2 Technology of closed-face tunnellingmachines

2.1 Machine types There are two basic types of pressurised closed-face tunnelling systems– slurry tunnelling machines (STMs) and earth pressure balancemachines (EPBMs).Slurry tunnelling machines were developed specifically for use in

cohesionless soils that contain little or no silt or clay. These machinesoperate with piped hydraulic spoil removal and the slurry is regeneratedin a separation plant that removes the excavated spoil from the slurry.Earth pressure balance machines were developed for use in weak

cohesive soils that are capable of filling completely the excavationchamber ahead of the pressure containing bulkhead or plenum inorder to provide positive pressure support to the tunnel face. Theground or soil is required to be plasticised during the excavation pro-cess so that it is capable of creating a plug or seal in the system’sspoil extrusion screw conveyor in order to maintain pressure supportof the face, resist ground water pressure, and discharge spoil at atmos-pheric pressure for loading into muck haulage skips or onto continuousconveyor haulage systems.Pure cohesionless soils and pure weak cohesive soils, however,

are rare. Consequently it is necessary to extend the application ofSTMs into cohesive soils and of EPBMs into cohesionless soils. Theresult is the necessity to provide a more extensive separation plantfor STMs in cohesive soils and for EPBMs the need to inject condition-ing agents such as chemical foams and polymers into the plenumand the screw conveyor housing to produce a more fluid/plasticisedspoil.Slurry tunnelling machines and EPBMs were developed initially in

Japan and Europe. In Japan, STMs were developed in the 1960s withEPBMs introduced in the mid to late 1970s. In Europe, STMs werein use in the 1970s with a similar gap of about 10 years before thefirst EPBMs were employed. Slurry tunnelling machines and EPBMshave seen numerous developments and improvements since their incep-tion and the modern machines, as typified by those used to excavate thetunnels of the Channel Tunnel Rail Link (CTRL) under east London,have continued the evolution of closed-face TBMs.This section describes recent developments of closed-face TBM

technology designed to improve the ability of the systems to controlground stability and operate safely.

2.1.1 Slurry machinesWith an STM, excavation is carried out by a rotating cutterhead fittedwith picks or disc cutters or a combination of both. The excavatedmaterial is mixed with the frictionless support fluid, usually a bentonitesuspension that is maintained at a predetermined positive pressure inthe plenum. This pressure reacts to any imbalances between thevolume of suspension supplied to the plenum and the amount ofsuspension combined with excavated material removed from it. Thesevolume variations are unavoidable and pressure is maintained eitherby controlling the volume differences with interlinked pumps andvalves or by the provision of either a compressed air reservoir or airbubble. The bentonite tunnelling machine developed in the early

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1970s in the UK (Bartlett et al., 1974) and the Hydroshield developed inGermany somewhat later (West, 1988) are examples of slurry machinetypes.There have been many developments and improvements to STMs

from their beginnings and modern STMs, such as the system used onthe CTRL to excavate the tunnels under the River Thames and thelarger-diameter machines in use in Europe, can excavate a considerablygreater range of soil types than before.

2.1.2 Earth pressure balance machinesExcavation using an EPBM is also by a rotating cutterhead fitted withpicks or disc cutters or a combination of both. The soil excavated fromthe face enters the plenum directly behind the cutterhead in a fluid/plasticised state having been mixed with varying proportions ofconditioning agents.The plasticised spoil is removed from the plenum via an Archime-

dean screw. While in the screw flights, the plasticised spoil forms aneffective ‘plug’ to ensure there is no loss of pressure in the plenum.Pressure is maintained on the face to balance the soil pressure by a

combination of propulsion thrust and removal of material at thecorrect rate to match the rate of advance. At the end of the Archi-medean screw there is a guillotine discharge gate that can be used inexceptionally fluid soils to assist in maintaining the correct earthpressure. In addition, some machines have been fitted with positive

Figure 2.1 Illustration ofan STM (slurry tunnellingmachine) system

Figure 2.2 Illustration ofan EPBM (earth pressurebalance machine) system

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displacement devices such as, for example, piston pumps, at the end ofthe screw to further control the discharge.

2.2 Cutterhead andcutters

Through experience, manufacturers have evolved cutterheads that havethe optimum open area in terms of the ability to give added mechanicalsupport to the ground at the tunnel face and to allow excavatedmaterial to flow through the cutterhead. Modern cutterheads arehighly abrasion-resistant leading to a reduced need for entry into thepressurised plenum for necessary maintenance.The ability of cutterheads to break up boulders and pass them

through the head has been greatly improved. This largely avoids theneed for entry to the pressurised plenum to break up or removeboulders. Cutter technology has also improved considerably. Allcutters are now back-loading which removes the need for personnelto go ahead of the cutterhead to undertake cutter changes. At leastone manufacturer is in the process of testing ‘intelligent’ disc cuttersthat give an early indication of wear. Wear indicators on picks arenow in common use.Copycutters, or cutters that can be moved in and out from the body

of the cutterhead to provide an overcut, can be useful in assisting thesteering of a TBM particularly where ground conditions are givingrise to problems. Modern copycutters are designed to overcut all orpart of the tunnel face. Copycutters are normally used as a last resortbut they can also act as gauge cutters in the event that the fittedgauge cutters become badly worn.A disadvantage of using a copycutter is the risk that the cutter

becomes jammed in the extended position. This risk, however, issmall as the copycutter is usually a very simple and robust item ofequipment.Figures 2.3, 2.4, 2.5 and 2.6 show cutterheads of the four types of

closed-face tunnelling machines used recently to excavate the ChannelTunnel Rail Link Section 2 tunnels.

2.3 Spoilconditioning

It is a fundamental requirement of EPBM operation to optimise theconditioning of the excavated material in the plenum. Modern practiceis to use surfactant foam and/or polymers. This greatly enhances theEPBM’s ability to maintain the correct pressure in the plenum andtransmit this pressure to the tunnel face. Its further effect is to improvethe ability of the spoil to flow from the plenum into and through the

Figure 2.3 KawasakiEPBM used on Contract220 of the CTRL Section 2tunnels

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Figure 2.4 Wirth EPBMused on Contract 240 ofthe CTRL Section 2tunnels

Figure 2.5 Lovat EPBMused on Contract 250 ofthe CTRL Section 2tunnels

Figure 2.6 HerrenknechtSTM used under the RiverThames on Contract 320of the CTRL Section 2tunnels

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screw conveyor. Together, these lead to greatly improved face stabilityand settlement control.Spoil conditioning also reduces cutterhead torque, wear of machine

components, and maintenance requirements.With STMs the traditional use of a bentonite slurry has been

improved by the addition of long chain polymers for more efficientapplication in certain soil types.

2.4 Dealing withboulders

Boulders dispersed randomly in the ground are difficult to identify withtraditional site investigation methods. Meeting boulders, of varyingsizes, in the path of a closed-face TBM tunnel heading is thereforeoften a surprise. As a result, it is important that TBMs are equippedwith the means to deal with unexpected as well as expected boulders.Earth pressure balance machines include the ability of the cutterheadto break down boulders to sizes suitable for passing through thescrew conveyor. On STMs the practice is to install powerful crushersat the intake of the out-bound slurry circulation pipe to reduce bouldersto fragments that can be pumped.In addition, the cutterhead should be capable of having disc cutters

fitted without the need for structural modification. There should be afacility to grade boulder fragments and screen their passage throughthe cutterhead openings. This can be achieved by practical design ofthe cutterhead openings and the addition of grizzly bars across theopenings. In the case of an EPBM this presents the screw conveyorwith small enough pieces for transportation and on STM systemswith pieces that will fit into the crusher.

2.5 Screw conveyordesign

Nowadays screw conveyors in EPBMs are usually of the Archimedeantype. Conditioning agents can be injected to reduce screw torque, how-ever care has to be taken that, in conditioning the spoil in the screw, thevital pressure-maintaining ‘plug’ is not destroyed. Screw conveyorsnormally enter the plenum at the invert of the pressure bulkhead.This aids pressure control and helps to clear the plenum of excavatedmaterial for required maintenance and repairs. Protection againstscrew wear has been improved but a screw used without a good spoilconditioning agent will wear rapidly in abrasive soils.

2.6 TBM articulation Various methods of articulation have evolved to improve alignmentcontrol. These include articulated cutterheads and active or passivearticulation of the shield body.

2.7 Seal systems The design, technology and manufacture of seals that protect the mainbearings on which the cutterheads of TBMs are assembled and drivenhas improved significantly in recent years and particularly to keeppace with increasing machine and cutterhead diameters and higheroperating pressures.

2.8 Tail seals The introduction of grease-fed wire brush tail seals, in sets of eitherthree or four, has reduced the risk of an in-rush of water and materialat the trailing end of STM and EPB TBMs. This is one of the moresignificant improvements in terms of ground control. For TBMssubjected to high hydrostatic heads an emergency inflatable seal canbe fitted if considered necessary.

2.9 Control systems Computers are at the heart of the control system on modern closed-faceTBMs. They provide the logic that is used to manage the differentfunctions of the machines and the interlocking facilities that prevent

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TBM functions activating if the parameters are not correct. Through aninterface they also receive many of the control instructions and displaythe information used to operate the machines. They are likely to belinked to computers on the surface that duplicate the display in theTBM operator’s cabin. They can also be linked to remote locations,such as the offices of the TBM manufacturer or the head offices ofthe contractor or the client.The underlying data streams can be analysed in a variety of locations

to monitor the performance of the machine, predict maintenancerequirements, provide data for early warnings of impending problems,and allow later back-analysis of any problems. A very large number ofcritical machine functions are likely to be monitored in real time andrecorded in a tabular format against time to give the raw informationthat can be examined in detail or presented graphically as an aid tomanagement or investigation. All critical data should be automaticallybacked up at the TBM and elsewhere.The list of functions that can and should be recorded may run into

hundreds. Every hydraulic pressure, every electrical circuit and everymoving part can generate data if fitted with a sensor.Often the raw data is processed to provide management information.

As such it is important that all data is produced and stored in its mostbasic form, as well as in processed forms to provide useful managementreports.Machine functions that can be reported graphically include cutter-

head thrust, torque and rotation, total power demand, annular groutvolumes and pressures, rates of advance etc. For an STM, monitoreddata will also include flows and pressures in the slurry circulationsystem and the operation of the separation plant. On EPBMs addi-tional monitored data would include spoil pressure, screw conveyorpressure, discharge gate opening, spoil quantity removal, operationof the belt conveyors that transfer spoil to the haulage loading station,injection of water and conditioning agent etc.

2.10 Improvedgeneral reliability

It is estimated that more than 3000 closed-face TBMs have beenmanufactured and used in the world up to 2005 and that growth overthe years up to 2005 has been exponential as the applicability of thesystems has expanded and urban soft-ground tunnelling has increased.This has resulted in considerable improvement in the quality and relia-bility of each new machine and an increase in the ability of each newsystem to maintain ground stability and improve operational safety.It has also expanded the pool of experienced personnel although thismust be maintained to continue to service this particular sector ofcivil engineering as the market grows.When excavating in unstable, water-bearing, granular and soft

cohesive materials, the best method for controlling ground stability isto use a closed-face TBM. When excavating in fissured rocks closed-face TBMs can be used as an effective method of controlling ground-water inflows.

2.11 Futuredevelopment

There has been considerable development and improvement of bothSTMs and EPBMs since their inception in the 1960s and 1970s. Mostrecent developments include a closed-face tunnelling machine thatuses compressed air pressure as well as earth pressure to support atunnel face. Such a machine system was used for construction of the1.3 km-long twin-bore 8.1m-internal diameter tunnel throughLondon Clay at Heathrow Airport in the UK (Sam et al., 2003). Atthe same time in Paris, France a dual-mode machine that can operate

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either as an EPBM or as an STM was used on the 10 km-long, 10.4m-internal diameter double-decked A86 highway tunnel through thechalk, clay and sands of the Seine River Basin (British TunnellingSociety, 2004). Development of closed-face systems to further refinethe advantages of these techniques can be expected with the ordering,manufacture and application of each new machine.As a result of this continuing technical development, it is important to

provide suitable training of both staff and operatives. This training andongoing development is necessary to ensure that there is a pool ofexperienced staff and labour available who understand the complexityof modern tunnelling machines and understand the requirements foroperating them to reduce the risks of instability and collapse.As will be seen throughout this document the management and

operation of a closed-face tunnelling machine is a task that requires ahighly skilled and educated staff and workforce – one that is motivatedand understands the requirements of the work.

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