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journalTunnelling

The international journal for the tunnelling industry

www.tunnellingjournal.com April/May 2016

DIFFICULT GROUNDSOLUTIONS:HOW TO KEEP YOURTBM MOVING FORWARD

IN MANY ROCK PROJECTS, theoccurrence of squeezing ground, highinrushes of water, blocky rock, and otherchallenges is a real possibility. Today’shighly adaptable TBMs are capable oftackling these tough conditions usingcutting-edge technology coupled withmodern ground investigation methods.The story of how these methods

concentrated our viewpoint and came intopractice involves hardship and difficultground conditions that tested the mettleof a shielded hard rock TBM on a recentproject. What came out of thosechallenges is a new way to deal with bothpredicted and unforeseen groundconditions in rock and mixed groundtunnels.

Solutions informed by field experienceThe project in question began itsunderground excavation in 2012 nearAnkara, Turkey using a 10m diameterDouble Shield TBM. Known as the KargiHydroelectric Project, the initial 11.8kmlong tunnel was expected to be bored insofter yet self-supporting geology for thefirst 2.5km, giving way to competent rock

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ENSURING ADVANCEEven in difficult ground, new TBM solutions can carve a path tosuccess says Lok Home, Robbins President

The 10m diameterRobbins Double Shieldmachine used forTurkey’s KargiHydroelectric project.

that would not require a tunnel lining forthe rest of the drive. However crewsimmediately ran into much more difficultground than they had anticipated -everything from running ground with sandand clay to blocky rock and water bearingzones. After boring just 80m the TBMbecame stuck in a section of collapsedground that extended more than 10mabove the crown, loaded onto and aroundthe cutterhead. As a countermeasure thatwas immediately put into place to avoid thecutterhead becoming stuck in the blockymaterial, crews began boring half strokesand half resets. Even with these measures,the machine encountered a section ofextremely loose running ground with highclay content. A collapse occurred in front ofthe cutterhead and the cathedral effectresulted in a cavity forming that extendedmore than 10m above the crown of thetunnel. The cathedral effect can be difficultto detect and control as loose rock can seton the shield and above the cutterhead,and the cathedral void can occur metersabove this material. Injection ofpolyurethane resins via lances insertedthrough the cutter housings and muckbuckets was the method utilized forconsolidation operations; however,injection locations were restricted to theavailable openings and subsequentattempts to restart the cutterhead provedto be unsuccessful. A bypass tunnel wassuccessfully used to free the TBM; however,this represented only the first of a total ofseven bypass tunnels that would beexcavated in the first 2km of tunnelling.

It became apparent that much largermeasures would be needed to get throughthe difficult conditions. In order to improveprogress, the contractor, owner, consultantsand Robbins engineers worked together toformulate a plan to improve advance bymodifying the TBM for the now-knownground conditions. The contractor, with theassistance of the Robbins field service team,installed a custom-built canopy drill andpositioner to allow pipe tube support

installation through the forward shield.Drilled to a distance of up to 10m ahead ofthe cutterhead, 90mm diameter pipe tubesprovided extra support across the top 120to 140 degrees at the tunnel crown.Injection of resins and grout protectedagainst collapse at the crown whileexcavating through soft ground. As a resultof successful use of the probe drillingtechniques, the contractor was able tomeasure and back-fill cavity heights abovethe cutterhead in some fault zones to over30m and, in addition, was able to helpdetect loose soil seams and fractured rockahead of the face.

To further mitigate the effects ofsqueezing ground or collapses, custom-made gear reducers were ordered andretrofitted to the cutterhead motors. Theywere installed between the drive motor andthe primary two-stage planetary gearboxes.

During standard rock boring operations thegear reducers operated at a ratio of 1:1,offering no additional reduction andallowing the cutterhead to reach designspeeds for hard rock boring. When themachine encountered loose or squeezingground the reducers were engaged, whichresulted in a reduction in cutterhead speedbut the available torque was increased bynearly double. The net effect of themodifications was to allow the DoubleShield TBM to operate much like an EPB infault zones and squeezing ground withhigh torque and low RPM—these methodseffectively kept the machine from gettingstuck. In addition, short stroke thrust jackswere installed between the normal auxiliarythrust to double total thrust capabilities.

Ultimately, after in-tunnel modifications,the machine at Kargi was able to excavatethe ground very efficiently, even boring723m in one month - more than twice therate of a drill and blast heading proceedingfrom the opposite end of the tunnel. TheTBM broke through in July 2014 afterboring 7.8km total.

The lessons learned at Kargi are nowbeing applied in multiple ways. Robbins’latest generation of Crossover machineshas been improved by the experiences inTurkey - for instance, the successful canopydrill design from Kargi was pre-installed onthe TBM for Mexico’s Túnel EmisorPoniente II (TEP II), providing another ringfor grout drilling or forepoling close to thecutterhead. As used at Kargi, the canopydrill operates in the top 120 degrees of thetunnel, while a second probe/grout drill islocated further back on the machine,allowing two different patterns of holes.

High torque/breakout torque is anotherfeature added to the TEP II machine, so thattwo-speed gear boxes can be activated toachieve high torque at a low speed, similarto how an EPB operates. With two-speedgear boxes, the cutterhead can be freed inbad ground where it might otherwisebecome stuck. The TEP II operating crewhave been obliged to use this feature onseveral occasions; as at Kargi the first few100m of tunnelling contained collapsingground.

Pathway to success in difficult groundAfter the lessons learned at Kargi, it wasdecided to create a comprehensive systemthat could tackle the various conditionsfaced by shielded machines in hard rock.Known as Difficult Ground Solutions (DGS),the system consists of a set of integratedfeatures tailored to a specific project’sgeology. The main components of DGSallow for ground investigation ahead of theTBM, increased monitoring, and methodsto keep a machine shield from becomingstuck. The main components are listedbelow.

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Left: Crews at Kargiencountered everythingfrom squeezing groundand blocky rock torunning ground andcathedralling, requiringin-tunnel machinemodifications.

Below: The RobbinsCrossover machine forTúnel Emisor Poniente IIis equipped with drillsincluding a probe drilland forepole drill forground consolidation.

Multi-Speed Cutterhead DrivesAs determined at Kargi, customizedcutterhead drives can be instrumental ingetting through difficult ground even whenthe TBM is designed for boring in mostlyhard rock conditions. Designing a machinewith high-torque, continuous boringcapabilities allows that machine’scutterhead to restart with break-out torquein difficult ground. The net effect is that themachine can keep boring in the event of aface collapse and can effectively borethrough fault zones and running groundwhere the potential for cutterheadjamming exists. Going one step further,multi-speed gearboxes give the machinethe ideal EPB torque if larger sections ofsoft ground are anticipated.

Continuous Advance Shield DesignWhen blocky rock or squeezing ground isexpected, using a shielded TBM can betricky. The risk of a machine’s shieldbecoming stuck is real and can be thesource of major delays. ContinuousAdvance Shields involve designing theshortest possible shield length, withstepped shields if necessary (particularly if aDouble Shield TBM is used). Stepped ortapered shields involve each successiveshield having a slightly smaller diameterthan the last to accommodate for anysqueezing or ground convergence as theTBM excavates. Radial ports in the shieldscan be used for application of Bentonite toprovide lubrication between the shield andtunnel walls, again to avoid a stuckmachine. Should the machine becomestuck, there are additional solutions:hydraulic shield breakout can be used inover-break conditions. The radial ports canbe made to inject pressurized hydrauliclubricants to free a shield that has alreadybecome stuck. Lastly, additional thrust jacksbetween the normal thrust cylinders cansupply added thrust in a short stroke tobreak loose a stuck shield.

Convergence Measuring SystemAgain for use in squeezing or blockyground, this system utilizes a hydrauliccylinder mounted on top of the shield andconnected to the TBM’s PLC. It measuresthe shield gap in the tunnel crown, so thatif squeezing or collapsing ground isdetected the crew can take measures tolimit it. These measures include usingbentonite lubrication, crown or face rockconditioning, or planning ahead to useanother system in the area before themachine can become stuck.

Cutterhead Inspection CameraThe cutterhead inspection camera can beused to remotely inspect the boring cavitywithout intervention, and to check waterlevels ahead of the TBM. While these

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Above: A sealablemuck chute withclosure doors (seenhere in red) caneffectively seal off amachine from severewater inflows.

Left: Peripheralports located 360degrees around themachine shield (seenin red) can be usedto inject bentoniteand prevent ashielded TBM frombecoming stuck insqueezing groundconditions.

An example of cementitiousgrouting ahead of the TBMfrom a probe drill located in therear shield. Robbins TBMsequipped with DGS can bemounted with multiple probedrills at different locations.

Forepole drills and spiles (seen here in red) arean integral part of the DGS and provideenhanced ground consolidation capabilitiesaround the shield periphery.

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cameras have been used to monitor mixingchambers and perform cutterheadinspections in soft ground TBMs, their use inhard rock machines has been much morelimited. With DGS, the probe and injectionsholes in the cutterhead and front shield arespecifically designed to accept thesecameras.

Water Inrush ControlIn the event of a large inrush of water, aguillotine gate on the muck chute caneffectively seal off the muck chamber to keepthe crew safe as well as keep the machinefrom becoming flooded out. Additionalinflatable seals can seal the gap between thetelescopic shield and outer shields of a DoubleShield TBM to keep everything water tight.This system is termed “passive” waterprotection because the TBM is stopped inplace (not actively operating). During thattime the crew can then work to grout offwater inflows and dewater the chamber tocontrol the flow before they begin boringagain. The grouting crew also have the addedassistance of back pressure to assist ingrouting.

Improved Ground Detection andConsolidationOne of the key lessons of Kargi was that

more drill ports, and more types of drills, isa necessary component of shieldedtunnelling in difficult ground. Multipleprobe drills can now be installed on theTBM, with ports to provide probing patternsin a 360-degree radius. High-pressure groutinjection can be done through these sameports to stabilize ground up to 40m aheadof the face (or more if using specializeddrills). The type of grout injected can alsobe specialized - for example chemical orpolymer grout can be used to seal offgroundwater. Lastly, a rotary forepole drillcan be installed just behind the cutterheadsupport to allow for ground consolidationaround the shield periphery. The forepoledrill is of particular use in fractured rock andfault zones.

Applications & The FutureThe DGS system will be used on several TBMprojects, including New York City’s RondoutWest Bypass Tunnel. Scheduled to begin in2017, that project will use a 6.6m diameterSingle Shield TBM to excavate a 4km longtunnel below the Hudson River. Groundconditions are expected to consist of shalewith possible zones of intense water inflowup to 30bar. The machine will be designedto passively hold the potentially high waterpressure using a cutterhead drive sealing

system and backfill grouting through thetailskin. The machine is equipped with highthrust to get through challenging groundand sophisticated drilling and pre-groutingequipment for detection. Water-powered,high-pressure down-the-hole hammers arecapable of accurate drilling 60 to 100mahead of the TBM, while blow-outpreventers enable drilling at high pressuresup to 20 bar. This is an example of acustomized DGS system for expectedgeological difficulties.

Of course the overall goal of DGS is toavoid making these modifications in thetunnel - a process that can cause significantdowntime. The system is an assurance thatthe machine will be able to handle expectedand unexpected conditions, and ifconditions are known the system can bemade all the more accurate. Contractorsand owners must strive to provide accurategeological reports so that DGS can be at itsmost effective. Owners and contractorsshould give full consideration to building inthese features when difficult conditions area possibility. With these features, we areconfident that a shielded machine can keepadvancing, whether the concern is highcover mountainous tunneling withsqueezing and rock bursting, water inflows,fault zones, or all of the above.