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ChoiceofTBMtypeformountaintunnellingunderverypoorgeologicalconditions:Hybrid,Slurry,Epb,Dsu,ConvertibleTbms

RemoGrandori1,AntonioDeBiase2,MarcoD’Ambrosio3

1PresidentSeliOverseasSpa,PiazzaF.DeLucia,Rome,Italy,r.grandori@selitunnelling.com2TechnicalDirectorSeliOverseasSpa,PiazzaF.DeLucia,Rome,Italy,a.debiase@selitunnelling.com3ChiefEngineerSeliOverseasSpa,PiazzaF.DeLucia,Rome,Italy,m.dambrosio@selitunnelling.com

ABSTRACT Rocktunnellingundermountains,highcoversandinverypoorgeologicalconditionsmightcausemajorproblemstotheadvanceofaTBM,especiallyinthemostcriticalconditions.Toimprove the capability of the TBMs to copewith the adverse conditions typical of theseapplications, the traditionalopengripper TBMsevolved toDouble Shield in the seventiesandtoDoubleShieldUniversalatbeginningofthe21stcentury.Inparallelthetechnologicalimprovements and innovations in soft and mix ground tunnelling of the EPB and SlurryTBMs in the last 30 years, extended the capability to control face instabilities andwaterinflowsunderawiderrangeofgeologies.Furthertotheabove,inanattempttocombineinone single TBM the capability toworkwith different operatingmodes, several hybrid orconvertible TBM types have been developed. Nowadays several different TBM types areavailable in the market and, understandably, the choice of the best type for a specificapplication became quite complicated. This choice is critical and does impacts the entiretunneldesign(typicalsection,support,lining,alignment,schedule)aswelltheprojectcostand construction risks. In an attempt to provide a basic guideline to select thebest TBMtypesforthedifferentpossibleapplicationsinmountaintunnelling,thispaperdescribesthemain design features of the different types of TBMs available on the market and theirpreferred field of application. The above based also on several recent applications undercriticalgeologicalconditions.

1. INTRODUCTIONTunnellingundermountainshasbeenthefieldofapplicationofthefirstTBMswhenthistechnologywasinitiatedmorethan60yearsago.

InitiallyTBMswereutilizedessentially toexcavate long tunnels ingood rockwith limited rocksupports required. For these application Open Grippers TBMs were the only type of machinesemployed.DoubleShieldTBMswereintroducedinthemarketintheearlyseventieswiththreemaintargetsandthechallengesthattheseapplicationmayposetodesignersandcontractors:(a)IncreasetheTBMperformancesintermofadvanceperdaythankstotheabilitytoerectasegmentalliningsimultaneouslywith theexcavationphase, (b)beable toadvance throughunstableanddisturbedrocks under the protection of the shields and (c) provide a finished tunnel by installing a precastliningastheTBMadvance.In2003SELIdevelopedthedesignoftheDoubleShieldUniversal(DSU)TBMasanevolutionoftheoriginalDoubleShieldTBMdesign.

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This new TBM design was targeted to improve: the TBM capability to advance throughsqueezing/converging rock formations under high cover, the telescopic joint design (allowing bysuchtooperatethemachineindoubleshieldmodeinveryweakrock),thecapabilityoftheTBMtoinvestigateandtreatthegroundaroundandaheadofthetunnelface.

Inthelatestyears,theapplicationofnewTBMtypeshaveappearedonthemarketandstartedbeing employed inmountain tunneling projects. Sometimes they are an enhanced version of the“classic”types,inothercasestheyborrowsignificantfeaturestypicalofthesoftsoilTBMs.

2. THECHALLENGESOFMOUNTAINTUNNELLING

Inmountaintunnelingthefollowingphenomenamightoccur:seismicity&asymmetricstresses,hardand/orabrasiverock,highwater inflow,wedgeinstabilityandrockfall,caving,squeezing,karsism,spallingandrockburst,presenceofharmfulgasesandgeothermalphenomena.Inadditiontothat,itisnecessarytodealwiththedifficultiesofeffectivegeologyreviewresultingfromtheproblematicprediction of the geology at depth,with few boreholeswith core recovering and often not deepenoughoratproperlocationandwiththeusuallackofmanyrockoutcrops.

The reasons above make rock tunneling under mountains, especially for long tunnels wherewide rangeof conditionswithvery severecriticalities canoccur, anactivity requiringnotonly therighttechnology,butalsoknowhowandexperienceinthisspecificfield.

ThispaperfocusesontheTBMchoiceinrelationtothecasesofpoorrockconditionsandlargewaterinflows.

3. TBMTYPES

Ingeneral,typicalhardrockTBMsworkin“openexcavationmode”attheatmosphericpressureandwithoutanybalancingpressure(soil/earth/slurry)tostabilizethefrontofexcavationwhileEPBandSlurry TBMs work in “close excavation mode” applying a balancing pressure to the front ofexcavationinordertostabilizethefrontofexcavation,limitingalsotheeffectsonthehydraulicrockmass conditions. Theexcavation in “openmode” is commonly carriedoutbyextracting the spoilfromtheCutterhead/ExcavationChamberthroughamuckconveyorwhiletheexcavationin“closemode”iscarriedoutbycontrollingtheextractionofspoilfromtheExcavationChamberwithscrewconveyorand/or througha slurrypumping system (thecontrolledvariabilityof the flowextractedfromtheExcavationChamberallowstheregulationofthebalancingpressure).

Inbetweenthetwomaintypes,thereareseveraltypeofmachinesabletoworkeitherin“openmode” or in “close mode” and according to the TBM manufacturer they are designated withdifferent names (Convertible, Dual Mode, Hybrid, Crossover, Variable Density, etc.), that oftengenerate confusion in the market. To approach the mountain tunnelling, in the past, wereimplementedspecialdesignfeaturesandspecialdevicesonthetypicalhardrockmachines(abletoexcavatein“openmode”only).Overthelastfewyearsinfact,thetrendofthemanufacturersistodevelop,alsoformountaintunnelling,TBMsforexcavationin“closemode”orabletoworkinbothways.

TBM types hereby considered are: Open Gripper, Single Shield, Double Shield, EPB, Slurry,Hybrid,Convertible.

3.1. OpenGripperTBMs

OpenGripperTBMs,allowfastadvancementwheninmediumtohighrockstrengthsandthereforeareanidealsolutionforunlinedtunnelsinstablerockformations.TBMentrapmentriskinsqueezingrockisverylowsincethereisnoclosedshieldandovercuttingispossible.Incaseoffracturedrock

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conditions, immediate rock support measures are installed behind the Cutterhead (rock bolts &nails,steelmesh,steelarches),oftenthroughafingershieldthatoffersprotectionforthepersonnel,while shotcrete is usually applied in the Back Up area (within a distance from the front facedependingontherockmassstability).

On the contrary, in case of soft and unstable rock, gripping operation becomes difficult andsafety for the personnel is reduced unless pretreatment is performed. Additional investigationthrough probe drilling, water drainage and pre-grouting treatment of the rock mass shall beactivatedfromthemachine.Incomingwatershallbedrainedandpumpedawayfromthemachinewheretherockmasspermeabilityisnotenoughreducedthroughpre-treatment.

3.2. SingleShieldTBMsOpenMode

Single Shield TBMs (SS TBMs) have the simplest possible configuration among the shieldedmachines.TBMobtainsthenecessarythrusttopressthecutterheadonthetunnelfacebypushingagainsttheprecast lining, installedinthetailshield inalternatephasewiththeexcavation.Duetotheirsimplicity,suchTBMshaveusuallyadvantagesfromtheoperationalandmaintenancepointofview,butincaseofsqueezingandconvergence,groundoverboringcapabilityislimited.Inpresenceof unstable front face blocking the cutterhead rotation, they have reduced/ no capacity to it pullback from the face to start again boring (after stabilization of the excavation front) and theiradvance rate in good/hard rock is limitedby the longer advance cycle (ring-build afterexcavationwithout overlapping). On the contrary, they allow quicker and more efficient probe drillinginvestigation anddrilling for rockmass consolidation in the rockmass through the shield and thefront.

A special SS TBM has been recently delivered by Robbins for Rondout West Branch BypassTunnel, where fractured and faulted rock mass with high water inflows at high pressures areexpected.TheTBM isdesigned tohandlehighwater inflowsatpressureover20barbymeansofextensiveprobingaheadandpre-excavationgroutingequipmentandisequippedwithacontinuousandanemergencypumpingsystemfor50l/sand150l/srespectively.ThisTBMismeanttobesealedquicklyintheeventofasuddeninrushofwaterandisprovidedwithanemergencyinflatablesealingsystem to protect the main bearing, a solution becoming always more frequent in projects withpotentialhighpressurewater.Basedon theexpectedwater inflowsandon thegeotechnical rockmassproperties,patternsforprobeholesandforpre-excavationgroutingwillbeapplied.

Figure1.DetailofSealableRobbinsTBMforRondout Figure2.–TypicalProbe/Groutholepattern

3.3. DoubleShieldTBM

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DoubleShieldTBMs(DSTBMs)canbeoperatedbothinDoubleshieldmodeformaximumadvanceratesand inSingleShieldmodewhengripping isnotpossible.DSTBMshave someadvantages inrespect of SS TBMs when boring under very critical rock conditions. Namely: can advance fasterwhengripping;telescopicshieldcanbeopenedorclosedtoreleasematerialpressureontheshields;forwardshieldcanbemovedbackward in caseof collapsingmaterialat the front toavoidcutter-headblockageandproceedwithrockmasstreatment;activedrivingsystemallowsbettersteeringcontrolinmixedfaceconditionsandveryweakrockmass.

ThedesignofthesemachineshasdevelopedinthedirectionoftheUniversalDSTBMtype,asinthe case of Abdalajis Tunnel East (Spain),where SELIwas actively involved in the construction aspartneroftheDragados-Tecsa-Seli-JagerJointVenture.The10mdiameterDSUMitshubishiTBMwasdesignedwithspecialfeatures:totalTBMlengthintherangeof11m, i.e.equaltothelengthofasingle shield TBM of same diameter; high conicity to allow the TBM advance also in squeezingground; new telescopic articulation design to eliminate the problem of packing the joint in looseground;overcuttingfacilitiesto increasethegapbetweentherockandthesegments insqueezingground.

A Universal DS TBM manufactured by SELI was employed in the challenging KishangangaHydroelectricProjectinIndiaintheHimalayanMountainsfacingexceptionalsqueezingconditionsaswell as heavy loadson the segmental liningdue topotential highwater pressure. The15km longTBM tunnel under high overburden (>1500m) was excavated by SELI using a 5,2m diameterUniversalDoubleShieldTBMwithexceptionalperformances.

3.4. EPBTBMs TheEPBTBMareSingleShieldTBMswherethefrontofexcavationissustainedintotheexcavationchamberbyapplyingabalancingpressurewiththesameconditionedexcavatedmaterial.EPBshavebeenoriginallyintroducedinthetunnellingindustrytoexcavateincohesivesoils.

Figure5.TypicalarrangementofEPBTBM

Figure 3. DS TBM 1-front shield; 2-Cutterhead; 3-gripper pad; 4-auxiliarythrustcylinders;5-segmentlining

Figure 4. Kishanganga project SELI DSTBM

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AnEPBtypeofTBM,ifproperlydesigned,willhaveadvantagesonopentypeTBMs(gripper,SS

andDSTBMs),whenboringthroughPoortoVeryPoorRockmassconditions(rockIV-VofBieniawskiRock Mass Classification), and in dealing with unstable faces in presence of low ground waterpressures.Ontheotherside,whenfacinghighlyunstablefacesassociatedtohighpressurewatersinflows,anEPBTBMwillbeun-capabletoadvancewithoutthepriorexecutionofpre-treatmentsofthegroundandingeneralwillbelessflexiblethanaDSTBMintheveryextremeconditions.

WhenboringinstablerockanEPBTBMcanoperateinopenmode,althoughitsperformanceislimitedbythelowercutterheadrevolutionspeedandespeciallyscrewconveyorwearisquitehigh.Whenboringinclosemodethroughunstableandweakrock,byutilizingfoamsandotheradditives,aproperlydesignedEPBTBMcanapplyapressure to the face, stabilize itandcontrol thegroundwater up to a pressure of 3-4 bars (depending on the capacity to create a “paste”with very lowpermeability inorder todissipate thepressure along the screw conveyor).Above thesepressuresthe EPB operation become very problematic.Water and fines flows through the screw in an un-controlled manner (without dissipating pressure). Consequently inside the excavation chamberbouldersandlargerchipsofrockstendstoaccumulateandblockthecutterheadrotationandshieldadvance.

EPBTBMshavea long single shield and their capacityofovercutting is limited. Their shield ismoreeasilystuckinconvergentrockthantheonesofDSTBMs.

Experience say that in hard rock conditions, the operational principle of the EPB cannot bereached in certain hydrogeological conditions due to the follows: 1)Not effective counterbalancepressureintheexcavationchamber/Cutterheadtosustainthefrontofexcavation;2)Impossibilitytoobtainareductionofthewaterpressurefromthefrontofexcavationtothedischargingpointofthescrewconveyorandconsequentlytosealthewateroutofthetunnel.

3.5. SlurryShieldTBMsOriginallydevelopedfornoncohesivesoils,inthelatestyearsapplicationofslurryshieldTBMshasbeenwidely expanded. These TBM type, use a bentonite suspension to pressurize the excavationchamberand stabilize the front.As inEPBTBMs, cutterheadcan rotatebothways.Cuttingwheelwith lowopeningratiodegreeshallbepreferredastheyallowahighernumberofcuttingtoolstoadvancefasterinhardrock.Acloseslurrycircuitisemployedasspoilremovalsystemandpumpstheexcavatedmaterialoutofthetunneltoaslurrytreatmentplantthatprovidestheseparationoftheexcavated material from the slurry mixture. Conventional Slurry TBMs and Mixshield (orHydroshield) TBMs, which use an automatic controlled air bubble to control the pressure), canexcavate in close mode in heterogeneous formation with water inflow at pressure higher than15bar. At low water pressure, they can manage potentially high groundwater inflow with nosignificanteffectsontheproduction.Furthermore,thesameslurrypumpingsystemcanbeusedforwater evacuation while bentonite lubrication has a cooling effect on cutters consumption. Theclosedmuckremovalsystem,ensurethecleanlinessof thetunnel, thedustabsence,and improvethesafetyincaseoftunnelingingassyareas.

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Figure6.MixedShieldTBM1--Cutterhead;2-ExcavationChamber;3-PressureBulkhead;4-Feed

Line;5-AirBubble;6-SubmergedWall;7-Segment;8-ErectorSlurryTBMsincludingtheauxiliaryequipmentrequireanoverallhigherinvestmentthanEPBs,a

large area available for the slurry treatment on site, and high power consumption. In case notreatment of the ground is applied, precast lining shall be dimensioned to bear the loadsaccordingly.

3.6. HybridTBMsHybrid TBMs include features that are typical of more than one conventional TBM type. In thischapterwillbebrieflyanalyzedthecaseoftheVishnugadPipalkotiTBM(hybridbetweenaDSandanEPBTBM)andthecaseoftheArroweldtunnelprojectTBMs(hybridbetweenaSSTBMandanEPB).BothareopentypeTBMsthatcannotexcavateinclosemodebutina“semi-EPB”mode.

TheTBMforVishnugadPipalkotiProject(India),designedjointlybySELIOverseaswithTerratecandMitsubishi,isprovidedwithanewtelescopicshieldsuitableforTBMoperationinDSmodeeveninunstableandravellingground,withveryshortshields forcontainingtotalTBMlengthsimilartotheoneofaSSTBM,conicalshielddesign,extendedovercuttingcapacity,possibilitytodrill/groutandinsertpilestopre-treat/stabilizethegroundattheface,veryhighmainandauxiliarythrust,veryhigh EPB like cutterhead torque capacity, capacity towork in SSmodewhen the required by thegeologywithasoilconditioningsystemandisprovidedofahighpressurewaterspraylineinstalledinthecenterofthecutterheadtoavoidmaterialclogging inthiscriticalarea. Inadditiona“flooddoor”systemhasbeeninstalledonthebulkheadoftheTBM.Thissystemregulatestheflowofthemuck&watertotheconveyorandeventuallysealsouttheexcavationchamber,simulatingalmostanEPB advancemode; it will be utilized when boring in special ground conditions to limit the faceinstabilityandpreventthefloodingoftheTBMandofthetunnel.

Figure7.MuckflowcontroldoorsinVishnugadTBMFigure8.ArroweheadTBMscheme

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In Arrowhead Project in San Bernardino (USA), difficult hard rock ground conditions with apotential for high groundwater inflows, highhydrostatic pressures and fractured and faulted rockmass were expected. Two Hard Rock SS TBMmanufactured by Herrenkencht withmucking by ascrew conveyor and specially equipped extensive pre-excavation grouting facilities have beenemployed. In order to cope with the heterogeneous ground conditions, such TBMs have beenoperatedinopenmodemuckingoutbyascrewconveyororinstandstillmodeapplyinganextensivegroundtreatment inverypoorrockconditions.Cutterheadrotationforexcavationwaspossible inonedirectiononly.Cutterheadwasclosedtypeandequippedwithcutters.Highwater inflowhavebeenmanagedbyadedicatedpumpingsystem.

3.7. ConvertibleTBMs

Convertible / Dual Modes TBMs are machines that can switch from one mode of excavation toanotherone.WewillrefertoconvertibleTBMswhenthemodificationrequiresthereplacementofsomemajorcomponent,whiletoDualModeTBMswhentherearenomajormodificationsrequiredastheyarealreadyprovidedwithfeatures“readytouse”of2ormoreTBMtypeandarerequiredmainly changes on the configuration and loading system of the cutterhead. Switching mode inConvertibleTBMstakesusually1-2weeks,whileittakes2-3daysinDualMode.

While convertible are more appropriated when two different rock mass sections are clearlyidentifiedandtheyarelocatedideallyatthestartorattheendofthedrive,withdualmodeTBMsrocksectionsareusuallyexpectedtochangemorefrequently.

AmongtheexperienceswithConvertibleTBMsare includedconversions fromDStoEPB–seeBelesHeadracetunnelSELITBMwherethemuckconveyorcouldbereplacedwithascrewconveyorand was provided with torque capacity activated by a simple modification – or EppenbergHerrenknencht TBM where conversion from SS TBM to Slurry type was requiring also thereplacementofthecontinuoustunnelbeltconveyortoaslurrymucksystemandtheinstallationofaSlurryTreatmentPlantoutofthetunnel.

An interesting case is the one of Lake Mead project in Las Vegas, where the dual modeHerrenknechtTBMwasdesignedtooperate inhardrockwithpotentialhighwater inflowunderamaximum hydrostatic pressure of 17 bar. The TBMwas a Single Shield TBM able toworkwith ahorizontalscrewconveyor inopenmodeorbymeansofaSlurrycircuit inclosedmode. Althoughnot initially expected, the TBM has been used mainly in closed mode due to the very criticalconditionencountered.

Figure10.LakeMeadinOpenMode(Leftside)andinClosedMode(Rightside)

4. INFLUENCEOF TUNNELDIAMETERONTHE SELECTIONOF THETYPEOF TBM,OF ITSDESIGNANDPERFORMANCES

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While, ingeneral, thehydro-geologyand thegeotechnicalparametersof the rock formationsplaythe most important rule for the selection of the most suitable type of TBM, another importantparameterdrivingtheTBMselectionisthediameterofthetunnel.

Diameter has an important influence on the selection of the TBM. When along the tunnelalignment,theprevailingrockmassqualityispoortoverypooralsowithexpectedhighamountofinrushwater,theselectionmovestoaTBMworkinginclosemode(orabletoworkinDualMode).

When the prevailing rock mass quality is good to fair and stable then for small to mediumdiameterTBM(4to7meters)inrocktunnelinginheterogeneousground,usuallyDSTBMsshallbepreferreddue their flexibility asmentioned in theparagraph6. SS TBMs for suchdiametermightsufferofthreeissues:

a. Cutterheadcannotberetractedfromthefrontincaseofcollapsesandsoitismoredifficulttorestarttheexcavationifcuttingwheelisblocked.ASSofsmall/mediumdiameterrequiresusuallya frontarticulationforbeingabletoberetracted,butthis featureresults in longershields similar toDSTBMs.Furthermore, retraction is limitedwhencompared to than theoneofaDSTBM.

b. Secondly, lack of accesses to the rock (in DS TBMs it is possible through the telescopicshields)donotallowinterventiontostabilize,treatordischargetherockaroundtheshieldsincaseofissues.

c. Furthermore, with SS TBMs is not possible to implement high conicity to the shields andtherefore thesemachines aremore subject toShield entrapment. This happensespeciallyfor small to medium diameters, where ratio TBM length/ Cutterhead diameter, is verysignificant.

Therefore,DSTBMformedium-smalldiameteraregenerallymoreflexible.Forlargediametersinstead,usuallyispreferredaSSTBMtoaDSTBM.Therearemanypositive

experiences on theAlpswith SS TBMsof largediameter (9 to 12m). This is due to the chance torealizeaCutterhead that canbemovedverticallyandhorizontallywith respect to the shield.ThispermitstounblocktheCutterheadincaseofunstablefront,tobetterdrivetheTBMincaseofmixedfront/low characteristic, and toprovide someovercuttingmoving theCutterhead vertically to thetopreducing the lowconicityshields issues.Furthermoreonsuchdiameters theadvantagesofDSTBMsintermsofrocktreatmentarelimitedsinceitisdifficulttotreattherockthroughtheshields.TheenhancedsimplicityintheDesignandfunctioningoftheSSTBMsonthesediametersmakethismachineeasierandthereforereducestheminorproductivityduetoalternateexcavationandlininginstallation.Inaddition,theassembly,startinganddisassemblytimeconsumingofasingleshieldarelower than the ones of a DS TBM. In terms of costs, the SS TBMs, being simpler, require a lessinvestment.

Often, even when the general rock mass quality does not strictly requires close mode TBM,diametersunderthe4mtendtocompromisethecharacteristicsoftheEPBTBMs,andcentralscrewconveyors are generally adopted. For Closemode application, the use of central screw is often acompromise that does not allow a proper EPB operation and will cause segregation muck, withbouldersandlargerchipsaccumulatinginthelowerportionoftheexcavationchamber.Accessibilityto the front is also often reduced requiring longer procedure and additional safety devices andprocedures to access to front of excavation formaintenancepurpose andother intervention intoandfromtheCutterhead.

Incaseofneed,evacuationof thewatercanapplyonly throughthescrewconveyorsincethedrainageholesprovidedforinthelowerhalfofthefrontshieldwillbeclogged,almostimmediately,by muck debris and stuck mud. This will make the mucking out and the control of water veryproblematicwhena lowpermeable “paste” cannotbe created into theCutterheadandalong thescrewconveyor.ThequitelongFrontShieldandthusthelongdistanceofthefrontarticulationfrom

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the face, may create steering problems in soft rock/ground due to the heavy Front Shield andCutterhead. Long shieldswill be stuck in rapidly convergent ground formation, the unidirectionalrotationoftheCutterheadwillmakedifficulttocontrolthetorqueandshieldroll

For small diameters (up to 5-6m), Dual Mode TBMs also are affected by the limited spaces,requiringcompromisesonthetechnicalchoices.Thiscouldresult inproblematicoperationinbothmodes,problemofaccessibility to the face, steeringandtorquecontrolproblems,waterdrainageproblems,longstandbytimestochangeconfiguration,lowadvancerateeveninnormalconditions.Theabovewithoutprovidinganyrealadvantageintermofcapacitytofacetheextremeconditionsexpectedalongthistunnel.

For Diameter above 6m, Dual Mode TBM applications are more suitable when, clearly, thegeologypresentsheterogeneous lithology.Withinsuchcases,thetunnelalignmentdevelopsalonghard rock formation stretches and very poor rock formation or loose soil stretches with similarextensions.

More frequents inmountain tunneling are the situationswhere,within hard rock formations,onlylimitedstretcheshavehightectonicstresseswithfaults.Inthesecases,hardrockTBMs(DSorSS for higher Diameters) are in any case preferable mitigating the risks of fault zones crossingthroughasystematicapplicationofaheadinvestigationandpre-excavationgroutingapplication.

5. INTERACTIONWITHDIFFERENTTYPEOFTBMSANDOPERATIONALMODEInteraction between TBM and geology is heavily depending on the features of the machines,especiallyfortheHybridandConvertibleduetotheirspecificvariablenature.Inthefollowingtablehowever we try to assign a general rating for the analyzed TBM types interacting with differentgroundconditionsinmountaintunneling.

OPEN

GRIPPERTBM

SINGLESHIELDTBM

DOUBLESHIELDTBM

EPBTBM

SLURRYTBM

HYBRIDTBM

CONV.TBM

EXCAVATIONMODES OPEN OPEN OPEN OPEN/CLOSE CLOSE SEMI

EPBOPEN/CLOSE

ADVANCERATEINVERYHARDROCK ++ 0 + -- - + ASPER

MODE

ADVANCERATEINGOODROCK + + ++ - 0 + ASPER

MODE

ADVANCERATEINPOORROCK -- 0 0 + ++ 0 ASPER

MODE

FINALMUCKDISPOSALCOST ++ ++ ++ - + + ASPER

MODE

EQUIPMENTWEAR ++ ++ ++-

INOPENMODE

0 + ASPERMODE

FACEACCESSIBILITYFOR

MAINTEN./INTERVENT.++ + ++ 0 0 + ASPER

MODE

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HIGHGROUNDWATERPRESSUREANDFLOW -- - - - ++ + ASPER

MODE

UNSTABLEFACE-LOWWATERPRESSURE -- 0 0 ++ ++ + ASPER

MODE

UNSTABLEFACE-HIGHWATERPRESSURE -- -- -- -- ++ + ASPER

MODE

CONVERGENCYORSQUEEZINGGROUND ++ - + -- --

VARIABLEON

CASES

ASPERMODE

BOULDERS ++ ++ ++ -- - + ASPERMODE

Table1.InteractionofTBMtypeswithdifferentrockconditions

6. SPECIAL MEASURES TO COMPLEMENT TBMS CAPABILITY TO FACE ADVERSEGEOLOGIES

In order to facedifficult geologies, TBMs shall be capable to:A. Recognize in advance apotentialgeology which may cause a TBM disruption/stoppage; B. Pre-treat the Rock Mass in order toimprovethegeotechnicalandhydrologicalconditions;C.Drainhighwaterinflows.

Therefore,independentlyfromthetypeofTBM,themachineshallbeequippedwith:-Asystematicinvestigationwithprobedrill-A largenumberofholes intheshieldtoallowtheexecutionof"micropilesofconsolidation"

veryclosetogetherandeffectivewithlimitedinclinationforgroundtreatment- Additional holes in the shield with higher inclination (15°-20°) having the scope of water

drainage-Possibilitytoexecutedrill-holesalsothroughtheTBMCutterheadandwithinthetunnelcross

section-Groutplantfortheapplicationofpre-excavationgrouting-OptimizeddewateringsystemintegratedwiththeTBMandwithhighcapacitypumpsallalong

thetunnel.

7. ACTUALLIMITATIONSOFTHETBMTECHNOLOGYANDNEWDEVELOPMENTS

ThereisnoanyTBMkindthatcancopewithallthevariableandextremesevereconditionsofpoorrock/largewater inflow inmountain tunneling. The direction of TBM technology anyway is goingtowards always more hybrid, convertible and dual mode solution, although sometimes theunconditionedresearchoftheoreticaluniversalitytoallrockconditionsresultsinreducedsimplicity.havingmorefeaturestoenhancethetheoreticalflexibilityresultsinareallesseffectivesolutions.

ImprovementinthegeologicalforecastisessentialinordertoreducetheriskofTBMblockageor performance reduction. Steps in this direction have been taken, but require additionalimprovementsintermsofdetailsandreliability.

8. CONCLUSIONS

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TheselectionofthemostsuitableTBMformountaintunnelingshallbetheresultofalogicalprocessbased on geology review, risk analysis for the excavation, capability to investigate and treat theground,performance(includingtimeandcostforsetup,start,assemblyanddisassembly),qualityofthetunnelintermsoffinalliningandoperation.Oncetheselectionisdone,theTBMdesignshallbedeeplyevaluatedintheintheconcept,specificationanddetailsasthiswillmakeaprojectsuccessfulornotinconjunctionwiththecontractorexperience.TheTBMconceptanddetaildesigncannotbeleft to the TBM supplier only,who rarely has real direct experience inoperating TBMs indifficultgeologies,buttheSpecializedContractorshallcontributeandcooperatewiththeTBMmanufacturerproviding its experience to the TBM characteristics and features to overcome also the worstconditions.

9. CITATIONANDREFERENCESGutter,W.,Romualdi,P.,2003.NewDesignfora10mUniversalDoubleShieldTBMforlongrailwaytunnelsincriticalandvaryingconditions.InRETC2003Proceedings

Ariza,L.,Mullerova,M.,Palmer,M.,Swannel,N.,DeBiase,A.,2015.DesignoftheheadracetunnelsegmentalliningfortheKishangangaHydroelectricProject.InRETC2015Proceedings

TylerD.Sandell,JacekStypulkowskiDualMode,2015.“Crossover”TypeTunnelBoringMachines:AUnique Solution for Mixed Ground in the Middle East. In Robbins Company websitehttp://www.therobbinscompany.com/

PeterKenyon,2014.PushingTBMdesignlimitsunderLakeMead.InTunnelTalkwebsitehttps://www.tunneltalk.comDavidTerbovic,MartinoScialpi,2016.RondoutWestBranchBypassTunnel—TBMBoringinHardRockAgainstHighWaterPressureandHighWaterInflowsBeneaththeHudsonRiverinNewYork.Brian Fulcher, et al., 2007. Piercing the Mountain and Overcoming Difficult Ground and WaterConditionswithTwoHybridHardRockTBMs.InRETC2007Proceedings.

Picturesno.5-6-7fromHerrenknechtAGwebsite.http://www.herrenknecht.com