedthattheCL1clayfor200mfromtheEasternBankisunder-consolidated. For the remainder of the drives, the clay was identified as normallyconsolidated.Fortunately,intheunder-consolidatedsectionadjacenttothecanal,thetunneltubesarewithintheSL2sandlayer,andthereforehasnodirectimpactonthetunnelstructure.

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4.3. DifferentialGroundMovementsinCL1

Amajorengineeringriskwasthetunnelstructureresponsetodifferentialmovementsbetweenthe‘hardpoints’ – the soil substitutionblocksof the intervention safehavens and launch / receptionshaftswithpanelsfoundedintheSL2sandlayer–andthefree-field,wherethetunneliseffectively“floating” in thesoftclay.Thehighstiffnesscontrastbetweenthe launchingshaft (founded in thedenserSL2sand),andtheverysoftclayhasasignificantimpactonthesegmentallining.

The expected heave of the tunnel lining (from uplift forces and the long-term dissipation of theexcessporepressuresgeneratedbythetunnelling)wouldbeapproximately60mm.Thismovementwould be detrimental to the segmental lining by distorting the tunnel tube in the longitudinaldirection, causing an expected gap opening between the rings > 10mm, which would adverselyimpact the liningwater tightnesswithsevereconsequences for thedurabilityandserviceabilityofthetubes.DrainingtheCL1couldconsequentiallyalsocauseuncontrolledsettlementsduetoclayconsolidation.

Tomitigatethis,atransitionzonecomprisingasystemofplastic-concretediaphragmwallpanelsandbarretteswasconstructed, tosmoothenthestiffnesscontrastandtunnel-soil founded in theSL2.Suchaconfigurationallowsasmootherdeformationandcurvatureofthesegmental liningtube inthelongitudinaldirection,whichdistributesthegapopeningovermorecircumferentialjoints.

Theresultsfromasensitivityanalysisshowedthemaindesignassumptionswereconservativeandresultedinupperlimitvaluesofheave.Figure7illustratestherecordedverticaldeformationoftheSouth tunnel tube,ofonly15mmheave,hasoccurredwithnoongoingdeformation for the last6months.Thelongertermbehaviorofthesoil-tunnelsystemwillbemonitoreduntilcommissioning,byusingthe3Dtargets,extensometersandvibratingwirepiezometers.Therequirementsforlongertermmonitoringwillbedeterminedbasedontheseresults.

Figure7.Tunnelheaveinthefreefieldinfrontofthestartingtransitionzone

4.4. TBMTiltinginCL1

ThesoftclaygavepotentialforTBMtilting,especiallysincethetunnelinvertatthelaunchingshaftwas just 22.5m below ground level, where the CL 1 clay is still very soft. The tilting risk wasexacerbatedasthetunneldriveslauncheddownagradientof3.3%.Thetransitionzonecontributedtothedistributionandsupportoftheweightoftheshieldandoutsidethiszone,itwasverifiedthatduringnormaloperationthesubmergedweightisverysmall,whileforunforeseenstoppages,tiltingcould be prevented if only 50% slurry drawdownwas allowed. The latter is also governed by the‘blow-out’checkverificationinthelowoverburdensections.

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4.5. TBMCutterheadCloggingPotential

ThecloggingpotentialwasinvestigatedforbothclaylayersCL1andCL3andtheresultsindicatedthatCL1hasgenerallylowConsistencyanddespiteitshighPlasticityIndex,lowcloggingpotential,howeverahighercloggingpotentialwasrecognisedforthedeeperpartofCL1.

ForCL3,alltestresultsindicatedahightoveryhighPlasticityIndex,whichcombinedwithastifftoverystiffConsistencygaveahighcloggingpotential.InsectionswhereCL3comprised>20%ofthetunnel face, the riskofmassivecloggingof thecutterheadandexcavationchamber is significantlyincreased.CL3was tobeencountered in thedeepest sectionsof the tunnel alignment, includingbelow the Suez Canal, where hyperbaric intervention pressures exceeded 6 bar, making manualcleaningaverytimeconsumingandriskierprocedure.

Given the potential for clogging, modifications were made on site, including the opening of thecutterhead(fromto31%to46%)andimprovingthefaceflushingwithadditionalnozzlesandhigherflushingcapacity.

4.6. TunnelDrivebelowtheSuezCanal

Amajor concernwas the geological uncertainty given the lack of intrusive site investigation databelowthecanal,which,alongwiththemethanegaspresence,theabrasivenessoftheSL2,andtherelativelylowcovertothecanalbottom,wereallconsideredcarefullyinthedesignandplanningofTBMoperations.

TominimizethepossibilityofTBMstopsunderthecanal,cutterheadmaintenancewascarriedoutatSH4(some70mbeforetheCanal)alongwithallcablesbeingextendedinadvance.

A recent bathymetric survey revealed a Canal bottom 2m deeper than anticipated during thefinalisationofthetunnelalignment,givingcoverbelowthecanalof18m.Thismeanttheindicatedmaximumallowed face support pressure is very close to theminimum required for face stability.CarefulcontrolofthefacepressurewasrequiredandspecialtrainingsessionswereheldtopreparetheTBMoperationteam.However,inevitablepressurespikesoccurredduringpipeextensionandtominimize these peaks, the slurry level was maintained at the man-lock access level, while theevacuatedslurrywasdischargeddirectlytotheslurrytreatmentplant(STP).

Duringtheshortstoppagesforringbuildingetc.,settlementofsandparticlescausedablockageoftheopeninginthesubmergedwall,resultinginslightpressurespikeswhenresumingexcavationastheslurrywassuppliedtothechamberbutthedischargelinewastemporarilyblocked.Tomitigatethis,priortoadvancingtheTBM,thecutterheadwasrotatedtomixthesettledsandparticlesandunblocktheopening.

ObservationsofthedischargeattheSTPshowedthatbelowthecanalbothTBMsexcavatedafacemainly inSL2,butwitheitherwithasmallportionofCL1 inthecrownorCL3at the invert.Theformer condition had potential for soil instability, but bymaintaining good bentonite quality andbeingpreparedtoinjectfreshslurryintotheshieldannulargapifnecessary,theriskwasmitigated.

BothTBMscrossedbelowthecanalwithoutanymajorincident.

4.7. HardpointsalongtheAlignment

The ground above the tunnels is largely undeveloped, being desert to the East of the Canal andmostlyagriculturallandtotheWest.TheWestsidehoweverhasfoursignificanthard-pointswhichare:thePortSaid–Ismailiasingle-trackrailway;themain4-lanehighway;anaturalgaspipeline;theAltina fresh-waterCanal. TheTBMs successfully traversedall thesepointswith settlementswithintheexpectedvalues.During thecrossings,additionalprecautionarymeasures included the railwayoperatorreducingthespeedoftrains,enhancedmonitoringofthegaspipelineandthepresenceofthepipelineowner,toallowswiftreactiontoadverseconditionsmitigatedtherisk.

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4.8. AggressiveGroundwaterandConcreteSegmentDurability

Thesalinegroundwaterpresenteddurabilityconcernsforthesingle-passconcretesegmentallining.ThedurabilityandconcretedesignmixwasanalyzedthoroughlytocomplywithbothBS8100andCS163-2008.

5. TBMINTERVENTIONS–SAFEHAVENS

A safe and efficient TBM operation is only ensured if regular inspections andmaintenance takesplace. Three stopswere planned (excluding the stop in the launch substitution block) at selectedlocationsformaintenanceofthecutterheadandbrushes.Ateachoftheseplannedstops,specialsoilsubstitution blocks were constructed from overlapping diaphragmwall panels. Thematerial usedwasalowstrength‘plasticconcrete’,ofstrength<7MPa.

Table1.TBMplannedinterventionsatSafeHavens

PlannedInterventions

Chainage GroundImprovement

SIze Scope

SAFEHAVEN6 23+222 Plastic concreteplug

14mTBMMaintenance andmodification formethane gas under atmosphericconditions

SAFEHAVEN4 22+330 Plastic concreteplug+dewatering

11.5 m and 24 mwatertightenclosure

TBMMaintenance70mbeforethecanalunderatmosphericconditions

SAFEHAVEN2 21+442 Plastic concreteplug+dewatering

4.6mTBMMaintenance after the canal withpressure of 2.9bars and controlleddewatering

ThefirststopatSH6,located350mfromthelaunchingshaft,waspriortoreachingtheabrasiveSL2andpotentiallystickyclay.TBMmodificationsforthemethanegasweremadeatthisstop,carriedoutunderatmosphericconditions.

The most critical and demanding maintenance stop was before the canal at SH4, where themachineswereinspectedthoroughlyandalltoolsreplaced.SH4islocatedatthelowestpointofthetunnelwithahydrostaticpressureupto6bar.Astherequiredmaintenancewasplannedinashortatimeaspossible,SH4wasdesignedtoallowaninterventioninfree-airbytheconstructionofaD-Wallcellaroundthesafehavenandde-wateringwellstolowerthegroundwaterlevel.

BothTBMinterventionswerecarriedoutsuccessfullyunderatmosphericpressure(Fig.8),despiteaslowerthanexpecteddewateringprocess, initiallyraisingconcernsoverthewater-tightnessofthesystem.

SH2islocatedattheWesternsideoftheCanalandisthefirstplannedstopformaintenanceafterthe 0.90 kmdrive throughmixed soil conditions and below the Canal. At SH2 the tunnels are inmixedfaceconditions (50-50CL1andSL2)andahydrostaticpressureof5bar.TheSH2block isonly4.6mandthestabilizingpluginfrontofthecutter-headis2.3m.Astheblockissmallandtheshield liesmostlywithinthemixedsoilformation,hyperbaric interventionswerecarriedoutunderanairpressureof2.9bar.Thispressurewaspermittedbyacontrolleddewateringsystemreducingtheconfinedwateraquiferheadby20m.

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Figure8.AtmosphericfreeairinterventionatSH4(~57mdepth)

6. CROSSPASSAGES–GROUNDFREEZING

Two Cross Passages (internal diameter 3.6m) are to be constructed (expected July 2018) byconventionalminingusinggroundfreezingforwatertightnessandgroundimprovement.

Laboratoryfreezingtestsshowedaverysensitivefrozensoilduetothehighsalinityandthevariableparticlesizedistribution.Testsshowedveryvariablestrengthandcreepparameters,andafreezingpointbelow-4oC.Duetothelowfrozensoilstrength,anincreasedfrozenringthicknessisrequiredto provide stability and control deformation of the ring. Themost critical area is adjacent to thetunneltubewherethedeformationmustbecontrolledtoavoiddamagetothefreezingpipes.Thefacewillalsobe frozen toenhancestabilityduringexcavation.Half-moonsteel frames,erected tosupport the tunnel segments,will allow free trafficmovement in the tunnel during cross passageconstruction.Thepermanentliningwillbeinstalledbeforethefreezingisturnedoff.

7. CONCLUSION

TBM excavation has concluded with both drives successfully completed in December 2017 andJanuary2018,without incident.The tunnels,at13.05mdiameter,areamong the largestdiametertunnelsundertakenintheMENAregion.

In regard to the potential methane gas hazard, excavation proceeded with carefully selectedmodificationstotheTBMandancilliaryequipment,andundertheplannedsafeworkngprocedures.In considering the gas hazard, an examination of the regulatory frameworks usually applicable totunnelling,indicatedthatnonewerewhollyappropriatetoaclosedfaceTBMinstallingasegmentallining in a ‘factory environment’. The risk analysis incorporated aspects from the three regulatoryframeworksoutlinedabove,andprovedtobethemosteffectiveinmanagingthehazardandgavetheconfidencetoundertakethetunnellinginasafeworkingenvironment.Onlyoccasionalelevatedgaswasencounteredinthetunnel,whichdidnotrequireevacuationofpersonnelandshutdowwnoftheTBM.

Theconstructionofpre-planned, soil substitutionblock ‘safehavens’, combinedwithde-watering,forTBMmaintenancestops,enabledmaintenancetobecarriedoutatatmospheric,orasignificantlyreduced hyperbaric intervention pressure. This contributed greatly to reducing un-plannedhyperbaric interventions, thereby allowing faster TBMprogress and reduced risk of high pressure(upto6bar)interventionsinthepresenceofmethanegas.

The risk of unacceptable tunnel structure deformation, in response to differential movementsbetween the soil substitution ‘plastic concrete’ blocks,and the very soft clays, was successfullymitigated by the use of ‘transition zones’ - soil substitution panels across the tunnel – whichsmoothed the stiffness contrast. Tunnel and ground monitoring has confirmed the tunneldeformationhasremainedwithintheacceptablelimits.

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The complex ground conditions, combined with the ‘fast-track’ nature of the project, (frompreliminarydesigntoconclusionofthedrivesinunder30months)presenteduniquechallengestoallthepartiesinvolved,whichweresuccessfullyovercomebyacollaborativeapproach.

8. REFERENCES

[1] Rizos, D., Williams, D., Amin, T., Aboudshiesh, M., Nicola, D., A., Foda, A., “TBM TunnellingundertheSuezCanal-PortSaidTunnelsinChallengingGroundConditions”,GeomechanicsandTunnelling1-2018(inprint).

[2] SUVA(SwissAccidentInsuranceInstitute)“PreventionofAccidentsduetoFiresandExplosionsduring the Construction of UndergroundWorks in Rock Formations Containing Natural Gas”.TechnicalDataSheet,March2002.

[3] ATEXWorkerProtection(Use)Directive1999/92/ECasBSEN60079-10:2015.

[4] BS6164:2011,BritishStandardforSafetyinTunnelling.

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